CSWIP 3.

1U NDT Inspection Diver

DIS1

Training and Examination Services

Granta Park, Great Abington
Cambridge CB21 6AL
United Kingdom
Copyright © TWI Ltd
CSWIP 3.1U – NDT Inspection Diver

Contents
Section Subject

Preliminary pages

1 Engineering Offshore Structures

1.1 General background
1.2 Design specifications
1.3 Construction activity monitoring system
1.4 Guidance on design and construction
1.5 General design considerations
1.6 Pipelines
1.7 Offshore oil terminals
1.8 Future trends
2 Offshore Structures and Installations
2.1 Introduction
2.2 Steel production platforms
2.3 Terminology
2.4 Concrete and steel gravity platforms
2.5 Terminology associated with different offshore structures
2.6 Floating production storage and offloading units (FPSO)
2.7 Inspection of FPSO systems
3 Loading on Offshore Structures
3.1 General introduction
3.2 Properties of materials
3.3 Mechanisms of fracture
3.4 Stress concentration
3.5 Residual stresses
3.6 Forces on a structure
4 Deterioration of Offshore Steel Structures
4.1 General comments
4.2 Categories of deterioration and damage
4.3 Accidental damage
4.4 Corrosion
4.5 Fatigue
4.6 Wear
4.7 Embrittlement
4.8 Structural deterioration
4.9 Repairs to offshore structures
4.10 Repair inspection

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5 Deterioration of Offshore Concrete Structures
5.1 The nature of concrete
5.2 Aggregate
5.3 Binder (cement and water)
5.4 Additives
5.5 Loading on concrete
5.6 Types of concrete
5.7 Weight coat
5.8 Organic polymers
5.9 Concrete construction techniques
5.10 Other features of offshore concrete structures
5.11 In-service deterioration of concrete structures
5.12 Imperfections of concrete
5.13 Inspection of concrete structures
5.14 General concrete terms
6 Marine Growth
6.1 Types of marine growth
6.2 Factors affecting the rate of marine growth
7 Corrosion
7.1 Energy considerations in corrosion
7.2 The corrosion process
7.3 The anodic reaction
7.4 The cathodic reaction
7.5 Electrochemical aspects of corrosion
7.6 Electric theory
8 Types of Corrosion
8.1 Corrosion cells
8.2 Dissimilar metal corrosion cell (galvanic corrosion)
8.3 Concentration cell corrosion
8.4 Pitting
8.5 Intergranular corrosion
8.6 Grain boundary corrosion
8.7 Stress Corrosion Cracking (SCC)
8.8 Corrosion fatigue
8.9 Erosion corrosion
8.10 Fretting corrosion
8.11 Biological corrosion
8.12 Other factors affecting corrosion rates
8.13 Temperature
8.14 Water flow rate
8.15 The pH value of the water

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9 Corrosion Protection
9.1 Cathodic protection
9.2 Sacrificial anode method
9.3 Impressed current method
9.4 Practical considerations for installing ICCP systems
9.5 Using coatings to protect the structure
9.6 Use of inhibitors (controlling the electrolyte)
9.7 Corrosion protection by design
10 Corrosion Protection Monitoring
10.1 Inspection requirements
10.2 Cathode potential measurement
10.3 High purity zinc electrodes (ZRE)
10.4 CP readings utilising silver/silver-chloride (Ag/AgCl) electrodes
10.5 Current density measurements
10.6 Calibration procedures for hand-held CP meters
10.7 Necessary equipment
10.8 Procedure
10.9 Calibration of the meter
10.10 Operating procedures
10.11 Normal cathode potential readings against Ag/AgCI
11 Welding and Welding Defects
11.1 Joining metal components
11.2 Fabricating offshore structures
11.3 Welding processes
11.4 Flux-shielded arc welding
11.5 Metal inert or metal active gas welding (MIG/MAG) welding
11.6 Tungsten Inert Gas (TIG) welding
11.7 Submerged arc welding (SAW)
11.8 Types of welded joint
11.9 Types of weld
11.10 Welding metallurgy
11.11 Further considerations for weld control
11.12 Welding terms
11.13 Plate preparation terms
11.14 Terms defining weld features
11.15 Welding process terminology
11.16 Weld defect terminology
11.17 Cracks
11.18 Cavities
11.19 Solid inclusions
11.20 Lack of fusion and penetration
11.21 Imperfect shapes
11.22 Miscellaneous
11.23 Reporting defects in welds

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12 Photography
12.1 Light and photography
12.2 The camera
12.3 How digital cameras compare with conventional cameras
12.4 Bracketing – getting the exposure right
12.5 Focusing
12.6 The lens focal length
12.7 Depth of field
12.8 Framing the subject
12.9 Light and underwater photography
12.10 Artificial light for underwater photography
12.11 Close-up weld mosaic photography
12.12 Specific applications for offshore photography
12.13 Picture grabbers
12.14 Specific requirements for inspection photographs
12.15 Recording photographs and care of equipment
12.16 Procedure for close-up mosaic photography of a weld
13 The Use of Video in Offshore Inspection
13.1 Introduction
13.2 Advantages of video
13.3 Disadvantages of video
13.4 Videography systems
13.5 Video cameras
13.6 Video transmission standards
13.7 Video recording and storage
13.8 Ancillary video equipment
13.9 Deployment of underwater video
13.10 Preparation for deployment of underwater video
13.11 Practical techniques for underwater video inspection
13.12 Video commentary
13.13 Video pointer
13.14 Post-inspection
14 Ultrasonics
14.1 Physics of ultrasound
14.2 Frequency
14.3 Velocity
14.4 Types of ultrasonic waves
14.5 Wavelength
14.6 Further effects of ultrasonic properties in materials
14.7 Acoustic impedance (Z)
14.8 Attenuation
14.9 The direction of propagation of an ultrasonic wave

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14.10 Generating ultrasound
14.11 Types of transducers (probes)
14.12 Couplant
14.13 The sound beam
14.14 Principles of ultrasonic testing
14.15 Ultrasonic test systems
15 Inspection Methods Available to Inspect Underwater
Structures
15.1 Visual inspection
15.2 Videography
15.3 Photography
15.4 Ultrasonic techniques
15.5 Flooded member detection (FMD)
15.6 Crack detection techniques
15.7 Taking measurements underwater
15.8 Crack depth measurement
15.9 Cathodic potential (CP) measurement
15.10 Recording shapes and surface profiles
16 Inspection Maintenance and Repair, Quality Assurance
and Control, Recording and Reporting
16.1 Legislation relating to inspection of offshore structures
16.2 Structural integrity management of ageing installations
16.3 The importance of QA and QC
16.4 Databases and trend analysis
16.5 The importance of documentation and record keeping
16.6 Types of reporting systems
16.7 Reasons why inspection is required
16.8 Continuity of inspection
16.9 Design stage
16.10 The Box Matrix System
16.11 Clock orientation and datum points
16.12 Safety Critical Elements (SCE)
16.13 Production of the raw materials
16.14 Fabrication stage
16.15 Launching and installation
16.16 Damage survey
16.17 How the Criteria of Non-Conformance System is applied
16.18 Documentation in an anomaly-based reporting system
16.19 Verbal reporting
16.20 Corrosion protection and coating inspection report
requirements
16.21 Procedure for the close visual inspection of a weld
16.22 Summary of other recording methods used underwater
16.23 Certification of personnel and equipment
16.24 Inspection activities in an anomaly based system
16.25 Decommissioning

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17 Cleaning for Inspection and Profile Grinding
17.1 Cleaning
17.2 Diving Medical Advisory Committee (DMAC) advice
17.3 Standard of surface finish
17.4 Profile grinding

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Contents Copyright © TWI Ltd
 The Certification Scheme for
Personnel

Organisation and Examination

The Certification Scheme for Personnel (CSWIP)

CSWIP is an accreditation body approved by the UK government’s board of

trade and industry. CSWIP is a subsidiary of TWI certification, which is
incorporated into the welding Institute (TWI).

TWI
Is a world centre for materials joining technology and is the parent organisation
for TWI certification.

Company profile
TWI Ltd, the operating arm of The Welding Institute, is one of the world's
foremost independent research and technology organisations. Based at Great
Abington near Cambridge since 1946, TWI provides industry with engineering
solutions in structures, incorporating welding and associated technologies
(surfacing, coating, cutting, etc.) through:

 Information.
 Advice and technology transfer.
 Consultancy and project support.
 Contract R&D.
 Training and qualification.
 Personal membership.

Single source of expertise

TWI Ltd is the only single source of expertise in every aspect of joining
technology for engineering materials - metals, plastics, ceramics and
composites.

Non-profit company
TWI is a non-profit distributing company, limited by guarantee and owned by its
Members; it is, therefore, able to offer independent advice. It is internationally
renowned for bringing together multidisciplinary teams to implement
established or advanced joining technology or to solve problems arising at any
stage - from initial design, materials selection, production and quality
assurance, through to service performance and repair.

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 Global benefits
There are over 450 skilled staff dedicated to helping industry apply all forms of
joining technology safely and efficiently. Some 3200 companies and
organisations – representing virtually all sectors of manufacturing industry,
from over 60 countries around the globe – benefit from TWI services.

Confidential consultancy
TWI undertakes contract R&D in confidence for both industry and governments.
As a consultant, it can offer individual experts or teams, able to help solve
problems of all kinds related to materials joining. It will send its specialists
anywhere in the world, at short notice, on troubleshooting missions.

TWI Certification Ltd

A TWI Group company formed in 1993.

Certification Management Board

The body with overall responsibility for the activities of TWI Certification Ltd is
the Certification Management Board.

Professional Board of Certification Management Board

TWI (TWI Certification Ltd)

Membership, Education & Membership, Education & Registration Committee

Registration Committee

CSWIP Welding Specialists & Practitioners

Management Committee

CSWIP Plastics Welders Certification Management

Committee

Welding Fabricator Certification Management

Committee

Certification Scheme for Welder Training

Organisations

CSWIP In-Service Inspection Management Committee

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 Responsibilities of the board
Thus the certification management board:

 Acts as the governing board for certification in keeping with the

requirements of the industries served by the scheme.
 In turn, appoints specialist management committees to oversee specific
parts of the scheme.
 Comprises 12 representatives of industry and other parties with a valid
interest in the certification schemes, for example, fabricators, client
organisations, design authorities and training associations. This ensures that
the certification schemes truly reflect the needs of industry.

The management committees

 Meet regularly and monitor the administration of the examinations.
 Recommend changes where they are needed if it means that the
examinations can be improved to meet the requirements of industry.
 Discuss new certification ideas.

It can, therefore be seen that CSWIP is a comprehensive scheme, which

provides for the examination and certification of individuals seeking to
demonstrate their knowledge and/or experience in their field of operation. The
scope of CSWIP includes welding inspectors, welding supervisors, welding
instructors and underwater inspection personnel.

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 CSWIP Certification for Underwater Inspectors
Requirement documents
All CSWIP examination requirement’s documents are available free of charge
and may be downloaded from the website www.cswip.com.

Inspector categories
There are four categories of certification in the underwater inspector scheme:

1 3.1 U Diver Inspector.

2 3.2 U Diver Inspector.
3 3.3 U ROV Inspector.
4 3.4 U Underwater inspection controller.

(This is an approved course for preparation for the 3.1U examination.)

The CSWIP 3.1U examination

This consists of two main elements, a theoretical examination and a practical
assessment.

Theory examination
This consists of two separate papers, with a total of 100 multiple choice
questions which will include questions on concrete.

One paper:
 Contains 50 sector specific questions on 3.1U subjects relating to sub-sea
applications.

The other paper:

 Contains 50 general theory questions.

The pass mark is 70% for each paper and the time allowed is 75 minutes per
paper.

Practical examination
This will consist of the following parts:

 Visual examination of an underwater steel structure.

 Cathodic potential measurements.
 Ultrasonic digital thickness measurements.
 Underwater photography.
 Use of video with oral commentary.

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 27/08/2015

DIS1 CSWIP 3.1U Introduction

NDT Inspection Diver

Copyright © TWI Ltd Copyright © TWI Ltd

Introduction Introduction

Copyright © TWI Ltd Copyright © TWI Ltd

Introduction Introduction

Copying of any course material

without permission by whatever
means is strictly forbidden

Copyright © TWI Ltd Copyright © TWI Ltd

1
 27/08/2015

Introduction Introduction

Course objectives: You MUST pass an end of course examination before

being eligible to take the CSWIP certification
examination.
 To explain theoretical principles of subsea
inspection.
This consists of:
 To be proficient in practical visual subsea
 50 multiple choice
inspection techniques.
questions.
 To gain eligibility to sit the CSWIP 3.1U  Time allowed: 1 hour.
examination.  Pass mark: 60%.

You are allowed ONE re-

test only should you fail
this examination.

Copyright © TWI Ltd Copyright © TWI Ltd

CSWIP 3.1u Certification Examination CSWIP 3.1u Certification Examination

 CSWIP 3.1u Theory Examination CSWIP 3.1u Practical Examination.

 2 x 50 Multiple choice question papers.  GVI of a steel structure.
 Time allowed: 75minutes per paper.  CVI of a circumferential weld.
 Pass mark: 70% (per paper).  Digital photography: Close up mosaic of weld,
 Paper (A): General knowledge all subjects. an area of damage and 3 stand-offs of a
 Paper (B): Sector specific questions on: subject.
 Welded structures, pipelines, risers and concrete  CP readings using hand-held Bathycorrometer.
terminology.
 Digital thickness measurements: Find and map
 Modes of failure.
an area of thinning.
 Welding technology and associated defects.
 In-service defects.
 Environmental influences on subsea structures.

Copyright © TWI Ltd Copyright © TWI Ltd

Any Questions?

Copyright © TWI Ltd

2
 Section 1

Engineering Offshore Structures

1 Engineering Offshore Structures
1.1 General background
In the initial stages of development of offshore oil platforms, the designs
evolved from land-based structures and were constructed on site. The
engineering design knowledge was either borrowed or extrapolated from
traditional fields of civil engineering and naval architecture.

During the 1950s, new technology began to be developed for this type of
structure. Since then many advances have been made, particularly in the field
of materials. Governments’ legislation in the various host countries with
offshore oil has also played a role in shaping the design of production platforms
and the various other structures seen offshore.

Economics are very important and play a leading role in platform design. For
example, it is only possible to justify the expenditure for a massive eight-legged
steel or a huge concrete gravity platform, when the hydrocarbon reserves in a
particular field are large enough to, not only warrant the initial capital cost but
will also guarantee a good income for a long period of time.

There is also a growing concern for the environment and this consideration
influences certain aspects of structural design. Another factor of prime
importance is safety of personnel.

There are two facets to this:

1.1.1 Safe to operate

The first facet is the usual concern of engineers to design a structure which is
elegant if possible, conservative in its use of materials, fit for the design
purpose, able to operate for the prescribed length of time, safe to operate and
within the allowed budget.

1.1.2 Government Legislation

The other facet is Government Legislation. This is put in place to ensure that
structures are fit in all aspects, including safety, for the purpose they are
designed to fulfil.

1.2 Design specifications

The requirements for an offshore platform will necessitate the consideration of a
number of factors and involve drawing up design specification. The full design
specification will contain many different factors, so by way of illustration, the
following list should serve to indicate some of those affecting load bearing and
cost.

1.2.1 Materials
Should be readily available from suppliers in the required form and should meet
the requirements of the design specification.

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1.2.2 Working life
This may typically be 25 years.

1.2.3 Loading
The platform should provide a safe working environment for the purpose of
recovering hydrocarbon reserves. It must be capable of withstanding the loads
imposed on it by the drilling and other works performed in and on the work
areas and it must withstand the forces imposed by wind and wave action.

1.2.4 Environment
Open sea conditions will impose very harsh conditions on the entire structure
but especially the Jacket. Due consideration must therefore be made to the
effects of corrosion because of this environment.

1.2.5 Maintenance
This should be kept to a minimum. Consideration must be given to the
underwater maintenance, being especially singled out with a view to not only
minimising it but also to use the most cost effective means of achieving any
necessary works.

1.2.6 Weight
The weight of the deck modules must be considered, so that the Jacket can be
designed to support this weight. The all-up weight will have ramifications on the
cost and on the seabed design of the foundations.

1.2.7 Dimensions
The size of the structure will be dictated by the work functions and will be
strongly affected by the requirements to keep the topside weight to the
minimum.

1.3 Construction activity monitoring system

At the same time as the design specification is drafted, it is possible for the
Quality Assurance (QA) function to be implemented. This can take the form of
an Activity Monitoring System that would comprise:

 Full certification for the location of all components, normally by way of as-
built drawings. This would usually include any concessions, repairs and the
actual location of J tube and temporary access holes.
 Full material certification.
 Non-destructive testing (NDT) and inspection certification, which would
include personnel qualifications.

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1.4 Guidance on design and construction
With these engineering requirements in mind as the basic starting point, design
and structural engineers will be able to obtain guidance as to what minimum
standards are acceptable to the appropriate authority or government body,
whatever country they are operating in.

Changes in UK regulations
In 1988, there was a substantial leakage of gas condensate on Piper Alpha,
which was a large North Sea oil platform. The leakage led to an explosion and
large fires, which engulfed the Piper Alpha platform and led to the loss of 167
lives.

A Public Inquiry was established with the aims of establishing the causes of the
disaster and making recommendations for changes to the safety regime. The
enquiry was chaired by Lord Cullen and made its recommendations in 1990.

There were 106 recommendations aimed at improving the regulation of Health

and Safety Offshore and all were accepted by the Government. A central
recommendation was subsequently developed and implemented by the UK
Regulator, Health and Safety Executive (HSE), in the form of ‘the Offshore
Installations (Safety Case) regulations’, which came into force in 1992.

These regulations require the operator/owner (known as the Duty Holder) of

every fixed and mobile installation operating in UK waters to prepare Safety
Cases for each installation on the UK continental shelf, which the HSE must
accept before operations are permitted.

The Safety Case is expected to demonstrate that the Duty Holder has the
ability and means to control major accident risks effectively. This requires that
the Duty Holder has identified the major accidents, hazards, assessed the
major accident risks, implemented control measures to ensure that the risks are
reduced to as low as reasonably practicable (ALARP), in compliance with
all relevant statutory provisions and made adequate arrangements for auditing
and reporting.

The new Safety Case regime effectively replaced the pre-1988 prescriptive
system for SIM with risk based and goal setting activities structured around the
management of safety critical elements (SCEs). SCEs are defined as the parts,
or components of an installation and its plant, whose failure could cause, or
contribute substantially to a major accident, or whose purpose is to prevent,
control or limit the effects of a major accident.

The Safety Case regulations were revised in 2005, to reflect 13 years of

experience. Under the 1992 regulations, a Safety Case lasted three years
before it had to be resubmitted for acceptance. According to the 2005
regulations, a Safety Case will last the life of the installation, without the need
for routine resubmissions.

However, the Safety Case is intended to be a living document, kept up-to-date

and revised as necessary to ensure it remains current and reflects actual
operational conditions on the installation.

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 The Duty Holder is required to conduct a thorough review of the current Safety
Case at least every five years, or as directed by the HSE. In addition, revisions
that make a material change are required to be resubmitted to the HSE for
acceptance (a material change is one that changes the basis on which the
original Safety Case was accepted, such as significant modifications or repairs,
introduction of new activities, extension of life beyond original design life or
major changes in technology).

In addition to the Safety Case Regulations, the new goal-setting regulation

(replacing the previous prescriptive regulations) were introduced, as outlined
below.

1.4.1 Guidance in UK regulations

The regulatory requirements for the asset integrity management of structures
operated on the UK continental shelf are specified in the following documents
(Stacey, OMAE-49089):

 The Offshore Installations (Safety Case) Regulations 2005 (SCR05), which

make preparation of a Safety Case a formal requirement (described above).

 The Offshore Installations And Wells (Design And Construction, etc.)

regulations 1996 (DCR), which require the Duty Holder to ensure that
suitable arrangements are in place for maintaining the integrity of the
installation, through periodic assessments and carrying out any remedial
work in the event of damage or deterioration.

The DCR place a requirement on the Duty Holder to design installations to

withstand such forces acting on it that are reasonably foreseeable and that
in the event of foreseeable damage it will retain sufficient integrity to enable
action to be taken to safeguard the health and safety of personnel on or
near it.

 The Offshore Installations (Prevention Of Fire And Explosion And Emergency

Response) Regulations 1995 (PFEER), which require that the duty holder
takes appropriate measures for the protection of persons on offshore oil and
gas installations from fire and explosion and for securing effective
‘emergency response’, which means action to safeguard the health and
safety of persons on such installations in an emergency.

The Duty Holder is expected to perform ‘an assessment’ which consists of:
(a) the identification of the various events which could give rise to a major
accident involving fire or explosion; or the need for evacuation, escape or
rescue to avoid or minimise a major accident; (b) the evaluation of the
likelihood and consequences of such events; (c) the establishment of
appropriate standards of performance to be attained by anything provided
by measures for ensuring effective evacuation, escape, recovery and rescue
to avoid or minimise a major accident. This requires inherent safety by
design, preventive, detection, control and mitigation measures (which
include plant and management systems).

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  The Pipelines Safety Regulations 1996 (PSR), which require the operator to
ensure that a pipeline has been designed adequately (it can withstand
internal and external forces arising from its operation); it is maintained in
an efficient state, in efficient working order and in good repair throughout
its service life; all hazards relating to the pipeline with the potential to cause
a major accident have been identified and the risks arising from those
hazards have been evaluated and that the Safety Management System is
adequate.

 The Provision And Use Of Work Equipment Regulations 1998 (PUWER),

requires employers to ensure that work equipment is fit for the purpose for
which it is used; where the safety of work equipment depends on the
installation conditions, inspection of the equipment after installation is
required to ensure correct installation and safe operation, where the work
equipment is exposed to conditions causing deterioration which is liable to
result in dangerous situations, inspection of the equipment during service at
suitable intervals is required to ensure that health and safety conditions are
maintained and that any deterioration can be detected and remedied in
good time.

In short, employers are required to identify and control potential risks from
hazards due to equipment failure; put in place appropriate inspection
procedures after installation; and appropriate inspection and maintenance
procedure during service to ensure that the work equipment is in efficient
working order and in good repair.

ISO standards for offshore structures

Standards have been (and are being) developed in the ISO 19900 series giving
guidance on the design, construction, transportation, installation, integrity
management and reassessment of offshore installations.

Structures covered by these standards include: bottom-founded fixed steel

structures; fixed concrete structures; floating structures, such as mono-hull
FPSOs, semi-submersibles and spar platforms, arctic structures and site-specific
assessment of jack-up platforms.

The following ISO offshore structures standards have been published:

 ISO 19900: 2002 General requirements.

 ISO 19901 Specific requirements.
 ISO 19901-1: 2005 Metocean design and operating considerations.
 ISO 19901-2: 2004 Seismic design procedures and criteria.
 ISO 19901-3: 2010 Topsides structures.
 ISO 19901-4: 2003 Geotechnical and foundation design considerations.
 ISO 19901-5: 2003 Weight control during engineering and
construction.
 ISO 19901-6: 2009 Marine operations.
 ISO 19901-7: 2005 Station keeping systems for floating offshore
structures and mobile offshore units.
 ISO 19902: 2007 Fixed steel offshore structures.
 ISO 19903: 2006 Fixed concrete offshore structures.
 ISO 19904-1: 2006 Floating offshore structures – mono-hulls, semi-
submersibles and spars.
 ISO 19905-1: 2013 Site specific assessment of jack-ups.
 ISO 19906: 2010 Arctic offshore structures.

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 ISO standards for oil and gas production are expected to have primacy in most
regions of the world, including the UK continental shelf, for the design of new
offshore installations and for modification to and reassessment of existing
offshore structures.

1.5 General design considerations

Typical design considerations are outlined below (extract from DNV-OS-C101:
2011 – General Design of Offshore Structures, LRFD method).

1.5.1 Aims of the design

Structures and structural elements are designed to:
 Sustain loads liable to occur during all temporary, operating and damaged
conditions, if required.
 Maintain acceptable safety for personnel and environment.
 Have adequate durability against deterioration during the design life of the
structure.

1.5.2 General design considerations

The design of a structural system, its components and details is required, as far
as possible, to account for the following principles:
 Resistance against relevant mechanical, physical and chemical deterioration
is achieved.
 Fabrication and construction comply with relevant, recognised, techniques
and practice.
 Inspection, maintenance and repair are possible.

1.5.3 Limit States

In a Load and Resistance Factor Design (LRFD) standard, load factors are
applied to characteristic reference values of the loads acting on the structure
and resistance factors are applied to characterise the resistance of the structure
or resistance of materials in the structure.

The design considers a number of limit states such as:

 Ultimate Limit States (ULS) corresponding to the ultimate resistance for

carrying loads.
 Fatigue Limit States (FLS) related to the possibility of failure due to the
effect of cyclic loading.
 Accidental Limit States (ALS) corresponding to damage to components
due to an accidental event or operational failure.
 Serviceability Limit States (SLS) corresponding to the criteria applicable
to normal use or durability.

Examples of limit states within each category:

Ultimate limit states (ULS)

 Loss of structural resistance (excessive yielding and buckling).
 Failure of components due to brittle fracture.
 Loss of static equilibrium of the structure, or of a part of the structure,
considered as a rigid body, eg overturning or capsizing.
 Failure of critical components of the structure caused by overloading (in
some cases reduced by repeated loads).
 Transformation of the structure, for example, due to buckling, plastic
collapse or excessive deformation.

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 Fatigue Limit States (FLS)
 Cumulative damage due to repeated loads.

Accidental Limit States (ALS)

 Structural damage caused by accidental loads.
 Ultimate resistance of damaged structures.
 Maintain structural integrity after local damage or flooding.
 Loss of station keeping (free drifting).

Serviceability Limit States (SLS)

 Deflections that may alter the effect of the acting forces.
 Deformations that may change the distribution of loads between supported
rigid objects and the supporting structure.
 Excessive vibrations producing discomfort or affecting non-structural
components.
 Motion that exceed the limitation of equipment.
 Temperature induced deformations.

1.6 Pipelines
Offshore pipelines are used to transport oil or gas from platform to loading
towers or to shore. They are fabricated from high-grade steel pipe (eg
API/5LX), which is bitumen wrapped for corrosion prevention and coated with a
layer of reinforced concrete to provide a weight coating, which gives additional
protection as well.

The sizes normally vary from 50mm (2 inch) to 914mm (36 inch) and the wall
thickness normally varies according to the pressure rating required.

1.6.1 Pipeline laying

The methods for laying pipe has evolved since the 1950s and uses lay barges
on which standard 12m lengths of pipe are welded together along the centre of
the specially designed and fitted out deck of the vessel.

Each joint is inspected by X-ray, then coated with bitumen and wrapped with a
protective sheathing. Modern day inspection involves automated ultrasonic
techniques.

As new lengths of pipe are added, the assembly is fed over the stern and the
barge is moved forward, usually by pulling on anchors, which have been laid by
an associated anchor-handling vessel.

An alternative approach is laying pipe from a reel barge. The earliest application
of this technique occurred during World War II when a 76mm (3”) diameter
pipe was laid across the English Channel in operation PLUTO (Pipeline Under the
Ocean). This early application used floating reels with the pipeline being
unwrapped from them as they were towed along.

The modern application requires the pipe to be prepared on land and then
wound onto the reel, which is mounted on the stern of the reel laying vessel,
which itself is moored at a specially designed pier. The vessel then proceeds to
the required site and lays the pipeline by un-reeling it over the stern as the
barge steams forward.

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 The welding and preparation work on land is carried out in a spooling yard,
where the pipe sections are supplied in 12m (40ft) lengths. These are welded
together to form stalks, usually about 518m (1700ft) long. All the welds are
X-rayed (or inspected by automated ultrasonic testing), coated and the stalks
are stowed in racks alongside the spooling dock.

At the start of spooling, the first stalk is moved into the roller system. The end
is welded to a stub of pipe on the reel and is pulled onto the reel. The second
length is then welded to the end of the first, the weld is X-rayed and coated and
the procedure is then repeated for subsequent stalks.

All welding and loading operations are performed at the shore facility and
therefore are less affected by weather conditions. The major area of criticality is
establishing and maintaining even tightness of the wraps on the reel, this is to
avoid potential breakthrough of one wrap into another, which would cause
damage to the pipe. The reeling and unreeling of the pipe actually causes
yielding of the steel and the maximum diameter pipeline that can be laid is
600mm (24 inch).

Figure 1.1 MSV Norlift, laying the 10 inch pipeline between the Neptune and
Mercury fields.

1.7 Offshore oil terminals

Large oil tankers are cheaper to run than small ones. This philosophy of building
large tankers was reinforced in the 1950s, when the Suez Crisis forced tankers
from the Gulf to detour around the South of Africa to reach Europe. As tanker
sizes increased, the number of ports that could handle these large vessels
decreased and public opinion was against allowing such tankers too close to
inhabited areas.

Many solutions were proposed to solve this problem of shrinking docking

facilities, which included artificial harbours, artificial offshore islands, multiple
buoy mooring systems, tower mooring systems and single point mooring (SPM)
or single buoy mooring (SBM) systems.

The SPM is the most widely used because of its relatively low operational cost,
reliability and flexibility and is shown in section 2.

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1.8 Future trends
We are likely to see continued development of current trends and techniques in
all areas of offshore engineering, with the probability of new techniques being
evolved to enable the exploitation of reserves, which are currently marginal or
beyond the range of present day techniques.

1.8.1 Drilling
This is a branch of engineering which has seen numerous developments, the
results of which have made recovery of reserves more efficient and effective.
Cost reduction and further development of marginal reserves will, no doubt,
cause a continuation of developments of the present techniques and trends.

There will surely be, for instance, increased use of:

 Horizontal drilling enables more formations to be exposed to production

and reduces reservoir problems, such as associated gas and water
production. It is useful for thin and tight, low permeability reservoirs. Fewer
wells are needed to achieve optimum reservoir production than with
conventional drilling.

 Extended reach drilling can reduce the number of platforms required to

develop a field as a greater reservoir area can be drained from one central
platform. Horizontal distances up to 7000m have been achieved.

 Slim-line well design involves cost-effective casing design around an

optimal production conduit; this can also reduce the number of wells needed
to achieve optimum reservoir drainage.

 Rig automation allows several labour intensive tasks, such as pipe

handling, to be carried out automatically. For instance, on the rig package
developed for Norske Shell’s Troll platform, only one driller and an assistant
man the rig floor.

On a conventional rig, between five and seven people would be needed to

carry out equivalent tasks. All pipe-handling operations are carried out from
a specially designed control cabin. Removing personnel from the drill floor
means more cost-effective and potentially safer operations.

 Temporary (lightweight) topsides on platforms can make production

platforms lighter and cheaper than traditional platforms, which include
permanent integrated drilling facilities. For example, Norske Shell’s Draugen
and Troll platforms are designed so that the derrick set can be removed at
the end of the drilling and completion phase.

By removal of the drilling derrick and modules the load is decreased on the
platform and the stress on the welds reduced. Shell’s Gannet Platform is
another lightweight design.

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 Figure 1.2 Gannet Platform North Sea central sector.

 Tender assisted operations also help to minimise the weight of the

production platform by providing most drilling support equipment on a
floating, anchored, ship-shaped tender in calm waters or an anchored semi-
submersible unit for deeper or harsher environments.

 Mobile drilling units are jack-up or semi-submersible rigs, depending on

water depth. They can be used to drill production wells (with well
completion on the seabed and production pipelines led to a nearby facility)
where size and economics of the reservoir do not justify the installation of a
platform.

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1.8.2 Design practices
Fixed platforms are now lighter, slimmer and simpler than the giant platforms
built in the 1970s. There is scope for further simplification, for example of
topsides, which account for more than half the capital cost of a platform.

Platform development Sub-sea Satellite at 20Km

Figure 1.3 Comparisons of capital costs.

Topside costs can be reduced, for instance by standardising designs and

reducing sparing (duplication of equipment).

Another option is to examine alternatives to conventional platform designs.

Studies of a purpose-built production jack-up unit, a concrete gravity structure
and a tripod tower platform have shown that all three are technically viable and
could offer cost saving for applications in water depths around 100m.

Greater use of sub-sea satellite technology instead of building a platform can

reduce costs, especially where infrastructure already exists nearby, which can
be used as a host platform.

As indicated by the relative sizes of the pie charts in Figure 1.3, the capital
costs of constructing a sub-sea satellite 20km from an existing platform are
much lower than the costs of constructing an additional platform.

However, in such an instance, the long term technical integrity of existing

facilities, platforms and pipelines must be ensured, given that they may be in
continued use beyond the original design life, which was probably in the order
of 20 years anyway.

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 Bibliography
‘API Recommended Practice for Planning; Designing and Constructing Fixed
Offshore Platforms’, American Petroleum Institute.

Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

DNV-OS-C101: ‘Design of offshore steel structures, General (LRFD Method)’,

DNV Offshore Standard, 2011, http://www.dnv.com.

HSE website: ‘Integrity of primary structure of fixed steel installations’,

http://www.hse.gov.uk/offshore/integrity-primary.htm.

Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office

Books), 1988, ISBN 13: 9780114129118.

SI 1995/743: ‘The Offshore Installations (Prevention of Fire and Explosion and

Emergency Response) Regulations 1995’,
http://www.legislation.gov.uk/uksi/1995/743/introduction/made.

SI 1996/825: ‘The Pipelines Safety Regulations 1996’,

http://www.legislation.gov.uk/uksi/1996/825/contents/made.

SI 1996/913: ‘The Offshore Installations and Wells (Design and Construction)

Regulations 1996’, HMSO.

SI 289: ‘The Offshore Installations (Construction and Survey) Regulations

1974’, BSI.

SI 998/2306: ‘The Provision and Use of Work Equipment Regulations 1998’,

http://www.legislation.gov.uk/uksi/1998/2306/contents/made.

Stacey A, Birkinshaw M, Sharp J V, May P, ‘Structural integrity management

framework for fixed jacket structures’, Proc. 27th Int. Conf. on Offshore
Mechanics and Arctic Engineering, Paper OMAE2008-57413, 2008, Portugal.

Stacey A, ‘KP4: Ageing and life extension inspection programme for offshore
installations’, Proc. 30th Int. Conf. on Ocean, Offshore Mechanics and Arctic
Engineering, Paper OMAE2011-49089, 2011, The Netherlands.

Stacey A, Sharp J V, ‘Ageing and life extension considerations in the integrity

management of fixed and mobile offshore installations’, Proc. 30th Int. Conf. on
Ocean, Offshore Mechanics and Arctic Engineering, Paper OMAE2011-49090,
2011, The Netherlands.

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 Stacey A, Sharp J V, ‘Structural integrity management framework for mobile
installations’, Proc. 30th Int. Conf. on Ocean, Offshore Mechanics and Arctic
Engineering, Paper OMAE2011-49656, 2011, The Netherlands.

Tapper P P, ‘Engineering Aspects of North Sea Operations - The Shell Approach’,

The Gatwick Press.

‘The Offshore Challenge’, Shell Briefing Service.

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 27/08/2015

Engineering Offshore Structures

General background:
 Initially, the designs used for offshore platforms
were borrowed from traditional fields of civil
engineering and naval architecture.
CSWIP 3.1U Course  During the 1950’s new technology developed
and many advances were made.
Engineering Offshore Structures  Government legislation in various countries with
Section 1 offshore oil played a role in shaping the design
of offshore structures.
 Economics, the environment and safety of
personnel have also had great influence on
structural design.

Copyright © TWI Ltd Copyright © TWI Ltd

Considerations in Design Design Specifications

Safe to operate: Materials:

 Designers strive to achieve a structure which  Should be readily available in the required form
is elegant if possible, conservative in and meet the design specification.
material, fit for purpose, able to operate for
the prescribed length of time, safe to Working life:
operate and within budget.
 This may typically be 25 years.

Government legislation:
Loading:
 This is put in place to ensure that structures
are fit in all aspects, including safety, for the  The platform should provide a safe working
purpose they were designed to fulfill. environment capable of withstanding all loads
imposed upon it such as drilling, wind and
waves.
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Design Specifications Design Specifications

Environment: Weight:
 Consideration must be given to the effects  The weight of the deck modules must be
of corrosion and to offshore conditions. considered so that the jacket can be
designed to support this weight. The all-up
Maintenance: weight will have ramifications on the cost
and on the design of the seabed
 This should be kept to a minimum. foundations.
Underwater maintenance being especially
singled out with a view to not only
minimising it but also to use the most cost Dimensions:
effective means of repair works.  Size is dictated by the work functions but
topside weight should be kept to a
minimum.

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Construction Activity Monitoring

Guidance on Design and Construction
System
At the same time as the design specification is drafted,  With these engineering requirements in mind
the quality assurance (QA) function can be as the basic starting point, design and
implemented. structural engineers will be able to obtain
This can take the form of an activity monitoring guidance as to what minimum standards are
system that would include: acceptable to the appropriate authority or
 Full certification for the location of all components, government body, whatever country they are
normally by way of as-built drawings, including operating in.
concessions, repairs and actual location of J tube
and temporary access holes.
 Full material certification.
 Non-destructive testing (NDT) and inspection
certification, including personnel qualifications.

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Changes in UK Regulations Changes in UK Regulations

In 1988 a gas condensate leak on the Piper Alpha The safety case is expected to demonstrate that the
caused an explosion and large fires which duty holder has the ability and means to control major
destroyed the platform and cost 167 lives. A accident risks effectively. This requires that the duty
public enquiry ensued and it’s chairman, Lord holder has identified the major hazards, assessed the
Cullen made 106 recommendations aimed at major accident risks, implemented control measures to
improving safety offshore. ensure that the risks are reduced to as low as
The Health and Safety Executive (HSE) developed reasonably practicable (ALARP), in compliance with all
‘The Offshore Installations (Safety Case) relevant statutory provisions, and made adequate
Regulations’ in 1992, which require the arrangements for auditing and reporting.
operator/owner known as the Duty Holder to
prepare safety cases for each installation in UK
waters, which the HSE must accept before
operations are permitted.
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Changes in UK Regulations Changes in UK Regulations

 The Safety Case Regulations were revised in However, the safety case is intended to be a
2005, to reflect 13 years of experience. Under living document, kept up-to-date and revised as
the 1992 regulations, a safety case lasted necessary to ensure it remains current and
three years before it had to be re-submitted reflects actual operational conditions on the
for acceptance. According to the 2005 installation.
regulations, a safety case will last the life of The duty holder is required to conduct a thorough
the installation, without the need for routine review of the current safety case at least every
re-submissions. five years, or as directed by the HSE.

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Changes in UK Regulations General Design Considerations

 In addition, revisions that make a material Aims of the design

change are required to be re-submitted to the Structures and structural elements are designed
HSE for acceptance (a material change is one to:
that changes the basis on which the original  Sustain loads liable to occur during all
safety case was accepted, such as significant temporary, operating and damaged
modifications or repairs, introduction of new conditions, if required.
activities, extension of life beyond original
design life, or major changes in technology).  Maintain acceptable safety for personnel and
environment.
 Have adequate durability against deterioration
during the design life of the structure.

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General Design Considerations General Design Considerations

Limit states
 The design of a structural system, its  ULS (Ultimate): Relating to the ultimate
components and details is required, as far as resistance for carrying loads.
possible, to account for the following  FLS (Fatigue): Related to the possibility of
principles: failure due to the effect of cyclic loading.
 Resistance against relevant mechanical,  ALS (Accidental): Corresponding to damage to
physical and chemical deterioration is components due to an accidental event or
achieved. operational failure.
 Fabrication and construction comply with  SLS (Serviceability): Corresponding to the
relevant, recognised, techniques and practice. criteria applicable to normal use or durability.
 Inspection, maintenance and repair are
possible.

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Pipelines Pipelines

Offshore pipelines are used to transport oil or gas from

platform to a loading tower or to shore. They are
fabricated from high grade steel pipe (eg API-5LX)
which is bitumen wrapped for corrosion prevention and
coated with a layer of reinforced concrete to provide a
weight coating which also gives physical protection.
 Pipeline laying from a lay barge.
 12m lengths are welded together on the deck of a lay
barge.
 Each joint is X-rayed and bitumen wrapped.
 As joints are made up the pipe is fed over the stern
as the barge is moved forward.

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Pipelines Pipelines

Pipeline laying from a reel barge:

 Lengths of pipe kilometres long are made

up ashore and wound onto reels mounted
on special vessels.

 The vessel then lays the pipe by un-reeling

it on location. (Maximum diameter of the
pipe is 600mm).

Reel barge MSV Norlift laying 10in pipe

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Offshore Oil Terminals Offshore Oil Terminals

 Large oil tankers are cheaper to run than  Many solutions were proposed to solve this
small ones. This philosophy of building large problem of shrinking docking facilities, which
tankers was reinforced in the 1950s when the included artificial harbours, artificial offshore
Suez Crisis forced tankers from the Gulf to islands, multiple buoy mooring systems, tower
detour around the South of Africa to reach mooring systems and single point mooring
Europe. As tanker sizes increased, the number (SPM) or single buoy mooring (SBM) systems.
of ports that could handle these large vessels
decreased and public opinion was against  The SPM is the most widely used because of
allowing such tankers too close inshore. its relatively low operational cost, reliability
and flexibility and is shown in section 2.

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Future Engineering Offshore Trends Future Engineering Offshore Trends

Drilling Temporary topsides on platforms.

 Horizontal drilling: Using directional drill assemblies.
 Extended reach drilling: Using longer drill strings. This design innovation
 Slim line well design: Using cost effective casing to allows for the removal of
minimise wall thickness and diameter of the well the derrick after drilling
casing. is completed.
 Rig automation: Using automatic pipe handling
equipment to remove personnel from the drill floor.
 Temporary (Lightweight) topsides on platforms:
Removable derrick after completion of drilling.
 Tender assisted operations: Drilling support
equipment kept on vessel anchored alongside. Gannet platform

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Future Engineering Offshore Trends Future Engineering Offshore Trends

Design practices: Comparison of capital costs for offshore

production methods:
 The thrust of design is to continue the trend
Platform development Subsea satellite at 20km
to make structures slimmer and simpler.
 The main emphasis for the future is likely to
be the topside modules.
 Concurrently there will be a greater use of
sub-sea satellite technology.
 Both of these trends will reduce production
costs.

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Any Questions?

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5
 Section 2

Offshore Structures and Installations

2 Offshore Structures and Installations
2.1 Introduction
Offshore hydrocarbon deposits may be gas, oil or a mixture of the two. They
are found at different depths in the seabed. They are of different sizes and the
recovery of the reserves can be easy or difficult depending on the actual
geology of the particular well.

These factors influence the design of offshore structures and combine to be one
of the basic reasons for there being different types of offshore installations.
Some of the biggest installations are in the North Sea and consist of concrete
gravity structures.

Then there are the more common steel platforms, which can be of the eight-
legged type or may be of a lightweight four-legged variety. There are also
jack-up rigs, which are mobile, and tension leg platforms (TLP), which float.

Apart from these production facilities, there are also seabed wells, manifold
centres and thousands of kilometres of pipelines. Another common structure
seen worldwide is the single point mooring (SPM) which comes in a variety of
designs, some of which incorporate storage facilities.

2.2 Steel production platforms

With steel fixed platforms the jacket supports the superstructure, which
contains all the necessary facilities. The jacket is built in a fabrication yard and
if it is a large six or eight-legged jacket designed to support full production
facilities, it may well have modified legs designed as floats, or additional ballast
tanks may be installed so that it can be floated out to the site.

Smaller steel structures, which have been designed and built as a result of
advances made in materials, better understanding of the forces imposed on
offshore structures and different design concepts are loaded onto a barge,
which carries them out to sea.

Both these types of platforms are sometimes referred to as steel piled

structures because the jacket is piled into the seabed, once it is in the upright
position, with piles either driven through the legs or positioned around the main
legs and driven through pile sleeves, so-called skirt piles.

One example of the large fixed production platform is the Brent A, which is
installed in Shell-Expro’s Brent Field in the North Sea (figure 2.1).

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 Figure 2.1 Shell-Expro’s Brent A Production Platform North Sea.

2.2.1 Brent A statistics

Water depth 140m

Substructure
Jacket type Self-floating steel construction
Number of legs 6
Number of piles 32 (skirt piles)
Weight of jacket 14,225 tonnes
Weight of piles 7,316 tonnes

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 Superstructure
Production capacity 100,000 bbl/d oil and 200-mmscfd gas
Height of deck above sea level 21.7m
Deck area 300sq.m
Deck construction Plate girder
Weight of deck 1,507 tonnes
Weight of deck facilities 2,354 tonnes
Weight of modules and equipment 14,762 tonnes

An example of a lightweight platform servicing seabed wellheads and facilities is

the Gannet A platform in Shell-Expro’s Gannet field in the central North Sea
(Figure 2.2). The structure is of the same basic design but not as massive as
the production platform.

Figure 2.2 Shell-Expro’s Gannet Platform.

2.3 Terminology
The production platforms are the most massive installations and they may be of
steel or concrete construction, steel being the most prevalent
(Figure 2.3). Both types have standard components and a thorough working
knowledge of this terminology is necessary to be able to communicate with
other engineers. Much of this terminology also applies to the other types of
structures and, therefore, a review of this topic for platforms forms the basis for
a comprehensive working technical vocabulary.

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 Figure 2.3 A steel platform.

Following is the common terminology for the components making up the steel
sub-sea structure:

Figure 2.4 Steel structure terminology.

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  Can
One of the sections making up a node, ie the main body of the node.

 Conductor guide frame

Horizontal sections of framework, which restrain and guide the conductors.

 Leg
Main vertical component, constructed from a number of sections welded
together, supporting the rest of the structure.

Figure 2.5 A four-legged Jacket built for the compression platform installed as
part of CMS 2.

 Member
One of the horizontal, vertical diagonal or horizontal diagonal braces of the
jacket.

 Node
Point on the welded steel structure where two or more members meet and
are joined.

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 Figure 2.6 A typical node.

 Pile guides
A steel cylinder that supports the pile while it is driven into the sea-bed. Pile
guides are mounted in clusters around each leg at various levels. They are
usually removed on completion of piling operations.

 Pile sleeves
Long steel cylinders, grouped around the base of the legs into which the
piles are located before being driven into the sea-bed. The tops of the piles
should be level with the tops of the sleeves on completion of piling.

Figure 2.7 Pile sleeves.

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 Additional to these components are a number of attachments. The more
important of these are:

 Caissons
Open-bottomed tubular components terminating at various depths for the
purpose of the intake or discharge of water or waste.

 Conductors
Tubes for drilling purposes connecting seabed wells to the topside.

Figure 2.8 Conductors.

 Flowline bundles
Pipework bringing oil or gas from satellite wellheads into the platform and
containing control lines, product lines and well injection lines.

 Oil and gas risers

Vertical sections of the pipeline extending up the full height of the jacket
that are used for transporting oil or gas. Production risers carry oil or gas up
from the seabed wellheads via the submarine pipelines. Export risers take
the processed hydrocarbons down to pipelines.

2.4 Concrete and steel gravity platforms

The first gravity structure was installed in the North Sea in the mid-1970s,
while the first steel gravity platform was installed offshore in the Congo in the
late-1970s. Steel gravity structures have not proliferated while concrete
structures have.

The concept of a concrete structure came about because of some of the

problems associated with steel structures, namely the necessity for a large
number of large, heavy piles and the corrosion problem with steel in a hostile
environment. Concrete gravity structures require no piles and are immune to
corrosion.

Initially, there were perceived additional advantages of storage space within the
base cells and potentially huge deck space, which could be fitted out in calm
sheltered water, which in turn would minimise on-site commissioning, therefore
reducing expensive offshore construction manpower.

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 Figure 2.9 Shell-Expro’s Brent D concrete gravity platform North Sea.

Apart from Brent D, illustrated in Figure 2.9, there are numerous examples of
this type of structure constructed to different designs, such as Condeep, CG
Doris and McAlpine Seatank. To give some idea of the scale of this type of
platform, the main statistics for ‘Cormorant A’, which is a four-legged design,
installed in the Cormorant Field in the North Sea are detailed here.

Cormorant A statistics
Water depth 150m

Substructure
Storage capacity 1,000,000 bbls
Caisson shape Square
Caisson height 57m
Number of legs 4
Weight in air 294,655 tonnes

Superstructure
Production capacity 60,000 b/d oil and 30-mmscfd gas
Height of deck above MSL 23m
Area of deck 4,200sq.m
Deck construction Box girder
Weight of deck 5,593 tonnes
Weight of deck equipment 3,593 tonnes
Weight of modules 19,011 tonnes

2.4.1 Disadvantages of concrete structures

In spite of the initial optimism for the design of concrete structures, there are a
number of disadvantages.

Stability problems during tow-out to site have to be counteracted by limiting

the topside weight, thus reducing the apparent advantage of large deck space
fitted out in sheltered waters.

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 The very heavy-lift derrick barges now operating are able to operate in
comparatively wide weather conditions, thus reducing the cost of offshore
installations, and therefore, limiting the apparent cost advantage of concrete
constructions.

Concrete, as a material, cannot withstand tensile forces, so the use of the base
storage cells must be carefully monitored at all times to avoid the storage of
crude oil causing a build-up of differential loadings between cells, thus causing
excessive tensile stresses.

Also, pressure must not be allowed to build up in the cells; vapour pressure
must not exceed 2bar (30psi). Oil temperature in the cells must not exceed
38°C to avoid thermal stresses. If crude oil is not carefully monitored,
emulsions formed by the interaction of oil and associated water can accumulate
permanently within the cells.

Reservoir sand must not be allowed to accumulate and steps have to be taken
to eliminate this from the crude before it reaches the cells.

At least one of the main shafts will house utilities, and because it is some 100m
tall and very narrow, ventilating it is difficult, so breathing apparatus is issued
to maintenance staff working there, making routine maintenance and
operations of the equipment difficult.

Stagnant water accumulated in the structure encourages the growth of

anaerobic bacteria, which generate ideal conditions for the formation of
sulphate reducing bacteria (SRB).

The growth of SRB’s leads to the production of Hydrogen Sulphide (H2S), which
necessitates the creation of safety zones and special procedures to avoid risk to
personnel.

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 Figure 2.10 Processing takes place on Statoil’s Gullfaks A platform (North
Sea).

2.4.2 Basic components of a concrete gravity structure

Gravity structures may be made of steel or a mixture of steel and concrete, but
the most common is concrete. They are anchored to the seabed by their own
mass, hence the term gravity. Common features of this type of design are the
large diameter columns supporting the deck module and numerous
ballast/storage cells making up the base.

Anchorage points (cachetage points)

Are an essential part of the tensioning components in pre and post-stressed
concrete structures. The anchorage point is cast into the concrete at the ends of
the tensioning tendon or bundle of tendons. It grips the tendon and thereby
transfers the load from it to the structural concrete. It is commonly encased in
protective concrete domes (see section 5 Figure 5.4).

Breakwater walls
Are concrete walls in the splash zone, containing cast in holes that dissipate the
wave energy and thus protect the structure within the walled area.

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 Jarlan holes
Term used to describe the cast in holes in the breakwater walls, used to break
up waves to reduce their force on the structure ahead.

Figure 2.11 Close-up of a Jarlan hole and its effect on waves.

Support columns
The concrete or steel columns supporting the deck module.

Figure 2.12 Support columns and support domes on a Condeep design concrete
platform.

Support domes
The tops of the tanks, at the base of the structure, which are used as buoyancy
during the launch of the structure, may then be used to store oil, water or
drilling mud (Figure 2.12).

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2.5 Terminology associated with different offshore structures
Having introduced the terminology associated with production platforms, the
discussion should be extended to other types of offshore structures and vessels.
There are a number of different configurations for structures that are designed
to fulfil different functions.

2.5.1 Jack-up rigs

Used for wildcat drilling, production drilling and work-overs (Figure 2.13).

Figure 2.13 A three-legged Jack-up drilling platform with tow still attached
starting to jack-up.

The Jack-up platform consists of a main deck, which is watertight (the hull) and
floats for transit. Attached to the hull, via a rack and pinion assembly, are the
tubular steel lattice frame legs. The gears lower the legs to the seabed and the
hull is then jacked up by this same method to clear the water. On completion of
the drilling, the whole operation is reversed and the rig is towed away to a
different site.

DIS1-30815
Offshore Structures and Installations 2-12 Copyright © TWI Ltd
2.5.2 Semi-submersible rigs
Used for the same tasks as Jack-ups and may be self-powered
(Figure 2.14).

Figure 2.14 The Åsgard B semi-submersible production platform is linked to

the semi-submersible accommodation flotel Safe Britannia.

In deeper water the legs of a Jack-up platform would be so long as to give

concern about its stability, unless they were much larger in section. Therefore,
this type of rig is not used in water deeper than about 60m. At greater water
depths, the semi-submersible platform is used. The rig has large hollow legs
and pontoons, which can be flooded or pumped dry at will, thus ballasting the
platform.

When moving from site to site the rig is ballasted up to reduce water drag
during transit and, when drilling, the rig is ballasted down to improve stability.
It does float at all times and when drilling is kept in place usually by anchors,
but it may keep position by dynamic positioning (DP).

That is, the main engines run all the time and computers specially programmed
for the task with current data on weather, tide, sea state and various navigation
inputs, control the thrust to the various thrusters (propellers) to keep the vessel
stationary directly over one point on the sea-bed.

DIS1-30815
Offshore Structures and Installations 2-13 Copyright © TWI Ltd
2.5.3 Drill ship
Used for the same tasks as a jack-up, but in deeper water and so is more
weather dependent, but more manoeuvrable and mobile (Figure 2.15).

Figure 2.15 Drill ship Discovery 1.

Either DP or anchors normally keep drill ships on station dependent on the

water depth, site and project parameters.

2.5.4 Compliant towers

These are tall, slim, steel structures designed to sway slightly so that they
comply with wave action. The design is conceived as a half-way house between
fixed and floating structures. It is possible to use this design in water depths up
to some 1000m in moderate environments.

The Baldpate GB 260 Platform is located in 499m (1,650ft) of water, in Garden

Banks (GB) block 260, 120 miles off the Louisiana coast. This is the first
freestanding offshore Compliant Tower and one of the tallest freestanding
structures in the world. The tip of the flare boom extends (575m 1,902ft) above
the seafloor (Figures 2.16 and 2.17).

DIS1-30815
Offshore Structures and Installations 2-14 Copyright © TWI Ltd
 Figure 2.16 The GB 260 Compliant Tower, Gulf of Mexico.

Figure 2.17 Artist impression of the GB 260 Compliant Tower.

DIS1-30815
Offshore Structures and Installations 2-15 Copyright © TWI Ltd
2.5.5 Tension Leg Floating Platforms
Tension Leg Platforms (TLPs) consist of a hull anchored to the sea-bed by
vertical tendons, as shown in Figure 2.18. Vertical movement is constrained by
the tendons, thus allowing production wells to be located on deck. This design is
suitable for deep-water production and some engineers believe the technology
could be extended to water depths of 3000m.

Sea

Steel piled
anchor
b

Seabed
Figure 2.18 Diagrammatic layout of a typical TLP.

2.5.6 Floating production systems (FPS)

Floating production systems consist of a floating vessel with production facilities
connected to seabed wells by flexible risers (Figures 2.19 and 2.20). Vessels
may be purpose built, or converted and may be mono-hulls or semi-
submersible. Tankers, for example can be converted for this task relatively
quickly and cheaply.

In this case they are usually known as Floating Production and Storage
operations (FPSO). These vessels are quite weather dependent, which is why
purpose built vessels, have been developed.

DIS1-30815
Offshore Structures and Installations 2-16 Copyright © TWI Ltd
 Figure 2.19 The Åsgard A FPSO is 278m long and has a displacement of
184,300t.

Figure 2.20 Terra Nova FPSO.

DIS1-30815
Offshore Structures and Installations 2-17 Copyright © TWI Ltd
2.6 Floating production storage and offloading units (FPSO)
FPSOs have been used since 1977 when the first was installed off Spain. A
schematic of an FPSO setup is shown in Figure 2.21.

Figure 2.21 FPSO layout.

2.6.1 Reasons for using an FPSO

 Water depth too great for installing a fixed platform.
 Shallow water locations, where the size of the hydrocarbon accumulations
are not large enough to make a fixed installation commercially viable.
 Remote locations where export pipelines would be too expensive.
 Where the hydrocarbon accumulations are dispersed so far as to make
drilling from one platform unviable.
 Where weather conditions are extreme, using purpose designed units.

2.7 Inspection of FPSO systems

Inspection programmes for FPSOs revolve around the dry docking programme.
As they are vessels, it is usual for them to be dry docked every 5 years. This is
not a hard and fast rule in every part of the world, as there are arrangements
for these vessels to be inspected on site over a longer period than this. Also,
extensions are possible even on a five year regime.

DIS1-30815
Offshore Structures and Installations 2-18 Copyright © TWI Ltd
 However, a typical inspection programme would include:

 Mooring and riser inspection.

 ROV visual hull survey.
 Specific inspection of all sea chests.
 Turret Inspection.
 Inspect mid-water arch and confirm the lazy S configuration.
 Measurement of individual mooring chain links for wear.

These inspections should be carried out when it can be videoed. The FPSO is
loaded so the entire wetted area of the hull is under water. More detailed
inspections are normally carried out during refit or dry-dock and include:

 Eddy current, ACFM or MPI of specified components.

 Wall thickness measurements in specified areas, CP survey and replacement
of anodes.

2.7.1 Seabed facilities

The first sub-sea production Christmas tree was installed by Shell offshore
California in 1961. Since then, there has been a steady increase in these
facilities with the early wellheads being installed and serviced by divers.

Developments now allow these tasks to be completed remotely, thereby

importantly, extending the depth range for installation and maintenance.

Apart from seabed wellheads, there are also manifold centres, such as the
Underwater Manifold Centre (UMC) in Shell’s Cormorant Field, Linear Block
Manifolds (LBM), as installed in Shell’s Osprey Field and sub-sea isolation valves
(SSIV) as installed throughout the North Sea.

Currently there are more than 650 sub-sea wells of which approximately one
third are installed on the UK Continental Shelf. These structures can offer
advantages over platforms to:

 Reach remote parts of the field inaccessible to the existing platform.

 Develop a field too small to warrant the cost of a fixed platform where
process facilities can be provided as required by a floating facility.
 Develop a wide spread field using dedicated FPS and linking the wells with
pipelines.
 Develop a number of smaller fields all in the same district again using FPS.

Some of these facilities are illustrated below (Figures 2.22-2.27).

DIS1-30815
Offshore Structures and Installations 2-19 Copyright © TWI Ltd
 Figure 2.22 Artist impression of Balder Field.

Figure 2.23 Machar Field Seabed Manifolds.

DIS1-30815
Offshore Structures and Installations 2-20 Copyright © TWI Ltd
 Figure 2.24 Dunbar sub-sea choke manifold.

Figure 2.25 Seabed four well layout.

DIS1-30815
Offshore Structures and Installations 2-21 Copyright © TWI Ltd
 Figure 2.26 Renee Field manifold and wellheads.

Figure 2.27 Snohvit seabed wellheads and protection frame.

DIS1-30815
Offshore Structures and Installations 2-22 Copyright © TWI Ltd
2.7.2 Pipelines
Are used extensively for the transport of crude oil and gas and there are many
thousands of kilometres of sub-sea pipelines. Figures 2.28-2.30 indicate some
of these facilities and how they are laid.

These structures appear simple on a cursory inspection, but are carefully

designed and a good deal of specialised design effort goes into their
construction. Traditional pipelines are constructed of steel and may be made up
of nominal 40 foot (12.0m) lengths of pipe sections up to 3 feet (0.9m) in
diameter which are welded together.

Alternatively, smaller diameter steel pipe up to 2 foot (0.6m) diameter can be

laid up onto special reels and then laid in long lengths off the back of specially
designed reel-laying vessels.

Modern developments in materials have led to the widespread use of composite

pipes made of a variety of polymers. One such pipe is known as Coflexip. This
type of pipe is commonly laid from specially designed reels off the back of
suitable vessels. Figure 2.30 shows a reel-laying operation.

Figure 2.28 Gullfaks flowline bundle.

DIS1-30815
Offshore Structures and Installations 2-23 Copyright © TWI Ltd
 Standard pipe length 12.0m (40ft)

Figure 2.29 Standard pipe length.

Figure 2.30 CSO Apache reel-laying on the Cook Project.

DIS1-30815
Offshore Structures and Installations 2-24 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

DIS1-30815
Offshore Structures and Installations 2-25 Copyright © TWI Ltd
 27/08/2015

Introduction

 Offshore hydrocarbon deposits may be gas, oil

or a mixture of both. They are found at
different depths in the seabed, they are of
different sizes and the recovery of the
CSWIP 3.1U Course reserves can be easy or difficult depending on
the actual geology of the particular well.
Offshore Structures and Installations
Section 2  These factors influence the design of offshore
structures and combine to be one of the basic
reasons for there being different types of
offshore installations.

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Steel Production Platforms Steel Production Platforms

Offshore production platforms With steel fixed platforms the

are massive structures. They Jacket supports the
are constructed of both steel superstructure, which contains
and concrete, with steel being all the necessary facilities.
more prevalent.
It may be floated out using
There are standard components floatation tanks or loaded onto
with both types. a barge, towed into position
and sunk to the sea floor where
All the standard components it is then piled into the seabed.
have names and, as these also
apply to many other structures, Once the jacket is piled into the
an appreciation of this seabed the superstructure is
terminology will be of benefit. constructed using heavy lift
crane barges. Brent Alpha

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Lightweight Platforms Terminology

An example of a Gannet A Platform Following is the common terminology for the components
lightweight platform making up the steel sub-sea structure, the jacket, which is
servicing seabed wellheads constructed of steel pipe work and piled into the seabed.
and facilities is the Gannet  Node: A point on the welded steel structure where two or
A platform in Shell Expro’s more members meet and are joined.
Gannet Field in the central
North Sea.  Can: Vertical component of leg where the node is located,
ie the main body of the node.
The structure itself is of  Conductors: These are tubes for drilling purposes
the same basic design but connecting seabed wells to the topside.
not as massive as the
production platform.
 Conductor guide frame: Horizontal sections of framework,
which restrain and guide the conductors.
 Leg: The main vertical component, constructed from a
number of sections welded together, supporting the rest of
the structure.
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Terminology Terminology

Pile guide: A steel cylinder that supports the pile

while it is driven into the seabed. Pile guides are
mounted in clusters around each leg at various
levels. They are often removed on completion of
piling.
Pile sleeves: These are long steel cylinders grouped
around the base of the legs into which the piles are
located before being driven into the seabed. The tops
of the piles should be level with the tops of the
sleeves on completion of piling.
Conductors Caissons: Open bottomed tubular components
terminating at various depths for the purpose of the
intake or discharge of water or waste.
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Terminology Terminology

 Member: One of the horizontal, vertical diagonal or

horizontal diagonal braces of the jacket.

 Flowline bundles: Pipe-work bringing oil or gas from

Can satellite wellheads into the platform and containing
control lines and well injection lines.

 Oil and gas risers: The vertical section of the pipeline

extending up the full height of the jacket that are used
for transporting oil or gas. Production risers carry oil
or gas up from the seabed wellheads via the
submarine pipelines. Export risers take the processed
Pile sleeves Node
hydrocarbons down to pipelines.
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Terminology Concrete Structures

The first concrete gravity structure Brent Delta

was installed in the North Sea in
the mid -1970’s.

The concept of a concrete structure

came about because of some of the
problems associated with steel
structures, namely, the need for a
large number of heavy piles and
the corrosion problem with steel in
a hostile environment.

Concrete gravity structures require

no piles and are immune to
corrosion.

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Disadvantages of Concrete Concrete Structures

Tow stability problems.

 Heavy lift barges reduce the cost of offshore
installation, so the advantage of large deck
space fitted out in calm waters is null and
void. Troll platform
 Base storage cells must be carefully under tow
managed to avoid both tensile and thermal
stresses.
 Shaft ventilation problems.
 Accumulation of stagnant water leads to
formation of (SRBs) Sulphate Reducing
Bacteria and H2S.

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Concrete Structures Concrete Structures

Gravity structures may be

made of steel or a mixture of
steel and concrete, but the
most usual material is
concrete itself.
They are anchored to the
seabed by their own mass,
hence the term gravity.
Common features of this type
of design are the large
diameter columns supporting
the deck module and the
numerous ballast/storage
tanks (cells) making up the Statoil’s Gullfaks A
base.

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Terminology - Different Offshore

Concrete Structures
Structures
Jarlan hole

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Jack-up Rigs Semi-submersible Rigs

These rigs are used for wildcat These rigs are used for the same
drilling, production drilling and work- tasks as Jack-ups and may be self-
overs. powered or not.
Asgard B
The Jack-up consists of the main Semi- The rig has large hollow legs and
submersible
platform which is watertight (the hull) pontoons, which can be flooded or
which floats for transit. Attached to pumped dry at will, thus ballasting
the hull via a rack and pinion the platform.
assembly are the tubular steel lattice When operational they may be kept
legs. in place by anchors or by Dynamic
The gears lower the legs to the positioning (DP). That is the engines
seabed and the hull is then jacked-up are running all the time and
by this same method to clear the computers are programmed with all
water. necessary data to keep the vessel
positioned accurately over one spot
This gives a stable platform for drilling on the seabed.
and other operations.

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Drill Ships Compliant Tower

This is a tall, slim steel structure that is
The drill ship is used for Drillship Discovery 1 designed to sway slightly with the effects of
the same tasks as a
the wave action.
Jack-up but in deeper
The design is conceived as a ‘halfway
water. It is more
house’ between fixed and floating
weather dependant but
structures. It is possible to use the design
it is also more
in depths up to 1000m in moderate
manoeuvrable and
environments.
mobile.
The Baldplate GB 260 Platform is located in
499m of water, 120 miles of Louisiana
Either DP or anchors
coast.
normally keep drill ships
It is the first freestanding offshore
on station dependent on
compliant tower, as well as being one of the
the water depth, site
tallest freestanding structures in the world.
and project parameters.
The tip of the flare boom extends to 575m
(1,902ft) above the sea floor. GB 260 Compliant Tower

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GB 260 Jacket Tension Leg Floating Platforms

A Tension leg platform (TLP) Deck

consists of a hull anchored to

the seabed with vertical Hull Mooring
tendons. systems

Vertical movement is Tendons

constrained by the tendons,
which allows production wells Producti
on risers
to be located on deck.
This design is suitable for
deep-water production and
some engineers believe the Foundation
technology could be extended templates

to water depths of 3000m.

Piles Wells

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Tension Leg Floating Platforms Floating Production Systems

Floating Production Systems
(FPS) are a variation on the theme
of TLP’s and consist of a floating
vessel with production facilities
connected to seabed wells by
flexible risers.

Vessels may be purpose built or

converted and may be mono-hull
or semi-submersible. Tankers can
be converted to this task relatively
quickly and cheaply.

In this case they are usually known Terra Nova FPSO

as Floating Production Storage
Snorre B TLP
and Offloading Units (FPSO).

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Floating Production Storage

Single Anchor Leg Moorings (SALM)
and Offloading Units
FPSO’s are used where: Where tankers can load a cargo of oil at sea. A tanker cannot
manoeuver close to a production platform so the oil is piped to
a loading column. In this example it is 2km from the
 The water depth is too great to install a production platform.
platform.
 The size of the field does not warrant a
platform.
 Remote locations where pipelines are too
costly.
 The oil deposits are too dispersed for drilling
from one platform.
 Sites are exposed to extreme weather.

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Floating Production Storage

Single Buoy Mooring (SBM)
and Offloading Units
These are used for the export of
oil from the field to a tanker or to
import from a tanker to a refinery.

They consist of a single buoy

incorporating a swivel mechanism
to which the tanker is moored.

The oil is passed from a pipeline

on Seabed Via a Pipeline End
Manifold (PLEM) and subsea
flexible hoses to the buoy on the
surface, then via floating hoses to
the tanker.

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Mid-Water Buoyancy Arch Deadman Anchor

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Mid-Water Buoyancy Arch Turret Details on Norne FPSO

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FPSOs Balder Field Schematic

In-service inspection tasks: Dry dock inspection:

 Moorings and risers.  Eddy current or MPI.

 Mooring chains.  Wall thickness of
 Visual hull survey hull.
(ROV).  Replace worn
 Turret inspection. anodes.
 Mid-water arch.
 Lazy ‘S’ configuration.

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Seabed Facilities Seabed Facilities

Seabed wellheads. These structures can offer advantages over platforms:

Manifold centres (UMC) Cormorant Field.  To reach remote parts of the field inaccessible to the
Linear block manifolds (LBM) Osprey Field. existing platform.
Sub-sea isolation valves (SSIV).  To develop a field too small to warrant the cost of a
Currently there are more than 650 seabed fixed platform.
wells worldwide of which about one third are  Process facilities can be provided by a floating
installed on the UK continental shelf. facility.
 To develop a wide spread field using dedicated
FPSO’s.
 Link wells with pipelines.
 To develop a number of smaller fields all in the same
district using FPSO’s.

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Seabed Facilities Seabed Facilities

Machar Field Seabed Manifolds

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Seabed Facilities Gullfaks Field Schematic

Seabed Four Well

Layout

Gannet SSIV

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Renee Field Schematic Seabed Facilities

Renee Field Manifold and Wellheads

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Seabed Facilities Pipelines

Pipelines may appear to be simple structures but

they are carefully designed with specialised
design effort in construction.

 Traditional pipelines are constructed of

steel.
 Made up of nominal 40ft (12.0m) lengths of
pipe sections up to 3ft (0.9m) in diameter.
 Welded construction.
 Laid from a lay barge.

Snohvit Seabed Wellheads and Protection Frame

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Pipelines Pipelines

Lay barge

Gullfaks Bundle

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Pipeline End Manifold (PLEM) Pipelines

Shore-side fabrication facilities and loading jetty for reel

laying vessel loading

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Pipelines Pipelines

Concrete weightcoat performs three distinct

functions:

1. Adds weight to ensure negative buoyancy.

2. Protects from physical damage.
3. Protects from corrosion.
CSO Apache laying pipe

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Any Questions?

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9
 Section 3

Loading on Offshore Structures

3 Loading on Offshore Structures
3.1 General introduction
When any structure is being designed, the engineers will consider the forces
exerted by wind, water, weight of equipment and working loads. The material
that the structure is built from supports these forces, but it is a combination of
the strength of the material and how much of it is available that defines the
ability to withstand the loads.

For example, no one would moor a platform using chains from a bicycle lock,
but would have no problem using mooring chains, even though the former can
be made of stronger material. The reason that the mooring chain can take
higher loads is that it is thicker.

To take into consideration the strength a material needs to have and how thick
it needs to be, stress, defined by the load a material can support by area of the
cross-section taking the load, is used.

Stress is used so that comparisons with loading on other structures, of different

sizes and shapes, can be made.

Stress is defined as the force (or load) divided by the cross-sectional area
carrying that load. Stress is denoted by the Greek letter  (sigma) and is
defined mathematically as:

F

A

Load or force
or stress =
Area

Types of stress

Tensile stress: Load trying to pull a component apart, as in Figure 3.1.

Figure 3.1 Tensile stress.

Compressive stress: The opposite of tensile stress, in compression, a

component or structure is being pressed together (Figure 3.2).

DIS1-30815
Loading on Offshore Structures 3-1 Copyright © TWI Ltd
 Figure 3.2 Compressive loading of a solid.

Bending: Structures are sometimes loaded so that they are subjected to a

mixture of tensile and compressive stresses. A simple beam supported at the
ends and loaded in the middle is a good example, Figure 3.3.

Figure 3.3. A simple beam with a point load.

The top surface shortens as it experiences compressive stresses and the bottom
surface lengthens as it experiences tensile stresses. This type of loading gives a
stress distribution that varies from maximum compressive stress on one side,
to zero at an unstressed layer called the neutral axis, to maximum tensile
stress at the other side.

In this type of structure, there are both tensile and compressive stresses. Most
braces in platform structures experience this mixture of stresses.

DIS1-30815
Loading on Offshore Structures 3-2 Copyright © TWI Ltd
 Shear stress: Parallel planes shifted in relation to each other, as shown in
figure 3.4.

Figure 3.4 Shear loading of a solid.

In general, fluids and gases cannot produce shear resistance when stationary,
so internal or external pressure result in either tensile or compressive stresses.

As well as the shearing action shown in Figure 3.4, most rotating motion is
transmitted by shear; for example, the drive shaft of a car, or the force to
tighten a valve, often referred to as torsion.

3.2 Properties of materials

Materials are identified by their characteristic qualities such as hardness,
rigidity, thermal or electric conductivity, magnetic or not and so on. In order
that engineers can compare one material with another, it is necessary to
quantify these material properties. Among those properties commonly
considered when selecting a material for a particular application are:

 Yield strength – The stress at which a material begins to

deform plastically.
 Elongation – Measure of ductility.
 Toughness – Resistance to fracture (crack extension).
 Electrical conductivity.
 Thermal conductivity.
 Density – Weight per volume of material.
 Hardness – Wear resistance.

There are others but these will serve to illustrate the principle. It is not
necessary here to consider all these properties, but some comments on those
affecting load bearing is beneficial.

3.2.1 Yield stress

When a component is loaded, the material initially behaves elastically. When
the load is removed, the component returns to its original size and shape. This
will continue while the component is in use, unless the yield stress is exceeded.

Yield stress is the stress at which the material will no longer behave wholly
elastically. If the loading is continued beyond the yield point, the material will
deform and some of that deformation will be permanent. If a structure or part
of it is dented or bent, this indicates that it has been loaded above the yield
stress.

DIS1-30815
Loading on Offshore Structures 3-3 Copyright © TWI Ltd
3.2.2 Ultimate Tensile Strength (UTS)
If loading is continued well into the yield region, the applied stress reaches a
maximum value known as the ultimate tensile strength (UTS).

3.3 Mechanisms of fracture

Ductile and brittle fracture: Fracture can occur in a brittle or ductile fashion.
A ductile fracture is characterised by significant plastic deformation prior to and
during crack extension, whereas brittle fracture is characterised by the rapid
unstable propagation of a crack and can occur at an applied stress significantly
below yield strength levels and is frequently catastrophic, with no prior warning.

Very little plastic deformation is associated with the propagation of brittle

fracture and so limited permanent deformation occurs. Susceptibility to brittle
fracture is enhanced by any factor that decreases the yield strength such as:
low temperature.

Fatigue fracture: Fatigue is a mechanism of failure experienced by materials

under the action of a cyclic stress. It involves initiation and growth of a crack
under applied stress amplitude, which may be well within the static capacity of
the material. Discontinuities, such as welds, changes in section or material
flaws are favoured sites for fatigue initiation.

During subsequent propagation, the crack often remains tightly closed and is
thus difficult to find by visual inspection during the majority of its life. If
cracking remains undiscovered, there is a risk that it may propagate across a
significant portion of the load-bearing cross section, leading to final separation
by fracture of the remaining ligament. Fatigue occurs in metals, plastics,
composites and ceramics. It is the most common mode of failure in structural
and mechanical engineering components.

3.4 Stress concentration

This is caused within a material because geometric irregularities locally magnify
applied stresses. These irregularities can be large or small including; holes,
notches, sharp corners, weld toes and cracks. What is vitally important about a
stress concentration is its shape, not its size.

A standard emergency remedial procedure if a crack is found is to drill holes at

the crack tips. This is referred to as crack stopping or blunting. This same
principal applies to the toes of welds to reduce undercutting and to crack-like
features that may then be ground out, Figure 3.5.

Stress concentrated

Stress spread out

Figure 3.5 Crack stopping or blunting.

DIS1-30815
Loading on Offshore Structures 3-4 Copyright © TWI Ltd
3.5 Residual stresses
The residual stresses in a component or structure are stresses caused by
incompatible internal permanent strains. They may be generated or modified at
any stage in the component life cycle, from original material production to final
disposal. Welding is one of the most significant causes of residual stresses and
typically produces large tensile stresses of the order of magnitude of the yield
strength of the materials being joined.

Tensile residual stresses may reduce the performance or cause failure of

manufactured products. They may increase the rate of damage by fatigue,
creep or environmental degradation. They may reduce the load capacity by
contributing to failure by brittle fracture, or cause other forms of damage, such
as shape change. Compressive residual stresses are generally beneficial, but
cause a decrease in the buckling load.

3.6 Forces on a structure

The stresses on the structure will be affected by the forces that the structure
experiences. These are of two types; steady and cyclic. Several different
effects produce these forces; for example the weight of the equipment, the
reaction of the drilling forces, the hydrodynamic forces due to wind and wave
action.

3.6.1 The steady force on a structure in a fluid flow

The steady force exerted by a fluid as it passes a stationary structure is known
as the Drag Force. Therefore, if a structure is placed in a current of water or air,
it will experience a force in the direction of the flow trying to move it in that
direction.

This can be illustrated in a simple way by placing a walking stick in a swiftly

flowing stream. A holding force must be exerted to keep the stick in position.
This holding force is equal and opposite to the drag force on the walking stick
caused by the stream (Figure 3.6).

Figure 3.6. Force on a cylinder in a steady flow.

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3.6.2 Cyclic forces on a structure in a fluid flow
Consider the example of a cylinder in a fluid flow as outlined in section 3.4.1
above, but this time look at the flow pattern behind the cylinder. Figure 3.7
illustrates this.

Note: The flow behind the cylinder is not symmetrical, but those vortices are
shed alternately from each side.

Figure 3.7 Von Karmen Vortices shed from a cylinder in a fluid flow.

The effect of this is to create on the cylinder an alternating force at right angles
to the fluid flow and drag force direction (Figure 3.8).

Figure 3.8 Variations in side forces on a cylinder in a fluid flow.

The cyclic forces generated by the wind and water flowing past the structure
cause the vibrations at right angles to the flow that are so important, when
considering the fatigue life of a structure.

3.6.3 Wave loadings

Waves provide an oscillatory motion to the structure, producing forces that act
in addition to the forces produced by tidal currents.

These forces deform or try to overturn the structure.

The waves have a predominant direction for their maximum effect, but can
come from any direction, since they are wind generated.

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 Waves produced in a storm are generally short and very confused. However,
when produced by winds blowing over a long distance, or fetch, the waves tend
to moderate into long, high swell waves with a long period (distance peak to
peak).

A period of 14 seconds produces a wavelength of about 300m.

The height of the waves is independent of the period but depends upon the
stability of the waves and the energy content.

For the purposes of classification by the Duty Holder and for insurance, there
are standards for any design; these are based on statistical data. Using wave
rider buoys, as much information as possible is collected over as long a period
as possible on:

 Wave heights.
 Wave directions.
 Wave periods.

The analysis of this data produces two main results:

1 The maximum wave to be expected in a given time span, generally a 100

year period, (this so called 100 year wave is only a statistical quality
adopted for design, so more than one of these waves, or even larger
waves, might actually occur).
2 An energy spectrum of the waves (ie the graph of the energy in the waves
at different periodic times).

Structures are therefore designed for two wave loading conditions:

1 Using the maximum 100 year storm wave data.

2 Using the energy spectrum graph.

Owing to the directional properties of the waves, the structure will be designed
and placed so that the largest waves from the predominant direction are taken
on its strongest orientation, but all other directions should be considered.

Inaccuracy in placing the structure can create loads greater than the design
loads in that direction.

3.6.4 Structural response to wave loading

When a structure is placed in the sea it will experience a range of wave
energies and frequencies causing the structure to deflect:

 As the frequency of the wave energy peak approaches the natural frequency
of the structure, so the deflection of the structure increases and with it the
stress.
 The further the peaks of wave energy, frequency spectrum and natural
frequency of the structure are separated, the lower the maximum deflection
of the structure.

The same analysis applies to diving and other floating vessels in heave, roll,
yaw and pitch. Thus vessels designed for use in one part of the world may be
unsuitable for use in another, where the frequency spectrum differs.

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 The natural frequency decreases as the depth of the structure increases. Thus
new designs of structures developed for open water applications, such as the
Compliant Tower and the TLP, have natural frequencies below the wave energy
peak.

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 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

‘Failure of Stressed Materials’, Open University Press, 1983,

ISBN 13: 9780335171545.

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 27/08/2015

General Introduction

 When any structure is being designed the engineers

will consider the forces exerted by wind, water, weight
of equipment and working loads. These forces set up
stresses in the structure material.
CSWIP 3.1U Course
Loading on Offshore Structures  For example, no one would moor a platform using
chains from a bicycle lock, but would have no problem
Section 3 using mooring chains, even though the former can be
made of stronger material. The reason that the
mooring chain can take higher loads is that it is
thicker.

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General Introduction Stress

 To take into consideration the strength a  Stress is defined as the Force (or load = ω)
material needs to have and how thick it needs divided by the cross-sectional area carrying
to be, stress, defined by the load a material that load.
can support by area of the cross-section  Stress is denoted by the Greek letter sigma
taking the load, is used. (σ) and is defined mathematically as:

When a material is required to support or transmit a

load, it does so by creating a force between the atoms
of the material by moving them from their equilibrium
position. This can occur in a number of ways.

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Types of Stress Types of Stress

Tensile stress: Compressive stress:

 This is created in a material when the atoms  This is the exact opposite of tensile stress. The
are pulled apart. atoms are pressed together.

Usually tensile stresses are thought of as positive (+)

stresses and compressive stresses as negative (-)
stresses

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Types of Stress Types of Stress

Bending stresses Load Shear stress:

 When layers of atoms are pushed past each
other.
Compression Compression
Often referred to
as torsion.

Tension
Shear

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Properties of Materials Loading

Materials are identified by their characteristic qualities such Yield stress

as hardness, rigidity, thermal or electric conductivity,  When a component is loaded the material initially behaves
magnetic or not and so on. In order that engineers can elastically. This means that when the load is removed, the
compare one material with another, it is necessary to component returns to its original size and shape. This will
quantify these material properties. continue while the component is in use, unless the yield
Some properties commonly considered are: load is exceeded.
 Yield strength: The stress at which a material begins to deform
plastically.  Yield stress is therefore the stress at which the material will
 Elongation: Measure of ductility. no longer behave wholly elastically.
 Toughness: Resistance to fracture (crack extension).
 If the loading is continued beyond the yield point, the
 Electrical conductivity.
material will deform and some of that deformation will be
 Thermal conductivity. permanent. Therefore, if a structure or part of it is dented or
 Density: Weight per volume of material. bent, this indicates that it has been loaded above the yield
 Hardness: Wear resistance. stress.

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Loading Mechanisms of Fracture

Ultimate Tensile Strength (UTS): Ductile and brittle fracture:

 If loading is continued well into the yield  Fracture can occur in a brittle or ductile fashion. A
region, it reaches a maximum value known as ductile fracture is characterised by significant plastic
the ultimate tensile strength (UTS). deformation prior to and during crack extension,
whereas brittle fracture is characterised by the rapid
unstable propagation of a crack and can occur at an
applied stress significantly below yield strength levels,
and is frequently catastrophic, with no prior warning.
 Very little plastic work is associated with the
propagation of brittle fracture, and so limited
permanent deformation occurs. Susceptibility to brittle
fracture is enhanced by any factor that decreases the
yield strength such as: low temperature.

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Loading Stress Concentration

Fatigue fracture:  This is caused within a material because geometric

 Fatigue is a mechanism of failure experienced by materials irregularities magnify applied stresses locally. These
under the action of a cyclic stress. It involves initiation and irregularities can be large or small including; holes,
growth of a crack under an applied stress amplitude, which notches, sharp corners, inclusions and cracks.
may be well within the static capacity of the material.
 What is vitally important about a stress
Discontinuities, such as welds, changes in section or
material flaws are favoured sites for fatigue initiation.
concentration is its shape not its size. A standard
emergency remedial procedure if a crack is found is
 During subsequent propagation, the crack often remains
to drill holes at the crack tips.
tightly closed and is, therefore, difficult to find by visual
inspection during the majority of its life. If cracking remains  This is referred to as crack stopping or blunting.
undiscovered, there’s a risk that it may propagate across a  This same principle is applied to the toes of welds
significant portion of the load-bearing cross section, leading to reduce undercutting and to crack-like features
to final separation of the remaining ligament. that may be ground out.

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Stress Concentration Residual Stress

Residual stresses
Stress concentrated  Residual stresses in a component or structure
are caused by incompatible internal
permanent strains. They may be generated or
modified at any stage in the component life
cycle. Welding is one of the most significant
causes of residual stresses and typically
produces large tensile stresses of the order of
magnitude of the yield strength of the
materials being joined.
Stress spread out

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Residual Stress Forces on a Structure

 Tensile residual stresses may reduce the Forces on a structure.

performance or cause failure of manufactured The stresses on the structure will be affected by the
products. They may increase the rate of forces that the structure experiences.
damage by fatigue, creep or environmental
degradation. They reduce the load capacity by
contributing to failure by brittle fracture, or These are of two types:
cause other forms of damage, such as shape  Steady.
change. Compressive residual stresses are  Cyclic.
generally beneficial, but cause a decrease in
the buckling load. Several different effects produce these forces; for
example the weight of the equipment, the reaction
of the drilling forces or the hydrodynamic forces due
to wind and wave action.
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Drag Force Drag Force

The steady force on a structure in a fluid flow The force on the stick in the flow varies with the
The steady force exerted by a fluid as it passes a square of the velocity:
stationary structure is known as the drag force.
Therefore, if a structure is placed in a current of
water or air, it will experience a force in the  Double the flow speed and the drag force is
direction of the flow trying to move it in that increased by four times.
direction.  Treble the flow speed and the drag force is
increased by nine times.
This can be illustrated in a simple way by placing a  Changing the shape of a submerged object
walking stick in a swiftly flowing stream. A holding will reduce the drag force – its drag co-
force must be exerted to keep the stick in position. efficient will be less.
This holding force is equal and opposite to the drag
force on the walking stick caused by the stream.

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Drag Force Drag Force

Force on a cylinder in a steady flow Consider the example of a cylinder in a fluid flow
as shown, but this time look at the flow pattern
behind the cylinder. Notice that the flow behind
the cylinder is not symmetrical, but those vortices
are shed alternately from each side.

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Vibration Forces on a Structure Wave Loading

Vortex shedding causes alternating forces at Waves provide an oscillatory motion to the structure, producing
right angles to the flow forces that act in addition to the forces produced by tidal
currents.
These forces deform or try to overturn the structure.
The waves have a predominant direction.
 Waves produced in a storm tend to be short and confused.
However, when produced by winds blowing over a long
distance, or fetch, the waves moderate into long, high swell
waves with a long period.

The cyclic forces generated by the wind and water Statistics are gathered over a long time period regarding:
 Wave heights.
flowing past the structure cause the vibrations at right
 Wave directions.
angles to the flow that are so important, when
 Wave periods – distance between wave peaks.
considering the fatigue life of a structure.
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Wave Loading Wave Loading

Structures are therefore designed for two conditions:  When a structure is placed in the sea it will
experience a range of wave energies and
Static loading
This is to do with extraordinary conditions and is based on the frequencies causing the structure to deflect.
theoretical 100 year wave.

Dynamic loading  As the frequency of the wave energy peak

Is to do with ‘normal’ waves and is based on an energy spectrum approaches the natural frequency of the structure,
derived from data gathered over a considerable time period in a
particular area.
so the deflection of the structure increases and with
it the stress.
So the structure is orientated to withstand the largest
wave action and the predominant wave direction.

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Wave Loading

 The further the peaks of wave energy, the

frequency spectrum and the natural frequency
are separated, the lower the maximum
deflection of the structure.

 The natural frequency decreases as the depth Any Questions?

of the structure increases. Therefore new
designs of structures developed for open
water applications such as the compliant tower
and the TLP have natural frequencies below
the wave energy peak.

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5
 Section 4

Deterioration of Offshore Steel Structures

4 Deterioration of Offshore Steel Structures
4.1 General comments
As soon as any piece of engineering equipment, such as an engine, pipeline,
bridge or offshore structure is brought into service, it starts to wear out
because of use and if it is not maintained, it will eventually cease to operate
satisfactorily, either by no longer carrying out the function for which it was
designed or by failing in a catastrophic manner. The possible causes of
deterioration of an offshore structure including accidental damage, corrosion,
fatigue, wear and embrittlement are discussed in this section.

4.2 Categories of deterioration and damage

Broadly speaking, the modes of deterioration may be classified into six groups:

1 Gross structural damage.

2 Corrosion and erosion.
3 Fouling defects.
4 Coating defects.
5 Scour.
6 Metal and weld defects.

Specific types of deterioration and damage within these groups may be

categorised as:

 Deformation of the structure caused by impact.

 Loss of concrete matrix through impact or internal flaws.
 Missing bolts.
 Coating damage through abrasion or impact or deterioration.
 Damaged cables or ducts caused by impact or deterioration.
 Unstable foundations through poor geology.
 Missing members caused by accidental damage or failure.
 Debris, which may cause impact damage or create fouling or overload the
corrosion protection system.

4.3 Accidental damage

Engineers will try to anticipate all the different modes of failure when they first
design a structure, but deterioration due to accidental damage is difficult to
design against (this does not prevent the guidance notes for offshore structural
design from recommending that engineers in fact do just that).

In the United Kingdom, Safety Cases have to be submitted to the HSE for
evaluation and assessment, in an attempt to prevent accidental damage from
being a threat to safety. Because of the difficulties associated with preventative
design, one of the prime methods for dealing with accidental damage is the
implementation and effective execution of reporting procedures for informing
the appropriate responsible persons as soon as any accidental damage has
occurred.

This type of damage is also likely to occur on any structure because of the
reliance placed on being serviced by boats and helicopters. This presents a real
possibility for damage caused by accidents, such as collisions and the dragging
of either anchors or trawls across seabed installations.

Ideally, as indicated above, should this type of accident happen it should be

reported and surveyed as soon as it happens. But in the past, at least, such
accidental damage was mainly discovered during routine inspections; the event
not having been reported.

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 There have been numerous examples of this type of damage and just by way of
illustration; the Northwest Hutton suffered an accident during installation that
resulted in a main leg suffering loss of member straightness. An accident
involving a stand-by vessel and Brae Bravo resulted in a horizontal member just
above the splash zone suffering a similar fate.

4.4 Corrosion
Because steel is placed in a hostile environment, namely salt water, one of the
ever-present deterioration mechanisms on the structure will be corrosion.

Corrosion takes place in two different ways:

 Firstly, uniform corrosion is the process whereby metal is removed from all
over the surface as anodic areas continually shift, so that progressive
thinning of the member or pipe wall goes on until the thickness is reduced,
necessitating renewal of the component.

 Secondly, pitting corrosion is a localised corrosion which takes place in an

otherwise corrosion free material, creating a pit in the surface of the
material.

These pits deepen with time and, if another failure mechanism does not take
over, the pit will penetrate the full thickness of the material, causing leakage in
the case of a pipeline or service duct and so necessitate local repair.

Corrosion attacks of both kinds are accelerated by erosion, increase in

temperature and increase in oxygen content, added chemical attack from
biological sources and loading on the member from either external loading or
residual stresses caused in manufacture.

The latter is known as stress corrosion. As corrosion is such an important

deterioration mechanism in the offshore environment. The entire subject is
more fully explained in future sections.

4.5 Fatigue
Fatigue is the local failure of the material by crack growth caused by cyclic
loading. The cracks can grow from flaws in the material, such as a welding
defects or notches caused by accidental damage. Alternatively, they can initiate
in regions of highly stressed material, which are brought about by residual
stresses or stress concentration. Fatigue cracks can also start from pits created
by corrosion. This condition is known as corrosion fatigue and is covered in
section 8.

Fatigue causes more in-service failures of machines, vehicles, bridges and

similar structures than any other mode of failure.

The main reason for fatigue failure being so prevalent and therefore so
important, is that it can occur when the applied stress is significantly lower than
the yield stress of the material. Indeed, even if the stress intensity is kept
below the fracture toughness limit, Kic-crack growth can occur.

Fatigue is a cumulative form of failure, in that a crack is initiated at some point

of stress concentration and then propagates through the material by acting
virtually as its own stress raiser.

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 The final fatigue crack is the result of the accumulation of the small-scale
events associated with each of a great many load cycles. The fatigue crack
eventually reduces the cross-sectional area to such an extent that final failure
occurs by rapid fracture, often with gross deformation of the remaining un-
cracked area.

Fatigue cracking does not affect the material properties (fracture toughness
remains unchanged). Different materials have varying resistance to fatigue,
although the experience of service failures and laboratory testing has
demonstrated that fatigue is difficult to predict. This is because the process is
sensitive to a large number of variables including:

 Number of load cycles.

 Stress or strain amplitude.
 Mean stress level.
 Temperature.
 Environment.
 Microstructure of the material.
 Surface condition.

For design purposes the metallurgist and the design engineer centre their
interest on the results of laboratory tests that assess the number of loading
cycles N of a given type that the sample survives before fracture occurs.
Measurements of N are made as a function of the stress amplitude. When N is
plotted on a logarithmic scale against stress amplitude the S-N curve for the
material is obtained.

4.6 Wear
Normally thought of as the loss of material from surfaces that have been
rubbed against one another, it is often measured in terms of the mass lost in a
given time under specified conditions. More precisely, wear involves a
redistribution of material that adversely alters the surface. In the offshore
environment, wear is the thinning of material due to uniform corrosion, erosion
or a combination of the two. In the wider sense wear can be caused by a
number of different mechanisms.

Adhesive
When two surfaces rub together it causes friction and to explain this it is
assumed that some welding of the two contact surfaces occurs within the
contact area.

The mechanism of adhesive wear follows directly from this. When the two
surfaces slide over each other, material breaks away at the weakest sections.
These are the hills which make contact, as indicated in the sketch in Figure 4.1.

Figure 4.1 Adhesive wear.

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 The junctions at which the surfaces are in contact have been strengthened by
work hardening and, therefore, the fractures take place within the materials, at
some distance away from the interfaces between the points of contact (the
shaded areas in Figure 4.1). Each surface tears out some material from the
other and both surfaces become roughened as they gouge and score one
another. Wear is rapid and for this reason, in good engineering practice, sliding
combinations of similar metals are usually avoided.

Abrasive wear
In the mechanism for abrasive wear, a hard particle in one surface indents,
grooves and then cuts material from the other surface. In service, the main
cause of abrasion between sliding metals is the presence on one of the two
surfaces of particles of hard materials, such as carbide in steels, work hardened
wear fragments or hard oxide films. The particles may also be air or waterborne
dirt such as grit.

Wear caused by fatigue

When there is relative motion between two surfaces in contact, the state of
stress at any given point on or near the surfaces varies with time and this may
cause fatigue, the slow growth of cracks. The development of such cracks may
eventually detach pieces of material from the surfaces, thereby constituting
wear.

Chemical and corrosive wear

Chemical effects are most commonly exemplified by the repeating cycle of the
formation, removal and reformation of oxides (Rust films).

4.7 Embrittlement
In service could come about due to incorrect welding procedures or by the
absorption of a gas, generally hydrogen. It has been encountered in natural gas
pipelines and could also come about from the absorption of hydrogen produced
by an overprotective impressed current corrosion protection system or
associated with sour service. The temperature of the environment affects the
brittle behaviour of steel, brittle fracture being more likely to occur at low
temperatures.

4.8 Structural deterioration

The foregoing paragraphs outline the modes of failure associated with any steel
structure and these failure systems will now be put into the context of offshore
steel structures. Concrete structures are considered in Section 5. A convenient
way of illustrating these types of failures is to divide the life of a structure into
four stages. At every stage defects leading to deterioration and then failure can
occur.

4.8.1 Stage one – production of the raw materials

During the manufacturing of the raw materials, several defects can be included
into what will become the parent plate, pipe or profile. Defects include
laminations, pores, segregation of undesirable inclusions, incorrect chemical
composition or heat treatment condition.

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Figure 4.2 Casting defects.

When the molten steel is poured into the mould, some of the material may
splash onto the sides where it cools quickly and hence does not meld with the
bulk of the molten metal. When subsequently rolled they form fishtails, a
thinning of the metal.

Figure 4.3 Close up of a fishtail.

If the mould is not at the correct temperature, impurities can be trapped in the
metal forming bands, which when later rolled out, form segregation layers.
These layers can cause problems if the material is stressed during welding,
when the segregated layer tears, this is known as lamellar tearing. Pipes may
also form if the mould temperature is incorrect, when rolled these pipes form
laminations.

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 Carbon
Silicon
Manganese
Sulphur
Phosphorus Incorrect cooling rate
pipe forming

When rolled forms

lamination

Correct cooling rate

Figure 4.4 Incorrect mould temperatures.

Figure 4.5 Lamination.

4.8.2 Stage two - fabrication

While the structure is being built there are a multitude of problems associated
with all the aspects of fabrication. Ensuring that the correct materials are being
used, verifying the correct fit-up and tolerances are applied and many other
specific construction details are important daily tasks for all construction staff
throughout the fabrication period of any structure.

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 Steel structures fabrication defects
With steel structures the major fabrication processes involve welding, therefore,
some of the problems associated with this process will be outlined.

There are numerous variables associated with welding and each of these can be
subjected to either human or system errors, some of which are listed:

 Incorrect machining of the angle of bevel.

 Improper pre-heat treatment.
 Poor fit-up.
 Using improper weld consumables.
 Incorrect storage of weld consumables.
 Incorrect post-heat treatment.

These possible faults have to be guarded against during the fabrication stage of
any offshore structure.

During the actual welding process, there are a number of possible weld defects
that must be avoided. These are fully explained in section 11 for illustration. A
short catalogue follows:

Lack of root penetration

This is a weld defect associated with both Submerged Arc and Manual Metal Arc
(MMA) welding. Setting too low a voltage with the submerged arc process
causes the defect. Incorrectly positioning the welding rod is the cause with
MMA. In either case, the result is a crack-like defect in a potentially very
sensitive part of the weld.

Slag inclusions
These defects are likely to occur when MMA welding is the weld method utilising
a multi-pass technique. The cause of the defect is when slag from the previous
run is not cleaned off completely. This leaves isolated pieces of slag that remain
and are over-welded by the next run. These inclusions form the sites for
potentially dangerous notches.

Porosity
This is a weld defect that must always be guarded against. There may be many
causes for this flaw such as:

 Air contamination of the weld pool.

 Dirt or damp finding their way into the weld.

These contaminants break down in the weld and produce:

 Nitrogen.
 Hydrogen.
 Carbon monoxide.

These gases dissolve in the weld pool and then, as it cools, they come out of
solution forming gas bubbles, which is porosity in the weld.

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 Hydrogen-induced cold cracking
This final example of a fabrication defect is a type of cracking normally formed
in the heat affected zone (HAZ) quite some time after the weld is completed.
The cracking may occur almost immediately, some hours later, or even days
after the weld is finished, which is why NDT is carried out forty eight hours after
completion of welding, when there has been a risk of Hydrogen-Induced
Cracking (HIC).

The cause of cracking is hydrogen, initially dissolved in the weld pool,

permeating through the weld into the HAZ in sufficient quantities to embrittle
the microstructure; causing cracking.

4.8.3 Avoiding problems by design

Designers are aware of the problems associated with fabrication and the
processes that accompany it and over time have evolved new designs to
minimise these problems. It is well known, for example, that stress is
concentrated at any site where there is a sharp change of geometry such as in
weld toes and traditional tubular joints (Figure 4.6).

Figure 4.6 Stress concentration areas.

As illustrated in Figure 4.6, the stress concentration may be lowered by profiling

the weld cap to make the geometry more contoured and less angular.

By alterations to the design concept the stress concentration areas may be

removed from the nodal areas by utilising cast joints, thus removing the
welding to less highly stressed regions (Figure 4.7).

Figure 4.7 Cast node.

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4.8.4 Stage three - installation
Steel structures are commonly constructed on their side and then floated into
position where they are rotated to the upright position by flooding ballast
compartments in the jacket legs. This rotation imposes a bending moment on
the structure that may impose stresses that are, briefly, greater than the
working stresses the structure will subsequently withstand.

Of course, the flooding operation is conducted as carefully as possible and some

modern steel structures have been positioned with the ballast tanks pre-flooded
to minimise the stresses involved. The method is illustrated in Figure 4.8 and
shows the sequence of events with a self-floating structure.

Arriving on Site Controlled Flooding

1 2 Controlled Flooding 3
Flooding Begins of Upper Legs

4 Upending Almost Completed 5 Positioning 6 In Place

Figure 4.8 Self-floating structure.

4.8.5 Stage four - in-service

This is the stage in the life of a structure where underwater inspection first
becomes dominant. The major categories of defects that cause concern are
outlined below.

Steel in-service defect categories:

Fouling
This covers both marine growth building up on the structure and debris
collecting on and around it. Fouling may cause structural damage, galvanic
corrosion (see section 8), overloading of the CP system or cause safety hazards
to divers and ROV’s.

Coating damage
All types of coatings; paint, bituminous, epoxy and metallic may suffer from
defects caused either when they were applied or subsequently because of
deterioration or accidental damage, see section 9.

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 Cracks
These may be caused by latent flaws initiated during any of the earlier stages in
the life of the platform. They are most often associated with welded joints,
especially on nodal areas and cracking may be the end result of a defect
initiated at the fabrication stage. As stated earlier in this section, fatigue is the
major cause of component failure in-service.

This type of failure may be avoided if the crack is identified at an early stage,
before it propagates. It can be considered to be a notch at this stage and profile
grinding will remove this defect. This will reduce the weld throat thickness and
the wall thickness, however, provided this is kept within design parameters and
a smooth profile is achieved, the possibility of failure is more remote. Profile
grinding is more fully discussed in section 17.

Corrosion
An important form of structural deterioration, which is covered in detail in
sections 7-10. A great deal of underwater inspection effort goes into monitoring
corrosion.

Physical damage
This form of deterioration is generally caused by either collision or impact
damage caused by components being dropped. As mentioned earlier, all
accidental damage, indeed any incident, should be reported immediately so that
it can be assessed.

Figure 4.9 illustrates damage caused to a horizontal diagonal member by a 24m

length of caisson pipe that had fallen off from three levels above and pierced
the member. No one on the platform was aware that anything had happened.

Figure 4.9 Sketch indicating damage caused by a caisson section that failed in
service.

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Deterioration of Offshore Steel Structures 4-10 Copyright © TWI Ltd
 The foundations of a structure are an obvious area susceptible to movement of
material on the seabed. Any movement is likely to weaken the foundations
which, of course, jeopardises the whole structure.

4.8.6 In-service defect categories that affect both steel and concrete
The in-service defects listed for steel can also affect concrete. Concrete
structures may suffer from cracks, the reinforcement may corrode and physical
damage and scour is also quite possible. Cracks in the concrete surface are less
serious than cracks in steel because offshore structures fall into the
pre-stressed category and major components are therefore kept in
compression.

There are other considerations that do affect both steel and concrete structures
and that may cause defects in service:

Inter-tidal and splash zones

The inter-tidal and splash zones on any structure are regions of particular
susceptibility to deterioration:

 Corrosion is more aggressive in this area and must be more carefully

monitored.
 Marine growth build-up is greater in the top 20m of the sea and is
particularly dense in the inter-tidal region. This will increase mass and drag
in a part of the structure most vulnerable to these effects. Marine growth
may also affect corrosion rates.
 The risk of physical damage is greater in this region due to the risk from
floating objects and, in those parts of the world that are susceptible,
icebergs may collide with the structures. Certainly this is possible in offshore
Canada for example.

Risers
Are components common to both types of structures, although on concrete
platforms they may be installed inside the shaft, it is not uncommon to have
them mounted externally as well, these items are considered as part of the
associated pipeline and therefore are inspected annually because they can
suffer the same deterioration as pipelines. The clamps, guides and flanges are
subjected to the same regime.

Conductors and conductor guide frames

As with risers, these components can be common to all platforms and they are
exposed to the same risk of failure as risers, perhaps more so as there are
greater vibrations possible with these components than the rest of the platform.
Furthermore, conductors are normally kept in place by guides rather than
clamps, which allow relative movement between the conductor and its guides;
hence wear must be monitored, as there is a real possibility for fatigue cracking
to occur.

Caissons
Caissons are another group of components carefully monitored on an annual
basis. There is a common problem with this component when it is used as a
pump caisson. The pump is commonly suspended from the surface inside the
caisson. It is common for the pump to be at about 18m water depth level inside
the caisson. Conditions at this point on the inside of the caisson are near
perfect for corrosion to progress at excessive rates. This has caused component
failure on more than one occasion.

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 Overloading
Changes in the working practices and other commercial factors may lead to
extra items of equipment being installed, such as a newer, bigger crane. This
may lead to overloading if not carefully monitored.

4.9 Repairs to offshore structures

When any defects are identified on offshore structures, the Duty Holder’s
engineering department will make a decision on what course of action to take
depending on the severity of the defect and its position on the structure. It may
be that increasing the inspection effort monitors the defect or it may be
repaired. The types of defect that may be identified are:

 Welding defects.
 Impact damage.
 Fatigue damage.
 Corrosion.

Welding defects will normally be the most sensitive items and remedial action is
therefore more likely with this type of anomaly.

Impact damage may well be monitored as indicated earlier in this section.

Fatigue damage is the most difficult to identify and component failure may well
occur before this type of defect is identified. Fatigue is also very difficult to
predict and may therefore be the subject of a repair.

Corrosion consumes metal in the corrosion process and reduces the wall
thickness of the structural members. Corrosion is such a serious consideration
for offshore structures that the subject is dealt with fully in sections 7-11.

4.9.1 Welding repairs

Underwater welding repairs may be completed by wet welding or by deploying a
hyperbaric chamber and using dry welding procedures.

Wet welding
This has been used underwater for at least the last 75 years. However, due to
the problems associated with it such as:

 Uncontrolled cooling rate.

 Brittleness caused by the quenching effects of the water.
 Lack of sidewall fusion.
 Lack of inter-run fusion.
 Hydrogen embrittlement.

This technique has, until recently, only been considered to be a temporary

repair or a non-structural repair to low stressed components.

However, in the last few years there have been advances in wet welding and
currently techniques are available that may be used for structural repairs.

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 Hyperbaric welding
Requires that a hyperbaric chamber is sealed onto the repair site and the weld
is then completed in dry conditions. Specially qualified welder divers are used to
complete the weld.

4.9.2 Clamp repairs

Two types of clamps may be used for repairs:

1 Grout clamps.
2 Friction clamps.

Grout clamps
These are used to repair pipeline leaks and nodal joints.

The clamp is positioned and, once it is in place with the bolts tightened, the
annulus is pumped full of grout which completes the repair.

Friction clamps
Fitted by bolting on and will be manufactured to close tolerance so that when
the bolts are tightened the repair clamp offers a proper stress path for the loads
imposed on the repaired area. This type of clamp is fitted to at least one
offshore structure, where it has been in place for some 20 years without further
deterioration of the structure.

4.10 Repair inspection

All repairs will be inspected to ensure compliance with the procedure, that the
repair work is to the required standard, the damage register is maintained up to
date and that engineering confidence in the structure is maintained.

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 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office

Books), 1988, ISBN 13: 9780114129118.

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Deterioration of Offshore Structures

General:

 As soon as any piece of engineering

equipment, such as an engine, pipeline, bridge
CSWIP 3.1U Course or offshore structure is brought into service it
starts to wear out because of use and, if it is
Deterioration of Offshore Steel Structure not maintained, it will eventually cease to
Section 4 operate satisfactorily, either by no longer
carrying out its designed function or by failing
in a catastrophic manner.

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Categories of Deterioration Gross Structural Damage

Deterioration of an offshore structure can be 1. Gross structural damage.

broadly classified into six categories:
 Deformation due to impact damage.
 Gross structural damage.  Missing bolts.
 Corrosion and erosion.  Damage or deterioration of cabling or cabling
 Fouling defects. ducts.
 Coating defects.  Missing members, anodes or other
 Scour. components.
 Metal and weld defects.  Engineers will try to anticipate all the different
modes of failure when they first design a
structure, but deterioration due to accidental
damage is difficult to design against.
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Corrosion Fouling Defects

2. Corrosion. 3. Fouling defects.

Uniform corrosion: Whereby metal is removed from all over

These consist of both marine growth and debris.
the surface reducing the member or pipe wall thickness.
Marine growth is dealt with later.
Pitting corrosion: Localised corrosion which takes place in
an otherwise corrosion-free material creating a pit in the Debris is any type of foreign body on or near the
surface. structure such as scaffolding bars, wire strops,
Both types of corrosion are accelerated by: soft line etc.
 Erosion.  The effects of debris can be as follows:
 Increase in temperature.  Structural damage.
 Increase in oxygen content.  Galvanic corrosion.
 Chemical attack from biological sources.  Overloading of the CP system.
 Loading (external or residual) known as stress corrosion.  Safety hazard to divers and ROV’s.

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Coating Defects Scour

4. Coating defects. 5. Scour.

 Scour is the undermining of the seabed from
Paint and bitumen coatings can have the following defects: around the structure’s foundation. This will be
 Poor surface adhesion. brought about due to an increase in the water
 Blistering of the coating. flow rate around the base of the structure, in
 Flaking. extreme cases this could lead to failure of the
 Sagging and wrinkling. structure. This would be especially serious in a
 Cracked surface coating. concrete structure.
 Metal and weld defects
The first three will be a progression, with flaking being the
worst lack of adhesion. Sagging and wrinkling will normally be  This is fully explained in the chapter on
associated with thicker coatings. Cracked surface coating will welding.
possibly allow water to come into contact with the metal.

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Fatigue Fatigue

 Fatigue is the local failure of the material by Fatigue cracks can also start from pits created
crack growth caused by cyclic loading. The by corrosion.
cracks can grow from flaws in the material,  Fatigue causes more in-service failure than
such as welding defects or notches caused by any other mode of failure.
accidental damage. They can also initiate in  Fatigue is so prevalent because it can occur
regions of highly stressed material, which are when the applied stress is well below the
brought about by residual stresses or stress yield stress of the material.
concentration.
 Fatigue is a cumulative form of failure, in
that a crack is initiated at some point of
stress concentration and then propagates
through the material by acting virtually as
its own stress raiser.

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Fatigue Wear

The final fatigue crack is the result of the accumulation of Wear is the thinning of a material due to corrosion,
the small-scale events associated with each of a great erosion or a combination of the two.
many load cycles. The fatigue crack eventually reduces the
cross-sectional area to such an extent the final failure It can also be caused by a number of different
occurs by rapid fracture, often with gross deformation of mechanisms:
the remaining un-cracked area.
 Adhesive wear: Friction between two surfaces.
Fatigue cracking does not affect fracture toughness and is
difficult to predict in any material, this is because the  Abrasive wear: Hard particles on one surface
process is sensitive to a large number of variables, gouge the other.
including:
 Wear by fatigue: Micro-cracks detach particles
 Number of load cycles.  Environment. of material that cause gouges.
 Stress or strain  Microstructure of the
amplitude. Material.
 Chemical and corrosive wear: Oxides form that
 Mean stress level.  Surface condition. are removed and then reformed wasting
 Temperature. material.

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Embrittlement (Brittle Failure) Structural Deterioration

 Embrittlement is where the material changes its Defects leading to failure may be caused at any
properties from being ductile to brittle. This can be a of the four stages in the life of a structure.
localised effect.
 Brittle materials fail due to crack propagation so that
they are susceptible to fatigue as well as to brittle  Production of the materials.
fracture.
 Fabrication.
 Embrittlement in service could come about due to
incorrect welding procedures or by the absorption of  Installation.
gas, generally hydrogen.  In-service.
 Embrittlement has been encountered in natural gas
pipelines and could also come from the absorption of
hydrogen produced by an overprotective impressed
current system.
 Brittle fracture is more likely at low temperatures.

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Production of the Raw Material Fishtails

Stage one: Casting defects

Rolled Material

Splashes

Fishtails

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Inclusions Lamination

Carbon
Silicon
Manganese
Sulphur
Phosphoru Incorrect cooling
s rate pipe forming

When rolled forms

lamination

Correct cooling
rate

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Fabrication Welding Defects

Stage two: Fabrication defects. During the welding process there are a number of
 With steel structures the major fabrication possible weld defects that must be avoided, such
processes involve welding, therefore some of as:
the problems associated with this process will  Lack of root penetration.
be outlined:  Slag inclusions.
 Porosity due to air contamination of weld pool or
 Incorrect angle of bevel.
dirt and damp finding their way into the weld.
 Improper pre-heat treatment.
These contaminants break down in the weld and
 Poor fit-up.
produce either:
 Wrong weld consumables.
 Nitrogen, hydrogen, carbon monoxide.
 Improper post-heat treatment.
 These gases dissolve in the weld pool, then as it
cools come out of solution forming gas bubbles
which is porosity in the weld.

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Welding Defects Welding Defects

Hydrogen-induced cold cracking:  The cause of cracking is hydrogen, initially

 This final example of a fabrication defect is a type dissolved in the weld pool permeating through
of cracking normally formed in the Heat Affected the weld into the HAZ in sufficient quantities
Zone (HAZ) quite some time after the weld is to embrittle the microstructure; causing
completed. The cracking may occur almost cracking.
immediately, some hours later, or even days after
the weld is finished, which is why NDT is carried
out 48 hours after completion of welding, when
there has been a risk of hydrogen- induced
cracking.

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Avoiding Stress by Design Installation

Stage three: Installation

 Steel structures are installed in two major
operations.
Stress concentration
areas are identified  The Jacket is positioned and piled in.
and welds are  The deck modules are craned into place and
dressed. commissioned.
 During all the individual processes associated
with these operations structural damage may
occur.
Cast nodes are a method
that is employed to  A post-installation base-line survey is always
‘design out’ stress undertaken on completion of the installation of
concentration areas. the structure.

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Installation Stage Four: In-Service

Defects that may occur during the service life of a

structure:
 Fouling – marine growth and debris can cause
structural damage, galvanic corrosion, overloading
of the CP system, diver and ROV safety concerns.
1
Arriving on Site
Flooding Begins
2 Controlled Flooding 3
Controlled Flooding
of Upper Legs  Coating damage – caused on application or by
deterioration or accidental damage.
 Cracks – caused by latent flaws, welding faults,
fatigue.
 Corrosion – can cause structural failure and is so
important it is considered separately.
 Physical damage – caused by collision or impact.
 Scour – may weaken seabed foundations.
4 Upending Almost Completed 5 Positioning 6 In Place

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In-Service

Fatigue cracking Physical damage

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In-Service

Specific types of in-service deterioration.

Inter-tidal zone deterioration:
 More aggressive corrosion.
 More excessive marine growth.
 High risk of physical damage through collision and
dropped objects.

Risers – carrying product at higher pressure and

temperature.
Conductor guide frame and conductors – vibration,
pressure and wear.
Caissons – vibration and wear.
Overloading – installation of extra or bigger equipment.

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In-Service Repairs

Repairs to offshore structures. Welding repairs:

When any defects are identified on offshore structures
the Duty Holder’s engineering department will make a  Underwater repairs may be completed by wet
decision on what course of action to take, either to welding or by deploying a hyperbaric chamber
monitor or repair. and using dry welding procedures.
The type of defects that may be identified are:
 Welding defects.
 Impact damage.
 Fatigue damage.
 Corrosion.
Welding defects will normally be the most sensitive
items and remedial action is more likely with this type
of anomaly.
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Repairs Repairs

Wet welding has been used underwater for at Hyperbaric welding:

least 70 years, however due to the problems
associated with it, such as:  A hyperbaric chamber is sealed onto the repair
 Uncontrolled cooling rate. site and the weld is then completed in dry
 Brittleness caused by quenching effect of the conditions.
water.
 Lack of sidewall or inter-run fusion.  The actual weld technique is frequently the
 Hydrogen embrittlement. TIG method.
 The technique has, until recently, only been
used for temporary repairs or non-structural  Specially qualified welder divers are employed
repairs to low stressed components. to complete the weld.

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Repairs

Clamp repairs.
Two types of clamps may be used for repairs:

 Grout clamps: These are used to repair pipeline leaks

and node joints. The clamp is positioned and once the
bolts are tightened the annulus is pumped full of grout
which completes the repair. Any Questions?
 Friction clamps: These are fitted by bolting on and will
be manufactured to close tolerance so that when the
bolts are tightened the repair clamp offers a proper
stress path for the loads imposed on the repair area.
 All repairs will be inspected to ensure compliance with
the procedure and the damage register is maintained up
to date.

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6
 Section 5

Deterioration of Offshore
Concrete Structures
5 Deterioration of Offshore Concrete Structures
Concrete is a man-made stone material which may be cast to form a wide
variety of components or structural elements. The ancient Romans called it
Liquid Stone and enacted the first ever building regulations aimed at controlling
the quality of concrete.

Concrete derives its strength from the stones and sands (coarse and fine
aggregates) that it contains and not from the cement binder. Indeed, on its
own, cement with no aggregate has roughly the same strength as a baked
biscuit.

The aggregate in concrete gives the material excellent compressive strength

and stiffness. This is exploited by engineers when designing concrete
foundations for structures or concrete columns.

Concrete is very durable but it needs to be regularly inspected and maintained.

Typically, concrete structures have long design lives. For example, the Troll
structure has a design life of seventy years.

Another advantage of concrete is that it is readily available throughout the

world as we may utilise local aggregates for the mix. It is also relatively cheap
to produce when compared to other structural materials like steel.

Unfortunately, concrete is relatively weak when loaded in tension or shear. If

we require a structural element to withstand such stresses, then we find that
simple concrete may not be the best material to use. However, higher-
performance concrete systems have been developed that overcome these
weaknesses, although they do increase complexity and cost of the structure.

5.1 The nature of concrete

Concrete is a composite material composed of three basic ingredients. Civil
Engineers will be able to specify the exact types of each ingredient and its
relative proportions in order to create a material with the properties desired.

There is often a requirement for some flexibility in concrete structures in order

to avoid in-service cracking. There will, therefore, always be a compromise that
needs to be met with the concrete properties; between stiffness and strength.

5.2 Aggregate
Aggregate is hard material that is responsible for the compression strength and
stiffness of the concrete. Typically, stone is used that is quarried locally to the
construction site. Sometimes, however, aggregate that is not native stone must
be transported to the site with attendant increases in cost.

Aggregates may be quarried from land sites, they may be manufactured by

crushing larger rocks or they may be harvested from beaches. In the latter case
the aggregate may contain significant levels of salt (sodium chloride).

In this case, it is important that the aggregate be washed thoroughly in fresh

water in order to remove the salt. If salt-contaminated aggregate is used in the
production of concrete then it may accelerate the corrosion of built-in
steelwork.

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 The presence of salt-contamination in concrete may be discerned by observing
the formation of salt crystals on the surface of the concrete. This effect is
termed efflorescence. It should be remembered though, that salt crystals will
not form underwater and their presence in the splash-zone is not definitive of
salt-contamination.

The size and shape of aggregate are important factors that determine the
mechanical properties of the concrete. Generally, we classify fine aggregate as
stones that will pass through a 5mm sieve (sand). If only sand is used in the
concrete mix then the concrete (termed mortar) will be more flexile but less
strong than concrete made with coarse aggregate.

Coarse aggregate is generally defined as stone that will not pass through a
5mm sieve (gravel). Concrete made with a mixture of sand and gravel will
have greater strength than mortar and will be stiffer (and thus less flexible). By
specifying the proportions of sand and gravel and in the concrete the material
properties can be precisely controlled.

5.3 Binder (cement and water)

There are two types of cement – hydraulic and non-hydraulic. By far the most
common type is hydraulic cement. This is formed by baking calcium (limestone
– calcium carbonate) and silicon (clay) until it forms nodules called clinker. This
clinker is then ground with gypsum to a fine powder to form the highly alkaline
cement.

Hydraulic cements react with water in a process called hydration. The cement
forms a solid binder that holds the aggregate together to form a monolithic
mass of material – ie the mixture is said to set.

For cement to hydrate properly around 25% water (by weight) is required. In
practice excess water is added (often a total proportion of 40% or more) to
improve workability of the liquid concrete. Better workability means that the
concrete flows more easily through pipes and into moulds. The pouring of the
concrete at this stage is called placement.

Excess water forms capillary voids within the concrete and must be lost from
the bulk of the material. This happens by migrating to the surface of the
concrete (bleed water) and by evaporation from the surface. It is true to say
that the less excess water in the concrete mixture; the stronger the final
material. Thus, it is beneficial in terms of material strength to limit the water to
the minimum possible whilst still maintaining the required workability.

If the water content is too high then proliferation of capillary voids will result in
a porous and weak material. Also, as the excess water migrates to the surface
to be lost by bleeding and evaporation, then volume shrinkage will occur in the
outer layers of the concrete. This shrinkage effectively tightens the surface of
the concrete over the sub-surface layers forming a skin of tensile stress. As
concrete is relatively weak in tension this often leads to plastic shrinkage
cracking.

If there is embedded steelwork like reinforcement close to the surface of the

concrete, then excessive water may also lead to settlement of aggregate
particles around the steel. This may result in plastic settlement cracking
which will mimic the pattern of the reinforcement.

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 Hydration is an exothermic reaction – that is, it generates heat. If a large
amount of concrete is poured all at once then the heat produced during the
hydration process causes the sub-surface material to expand. Cooling at the
surface causes contraction and a skin of tensile stress forms. This may cause
thermal cracking. Thermal cracking may be avoided by limiting the bulk of
each pour or by laying sacrificial chilled-water pipes within the mould.

As the concrete hydrates it moves from a liquid state, gradually becoming

increasingly viscous until it sets as a solid. During this time the alkalinity
stabilises at around pH 12.5.

After the concrete has set to a solid a period of curing continues during which
silicon polymers form. The initial cure phase is considered to last from a number
of days to a number of weeks, during which time the temperature and
evaporation must be carefully controlled.

During the cure the concrete continues to harden and gain in strength. In fact,
concrete reaches around 70% of its final strength after the initial cure and
around 90% of its final strength after about one month. After that the material
very slowly becomes harder and stronger, reaching full maturity after around
27 years.

To control temperature and evaporation during the post-placement cure, the

concrete is often protected from the environment by covering with fabric or
plastic sheeting. Sometimes a water misting or spray system is used to slow the
rate of evaporation and reduce the likelihood of cracking.

Concrete slabs may be flooded with water during this time for the same reason
(called ponding). Vertical walls such as slip-formed structures may be painted
with curing compounds that form an evaporation-retarding membrane over the
surface of the concrete. Curing compounds may be removed following the initial
cure or may be left in place.

5.4 Additives
Additives (termed admixtures) are chemicals that are added to the concrete
mix to enhance its properties or to change its workability or rate of hydration:

Plasticizers: By adding a plasticizer we may reduce the water content of the

mix yet still retain adequate workability during construction. For this reason,
plasticizer is sometimes referred to as water-reducer.

Accelerators/retarders: These chemicals change the rate of hydration to suit

the construction method used. It is sometimes critical to accurately control the
rate at which the concrete sets, eg when slip-forming.

Others: The final properties of the concrete may be enhanced by the addition
of other admixtures. For example, a water-retardant may be added to enhance
resistance to penetrating water.

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5.5 Loading on concrete
Concrete is extremely strong when loaded in compression. It also has very
good compression stiffness. Unfortunately, it is relatively weak when loaded in
tension or shear.

Engineers may design a structure or a component to avoid loading the concrete

in tension or shear. For example, a concrete dam is often curved in order to
translate the hydrostatic pressure on the convex side of the dam into
compressive stress within the dam wall.

However, tension and shear cannot always be avoided by design. For example,
when a beam is supported at either end and is loaded from above, we see that
the lower face will be in tension and that shear stress is imposed at either end.
In this case, the concrete can be strengthened by embedding reinforcement or
by using a pre-stressed beam.

5.6 Types of concrete

There are several types of concrete that are commonly used in construction:

5.6.1 Grout
Grout is a mixture of cement and water. It may contain admixtures but does
not contain any aggregate.

It is obvious when we consider that the strength of concrete comes from its
aggregate that grout is not a strong material. It is, however, flexible and so is
used where we require some movement in the structure.

Typical applications for grout include:

 Sealant, to seal construction joints.

 Filler or packing material, between steel piles and the pile-sleeves.
 Repair material, typically for relatively small areas of repair.

Grout is never used as a structural material.

5.6.2 Mortar
Mortar is a mixture of fine aggregate (sand), cement and water. It often also
contains admixtures.

Since mortar contains fine aggregate, it is stronger than grout but is still
significantly weaker than concrete. Like grout, it is used when some flexibility is
required. It is often used as a jointing material – effectively forming a flexible
interface between two stiffer, stronger components.

Typical applications for mortar include:

 Jointing.
 Sealant.
 Filler or packing material.
 Repair material.

The same as for grout, mortar is not used as a structural material.

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5.6.3 Plain concrete
Plain concrete is a mixture of coarse and fine aggregate, cement and water.
Again, it often contains admixtures.

The term plain concrete refers to the fact that this type of concrete does not
contain any reinforcement. Thus, plain concrete is very strong and stiff in
compression but relatively weak in tension and shear.

Typical applications for plain concrete include:

 Structures and structural elements that are principally loaded in

compression.
 Repair material, typically for relatively large areas.

5.6.4 Reinforced concrete

Reinforced concrete is plain concrete that has embedded steelwork. The
function of the steel is to take any tensile or shear loading that the material is
subjected to. As such, the reinforcement is only laid in locations within the
concrete which will be subject to tension and/or shear – see Figure 5.1.

Figure 5.1 Placement of reinforcement within a beam.

The steel reinforcement most commonly takes the form of bars

(reinforcement-bar or re-bar). Re-bar is usually round-section, ribbed bar
with a diameter of between 10-50mm.

Figure 5.2 Steel reinforcement bars on a construction site.

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 Typical applications of reinforced concrete include:

 Structures and structural elements that may be subjected to tension and/or

shear stresses.

It is important to note that when first loaded, a reinforced concrete component

will deform as it takes up the load (strain). At this stage, the concrete may
crack as the steel reinforcement takes up any tensile or shear stresses. This is
known as settlement cracking and is common in reinforced concrete
components. This type of settlement should stabilise and the cracking should
not progress after the initial loading of the component.

The steel reinforcement is protected from corrosion whilst within the concrete
because of the high pH (around 12.5) of the material. This stimulates the
formation of a passive skin of iron (3) oxide around any embedded steelwork.
As a result, the steel is passivated. It is very important that the passive
environment around the reinforcement is maintained and that no water is
allowed to seep in around the steel.

The current standard relevant to concrete structures is EN 1992: Design of

Concrete Structures, (Eurocode 2). This standard replaces CP110 under which
many existing structures were built. Both standards require a minimum cover of
60mm over any embedded steel reinforcement (between the steelwork and the
outside environment) in order to protect against water ingress causing loss of
passivation.

If passivation is lost by water ingress causing chloride attack or by carbonation,

then the iron (3) oxide skin will be lost and replaced by iron (2) oxide. This
form of oxide expands as it forms, creating an increase in volume of the oxide
layer around the re-bar. This effectively pushes the concrete apart in the area
local to the corroding reinforcement.

A shear zone is created around the expanding oxide and the concrete will
eventually fail in shear. This failure is characterised by a crack along a 45° line
to the action of the shear stress. The resulting defect is therefore characterised
by a 45° crack surrounding material that has been pushed away from the
surface of the concrete.

Depending upon the orientation of the re-bar relative to the surface of the
concrete, this process will result in a popout or delamination – see Figure 5.3.

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 Figure 5.3 Loss of passivation causes expansive oxidation of the re-bar.

The case on the left will eventually result in a popout as the end of the re-bar
pushes a cone of concrete away from the surface. The case on the right will
most likely lead to delamination where a plate of material comes away from
the surface. Both are forms of spalling – a general term used to describe the
loss of material from the surface of concrete.

5.6.5 Pre-stressed concrete

Pre-stressed concrete contains steel rods or tendons that run within the
component. The tendons are pulled to a predetermined tension and locked into
anchor points (Cachetage Points), effectively compressing the concrete
component. The tension of the tendon is calculated to be greater than any in-
service tensile stress and the concrete is maintained in its strongest loading
state – see Figure 5.4.

Figure 5.4 Pre-stressed concrete beam is held in compression by the tendon.

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 If the tendon is stressed before the concrete is placed it is termed
pre-tensioned.

Post-tensioned components are built with conduits (ducts) cast within them.
The tendon is placed within the conduit and stressed using screw-threads or a
stressing ram after the concrete has been placed – see Figures 5.5, 5.6 and
5.7.

Figure 5.5 Conduits in place (and reinforcement) prior to placing concrete.

Figure 5.6 Cachetage points with tendon bundles locked in place.

Figure 5.7 A Hydraulic Ram.

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 Once the tendons are locked into the cachetage points, grout is injected into the
conduit to protect the steel by passivation.

As the concrete is maintained in a compressive load condition then this type of

concrete should not exhibit any settlement cracking or in-service cracking.

5.7 Weight coat

Weight coat is a generic term applied to the concrete-type covering of a
pipeline. Its primary function is to control the buoyancy of the pipeline and
ensure sufficient on-bottom mass to maintain stability. It also has the
secondary functions of impact protection and corrosion protection.

Although often called concrete weight coat, the actual material is commonly a
form of mortar called Gunite. The coating is commonly sprayed onto a pre-laid
wire mesh or reinforcement web. As such, it is often a very viscous mix with
very low water content.

5.8 Organic polymers

Although strictly not a form of concrete, organic polymers are often found
associated with concrete structures. The term organic refers to the carbon-
chain chemical nature of these materials, eg Epoxy.

Typical applications for organic polymers are:

 Sealant.
 Repair material for relatively small areas.
 Bonding material.

Organic polymers are very commonly used for defect repairs and may be
injected into the surface of a concrete structure to stabilise the material. They
are often activated by mixing with a chemical hardener just prior to application.
They may be readily identified by their hard, glassy or plastic-like texture.

5.9 Concrete construction techniques

Before the concrete can be placed it must me produced, usually in bulk in a
concrete plant. For large projects, such plant is sited at the construction yard.

Initially, the dry ingredients are dry-mixed and then water and any admixtures
are added. Quality control of the proportions of the ingredients and water are
essential at this stage. After a thorough mixing the concrete is given a unique
batch number, a time-stamp and any samples taken for testing. With projects
such as offshore structures, each batch of concrete has a use-by time
associated with it – if it is not placed by the expiry of that use-by time then it is
rejected.

Offshore concrete structures are built in a shore-based construction yard. The

structure is built in the upright orientation, ie from the ground upwards. Smaller
structures may be entirely constructed in the yard and then towed out when
completed. With larger structures, once they reach a certain height the yard is
flooded and the structure moved to sheltered water for completion.

The basic components of a concrete structure are illustrated in Figure 5.8.

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 Figure 5.8 Terminology used to describe components of a concrete structure.

There are three basic methods used to place the concrete to form a structure;
pre-casting, fixed shuttering and slip-form shuttering.

5.9.1 Pre-cast concrete

Pre-cast concrete is manufactured in a factory and is commonly used to
produce multiple items of a certain design of component – eg pipe sections.
Because of economies of scale, high-quality, re-usable moulds are used which
give good surface finish without the need for tie-bolts.

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5.9.2 Fixed shuttering
With fixed shuttering construction, walls (formwork) are built to contain the
concrete whilst it sets. The height of the formwork is limited by the hydrostatic
pressure exerted on the bottom of the formwork by the wet concrete. When the
concrete is placed within the formwork, it is consolidated by vibration or
tamping. When the concrete has set, the formwork may be removed and
reassembled higher up ready for the next pour of concrete.

There are various formwork systems available:

 Wooden formwork: Versatile and useful for creating complex shapes.

 Engineered formwork: Re-usable steel or aluminium modules.
 Plastic formwork: Re-usable plastic modules.
 Structural formwork: Not removed after the concrete has set – it forms part
of the reinforcing system.

Figure 5.9 Wooden formwork.

The use of fixed shuttering necessitates that each successive layer of concrete
sets sufficiently to allow for removal and reassembly of the formwork. This
gives rise to a series of horizontal construction joints in the structure.

Construction joints are joints made between successive placements of concrete

in the fixed shuttering. They are weak points in the structure in terms of lateral-
shear loading and are also potential places where water may penetrate the
structure. To help seal the joint, it is often rebated (set back) and filled with
sealant, such as grout, mortar, bitumen or epoxy – see Figure 5.10.

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 Figure 5.10 Rebated construction joint with sealant.

5.9.3 Slip-form shuttering

Slip-form shuttering is useful when constructing tall components with
unchanging shape such as the cells and columns of offshore structures.

The formwork is built onto a jacking system. The concrete is continually placed
within the slip-form as it fills with material, it is jacked upwards using hydraulic
rams. The typical rate of climb is 200-250mm per hour, see Figure 5.11.

Figure 5.11 Slip-form shutters with work-platform supported on a sacrificial

jacking-rod.

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 Because the slip-form is continually moving and the concrete is continuously
being placed, there are no construction joints ie the structure is monolithic with
no structural weak points.

As the slip-form creeps upwards, the concrete that is exposed at the lower skirt
of the formwork tends to bulge slightly. This gives rise to a characteristic
feature of slip-formed structures – a set of regular horizontal ridges known as
weatherboarding, see Figure 5.12.

Figure 5.12 Weatherboarding and vertical drag mark on a slip-formed structure.

Other features characteristic of slip-formed structures include irregular

horizontal ridges caused by slight variations in the concrete and the
construction process, and vertical drag marks caused by minor debris being
trapped between the formwork and the structure.

Without the slip-forming method of construction, offshore structures would

simply take too long to build. Slip-forming is a rapid and efficient method of
constructing extremely tall structures.

5.10 Other features of offshore concrete structures

5.10.1 Jarlan holes
A feature sometimes found set into concrete breakwater walls and anti-scour
fences – see Figures 5.13 and 5.14.

Figure 5.13 Jarlan holes set into walls to dissipate fluid energy.

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 The Jarlan hole is a nozzle that forces water to speed up as it is forced through
by tidal or wave action. At a critical velocity the flow becomes turbulent,
dissipating the fluid energy and reducing hydrodynamic loading.

Figure 5.14 Jarlan nozzles force water to speed up creating turbulence.

5.10.2 Columns
Columns of concrete structures may be dry or free-flooding. Indeed, there may
be both types on the same structure with one column acting as a utility shaft
containing risers and conductors. In such shafts, bacterial attack may lead to
the release of hydrogen sulphide gas and great care must be taken to ensure
that the column is sufficiently ventilated.

5.10.3 Expansion joints

Expansion joints are narrow gaps, which are sometimes left between large
masses of concrete to allow for thermal movement. Even if in-service thermal
variations are not significant, expansion joints may be required to allow for
movement during the hydration phase of the construction.

On offshore structures, expansion joints may allow for thermal movement

caused by storage of hot production fluids or to allow some structural flexibility.

Often, expansion joints will be sealed with a flexible sealant such as bitumen –
see Figure 5.15.

Figure 5.15 Expansion joint with sealant.

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5.10.4 Tow-out, placement and commissioning of concrete structures
Once completed, the structure is towed to location by tugs. When positioned
correctly, it is ballasted to sink gently to the seabed. The structure is
maintained in an upright orientation during this phase, limiting the imposed
bending moments. Depending on the nature of the substrate and underlying
rock, the structure may sit directly on the bed of the sea or on a prepared
foundation (Base raft).

Once on the seabed, on-bottom mass is increased by further water ballasting

and possibly also solid ballasting using dense material such as iron ore. Suction
anchors or skirt piles may also be employed to aid stability.

It should be noted that not all of the cells in the caisson of the structure will be
used for ballast – cells are also available for storage of production fluids and
product.

Loading of the deck, modules and superstructure may take place at the
construction site or at the final location utilising heavy-lift barges see Figures
5.16 and 5.17.

Figure 5.16 Structure with topsides being towed to site.

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 Figure 5.17 Topsides being loaded at final location.

5.11 In-service deterioration of concrete structures

Concrete structures deteriorate by two means – physical and chemical attack:

5.11.1 Physical attack

Physical attacks include the following assaults:

 Impact – the inter-tidal zone is most at risk of impact damage from floating
objects.
 Overload – eg storage cells need careful management to avoid overloading
diaphragm walls with hydrostatic imbalance.
 Freeze/thaw cycles can attack the exposed part of the structure forcing
small crevices and cracks to widen and eventually lead to spalling.
 Expansion of reinforcement caused by corrosion can lead to cracking and
spalling.
 Abrasion – concrete is relatively easily abraded and care must be taken to
avoid any rubbing or fretting of concrete surfaces.

5.11.2 Chemical attack

Chemical attacks include the following:

 Alkali-aggregate reaction (AAR).

 Sulphate attack.
 Chloride attack.
 Carbonation.
 Corrosion of inset steelwork.
 Biological attack.

The degradation of concrete by environmental action is termed weathering.

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5.11.3 Alkali-aggregate reaction
Most types of aggregate used in the production of concrete are chemically
stable. Some aggregate, however, reacts with the high pH of concrete. In
particular, aggregate containing silica will undergo alkali-silica reaction.

During the alkali-silica reaction, silica forms a gel and begins to absorb water.
The gel swells and starts to exert an internal pressure within the concrete. Gel
may weep from the surface of the concrete (exudation) and can lead to
expansion, cracking and spalling.

5.11.4 Sulphate attack

Seawater contains sulphates that, if allowed to penetrate into the concrete, will
react with the hydrated cement to form a crystalline solid called ettringite
(calcium-aluminium sulphate). This causes a volume expansion, leading to
cracking and spalling.

5.11.5 Chloride attack

If seawater seeps next to the steel reinforcement within the concrete then
chlorides will destroy the passive iron (3) oxide skin. This will initialise the
reinforcement corrosion process described in Section 5.6.4 – Reinforced
concrete, eventually resulting in cracking and spalling.

5.11.6 Carbonation
If atmospheric carbon dioxide reaches the steel reinforcement within the
concrete then passivation will be lost in a similar way to chloride attack. This
will eventually result in reinforcement corrosion, cracking and spalling.

Although carbonation is a theoretical risk for an offshore structure, if it has

been built according to either EN 1992 or CP110, then the process would be too
slow to pose any real threat to an offshore structure, although a crack could
theoretically speed up the process.

5.11.7 Biological attack

Under anaerobic conditions, sulphate-reducing bacteria living in the small voids
in the concrete produce hydrogen sulphide. Some of this gas may escape to the
surface of the material and if allowed to build up, can pose a significant health
risk.

However, some of the hydrogen sulphate may be captured by aerobic bacteria

that oxidise it to form sulphuric acid. This will dissolve away the carbonates
from the hydrated cement and weaken the material. Eventually this will lead to
cracking and spalling.

5.11.8 Corrosion of inset steelwork

If, for example, the anchor point for an external fixing such as a riser clamp
should make electrical contact with the embedded reinforcement, then a
corrosion cell will form. In this case, the embedded part of the inset steelwork
becomes cathodic and the external part of the steelwork becomes anodic,
leading to accelerated corrosion.

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5.12 Imperfections of concrete
The U.K. government has published an offshore technology report (OTH-
84-206) that categorises twenty six imperfections that may be seen on
concrete structures. This report defines three categories:

1 Category A: Defects.
2 Category B: Areas of Concern.
3 Category C: Blemishes.

It can be said that each of these imperfections may be either a construction

imperfection or an in-service imperfection - Table 5.1 summarises this
information.

5.12.1 Category A: Defects

Category A (defects) usually require detailed investigation and are likely to
generate an intervention to repair the defect.

The category A (defects) are:

 Cracks.
 Impact damage.
 Popouts.
 Delamination.
 Variable cover.
 Exposed reinforcement.
 Tearing.
 Poor repairs.

Popouts and delamination may be referred by the general term of spalling –

the loss of material from the surface of the concrete.

5.12.2 Cracks
Cracks may form because of:

 Plastic shrinkage cracking.

 Plastic settlement cracking.
 Thermal cracking.
 Overload.
 Freeze/thaw cycles.
 Alkali-aggregate reaction.
 Settlement cracking.
 Internal corrosion.
 Sulphate attack.

Cracks may take the form of individual, distinct general cracks, or of a

network more fine pattern cracking – see Figure 5.18.

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 Figure 5.18 Two horizontal general cracks and an area of pattern cracking.

Figure 5.19 Impact damage showing the colours of the broken aggregate.

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 Figure 5.20 A popout - a conical depression with walls sloped at 45°.
Note: The corroding end of the re-bar in the centre.

Figure 5.21 Delamination caused by reinforcement corrosion.

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 Figure 5.22 Variable cover.

A construction defect caused by laying the reinforcement too close to the

shutter.

Figure 5.23 Exposed reinforcement.

An in-service defect, in this case caused by abrasion.

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 Figure 5.24 Tearing.

Caused by premature movement of the shutter or slumping of the concrete

whilst in a semi-liquid state.

Figure 5.25 Poor repair showing multiple defects.

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5.12.3 Category B: Areas of concern
These will usually be monitored but may require an intervention to repair.

The category B (areas of concern) are:

 Embedded objects.
 Recessed metal plates.
 Cast-in sockets.
 Abrasion.
 Water-jet damage.
 Honeycombing.

Figure 5.26 Embedded objects (wood and wire-ties).

Note: The construction joint below.

Figure 5.27 A recessed metal plate, painted for corrosion protection.

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 Figure 5.28 Cast-in socket.

Soft edges indicate that this is a cast-in socket and not one that was drilled
after the concrete had set.

Figure 5.29 Abrasion damage has cut into the surface and exposed the
aggregate.

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 Figure 5.30 Water-jet damage has exposed colours of the aggregate.

Figure 5.31 Honeycombing is caused by lack of consolidation during placement.

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5.12.4 Category C: Blemishes
Category C are surface imperfections that have no structural significance.

Category C (blemishes) are:

 Construction joint.
 Blowholes.
 Scabbling.
 Good repair.
 Resin mortar repair.
 Sealant run.
 Rubbing-down marks.
 Formwork misalignment.
 Regular horizontal ridges.
 Irregular horizontal ridges.
 Vertical drag marks.
 Curing compound.

Figure 5.32 Construction joint.

Figure 5.33 Blowholes.

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 When the concrete is vibrated during consolidation, air tends to migrate
outwards until it meets the face of the shutter. Upon solidification the bubbles
of air form blowholes.

Figure 5.34 Scabbling.

The intentional roughening of the surface and when it is done using a machine,
the pattern will be regular - as above.

Figure 5.35 A good repair.

A good repair does not refer to the neatness of the repair; it refers to the
integrity of the repair, eg is the material bonded well?

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 Figure 5.36 A resin mortar compound.

Resin mortar compounds often have a hard glassy or plastic texture, eg epoxy.

Figure 5.37 Sealant runs beneath a construction joint.

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 Figure 5.38 Rubbing-down marks.

Rubbing-down marks are caused by troweling or wiping the surface during

construction.

Figure 5.39 Formwork misalignment.

Formwork misalignment is caused during construction.

Typically, the slip-form spends a period of time paused and then creeps
upwards at set intervals – often every one hour. This results in a series of
regular horizontal ridges, formed when the concrete bulges slightly as it departs
the lower skirt of the formwork.

Irregular horizontal ridges are caused by minor variations in the construction

process and the material.

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 Figure 5.40 Vertical drag marks and horizontal ridges.

Vertical drag marks are formed by minor debris becoming trapped between the
formwork and the surface of the structure. Often the debris consists of a piece
of concrete adhered to the shutter. As the slip-form moves upwards then the
debris is dragged with it.

Figure 5.41 Curing compound on concrete.

Curing compound is painted on the surface of the concrete to control

evaporation. It may or may not be removed after the initial curing period.

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5.12.5 Uncategorised imperfections
There are a few additional features and imperfections that OTH-84-206 does
not categorise.

These include the following:

 Expansion joints.
 Missing or damaged sealant.
 Surface staining.
 Corrosion of inset steelwork.
 Biological attack.

Figure 5.42 Expansion joint with a longitudinal crack.

This expansion joint has a longitudinal crack running along the sealant.

Note: The pattern cracking on either side.

Figure 5.43 Construction joint with some sealant missing.

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 Surface staining may be the result of:

 Exudation (from alkali-aggregate reaction).

 Efflorescence (salt crystals formed during evaporative loss of water).
 Lime encrustation.
 Corrosion.

Corrosion is the only stain that is likely to be seen during an underwater survey.

It is important when photographing staining to include a colour bar so that the

photograph may be presented in true colour. The engineer may then be able to
gauge the age of the defect by the colour of the stain.

Figure 5.44 Corrosion staining (this photograph should contain a colour bar and
scale).

Figure 5.45 Brackets with corrosion.

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 The corrosion of the brackets could be caused by earthing of the inset steel
against the reinforcement.

Figure 5.46 Marine growth on a concrete structure.

Barnacles do not attack concrete; however some species of tube-worms will

bore into the surface of the material.

5.13 Inspection of concrete structures

5.13.1 Cleaning for Inspection
Concrete is prone to abrasion injury and must be cleaned with care. Indeed, a
high pressure water-jet will effectively cut into the concrete surface and such
damage is classified as an area of concern.

The acceptable methods of cleaning concrete structures include:

 Low pressure water-jet.

 Hand brushing with nylon brushes.
 Plastic hand scrapers.

Brush-carts may be used so long as they are fitted with nylon brushes.

5.13.2 Navigation and positioning

It is the nature of concrete structures that they are relatively featureless and
are usually very large. This makes navigation and location reporting particularly
difficult. An ideal solution to this problem is to equip the inspector with an
underwater transponder; however, this is not always possible.

One solution is to mark up a grid on the surface of the structure. The grid must
be referenced to a known datum, such as a riser. The grid squares may be
named according to an alpha-numeric system as shown below in Figure 5.47.
Features and imperfections may be reported by their alpha-numeric grid
coordinate.

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 Figure 5.47 An alpha-numeric grid marked on the surface of the structure.

Note: The grid is referred to a riser as a datum.

A major drawback of the alpha-numeric grid system is that it is time-consuming

to set up. A quicker solution is to use a down-line and distance-line technique,
although the down-line may be replaced with a datum such as a riser – see
Figure 5.48. Using this method, the feature positions may be reported using
their depth and distance-from-datum. When depth is quoted then the inspection
controller must be aware of the tidal height at the time of the inspection in
order to convert the depth to an elevation according to the structural height
datum.

Figure 5.48 The down-line and distance-line technique.

Due to the problems in navigating and location reporting when inspecting

concrete structures, very often ROVs are used in preference to divers.

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5.13.3 Reporting requirements
The specific reporting requirements for any inspection will be specified by the
client’s procedure and should be made explicit in the inspection controller’s
briefing. As a general guideline it would be good practice to include the
following data in the verbal report:

 Location of imperfection.
 Type of imperfection.
 Severity.
 Orientation and/or pattern.
 Extent of imperfection (length, width or percentage area).
 Maximum depth (penetration into the surface).

When describing cracks, we can classify the appearance of the cracking into two
types:

General cracking: These appear as well-defined, distinct cracks where a

length, width and shape will be apparent. – see Figure 5.49.

Pattern cracking: These cracks form a network that are best described by
their pattern and the area that they cover – see Figure 5.49.

When describing the width of cracks, the convention below may be used:

 Fine cracks: Less than 1mm in width.

 Medium cracks: Between 1mm and 2mm in width.
 Wide cracks: Over 2mm in width.

A full summary of typical reporting requirements is contained within table 5.1.

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 Table 5.1 General reporting requirements for concrete inspection.
Imperfection OTH-84- Type Typical reporting
206 requirements
General cracks A In-service Length, orientation, width,
pattern, depth
Pattern cracks A In-service As above plus length and width of
pattern area
Impact damage A In-service Length, width and depth
Popouts A In-service Diameter, depth
Delamination A In-service Length, width and thickness
Exposed A In-service Length, width, amount of
reinforcement reinforcement affected
Tearing A Construction Length, orientation, width, depth
Variable cover A Construction Severity
Poor repair A Construction Length, width, type
Embedded B Construction Size, number, type
objects
Recessed metal B Construction Length, width, condition
plates
Cast-in sockets B Construction Size, depth, type, condition
Abrasion B In-service Length, width, depth, pattern
Water-jet B Construction Length, width, depth, pattern
damage
Honeycombing B Construction Length. width and maximum
depth
Construction C Construction Width (height), condition
joints
Blowholes C Construction Area, maximum depth
Scabbling C Construction Length, width
Good repair C Construction Length, width, type
Resin mortar C Construction Length, width, type
repair
Sealant run C Construction Extent, maximum length, type
Rubbing-down C Construction Area, pattern
marks
Formwork C Construction Extent, misaligned offset
misalignment
Regular C Construction Separation, maximum ridge
horizontal ridges height
Irregular C Construction Length, maximum ridge height
horizontal ridges
Vertical drag C Construction Width, depth, length (height) if
marks possible
Curing C Construction Area, colour
compound
Expansion joints --- Construction Length, width, condition of
sealant
Missing/damaged --- In-service Extent (length), type, detail of
sealant damage
Staining --- In-service Colour, texture, area, thickness
Corrosion of --- In-service Extent, depth if pitting, anode
inset steelwork condition if present
Biological attack --- In-service Extent (area), maximum depth

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5.14 General concrete terms
Spalling
Spalling is considered to be a symptom of something more serious. A spall is a
loose piece of concrete, which must have come from a spalled area.

Grout
Grout is semi-fluid slurry consisting of cement and water.

Gunite
Concrete sprayed by compressed air. It will have high strength and density,
used to repair walls and as weight coat on pipelines. It has a darker colour than
normal concrete.

Cable duct
Cast tubular duct through which the pre-stressing tendons will run. Normally
grout filled after tensioning.

Pre-stressed concrete
Concrete that has all the tensile and shear stresses relieved by the introduction
of compressive stress on the structure.

Base raft
The foundation slab bearing on the seabed.

Caisson
Large cylindrical structure often referred to as a cell.

Cell
Void bounded by diaphragm walls, term used synonymously with caisson for the
base cells of a structure.

Invert
The lowest point of an opening or tunnel.

Soffit
The underside of a concrete beam.

Jarlan hole
Perforation in a breakwater wall, used to dissipate the forces from wave action,
some of the force will be repelled and some will be admitted through the wall
where the Venturi principle dissipates the energy thus reducing the forces
acting on the wall.

Laitance
This is a fine powdery substance, which accumulates on the surface of concrete
as it sets; it will need to be removed prior to any new pour being applied.

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Deterioration of Offshore Concrete Structures 5-37 Copyright © TWI Ltd
 Exudation
Exudation consists of salts, which dissolve, in the concrete when fluid is passing
through a crack; it shows on the surface of the concrete as a whitish semi-fluid,
which accumulates around the crack. Note: On the surface it will always run
downwards, however in water it may drift sideways or even upwards, owing to
the fact that its density may be less than the water around it.

DIS1-30815
Deterioration of Offshore Concrete Structures 5-38 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office

Books), 1988, ISBN 13: 9780114129118.

Weidmann G, Lewis P, Reid N, ‘Structural Materials’, The Open University and

Butterworth’s, 1990, ISBN 13: 9780408046589.

DIS1-30815
Deterioration of Offshore Concrete Structures 5-39 Copyright © TWI Ltd
 27/08/2015

Introduction

 Concrete is a man-made stone material which

may be cast to form a wide variety of
components or structural elements. The
ancient Romans called it Liquid Stone and
CSWIP 3.1U Course enacted the first ever building regulations
Deterioration of Offshore Concrete Structures aimed at controlling the quality of concrete.
Section 5
 Concrete derives its strength from the stones
and sands (coarse and fine aggregates) that it
contains and not from the cement binder.
Indeed, on its own, cement with no aggregate
has roughly the same strength as a baked
biscuit.
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Introduction Introduction

 The aggregate in concrete gives the material  Another advantage of concrete is that it is readily
excellent compressive strength and stiffness. available throughout the world as we may utilise
This is exploited by engineers when designing local aggregates for the mix. It is also relatively
concrete foundations for structures or concrete cheap to produce when compared to other
columns. structural materials like steel.
 Unfortunately, concrete is relatively weak when
 Concrete is very durable but it needs to be loaded in tension or shear. If we require a structural
regularly inspected and maintained. Typically, element to withstand such stresses then we find
concrete structures have long design lives. For that simple concrete may not be the best material
example, Troll structure has a design life of to use. However, higher-performance concrete
seventy years. systems have been developed that overcome these
weaknesses, although they do increase complexity
and cost of the structure.
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The Nature of Concrete The Nature of Concrete

 Concrete is a composite material composed of  Aggregate is hard material that is responsible for
three basic ingredients. Civil Engineers will be the compression strength and stiffness of the
able to specify the exact types of each ingredient concrete. Typically stone is used that is quarried
and its relative proportions in order to create a locally to the construction site. Sometimes,
material with the properties desired. however, aggregate that is not native stone must
be transported to the site with attendant increases
in cost.
 There is often a requirement for some flexibility
in concrete structures in order to avoid in-service
cracking. There will, therefore, always be a  Aggregates may be quarried from land sites, they
compromise that needs to be met with the may be manufactured by crushing larger rocks or
concrete properties; between stiffness and they may be harvested from beaches. In the latter
strength. case the aggregate may contain significant levels
of salt (sodium chloride).
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The Nature of Concrete The Nature of Concrete

 In this case it is important that the aggregate be The size and shape of aggregate are important
washed thoroughly in fresh water in order to factors that determine the mechanical properties
remove the salt. If salt-contaminated aggregate is of the concrete. Generally, we classify fine
used in the production of concrete then it may aggregate as stones that will pass through a
accelerate the corrosion of built-in steelwork. 5mm sieve (sand). If only sand is used in the
concrete mix then the concrete (termed mortar)
 The presence of salt-contamination in concrete will be more flexile but less strong than concrete
may be discerned by observing the formation of made with coarse aggregate.
salt crystals on the surface of the concrete. This
effect is termed efflorescence. It should be
remembered though, that salt crystals will not form
underwater and their presence in the splash-zone
is not definitive of salt-contamination.
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The Nature of Concrete Binder (Cement and Water)

Coarse aggregate is generally defined as stone There are two types of cement – hydraulic and
that will not pass through a 5mm sieve (gravel). non-hydraulic. By far the most common type is
Concrete made with a mixture of sand and hydraulic cement. This is formed by baking
gravel will have greater strength than mortar calcium (limestone – calcium carbonate) and
and will be stiffer (and thus less flexible). By silicon (clay) until it forms nodules called clinker.
specifying the proportions of sand and gravel This clinker is then ground with gypsum to a fine
and in the concrete the material properties can powder to form the highly alkaline cement.
be precisely controlled.
Hydraulic cements react with water in a process
called hydration. The cement forms a solid
binder that holds the aggregate together to form
a monolithic mass of material – ie the mixture is
said to set.

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Binder (Cement and Water) Binder (Cement and Water)

For cement to hydrate properly around 25% Excess water forms capillary voids within the
water (by weight) is required. In practice excess concrete and must be lost from the bulk of the
water is added (often a total proportion of 40% material. This happens by migrating to the
or more) to improve workability of the liquid surface of the concrete (bleed water) and by
concrete. Better workability means that the evaporation from the surface. It is true to say
concrete flows more easily through pipes and that the less excess water in the concrete
into moulds. The pouring of the concrete at this mixture; the stronger is the final material. Thus,
stage is called placement. it is beneficial in terms of material strength to
limit the water to the minimum possible whilst
still maintaining the required workability.

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Binder (Cement and Water) Binder (Cement and Water)

If the water content is too high then proliferation If there is embedded steelwork like
of capillary voids will result in a porous and weak reinforcement close to the surface of the
material. Also, as the excess water migrates to concrete, then excessive water may also lead to
the surface to be lost by bleeding and settlement of aggregate particles around the
evaporation, then volume shrinkage will occur in steel. This may result in plastic settlement
the outer layers of the concrete. This shrinkage cracking which will mimic the pattern of the
effectively tightens the surface of the concrete reinforcement.
over the sub-surface layers forming a skin of
tensile stress. As concrete is relatively weak in Hydration is an exothermic reaction – that is, it
tension this often leads to plastic shrinkage generates heat. If a large amount of concrete is
cracking. poured all at once then the heat produced during
the hydration process causes the sub-surface
material to expand.

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Binder (Cement and Water) Binder (Cement and Water)

Cooling at the surface causes contraction and a After the concrete has set to a solid a period of
skin of tensile stress forms. This may cause curing continues during which silicon polymers
thermal cracking. Thermal cracking may be form. The initial cure phase is considered to last
avoided by limiting the bulk of each pour or by from a number of days to a number of weeks,
laying sacrificial chilled-water pipes within the during which time the temperature and evaporation
mould. must be carefully controlled.
During the cure the concrete continues to harden
As the concrete hydrates it moves from a liquid and gain in strength. In fact, concrete reaches
state, gradually becoming increasingly viscous around 70% of its final strength after the initial
until it sets as a solid. During this time the cure and around 90% of its final strength after
alkalinity stabilises at around pH 12.5. about one month. After that the material very
slowly becomes harder and stronger, reaching full
maturity after around 27 years.
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Binder (Cement and Water) Additives

To control temperature and evaporation during the Additives (termed admixtures) are chemicals
post-placement cure, the concrete is often protected that are added to the concrete mix to enhance
from the environment by covering with fabric or its properties or to change its workability or rate
plastic sheeting. Sometimes a water misting or of hydration:
spray system is used to slow the rate of evaporation
and reduce the likelihood of cracking. Plasticisers: By adding a plasticiser we may
Concrete slabs may be flooded with water during reduce the water content of the mix yet still
this time for the same reason (called ponding). retain adequate workability during construction.
Vertical walls such as slip-formed structures may be For this reason, plasticiser is sometimes referred
painted with curing compounds that form an to as water-reducer.
evaporation-retarding membrane over the surface of
the concrete. Curing compounds may be removed
following the initial cure or may be left in place.
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Additives Loading on Concrete

Accelerators/retarders: These chemicals change Concrete is extremely strong when loaded in

the rate of hydration to suit the construction compression. It also has very good compression
method used. It is sometimes critical to stiffness. Unfortunately, it is relatively weak
accurately control the rate at which the concrete when loaded in tension or shear.
sets, eg when slip-forming.
Engineers may design a structure or a
Others: The final properties of the concrete may component to avoid loading the concrete in
be enhanced by the addition of other tension or shear. For example, a concrete dam is
admixtures. For example, a water-retardant may often curved in order to translate the hydrostatic
be added to enhance resistance to penetrating pressure on the convex side of the dam into
water. compressive stress within the dam wall.

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Loading on Concrete Types of Concrete

However, tension and shear cannot always be Grout is a mixture of cement and water. It may
avoided by design. For example, when a beam is contain admixtures, but does not contain any
supported at either end and is loaded from aggregate.
above, we see that the lower face will be in
tension and that shear stress is imposed at It is obvious when we consider that the strength
either end. In this case, the concrete can be of concrete comes from its aggregate that grout
strengthened by embedding reinforcement or by is not a strong material. It is, however, flexible
using a pre-stressed beam. and so is used where we require some
movement in the structure.

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Types of Concrete Types of Concrete

Typical applications for grout include:  Mortar is a mixture of fine aggregate (sand),
cement and water. Again, it often also
 Sealant, eg to seal construction joints. contains admixtures.
 Filler or packing material eg between steel
piles and the pile-sleeves.  Since mortar contains fine aggregate, it is
 Repair material, typically for relatively small stronger than grout, but is still significantly
areas of repair. weaker than concrete. Like grout, it is used
when some flexibility is required. It is often
 Grout is never used as a structural material. used as a jointing material – effectively
forming a flexible interface between two
stiffer, stronger components.

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Types of Concrete Types of Concrete

Typical applications for mortar include:  Plain concrete is a mixture of coarse and fine
aggregate, cement and water. Again, it often
 Jointing. contains admixtures.
 Sealant.  The term plain concrete refers to the fact that this
type of concrete does not contain any
 Filler or packing material. reinforcement. Thus, plain concrete is very strong
 Repair material. and stiff in compression but relatively weak in
tension and shear.
 Same as for grout, mortar is not used as a  Typical applications for plain concrete include:
structural material.  Structures and structural elements that are
principally loaded in compression.
 Repair material, typically for relatively large areas.

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Types of Concrete Types of Concrete

 Reinforced concrete is plain concrete that has  The steel reinforcement most
embedded steelwork. commonly takes the form of bars
(reinforcement-bar or re-bar).

 The function of the steel is to take any tensile or  Re-bar is usually round-section,
shear loading that the material is subjected to. As ribbed bar with a diameter of
such, the reinforcement is only laid in locations between 10mm and 50mm.
within the concrete which will be subject to tension
and/or shear.  Typical applications of reinforced
concrete include:
 Structures and structural
elements that may be subjected
to tension and/or shear stresses.

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Types of Concrete Types of Concrete

 It is important to note that when first loaded, The steel reinforcement is protected from
a reinforced concrete component will deform corrosion whilst within the concrete because of
as it takes up the load (strain). At this stage, the high pH (around 12.5) of the material. This
the concrete may crack as the steel stimulates the formation of a passive skin of iron
reinforcement takes up any tensile or shear (3) oxide around any embedded steelwork. As a
stresses. This is known as settlement cracking result, the steel is passivated. It is very
and is common in reinforced concrete important that the passive environment around
components. This type of settlement should the reinforcement is maintained and that no
stabilise and the cracking should not progress water is allowed to seep in around the steel.
after the initial loading of the component.

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Types of Concrete Types of Concrete

The current standard relevant to concrete If passivation is lost by water ingress causing
structures is EN 1992: Design of Concrete chloride attack or by carbonation, then the iron
Structures, (Eurocode 2). (This standard (3) oxide skin will be lost and replaced by iron
replaces CP110 under which many existing (2) oxide. This form of oxide expands as it
structures were built.) Both standards require a forms, creating an increase in volume of the
minimum cover of 60mm over any embedded oxide layer around the re-bar. This effectively
steel reinforcement (between the steelwork and pushes the concrete apart in the local area.
the outside environment) in order to protect
against water ingress causing loss of
passivation.

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Types of Concrete Types of Concrete

A shear zone is created around the expanding The situation on the left will
eventually result in a popout as the
oxide and the concrete will eventually fail in end of the re-bar pushes a cone of
shear. This failure is characterised by a crack concrete away from the surface.
along a 45° line to the action of the shear stress.
On the right it will most likely lead to
The resulting defect is therefore characterised by delamination where a plate of
a 45° crack surrounding material that has been material comes away from the
pushed away from the surface of the concrete. surface.
Both are forms of spalling – a general
Depending upon the orientation of the re-bar term used to describe the loss of
relative to the surface of the concrete, this material from the surface of concrete.
process will result in a popout or delamination

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Types of Concrete Types of Concrete

Pre-stressed concrete contains steel rods or tendons that If the tendon is stressed before the concrete is placed it is
run within the component. The tendons are pulled to a termed pre-tensioned.
predetermined tension and locked into anchor points
Post-tensioned components are built with conduits (ducts)
(cachetage points), effectively compressing the concrete
cast within them. The tendon is placed within the conduit
component. The tension of the tendon is calculated to be
and stressed using screw-threads or a stressing ram after
greater than any in-service tensile stress and the concrete
the concrete has been placed.
is maintained in its strongest loading state.

Conduits in
place (and
reinforcement)

Cachetage
points

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Types of Concrete Types of Concrete

 Once the tendons are locked into the Weight coat is a generic term applied to the
cachetage points then grout is injected into concrete-type covering of a pipeline. Its primary
the conduit to protect the steel by passivation. function is to control the buoyancy of the
pipeline and ensure sufficient on-bottom mass to
As the concrete is
maintain stability. It also has the secondary
maintained in a
functions of impact protection and corrosion
compressive load then
protection.
this type of concrete
should not exhibit any Although often called concrete weight coat, the
settlement cracking or actual material is commonly a form of mortar
in-service cracking. called Gunite. The coating is commonly sprayed
onto a pre-laid wire mesh or reinforcement web.
As such, it is often a very viscous mix with very
An hydraulic stressing ram
low water content.
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Types of Concrete Types of Concrete

Although strictly not a form of concrete, organic Organic polymers are very commonly used for
polymers are often found associated with defect repairs and may be injected into the
concrete structures. The term organic refers to surface of a concrete structure to stabilise the
the carbon-chain chemical nature of these material. They are often activated by mixing
materials, eg epoxy. with a chemical hardener just prior to
application. They may be readily identified by
Typical applications for organic polymers are: their hard, glassy or plastic-like texture.

 Sealant.
 Repair material for relatively small areas.
 Bonding material.

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Concrete Construction Techniques Concrete Construction Techniques

 Before the concrete can be placed it must me Deck &

produced, usually in bulk in a concrete plant. For modules
large projects, such plant is sited at the
construction yard.
Columns
 Offshore concrete structures are built in a shore-
based construction yard. The structure is built in Diaphragm wall
the upright orientation, ie from the ground Domes between cells
upwards. Smaller structures may be entirely Cells
constructed in the yard and then towed out when Base-raft Caisson
completed.
 With larger structures, once they reach a certain
height the yard is flooded and the structure moved
to sheltered water for completion.
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Concrete Construction Techniques Concrete Construction Techniques

There are three basic methods used to place the With fixed shuttering construction, walls
concrete to form a structure; pre-casting, fixed (formwork) are built to contain the concrete
shuttering and slip-form shuttering. whilst it sets. The height of the formwork is
limited by the hydrostatic pressure exerted on
Pre-cast concrete is manufactured in a factory the bottom of the formwork by the wet concrete.
and is commonly used to produce multiple items
of a certain design of component – eg pipe When the concrete is placed within the
sections. Because of economies of scale, high- formwork, it is consolidated by vibration or
quality, re-usable moulds are used which give tamping. When the concrete has set, the
good surface finish without the need for tie- formwork may be removed and reassembled
bolts. higher up ready for the next pour of concrete.

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Concrete Construction Techniques Concrete Construction Techniques

There are various formwork systems The use of fixed shuttering necessitates that each
available: successive layer of concrete sets sufficiently to allow for
removal and reassembly of the formwork. This gives rise to
 Wooden formwork: Versatile and a series of horizontal construction joints in the structure.
useful for creating complex shapes.
 Engineered formwork: Re-usable
steel or aluminium modules. Construction joints are joints made
 Plastic formwork: Re-usable plastic between successive placements of
modules. concrete in the fixed shuttering. They
 Structural formwork: Not removed are weak points in the structure in
after the concrete has set – it forms terms of lateral-shear loading and are
part of the reinforcing system. also potential places where water may
penetrate the structure. To help seal
the joint, it is often rebated (set back) Rebated
and the filled with sealant such as construction joint
Wooden formwork with sealant
grout, mortar, bitumen or epoxy.

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Concrete Construction Techniques Concrete Construction Techniques

Slip-form shuttering is useful when constructing Because the slip-form is continually moving and
tall components with unchanging shape, such as the concrete is continuously being placed, there
the cells and columns of offshore structures. are no construction joints ie the structure is
monolithic with no structural weak points.
The formwork is built onto a As the slip-form creeps
jacking system. The upwards then the concrete
concrete is continually that is exposed at the lower
placed within the slip-form – skirt of the formwork tends to
as it fills with material, it is bulge slightly. This gives rise
jacked upwards using to a characteristic feature of
hydraulic rams. The typical slip-formed structures – a set
rate of climb is of regular horizontal ridges
200-250mm per hour. known as weatherboarding.

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Other Features of Offshore

Concrete Construction Techniques
Concrete Structures
Other features characteristic of slip-formed A feature sometimes found set into concrete
structures include irregular horizontal ridges breakwater walls and anti-scour fences are
caused by slight variations in the concrete and Jarlan holes. These are nozzles that forces water
the construction process, and vertical drag to speed up as it is forced through by tidal or
marks caused by minor debris being trapped wave action.
between the formwork and the structure.
At a critical velocity
the flow becomes
Without the slip-forming method of construction,
turbulent, dissipating
offshore structures would simply take too long to
the fluid energy and
build. Slip-forming is a rapid and efficient
method of constructing extremely tall structures. reducing
hydrodynamic
loading.

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Other Features of Offshore Other Features of Offshore

Concrete Structures Concrete Structures
Columns of concrete structures may be dry or  Expansion joints are narrow gaps, which are sometimes left
free-flooding. Indeed, there may be both types between large masses of concrete to allow for thermal
movement. Even if in-service thermal variations are not
on the same structure with one column acting as significant, expansion joints may be required to allow for
a utility shaft containing risers and conductors. movement during the hydration phase of the construction.
 On offshore structures, expansion joints may allow for
In such shafts, bacterial attack may lead to the thermal movement caused by storage of hot production
release of hydrogen sulphide gas and great care fluids or to allow some structural flexibility.
must be taken to ensure that the column is  Often, expansion joints will be sealed with a flexible sealant
sufficiently ventilated. such as bitumen.

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Commissioning of Concrete Structures Commissioning of Concrete Structures

Once on the seabed, on-bottom mass is increased by

Once completed, the structure is
further water ballasting and possibly also solid ballasting
towed to location with tugs. using dense material such as iron ore.
When positioned correctly it is
ballasted to sink gently to the
Not all of the cells in the caisson
seabed. The structure is
of the structure will be used for
maintained in an upright
ballast – cells are also available
orientation during this phase,
for storage of production fluids
limiting the imposed bending
and product.
moments. Depending on the
nature of the substrate and
Loading of the deck, modules and
underlying rock, the structure
superstructure may take place at
may sit directly on the bed of the
the construction site or at the final
sea or on a prepared foundation.
location utilising heavy-lift barges.

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In-Service Deterioration In-Service Deterioration

Concrete structures deteriorate by two means – Chemical attacks include the following:
physical and chemical attack:
• Alkali-aggregate reaction (AAR).
Physical attacks include the following assaults: • Sulphate attack.
• Chloride attack.
• Impact. • Carbonation.
• Overload. • Corrosion of inset steelwork.
• Freeze/thaw cycles. • Biological attack.
• Expansion of reinforcement caused by
corrosion. The degradation of concrete by environmental
• Abrasion. action is termed weathering.

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In-Service Deterioration In-Service Deterioration

Alkali-aggregate reaction. Sulphate attack.

Most types of aggregate used in concrete are Seawater contains sulphates that, if allowed to
chemically stable. Some aggregate, however, penetrate into the concrete, will react with the
reacts with the high pH of concrete. In hydrated cement to form a crystalline solid
particular, aggregate containing silica will called ettringite (calcium-aluminium sulphate).
undergo alkali-silica reaction. During the alkali- This causes a volume expansion, leading to
silica reaction, silica forms a gel and begins to cracking and spalling.
absorb water. The gel swells and starts to exert
an internal pressure within the concrete. Gel
may weep from the surface of the concrete
(exudation) and can lead to expansion, cracking
and spalling.

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In-Service Deterioration In-Service Deterioration

Chloride attack. Carbonation.

If seawater seeps next to the steel If atmospheric carbon dioxide reaches the steel
reinforcement within the concrete then chlorides reinforcement within the concrete then
will destroy the passive iron (3) oxide skin. This passivation will be lost in a similar way to
will initialise the reinforcement corrosion chloride attack. This will eventually result in
process, eventually resulting in cracking and reinforcement corrosion, cracking and spalling.
spalling. Although carbonation is a theoretical risk for an
offshore structure, if it has been built according
to either EN 1992 or CP110, then the process
would be too slow to pose any real threat to an
offshore structure, although a crack could
theoretically speed up the process.
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In-Service Deterioration In-Service Deterioration

Biological attack. Corrosion of inset steelwork.

 Under anaerobic conditions, sulphate-reducing
bacteria living in the small voids in the  This form of deterioration is perhaps more
concrete, produce hydrogen sulphide. Some of likely than reinforcement corrosion. There are
this gas may escape to the surface of the a number of different types of steel
material and, if allowed to build up, can pose a components cast into the structure, such as
significant health risk. riser clamp supports, steel skirts and towing
 However, some of the hydrogen sulphate may eyes.
be captured by aerobic bacteria that oxidise it  Should any of this steelwork be in contact with
to form sulphuric acid. This will dissolve away the internal reinforcement the exposed
the carbonates from the hydrated cement and steelwork acts as an active anode and the
weaken the material. Eventually this will lead to reinforcement becomes the cathode.
cracking and spalling.
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In-Service Deterioration Imperfections of Concrete

The U.K. government has published an offshore

Components in technology report (OTH-84-206) that categorises
contact with twenty six imperfections that may be seen on
Reinforcement
concrete structures.
This report defines three categories:
 Category A: Defects.
 Category B: Areas of concern.
 Category C: Blemishes.

Electron flow
Ion flow
It can be said that each of these imperfections
may be either a construction imperfection or an
Corrosion of inset steelwork in-service imperfection.
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Imperfections of Concrete Category A (Defects)

Category A (defects) usually require detailed investigation Cracks may take the form of individual, distinct general
and are likely to generate an intervention to repair the cracks, or of a network more fine pattern cracking.
defect. Cracks may form because of:
 Cracks.
 Impact damage.  Plastic shrinkage cracking.
 Popouts.  Plastic settlement cracking.
 Delamination.  Thermal cracking.
 Variable cover.  Overload.
 Exposed reinforcement.  Freeze/thaw cycles.
 Tearing.  Alkali-aggregate reaction.
 Poor repairs.  Settlement cracking.
 Popouts and delamination may be referred by the  Internal corrosion.
general term of spalling – the loss of material from the Two horizontal general cracks
 Sulphate attack. and an area of pattern cracking.
surface of the concrete.

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Category A (Defects) Category A (Defects)

Impact damage showing the colours of the broken aggregate Conical recess left from material being pushed out

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Category A (Defects) Category A (Defects)

Variable cover is
a construction
defect caused
by laying the
reinforcement
too close to the
shutter.

Delamination caused by reinforcement corrosion

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Category A (Defects) Category A (Defects)

Tearing is caused by
Exposed reinforcement is premature movement of
an in-service defect, in this the shutter or slumping of
case caused by abrasion. the concrete whilst in a
semi-liquid state.

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Category A (Defects) Category B (Areas of Concern)

 These will usually be monitored but may

require an intervention to repair.

 Embedded objects.
 Recessed metal plates.
Poor repair
showing
 Cast-in sockets.
multiple  Abrasion.
defects.
 Water-jet damage.
 Honeycombing.

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Category B (Areas of Concern) Category B (Areas of Concern)

Embedded objects (wood and wire-ties).

Note: The construction joint below. A recessed metal plate, painted for corrosion protection

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Category B (Areas of Concern) Category B (Areas of Concern)

Soft edges indicate that this is a cast-in socket and not one
that was drilled after the concrete had set. Abrasion damage has cut into the surface and exposed the
aggregate

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13
 27/08/2015

Category B (Areas of Concern) Category B (Areas of Concern)

Water-jet damage has exposed colours of the aggregate Honeycombing is caused by lack of consolidation
during placement

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Category C (Blemishes) Category C (Blemishes)

Category C are surface imperfections that have no

structural significance.

 Construction joint.  Rubbing-down marks.

 Blowholes.  Formwork misalignment.
 Scabbling.  Regular horizontal
 Good repair. ridges.
 Resin mortar repair.  Irregular horizontal
 Sealant run. ridges.
 Vertical drag marks.
 Curing compound.
Construction joint

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Category C (Blemishes) Category C (Blemishes)

When the concrete is vibrated during consolidation, air tends

to migrate outwards until it meets the face of the shutter. Scabbling is the intentional roughening of the surface.
Upon solidification the bubbles of air form blowholes. If done using a machine, the pattern will be regular -
as above.

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14
 27/08/2015

Category C (Blemishes) Category C (Blemishes)

Good repair does not refer to the neatness of the repair; it

A resin mortar compounds often have a hard
refers to the integrity of the repair, eg is the material bonded
glassy or plastic texture, eg epoxy.
well?
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Category C (Blemishes) Category C (Blemishes)

Sealant runs beneath a construction joint Rubbing-down marks caused by troweling

or wiping the surface during construction

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Category C (Blemishes) Category C (Blemishes)

Typically, the slip-form spends a

period of time paused and then
creeps upwards at set intervals –
often every one hour.

This results in a series of regular

horizontal ridges, formed when
the concrete bulges slightly as it
departs the lower skirt of the
formwork.

Irregular horizontal ridges are

caused by minor variations in the
Formwork misalignment is caused during construction
construction process and the
material.

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15
 27/08/2015

Category C (Blemishes) Uncategorised Imperfections

There are a few additional features and

imperfections that OTH-84-206 does not
categorise.

These include the following:

 Expansion joints.
 Missing or damaged sealant.
 Surface staining.
 Corrosion of inset steelwork.
Curing compound is painted on the surface of the  Biological attack.
concrete to control evaporation. It may or may not be
removed after the initial curing period.

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Uncategorised Imperfections Uncategorised Imperfections

This expansion joint has a longitudinal crack running along the sealant. Some of the sealant is missing from this construction joint
Note: The pattern cracking on either side.

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Uncategorised Imperfections Uncategorised Imperfections

Surface staining may be the result of:

 Exudation (from alkali-aggregate reaction).
 Efflorescence (salt crystals formed during evaporative loss of
water).
 Lime encrustation.
 Corrosion.
Corrosion is the only stain that is likely to be seen during an
underwater survey.
It is important when photographing staining to include a colour
bar so that the photograph may be presented in true colour.
The engineer may then be able to gauge the age of the defect
by the colour of the stain.
Corrosion staining: This photograph should contain a colour bar and scale

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16
 27/08/2015

Uncategorised Imperfections Uncategorised Imperfections

The corrosion of these brackets could be caused by earthing of the Barnacles do not attack concrete, however some species
inset steel against the reinforcement. of tube-worms will bore into the surface of the material.

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Inspection of Concrete Structures Inspection of Concrete Structures

Cleaning for Inspection. Navigation and positioning:

 Concrete is prone to abrasion injury and must be  It is the nature of concrete structures that
cleaned with care. Indeed, a high pressure water-jet they are relatively featureless and are usually
will effectively cut into the concrete surface and very large. This makes navigation and location
such damage is classified as an area of concern. reporting particularly difficult. An ideal solution
 The acceptable methods of cleaning concrete to this problem is to equip the inspector with
structures include: an underwater transponder; however, this is
 Low pressure water-jet. not always possible.
 Hand brushing with nylon brushes.
 Plastic hand scrapers.
 Brush-carts may be used so long as they are fitted
with nylon brushes.
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Inspection of Concrete Structures Inspection of Concrete Structures

 One solution is to mark up a grid on the

surface of the structure. The grid must be
referenced to a known datum such as a riser.
The grid squares may be named according to
an alpha-numeric system as shown in figure
X.47, below. Features and imperfections may
be reported by their alpha-numeric grid
coordinate.

An Alpha-Numeric grid marked on the surface of the

structure. Note: The grid is referred to a riser as a datum.

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17
 27/08/2015

Inspection of Concrete Structures Inspection of Concrete Structures

 A major drawback of the alpha-numeric grid Due to the problems in

system is that it is time-consuming to set up. navigating and
A quicker solution is to use a down-line and location reporting
distance-line technique, although the down- when inspecting
line may be replaced with a datum such as a concrete structures,
riser. very often ROVs are
 Using this method, the feature positions may used in preference to
be reported using their depth and distance- divers.
from-datum. When depth is quoted then the
inspection controller must be aware of the
The down-line
tidal height at the time of the inspection in and distance-line
order to convert the depth to an elevation technique
according to the structural height datum.
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Inspection of Concrete Structures Inspection of Concrete Structures

Reporting requirements. When describing cracks, we can classify the appearance of the
 The specific reporting requirements for any inspection cracking into two types:
will be specified by the client’s procedure and should be
made explicit in the inspection controller’s briefing. As a  General cracking: These appear as well-defined, distinct cracks
general guideline it would be good practice to include where a length, width and shape will be apparent.
the following data in the verbal report:  Pattern cracking: These cracks form a network that are best
described by their pattern and the area that they cover.
 Location of imperfection.  When describing the width of cracks, the convention below
 Type of imperfection. may be used:
 Severity.
 Orientation and/or pattern. Fine cracks: Less than 1mm in width.
 Extent of imperfection (length, width or percentage Medium cracks: Between 1-2mm in width.
area). Wide cracks: Over 2mm in width.
 Maximum depth (penetration into the surface).

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Inspection of Concrete Structures General Concrete Terms

Imperfection OTH-84-206 Type Typical Reporting Requirements Spalling:

General cracks A In-service Length, orientation, width, pattern, depth
Is considered to be a symptom of something
Pattern cracks A In-service
As above plus length and width of pattern more serious.
area
Impact damage A In-service Length, width and depth

Popouts A In-service Diameter, depth Grout:

Delamination A In-service Length, width and thickness Is semi-fluid slurry consisting of cement and
Exposed reinforcement A In-service
Length, width, amount of reinforcement
affected water.
Tearing A Construction Length, orientation, width, depth

Variable cover A Construction Severity

Gunite:
Poor repair A Construction Length, width, type
Is concrete sprayed by compressed air. It will
have high strength and density, used to repair
Embedded objects B Construction Size, number, type

Recessed metal plates B Construction Length, width, condition

walls and as weight coat on pipelines. It has a
See the full table in your notes darker colour than normal concrete.

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18
 27/08/2015

General Concrete Terms General Concrete Terms

Cable duct: Caisson:

A cast tubular duct through which the pre-  Is a large cylindrical structure often referred to
stressing tendons will run. Normally grout filled as a cell.
after tensioning. Cell:
Pre-stressed concrete:  A void bounded by diaphragm walls, term
Is concrete that has all the tensile and shear used synonymously with caisson for the base
stresses relieved by the introduction of cells of a structure.
compressive stress on the structure. Invert:
 Is the lowest point of an opening or tunnel.
Base raft: Soffit:
Is the foundation slab bearing on the seabed.  Is the underside of a concrete beam.

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General Concrete Terms General Concrete Terms

Jarlan hole: Exudation:

Exudation consists of salts, which dissolve, in
Is the perforation in a breakwater wall, used to the concrete when fluid is passing through a
dissipate the forces from wave action. crack; it shows on the surface of the concrete as
a whitish semi-fluid, which accumulates around
Laitance: the crack. Note that on the surface it will always
run downwards, however in water it may drift
This is a fine powdery substance, which sideways or even upwards, owing to the fact
accumulates on the surface of concrete as it that its density may be less than the water
sets; it will need to be removed prior to any new around it.
pour being applied.

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Any Questions?

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19
 Section 6

Marine Growth
6 Marine Growth
Once any structure is placed into the sea, planktonic marine growth will
colonise it. This build-up will have two effects:

1 The profile area of any component presented to the water flow will be
increased. This will increase the force on the structure overall.

2 Marine growth will change the texture of the surface from a smooth, round
steel or painted surface, to a surface made much rougher by the presence
of the marine growth on it. This roughness will increase with time as the
surface becomes more irregular due to parts of the dead marine growth
sloughing off. The effect of this is to increase the drag coefficient.

Both these effects increase the force on the structure. Information on the types
and amounts of marine growth build-up is required to confirm or modify the
design-predicted loads on the structure (Figure 6.1).

Figure 6.1 Flow conditions around a cylinder.

DIS1-30815
Marine Growth 6-1 Copyright © TWI Ltd
 These two effects of marine growth will have a knock-on effect on the structure
that will manifest itself by:

 Producing an increase in mass without any significant change in stiffness.

This causes a reduction in the structure’s natural frequency.
 Increasing the mass, there will be added slam effect and drag forces on the
structure. Marine growth, being most abundant at and just below the water
level, coincides with the zone of maximum wave and water force. Therefore,
the effects on the structure are increased in the region of maximum water
force.
 Affecting the corrosion rate, either by accelerating or retarding it.
 Reducing the effective area of the service inlets and outlets, hence reducing
system efficiency.
 Obscuring the important features on the structure, such as diver orientation
marks, valve handles, anodes and similar objects.
 Making inspection impossible before cleaning.

These effects give marine growth such an importance that it is necessary to

examine the problem in a little more detail.

6.1 Types of marine growth

From the engineering standpoint there are two main categories of fouling; soft
and hard.

 Those organisms that have a density approximately the same as seawater

cause soft fouling. They are important because of their bulk, but are
generally easy to remove.
 Organisms causing hard fouling are much denser (1.4 greater than
seawater) and more firmly attached to the structure and, therefore, are
more difficult to remove.

Both categories of marine growth have the potential to damage coatings.

These organisms will colonise the structure at different rates and at different
depths, dependent on the natural propensity of the particular species. Some
guidance is available to designers, as indicated in Table 6.1. Using this and
other data, designers can predict the most suitable time of the year to launch
and install a structure.

DIS1-30815
Marine Growth 6-2 Copyright © TWI Ltd
 Table 6.1 Typical distribution of marine growth in the North Sea,
(extract from Offshore Installations: Guidance on Design, Construction
and Certification, Fourth Edition.
Typical Typical Typical Depth
Settlement
Type growth coverage terminal (relative Comments
season
rate % thickness to MSL)
Hard Fouling
25mm/1yr Faster growth rates
150 -
Mussels July - Oct 50mm/3yr 100 0-50m found on installations
200mm
75mm/7yr in the North Sea
Giant barnacles in the
Barnacles Apr - July 5mm/yr 100 0-30mm 0-120m
tropics
About Coverage often 100%
10mm especially on new
Length structures 1-2 years
Calcereous 0 to
May - Aug 30mm per 50-70 tubeworms after installation.
Tubeworm seabed
3mth lay flat on Tubeworms remain as
the steel a hard, background
surface layer when dead
Soft Fouling
A permanent hydroid
Summer
turf may cover an
50mm per 30-70mm 0 to
Hydroids Apr - Oct 100 installation and
3mth Winter seabed
obscure the surface
20-30mm
for many years
Usually settle 4-5
years after installation
Plumose and can then cover
Jun - Jul 50mm/1yr 100 300mm 0-120m
Anemone surface very rapidly.
Live for up to 50
years
Often found in
About
Soft coral Jan - Mar 50mm/1yr 100 0-120m association with
200mm
anemones
May be several years
before colonisation
begins but tenacious
Variable holdfast when
Kelp Feb - Apr 2m/3yrs 60-80 0-20m
up to 6m established. Present
on some installations
in Northern and
Central North Sea

DIS1-30815
Marine Growth 6-3 Copyright © TWI Ltd
6.1.1 Soft fouling
Organisms in this group include:

Algae
Often referred to as slime and is generally the first organism to inhabit an
offshore structure. As it is very light sensitive, it is seldom observed in any
quantity below 20m (67feet). This is a very large family of plants which exist in
forms ranging from the microscopic to giant kelp species.

Bacteria
These are microscopic organisms which are also amongst the first inhabitants of
an offshore structure and will be present in depths well in excess of 1000m
(3333 feet).

Sponges
Are colonial animals found as a fouling species on offshore platforms and are
present at depths greater that 1000m (3333 feet).

Figure 6.2 Different species of sponges.

Sea squirts
These are soft-bodied animals and sometimes grow in large colonies and can be
found down to 1000m (3333 feet).

Figure 6.3 Sea squirts.

DIS1-30815
Marine Growth 6-4 Copyright © TWI Ltd
 Hydroids
These grow in colonies and from their appearance can be mistaken for
seaweed, but they are in fact animals related to sea anemones. The colonies
can produce dense coverage to depths of 1000m (3333 feet).

Figure 6.4 Close up photograph of a hydroid.

Figure 6.5 Different species of hydroids.

DIS1-30815
Marine Growth 6-5 Copyright © TWI Ltd
 Seaweeds
There are many types of seaweed that attach themselves to underwater
structures, but of these, kelp produces the longest fronds, which in the North
Sea, grow up to 6m in length under favourable conditions.

Figure 6.6 (Left) Sargassum (right) Bladder wrack.

Figure 6.7 (Left) Kelp Laminaria digitalis (centre) Holdfast (right) Laminaria
saccharina.

Bryozoa
Moss-like appearance and is really an animal with tentacles.

Figure 6.8 Different species of Bryozoa.

DIS1-30815
Marine Growth 6-6 Copyright © TWI Ltd
 Anemones
Sometimes called anthozoans, which mean flowering animals. The cylindrical
body is surmounted by a radial pattern of tentacles and looks a bit like broccoli.
It attaches itself to the structure by a basal disc and this attachment is so firm
that attempts to remove it often result in tearing the body of the anemone. The
colours and shapes are extremely variable even within the same species.

Figure 6.9 Anemones.

Dead Men’s Fingers (Alcyonium Digitatum)

Colonies have been observed on pier piles, rocks on the foreshore and offshore
structures. These colonies often grow to 150mm (6 inches) in length. When
submerged, many small polyps arise from the finger-shaped, fleshy main body,
each polyp having eight feathery tentacles. It is white to yellow or pink to
orange in colour, but when out of the water it is flesh coloured and the
similarity to the human hand gives it its common name.

Figure 6.10 Dead Men’s Fingers.

DIS1-30815
Marine Growth 6-7 Copyright © TWI Ltd
6.1.2 Hard fouling
Composed of calcareous or shelled organisms, the common types in this group
include:

Barnacles
The common species is Balanus balanoides. These grow in dense colonies to
depths of 120m (394 feet).

Figure 6.11 Barnacles.

Mussels
The main species is Mytilus edulis. The hard-shelled mollusc attaches itself to
the structure by byssal threads at the hinge of the shell. These threads are very
strong and mussels generally form dense colonies. Main colonisation is to
depths of 20m (67 feet), but mussels are found to depths of about 50m (164
feet).

Figure 6.12 Blue mussels (Mytilus edulis).

DIS1-30815
Marine Growth 6-8 Copyright © TWI Ltd
 Tubeworms
Calcareous tubeworm, often forms on flat surfaces, is white in colour, very
firmly attached to the surface of the metal and difficult to remove. It also grows
in colonies and these have been known to fill a warm water outlet, arranging
themselves parallel to the flow to obtain maximum nutriments.

Power cleaning is required to remove this growth, so firmly is it attached.

Although the main growth occurs to depths of 50m (164 feet), tubeworms are
found to depths of 100m (328 feet).

Figure 6.13 Tubeworm.

6.2 Factors affecting the rate of marine growth

If no steps are taken to prevent growth, such as application of an anti-fouling
solution or paint, the formation of bacterial slime occurs in two to three weeks.

As indicated in table 6.1 marine growth can mature very rapidly with barnacles
and soft fouling having been known to attach themselves and reach maturity in
three to six months. It generally takes two seasons for mussel colonies to
develop, often on top of the dead earlier fouling.

The type of organism, its development and growth rate will depend on several
factors, including the following:

DIS1-30815
Marine Growth 6-9 Copyright © TWI Ltd
6.2.1 Depth
Figure 6.14 gives a generally accepted representation of the combined effects
of weight and volume on the various types of marine fouling in British waters.
This should be read in conjunction with Table 6.1, which contains information
more specific to the design function.

The diagram shows clearly that the most weight is added in the vicinity of the
surface, which is the region of highest water-induced loading. The total column
in the diagram is not the sum of the others, but an estimate of a balanced
colony.

Note: The long lengths of seaweed have not been included.

Increase in depth reduces light intensity, which therefore reduces the ability of
organisms such as algae to photosynthesise. Algae therefore, gradually
disappear with depth and there is also a change in species to red algae at the
greater depths. Algae growth at depths below 30m (98 feet) have been
observed in the North Sea due mainly to the clarity of the water.

Plants such as seaweed, also photosynthesise to enable growth and the area
between the surface and approximately 20m is known as the Photic Zone.
Below the Photic zone plants will not flourish.

Figure 6.14 Representation of the distribution of marine growth with depth.

DIS1-30815
Marine Growth 6-10 Copyright © TWI Ltd
6.2.2 Temperature
A rise in water temperature will increase the growth rate of a colony; the
growth rate approximately doubles with a 10°C rise in temperature. There will
of course be a limit and most organisms cease growth at 30-35°C. As the
temperature variation is greatest near the surface, there is seasonal growth in
the marine colonies near the surface and continuous, slower growth as the
depth increases.

6.2.3 Water current

The speed at which the water flows over the surface plays an important part in
the type of fouling colony that develops. There are two aspects to consider, the
first being that of the larvae attaching themselves to the structure.

It is suggested that at speeds greater than 1 knot, many larvae are unable to
attach themselves. However, once attached, most fouling can withstand water
currents of more than 6 knots. At high water velocities, weakly attached fouling
is removed leaving only the firmly attached hard fouling.

Colonies growing on dead or dying fouling become loose and may be sloughed
off. The larvae can attach themselves to structures during slack flow periods, or
in localised spots of slower flow or dead water, such as crevices and locations
between hard fouling.

The second aspect to consider is that in general, once the organism is

established, a strong current brings more food and growth is accelerated.

6.2.4 Salinity
In nearly fresh water, fouling is usually confined to algal slime. As the salinity
increases, so the amount and type of fouling increases. First hydroids and
barnacles and finally mussels occur. The normal salinity of seawater is about 3-
3.5% and the size of mussels, for example, increases five-fold from a salinity of
0.6-3.5%.

6.2.5 Food supply

Growth of the fouling is obviously dependent on the quantity of nutriment
available. Growth rates seem to be faster in coastal waters than those a few
miles offshore where the water is deeper. Investigations suggest that the slow
currents that circulate around platforms become enriched with nutriments from
sewage and other waste that will increase the growth rate.

6.2.6 Cathodic protection

There are two types of corrosion protection widely used on North Sea structures
on those that use sacrificial anodes, the patterns of marine growth on the
structures themselves seem normal, but the anodes generally remain clear of
growth.

The other system, which uses an impressed current to cancel the corrosion-
induced ionic currents between the structure and the sea, suggests, on a
limited amount of evidence, that the marine growth rate is increased.

Currently the mechanism that encourages an increased growth rate is not

understood, more data is required before the observations of increased growth
can be confirmed.

DIS1-30815
Marine Growth 6-11 Copyright © TWI Ltd
 Note: The species listed here are the main types encountered in North East
Atlantic waters and is by no means exhaustive. West African, South American,
Arctic and Far Eastern will differ but the main groups and zoning will remain
similar.

DIS1-30815
Marine Growth 6-12 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office

Books), 1988, ISBN 13: 9780114129118.

Weidmann G, Lewis P, Reid N, ‘Structural Materials’, The Open University and

Butterworth’s, 1990, ISBN 13: 9780408046589.

DIS1-30815
Marine Growth 6-13 Copyright © TWI Ltd
 27/08/2015

Marine Growth

 Once any structure is placed into the sea, planktonic

marine growth will colonise it and this build-up will
have two effects:
 The profile area of any component presented to the water
CSWIP 3.1U Course flow will be increased. This will increase the force on the
structure overall.
Marine Growth  Marine growth will change the texture of the surface from a
Section 6 smooth, round, painted or steel surface, to a much rougher
surface. The effect of this is to increase the drag coefficient.

 Both these effects increase the force on the structure, so

Information on the types and amounts of marine growth is
required to confirm or modify the design-predicted loads on
the structure.

Copyright © TWI Ltd Copyright © TWI Ltd

Flow Conditions Around a Cylinder Marine Growth

These two effects of marine growth have a knock-on effect on

the structure that will manifest itself by:

 Producing an increase in mass without changing stiffness.

This causes a reduction in the structure’s natural
frequency.
 Increasing the drag forces on the structure. Marine growth
being most abundant at the water level coincides with the
region of maximum wave and water force.
 Affecting the corrosion rate, by accelerating or retarding it.
 Reducing the effective area of service inlets and outlets.
 Obscuring important features on the structure.
 Making inspection impossible before cleaning.

Copyright © TWI Ltd Copyright © TWI Ltd

Types of Marine Growth Types of Marine Growth

From an engineering standpoint there are two Both categories of marine growth have the
main categories of marine growth fouling: potential to damage coatings.

1. Soft: These organisms will colonise the structure at

Organisms that have a density approximately the different rates and at different depths dependent
same as seawater cause soft fouling. They are on the natural propensity of the particular
important because of their bulk, but are generally species.
easy to remove.
Some guidance is available to designers. Using
2. Hard: this and other data, designers can predict the
Organisms causing hard fouling are much denser (1.4 most favourable time of the year to launch and
greater than seawater) and more firmly attached to
the structure and therefore are more difficult to
install a structure.
remove.

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1
 27/08/2015

Marine Growth Soft Fouling

Type
Settlement
season
Typical growth
rate
Typical %
coverage
Typical thickness Depth Comments Algae:
Hard Fouling This is often referred to as slime and is generally
Mussels July – Oct
25mm/1yr
50mm/3yr 100 150 – 200 mm 0- 50m
Faster growth rates found on installations
in the North Sea
the first organism to inhabit an offshore
structure. As it is very light sensitive, it is
75mm/7yr

Barnacles Apr - July 5mm/yr 100 0 – 30 mm 0-120m Giant barnacles in the tropics

About 10 mm
Coverage often 100% especially on new seldom seen in any quantity below 20m. This is
a very large family of organisms and even
Solitary Length 30mm structures 1 to 2 years after installation.
May – Aug 50-70 tubeworms lie flat on the 0 to seabed
tubeworm Per 3mth Tubeworms remain as a hard, background
steel surface
layer when dead

50mm
Soft Fouling
Summer
30 – 70mm
A permanent hydroid turf may cover an
includes kelp. Therefore, it goes from the very
Hydroids Apr – Oct
Per 3mth
100
Winter
20 – 30mm
0 to seabed installation and obscure the surface for
many years small to the very large.
Usually settle 4 to 5 years after installation
Plumose
Jun – Jul 50mm/1yr 100 300mm 0-120m and can then cover surface very rapidly.
Anemone
Live for up to 50 years

Soft coral
About
Bacteria:
Dead men’s Jan – Mar 50mm/1yr 100 0-120m Often found in association with anemones
fingers
200mm
This, like algae, will be amongst the first
inhabitants of an offshore structure and will be
May be several years before colonisation
Variable begins but tenacious holdfast when
Kelp Feb – Apr 2m/3yrs 60-80 3-15m
up to 6m established. Present on some installations

present in depths well in excess of 1000m.

in Northern and Central North Sea

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Soft Fouling Soft Fouling

Sponges
Often found as a fouling Sea squirts
species on offshore These are soft-bodied animals
platforms and are present and sometimes grow in large
at depths greater than colonies down to depths of
1000m. 1000m.

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Soft Fouling Soft Fouling

Seaweeds
Hydroids There are many types of seaweed that attach themselves
Grow in colonies and from their appearance can be to underwater structures, but of these, kelp produces the
mistaken for seaweed, but they are in fact animals longest fronds which, in the North Sea, grow up to 6m in
related to sea anemones. The colonies can produce length under favourable conditions.
dense coverage to depths of 1000m.

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2
 27/08/2015

Soft Fouling Soft Fouling

Bladder wrack

Bryozoans
These have a moss-like
appearance, but are actually
animals with tentacles.

Found down to depths of

Sargassum
around 1000m.

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Soft Fouling Soft Fouling

Anemones:
These are sometimes called anthozoans, which
means flowering animals. The cylindrical body is
surmounted by a radial pattern of tentacles and
looks a bit like broccoli.

Anemones
It attaches itself to the structure by a basal disc and
this attachment is so firm that attempts to remove it Found at
often result in tearing the body of the anemone. depths
down to
150m
The colours and shapes are extremely variable even
within the same species.

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Soft Fouling Hard Fouling

Dead Men’s Fingers: Barnacles

 Colonies have been seen on pier piles,  The common species is Balanus Balanoides.
rocks on the foreshore and offshore These grow in dense colonies to a depth of 15-
structures. These colonies often grow
to 150mm in length, at depths down
20m but, are observed to depths of 120m.
to 300m.
 When submerged, many small polyps
arise from the finger-shaped, fleshy
main body, each polyp having eight
feathery tentacles.
 It is white to yellow or pink to orange
in colour, but when out of the water it
is flesh coloured and the similarity to
the human hand gives it its common
name. Common barnacle Tropical barnacle

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3
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Hard Fouling Hard Fouling

Mussels Tubeworms
This hard-shelled mollusc attaches itself to the  This often forms on flat surfaces. It is white in colour, very
firmly attached and difficult to remove. It also grows in
structure by byssal threads at the hinge of the shell. colonies and these have been known to fill a warm water
These thread attachments are very strong and outlet arranging themselves parallel to the flow to obtain
mussels generally form dense colonies. Main maximum nutriments.
colonisation is to depths of 20m, but mussels are  Power cleaning is required to
found to depths of about 50m. remove this growth. Although the
main growth occurs to depths of
50m, tubeworms are found to
Blue
mussels depths of 100m.

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Summary of Depths Summary of Depths

Soft fouling: Hard fouling:

 Algae 0-20m.  Barnacles 0-120m.

 Bacteria 0-1000m.  Mussels 0-50m.
 Sponges 0-1000m.  Tubeworms 0-100m.
 Sea Squirts 0-1000m.  Bryozoans 0-1000m.
 Hydroids 0-1000m.
 Seaweeds 0-20m.
 Anemones 0-120m.
 Dead men’s fingers 0-
300m.

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Factors Affecting the Rate of Marine Factors Affecting the Rate of Marine
Growth Growth
Depth: Temperature:
 From the previous diagram it can be seen that most  In general, a rise in water temperature will
weight is added in in the vicinity of the surface, increase the growth rate of a colony. There is
which is the region of highest water-induced of course a limit and most organisms cease
loading. growth at between 30-35°C.
 Increase in depth reduces light intensity, which
therefore, reduces the ability of organisms such as
algae to photosynthesise. Algae therefore, gradually
disappear with depth.
 Plants such as seaweed also photosynthesise to
enable growth and the area between the surface
and approximately 20m is known as the Photic
zone. Below the Photic zone plants will not flourish.
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4
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Factors Affecting the Rate of Marine Factors Affecting the Rate of Marine
Growth Growth
Water current: Salinity:
 The speed at which the water flows over the  In nearly fresh water, fouling is usually
surface plays an important part in the type of confined to algal slime. As the salinity
fouling colony that develops. increases, so the amount and type of fouling
 There are two aspects to consider, the first increases. First hydroids and barnacles and
being that of the larvae attaching themselves finally mussels occur. The normal salinity of
to the structure. A 1 knot current may be too seawater is about 3-3.5% and the size of
much at first but, once established, 6 knots mussels, for example, increases five-fold from
can be tolerated. a salinity of 0.6-3.5%.
 Secondly, in general, once the organism is
established, a strong current brings more food
and growth is accelerated.

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Factors Affecting the Rate of Marine

Growth
Food supply:
 Growth of fouling is obviously dependent on
the quantity of nutriment available.
Investigations suggest that the slow moving
currents circulating around platforms become
enriched with nutriments from sewage and Any Questions?
other waste that will increase growth rate.

Cathodic protection:
 Currently it is not understood why, but ICCP
systems seem to have the effect of increasing
the marine growth.

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5
Section 7

Corrosion
7 Corrosion
7.1 Energy considerations in corrosion
With time, most materials react with their environment to change their
structure. The reaction in metals is called corrosion, in polymers (plastics)
degradation and in concrete weathering.

Corrosion in metals is defined as the chemical or electrochemical reaction

between a metal and its environment, which leads to one of three
consequences:

1 The removal of the metal.

2 The formation of an oxide.
3 The formation of another chemical compound.

This change in the metal will be expected if the thermodynamics (energy state)
of the system is considered.

The First Law of Thermodynamics states:

 Energy can neither be created nor destroyed.

As a direct consequence of this law, when spontaneous changes occur they

must follow a rule, which is:

 Whenever a spontaneous change occurs it must release free energy from

the system to the surrounding at constant temperature and pressure.

Which is a way of stating the Second Law of Thermodynamics:

 When corrosion occurs naturally it releases free energy, as it is a

spontaneous process.

Take the case of a metal, such as iron or aluminium as an example; both are
found in nature as ores and when analysed, are found to be chemical
compounds including oxygen and carbon amongst other elements. This
necessitates the extraction of the metal itself from the other elements before it
can be used in fabrication.

The process whereby the metal is extracted requires either the smelting of the
ore (iron) or an electrolysis process (aluminium). The final metal produced is,
therefore, at a higher thermal energy level than the ore from which it was
extracted ie energy is added to the system.

One of the fundamental laws of equilibrium is that all systems try to reduce
their energy level to a minimum. This is why water runs downhill, reducing its
potential energy level as it flows. In similar fashion, metals tend to reduce their
energy and, therefore, obey the rule imposed by the second law. So free energy
is released.

There are numerous forms of energy but the energy causing corrosion is
chemical energy that is utilised to form lower energy chemical compounds,
like metal oxide, which resemble the original ore. Because steel (iron alloys of
various types) is such an important material in building and industry the
corrosion of iron has a special term, rust.

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Corrosion 7-1 Copyright © TWI Ltd
 RUST

Figure 7.1 Changes in energy levels of a typical metal extracted from ore.

7.2 The corrosion process

Knowing there is a driving force for the process, it is necessary to consider the
mechanism by which corrosion can take place. Firstly, a reminder of the basic
structure of the atom will assist in the understanding of the topic.

In its simplest form an atom is a positive nucleus surrounded by negatively

charged electrons. Figure 7.2 shows a simplified diagram of the structure of an
atom that is adequate for the purposes of this discussion.

Figure 7.2 Simple structure of an atom.

The overall charge on the atom is zero and an atom, so composed, has a
negative charge of electrons equal to the positive charge of the nucleus.
However, electrons can be added to or taken from the group that surrounds
each atom. When this happens, the overall charge on the atom is no longer
zero. This condition of the atom is called ionic.

So, if the atom loses an electron it becomes a positive ion, which means that
the atom now has a positive charge. If the atom gains an electron it becomes a
negative ion and now has a negative charge.

The first step in the corrosion process is that metal atoms change their state
from being metallic (that is no charge on the atom) to being ionic (that is

DIS1-30815
Corrosion 7-2 Copyright © TWI Ltd
 having a charge on the atom) by losing at least one electron from the outer
shell. The process of corrosion then goes on at the atomic level, each atom
losing one or more (usually no more than three) electrons to become an ion.

7.3 The anodic reaction

The reaction in which the metal is changed from its metallic state to its ionic
state is known as the anodic reaction. It is part of an overall reaction
involving the metal and other species present in the environment. This process
is also called oxidation.

Figure 7.3 The anodic reaction.

The anodic reaction for iron releases two electrons, as shown in figure 7.3
which represents a freely rusting iron surface immersed in seawater (the
electrolyte).

Figure 7.4 Anodic sites on surface of iron exposed to seawater.

DIS1-30815
Corrosion 7-3 Copyright © TWI Ltd
 This is one part of the reaction in electrochemical corrosion that takes place in
the presence of an electrolyte; that is often water or a water-based solution of
ionic compounds, such as acids, bases or salts. The metal ion passes into
solution and the electron passes through the metal, that is not actually being
corroded, that is; an electric current flows as indicated in Figure 7.4.

7.4 The cathodic reaction

These free electrons, formed in the anode reaction, must be used up if the
reaction is to proceed. This part of the reaction in the electrochemical corrosion
process, therefore takes place at the site where the free electrons are
neutralised and is known as the cathodic reaction. Alternatively, reactions such
as this that consume electrons are also known as reduction reactions.

Figure 7.5 Cathodic reaction.

Typically, a complete reaction is for the free electrons to be taken up by

positive ions and atoms of oxygen in the electrolyte. This gives the oxygen a
negative charge. Oxygen, however, readily accepts the free electrons because
its electron stability needs eight electrons in its outer valence shell, yet occurs
naturally with only six.

Figure 7.6 Cathodic reaction (reduction) into the electrolyte.

DIS1-30815
Corrosion 7-4 Copyright © TWI Ltd
 Free electrons move through the metal cathode to its surface where negative
ions form and subsequently emit free electrons into the electrolyte where they
combine with elements creating different compounds. The site of this reaction is
known as the cathode.

The actual reduction reaction at the cathode will vary according to the
composition of the electrolyte. Hydrogen evolution is a common reaction when
the electrolyte is acidic. Oxygen reduction is also very common, since any
aqueous solution in contact with air is capable of producing this reaction.

It is, of course, the reaction encountered in seawater. Metal ion reduction is less
common and is normally found in chemical process streams. The common
denominator with all these reactions is that they consume electrons and this is
the most important point to note.

7.5 Electrochemical aspects of corrosion

A fundamental definition for corrosion is:

Corrosion is the degradation of a metal by an electrochemical reaction

with its environment.

For corrosion to take place four criteria must apply:

1 There must be an anode: This normally corrodes by loss of electrons.

2 There must be a cathode: This does not normally corrode.
3 There must be an electrolyte: This is the name given to a solution that
conducts electricity.
4 Pure distilled water is not an electrolyte while seawater is.
5 There must be an electrical connection between the anode and the cathode
for the free electrons to travel along.

These four elements are shown diagrammatically in Figure 7.7 and all
electrochemical corrosion takes place by setting up cells like this.

Figure 7.7 Corrosion circuit.

As this is an electrochemical reaction and the chemistry has been touched on

already a few basic electrical definitions will round off this section.

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Corrosion 7-5 Copyright © TWI Ltd
7.6 Electric theory
Electricity is the passage of electrons between two defined points. This normally
occurs through a metal wire connecting the two points and is called a current.

Electricity can also pass through suitable aqueous solutions but the electrical
charge is then carried by ions.

The amount of charge carried by an electron is known and when a given

electron flow is passed at a constant rate it is measured in amperes and is
given the symbol I.

 In the MKS (metre, kilogram, second) system, one ampere is defined as

that constant current which, if maintained in each of two infinitely long
straight parallel wires of negligible cross-section; placed one metre apart, in
a vacuum, will produce between the wires a force of 2 x 10-7 Newtons per
metre length.

The driving force causing this current to flow is the potential difference between
two points and is measured in volts, which has the symbol V.

 In the MKS system this is defined as that difference of electrical potential

between two points of a wire carrying a constant current of 1 ampere when
the power dissipation between those points is 1 watt.

The flow of an electric charge is impeded by a quantity called resistance and

between any two points there is always some resistance to the passage of the
current. The unit of resistance is the ohm which has the symbol Ω.

 The MKS system defines the unit of electrical resistance as being the
resistance between two points of a conductor when a constant potential
difference of 1V, applied between these points, produces in the conductor a
current of 1A.

During the majority of this section all discussion and illustrations will be in
terms of electron or ion flow and as far as possible, positive and negative
notations will be avoided so as to avoid confusion, which often occurs when
corrosion is studied.

This confusion arises because of an historical accident that resulted in producing

what is now called conventional current. Electron flow is exactly opposite to
conventional current, this is what causes the confusion as studies in corrosion
so often involve discussion on electron or ion flow.

To avoid such problems on corrosion only electron flow will be considered.

Figure 7.8 illustrates the two types of flow.

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Corrosion 7-6 Copyright © TWI Ltd
 Conventional Flow Electron Flow
Conventional Electron
flow flow
Battery
Anode
A Cathode
+ ‐
Anode Cathode

+ A ‐

Figure 7.8 Conventional and electron flow.

DIS1-30815
Corrosion 7-7 Copyright © TWI Ltd
 Bibliography
Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office
Books), 1988, ISBN 13: 9780114129118.

Trethewey K R, Chamberlain J, ‘Corrosion for Students of Science and

Engineering’, Longman Scientific & Technical, 1988,
ISBN 13: 9780582450899.

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Corrosion 7-8 Copyright © TWI Ltd
 27/08/2015

Energy Considerations in Corrosion

With time, most materials will react with their environment

to change their structure. In metals this is called corrosion,
in polymers (plastics) degradation and in concrete
weathering.

Corrosion in metals is defined as the chemical or electro-

CSWIP 3.1U Course chemical reaction between a metal and its environment
Corrosion which leads to one of three consequences:

Section 7  Removal of the metal.

 Formation of an oxide.
 Formation of another chemical compound.

 This change in the metal will be expected if the

thermodynamics (energy state) of the system is considered.

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Energy Considerations in Corrosion The Corrosion Circuit

The first law of thermodynamics states:  Take the case of a metal, such as iron or aluminium, both
are found in nature as ores, which are chemical compounds
‘Energy can neither be created nor destroyed’ including oxygen, carbon and other elements. It is
necessary to extract the metal itself from the other
compounds before it can be used in fabrication.
Therefore, when spontaneous changes occur they
must follow this rule:  The process whereby the metal is extracted requires the
 ‘Whenever a spontaneous change occurs it must release smelting of the ore. The final metal produced is now at a
free energy from the system to the surrounding at constant higher energy level than the ore from which it was
temperature and pressure’ extracted (ie energy is added to the system).

 Which is a way of stating the second law of  A fundamental law of energy is that all systems try to
thermodynamics reduce their energy level to a minimum. Metals do this and
‘When corrosion occurs naturally it releases free therefore obey the rule imposed by the second law. So,
energy, as it is a spontaneous process’. free energy is released.

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Corrosion Circuit Corrosion Process

There are numerous forms of energy but the energy Knowing there is a driving force for the process, it is
causing corrosion is chemical energy that is utilised necessary to consider the mechanism by which corrosion
to form lower-energy chemical compounds, like the can take place.
metal oxide, which resembles the original ore. An atom is a positive
nucleus surrounded by
negatively charged
electrons.
The overall charge on the
atom is zero, as an atom
has a negative charge of
electrons equal to the
positive charge of the
nucleus.
RUST

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 27/08/2015

Corrosion Process Corrosion Process

However, electrons can be added or taken from The first step in the corrosion process is that
the group that surrounds each atom. When this metal atoms change their state from being
happens, the overall charge on the atom is no metallic (no charge on the atom), to being ionic
longer zero. This condition of the atom is termed (having a charge on the atom) by losing at least
ionic. one electron from the outer shell.

So if the atom loses an electron it becomes a The process of corrosion then goes on at the
positive ion and now has a positive charge. If atomic level, each atom losing one or more
the atom gains an electron it becomes a electrons (usually no more than three) to
negative ion and now has a negative charge. become an ion.

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Anodic Reaction Anodic Reaction

The reaction in which the metal is changed from its The anodic reaction for iron releases two
metallic state to its ionic state is known as the anodic electrons.
reaction. It is part of an overall reaction involving the
metal and other elements present in the
environment. This process is also called oxidation.

The site at which this takes place is the anode This represents a freely rusting iron surface immersed in seawater

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Cathodic Reaction Cathodic Reaction

The free electrons formed in the anode reaction Typically, a complete reaction is for the free
must be used up if the reaction is to proceed. electrons to be taken up by positive ions and
atoms of oxygen in the electrolyte.
This part of the reaction takes place at the site
where the free electrons are neutralised and is
known as the cathodic reaction. This gives the oxygen a negative charge.

Oxygen however, readily accepts these free

electrons because its electron stability needs
eight electrons in its outer valence shell, yet
occurs naturally with only six.

Reactions that consume electrons are also known as reduction reactions

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Cathodic Reaction Electrochemical Aspects of Corrosion

Free electrons move through the metal cathode A fundamental definition for corrosion is:
to its surface where negative ions form and Corrosion is the degradation of a metal by an
subsequently emit free electrons into the electrochemical reaction with its environment.
electrolyte, where they combine with elements
creating different compounds. For corrosion to take place four criterion must apply
there must be:

1. An anode.
The site of this 2. A cathode.
reaction is known as 3. An electrolyte.
the cathode. 4. An electrical connection between the cathode and the
anode.

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Electrochemical Aspects of Corrosion Electric Theory

These four elements are shown here and all Electricity is the passage of electrons between
electro-chemical corrosion takes place by setting two defined points. This normally occurs through
up cells like this. a metal wire connecting the two points and is
called a current.

Electricity can also pass through suitable

aqueous solutions, but the electrical charge is
then carried by ions.

The amount of charge carried by an electron is

known and when a given electron flow is passed
at a constant rate it is measured in amperes and
is given the symbol (I).

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Electric Theory Electric Theory

In the MKS (Metre, Kilogramme, Second) The driving force causing this current to flow is
system, one ampere is defined as that constant the potential difference between two points and
current which, if maintained in each of two is measured in volts, which has the symbol (V).
infinitely long straight parallel wires of negligible
cross-section; placed 1m apart, in a vacuum, will In the MKS system, this is defined as that
produce between the wires a force of 2 x 10-7 difference of electrical potential between two
Newtons per metre length. points of a wire carrying a constant current of 1
ampere when the power dissipation between
those points is 1watt.

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3
 27/08/2015

Electric Theory Electric Theory

The flow of an electric charge is impeded by a During the majority of this chapter all discussion
quantity called resistance and between any two and illustrations will be in terms of electron or
points there is always some resistance to the ion flow and as far as possible positive and
passage of the current. The unit of resistance is negative notations will be avoided so as to avoid
the ohm which has the symbol (Ω). confusion, which often occurs when corrosion is
studied.
The MKS system defines the unit of electrical
resistance as being the resistance between two This confusion arises because of an historical
points of a conductor when a constant potential accident that resulted in producing what is now
difference of 1V, applied between these points, called conventional current.
produces in the conductor a current of 1A.

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Electric Theory Electric Theory

Electron flow is exactly opposite to conventional

current, this is what causes the confusion as Conventional flow Electron flow
studies in corrosion so often involve discussion
on electron or ion flow.

To avoid such problems for the rest of this

discussion on corrosion only electron flow will be
considered.

Conventional and electron flow

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Any Questions?

Copyright © TWI Ltd

4
 Section 8

Types of Corrosion
8 Types of Corrosion
8.1 Corrosion cells
Corrosion cells, using the corrosion process outlined in the previous section, can
be set up by many different means but they all operate because there is some
dissimilarity between the anode and the cathode, such as:

 Dissimilar metals.
 Dissimilar phases in the grains of the metal.
 Dissimilar energy levels between the grain and the grain boundary of the
metal.
 Dissimilar ion concentrations.
 Dissimilar oxygen concentrations.

8.2 Dissimilar metal corrosion cell (galvanic corrosion)

It is found that when dissimilar metals are placed in the same fluid (electrolyte)
a potential difference (voltage) exists between them.

This can be demonstrated easily by placing two rods of different metals in water
and connecting a voltmeter between them. The voltmeter measures a voltage
and current flows from the anode to the cathode via the outside connection.

The cell acts as a very basic, low powered battery and in battery terms the
anode is the negative and the cathode the positive. Electrons flow from the
negative terminal to the positive terminal in the external circuit. Figure 7.8
Section 7 refers.

It is possible to determine which of the two metals will be the cathode and
which the anode by reference to the Galvanic Series. The rule is that metals
found lower in the series are anodic to any metal above them. For example,
zinc is lower in the series than mild steel; therefore, if zinc is connected to mild
steel and immersed in seawater, zinc will be the anode and corrode and mild
steel will be the cathode and not corrode.

If, on the other hand, mild steel, in the form of a ship’s hull is connected to
manganese bronze, the ship’s propeller, the mild steel now becomes the anode
and corrodes and the propeller is the cathode, which does not corrode.

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Types of Corrosion 8-1 Copyright © TWI Ltd
 Table 8.1 Galvanic Series in seawater.
Gold
Silver
18-8 (3%Mo) Stainless steel (passive)
Monel
Nickel (passive)
Copper
Red brass
Aluminium bronze
Admiralty brass
Yellow brass
Nickel (active)
Manganese bronze
Lead
18-8 Stainless steel (active)
50-50 Lead-Tin solder
Cast iron
Wrought iron
Mild steel
Cadmium
Alcad
Aluminium 52Sh
Galvanised iron
Zinc
Magnesium alloys
Magnesium

8.3 Concentration cell corrosion

Corrosion of this type is associated with crevices in the order of 20-100μm wide
(1 mircometre, μm = 1000 nanometres, nm) and commonly involves chloride
ions in the electrolyte.

The stages in the process are:

Corrosion will at first occur over the entire surface of the exposed metal at a
slow rate, both inside and outside the crevice. During this period of time the
electrolyte may be assumed to have a uniform composition and normal anodic
and cathodic processes take place. Under these conditions positive metal ions
and negative hydroxyl ions are produced, so as to maintain equilibrium within
the electrolyte.

This process consumes the dissolved oxygen, which results in the diffusion of
more oxygen from the atmosphere at any surface where the electrolyte is in
contact with air. In turn, the oxygen in the bulk of the electrolyte is replaced
more easily at metal surfaces rather than in any small crevices. This creates a
low oxygen situation within the crevice, that in turn impedes the cathodic
process and the production of hydroxyl ions is therefore reduced.

This results in excess positive ions accumulating in the crevice, which causes
negative ions to diffuse there from the bulk of the electrolyte outside in order to
maintain minimum potential energy overall. The metal ions, water molecules
and chloride all react in complicated chemical reactions forming complex ions,
which it is thought, react with water in an hydrolysis reaction, resulting in
corrosion products.

The increase of hydrogen ion concentration accelerates the metal dissolution

process, which in turn, makes the problem worse, as does the accompanying
increase of chloride concentration within the crevice.

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 An important feature of active crevice corrosion cells is that they are
autocatalytic, that is once started they are self-sustaining. It is worth
underlining the fact that the electrolyte in an active crevice can become very
acidic. This is the situation shown in Figure 8.1. The metal inside the crevice is
corroding rapidly while that outside is cathodically protected.

Final conditions
Initial conditions
Corrosion is accelerated in
crevice.

Figure 8.1 Crevice corrosion.

8.4 Pitting
Pitting is localised corrosion that selectively attacks areas of a metal surface.
Once formed, corrosion pits propagate in the same way as crevice corrosion.

Consider the case of a water drop laying on the surface of a sheet of clean mild
steel.

 The corrosion process initiates uniformly on the surface of the steel under
the water. This consumes oxygen by the normal cathode reaction in what is
a neutral solution at this stage.

 This causes an oxygen gradient to form within the water drop. It is obvious
that the wetted area around the water/air interface has more oxygen
diffusion from the air than the centre of the drop.

 This concentration gradient anodically polarises the central region, which

dissolves.

 The hydroxyl ions generated in the centre of the drop at the cathode diffuse
inwards and react with iron ions diffusing outwards, causing the deposition
of insoluble corrosion product around the depression, or pit.

 This further retards the diffusion of oxygen, accelerates the anodic process
in the centre of the drop and causes the reaction to be autocatalytic.

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Types of Corrosion 8-3 Copyright © TWI Ltd
 Long Path Air

Short Path Oxygen Diffusion Short Path

Water

Oxygen Depletion Layer

General corrosion with many local anodes and cathodes

Air
Water

Short Path
OH‐ OH‐
Fe2+ Fe2+

Rust Rust
Cathode Cathode
e‐ Anode e ‐

Figure 8.2 Pitting beneath a water drop.

As the process continues, the corrosion products accumulate over the pit and its
immediate surroundings, forming a scab and isolating the environment within
the pit from the bulk electrolyte.

It is thought that the autocatalytic process is assisted by an increased

concentration of chloride ions within the pit. This type of corrosion would be
possible in the splash zone of a structure, if it were not protected with a coating
such as paint or Monel sheathing.

8.5 Intergranular corrosion

Intergranular corrosion occurs between the grain boundaries in a material
because of intrusions in these regions. This is, primarily, because grain
boundaries are the preferred sites for the precipitation and segregation
processes, which occur in many alloys.

These intrusions are of two types:

Intermetallics (intermediate constituents)

 Compounds that are formed from metal atoms and having identifiable
chemical formulae can be either anodic or cathodic to the parent metal.

Compounds
 These are formed between metals and non-metallic elements, such as;
hydrogen, carbon, silicon, nitrogen and oxygen.

 Iron carbide and manganese sulphide, which are both important

constituents of steel, are both cathodic to ferrite (iron).

In principle, any metal that has inter-metallics or compounds at grain

boundaries will be susceptible to intergranular corrosion. Plain carbon steel is a
two phase metal and some grains are cathodic, while others are anodic and
intergranular corrosion initiates, as indicated in Figure 8.3

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Types of Corrosion 8-4 Copyright © TWI Ltd
 Figure 8.3 Corrosion in two phase metal.

8.6 Grain boundary corrosion

The driving force behind grain boundary corrosion is the area of higher energy
found at the grain boundary itself. These higher energy regions become the
anodic sites, while the bulk of the grain itself becomes the cathode. This
situation results in the loss of material in the anodic reaction at the grain
boundaries themselves, in the form of a line.

Figure 8.4 Grain boundary corrosion.

Weld decay, or preferential corrosion, is an example of this type of decay. In

this case the boundary is the fusion boundary that forms along the toe of the
weld and is a region of higher energy. This region becomes the anode and
corrosion sets in, often giving quite significant visual indications of its presence.

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Types of Corrosion 8-5 Copyright © TWI Ltd
 Figure 8.5 Weld decay or preferential corrosion.

8.7 Stress corrosion cracking (SCC)

SCC is the combined action of stress and a corrosive environment, which leads
to the formation of a crack, which would not have developed by the action of
the stress or environment alone.

The conditions necessary for SCC to occur are:

 Tensile stress.
 Suitable environment.
 Sensitive metal.
 Appropriate temperature and pH range.

Some examples of alloy system and environment combinations which cause

SCC are given in Table 8.2. Depending on the alloy system and corrodent
combination, the cracking can be intergranular or trans granular. SCC cracks
have the appearance of a brittle mechanical fracture, which is readily observed
in a scanning electron microscope, Figure 8.6.

Table 8.2 Combinations of alloy and environment susceptible to SCC.

Material Environment
Brass Ammonia
Austenitic stainless steels Chloride-containing solutions
High strength steels Hydrogen

A common feature of SCC that repeatedly occurs is the unexpectedness of its

manifestation. Often, a material that has been chosen for its corrosion
resistance is found to fail at a stress level well below its normal fracture stress.

DIS1-30815
Types of Corrosion 8-6 Copyright © TWI Ltd
 Figure 8.6 Example of SCC.

Problems with pipes and tubes are common because of the hoop residual
stresses that are the result of the fabrication process. Stress-relieving heat
treatments are a vital part of the quality control for these components because
of this.

8.8 Corrosion fatigue

There are many similarities between corrosion fatigue and stress corrosion
cracking, but the most significant difference is that corrosion fatigue is under
dynamic stress, whereas SCC is under static stress.

Fatigue affects all metals causing failure at stress levels well below the UTS.

In aqueous environments it is frequently found that a metal’s fatigue resistance

is reduced, or even that it no longer has a fatigue limit.

Summarising the stages in the development of a fatigue crack yields:

 Firstly the formation of slip bands.

 Next very small cracks form in the order of 10nm long.
 Then the extension of this crack along favourable paths.
 Finally, macroscopic, 0.1 to 1mm crack propagation; in a direction at right
angles to the maximum principal stress that leads to failure.

Corrosion fatigue can occur in any of the three states indicated by the
Pourbaix diagram. It can also occur at stress levels much lower than those for
stress corrosion cracking (SCC). It is also true that, while SCC growth rates are
independent of the stress intensity factor during much of the crack growth,
fatigue crack growth is always effected by it.

It is thought that the use of cathodic protection systems that place the metal in
the immune state and over time cause calcareous deposits to form, tend to
inhibit crack growth, ensuring that the structures are resistant to corrosion
fatigue.

DIS1-30815
Types of Corrosion 8-7 Copyright © TWI Ltd
8.9 Erosion corrosion
This is a self-explanatory name for a form of deterioration that results from a
metal being attacked because of the relative motion between an electrolyte and
a metal surface, which accelerates the rate of corrosion. Examples of this type
of corrosion are attributable to mechanical effects, such as, wear, abrasion and
scouring.

Soft metals, such as copper, brass, pure aluminium and lead are particularly
vulnerable.

Two main forms of erosion corrosion are:

1 Corrosion associated with laminar (fluid) flow.

2 Damage caused by impingement in turbulent conditions.

A laminar flow will cause several effects:

 Where the increased flow replenishes aggressive ions, such as chloride and
sulphide, this has a detrimental effect and corrosion rates increase.

 If the flow contains any solid particles, protective layers may be scoured
away causing excessive corrosion.

 The alternative to this is that it is sometimes possible in pipes for the

deposit of silt to be prevented, thus preventing the formation of any
differential-aeration cells in the crevices beneath.

 A possible beneficial effect is that more oxygen is carried to the area, which
minimises the formation of differential-aeration cells that are normally a
common cause of attack.

 Another possible beneficial effect is where a steady supply of inhibitor is

concentrated within the flow, as in a pipeline for example.

These combined circumstances make the effects of laminar flow unpredictable.

Taking the case of turbulent flow, however, the situation is much more
straightforward. The fluid molecules now impinge directly on the metal causing
wear. This obviously increases the corrosion rate.

This effect can easily occur inside a pipe because turbulence can be caused by
sudden changes in bore diameter, or direction (ie pipe bends), a badly fitted
joint or gasket, circumferential welds or silt deposits.

DIS1-30815
Types of Corrosion 8-8 Copyright © TWI Ltd
 Figure 8.7 Effects of flow in pipes.

8.10 Fretting corrosion

Fretting corrosion describes corrosion occurring at contact areas between
materials under load; subjected to vibration and slip. In appearance, it shows
pits and grooves in the metal surrounded by corrosion products.

It has been observed in a number of different components in machinery and in

bolted parts. In essence this is a form of erosion corrosion.

Fretting corrosion is very detrimental due to the destruction of metallic

components and the production of oxide debris. This leads to loss of tolerance
and may result in fatigue fracture due to the excessive strain caused by the
extra movement and the pits acting as stress raisers. A classic case on land of
fretting occurs at bolted tie plates on railway tracks.

The basic requirements for the occurrence of fretting corrosion are:

 The interface must be under load.

 Vibration or repeated relative motion between the interfaces must be

sufficient to produce slip or deformation on the surfaces.

 The load and relative motion of the interface must be sufficient to produce
slip or deformation on the surfaces.

This type of corrosion could occur in the metal adjacent to clamps and collars of
risers, conductors and caissons if there is the slightest movement underneath
them.

DIS1-30815
Types of Corrosion 8-9 Copyright © TWI Ltd
 Figure 8.8 Possible fretting corrosion between riser and riser clamp.

8.11 Biological corrosion

Biological corrosion is also referred to as microbiologically-induced corrosion
(MIC), emphasising the effect of living organisms (referred to as bacteria).

Sulphate-reducing bacteria (SRB) is one of many types of bacteria, which can

be found in drilling and pumping machinery, storage tanks, pipelines for water
injection, oil recovery and multiple production. SRB metabolise sulphates and
produce sulphuric acids or H2S, thus introducing hydrogen sulphide into the
system, resulting in pitting or sulphite stress corrosion cracking (SSCC).

Corrosion by marine biological action can be initiated in various ways, by:

 The production of corrosive substances like hydrogen sulphide or ammonia,

which result in direct chemical attack on the metal.

 Producing or actually being a catalyst in the corrosive action.

 The reaction of sulphate-reducing bacteria (SRB) under anaerobic (no

oxygen present) conditions.

- The most important of these are the bacteria Sporovibrio

desulfuricans. These thrive in the reduced oxygen conditions created
under heavy accumulations of marine growth, under thick deposits of
corrosion products, or under mud.

- There are indications that, because oxygen is unable to diffuse through

the heavy marine growth, the effect of this organism is to take the place
of oxygen in the usual cathodic reaction.

 By the formation of concentration cells around and under the organisms.

 Fretting corrosion may also occur due to the continued movement of hard
shelled creatures on the structure’s surface.

DIS1-30815
Types of Corrosion 8-10 Copyright © TWI Ltd
8.12 Other factors affecting corrosion rates
The corrosion rate is predictable within certain parameters and corrosion
engineers work this out when designing a protection system. There are
however, environmental factors that affect the overall corrosion reaction.

Specifically these are:

 Temperature.
 Water flow rate.
 The pH of the water.

8.13 Temperature
Most chemical reactions are speeded up by an increase in temperature. Hot
risers, exhaust and cooling-water dumps are all sites that can and do corrode
more quickly than other sections of offshore structures. Therefore, these
components being more susceptible must be inspected more regularly.

8.14 Water flow rate

In general, if the water flow is increased then the rate at which the metal is
removed is also increased. If there is impingement of the flow on the metal or
aeration takes place near the surface, then a very much larger rate of metal
removal is experienced locally. The pitting of ship’s propellers and pump and
dredger impellers are general examples of this.

8.15 The pH value of the water

The corrosion rate of metals is directly affected by the pH value of the
electrolyte. Steel, for example, corrodes least when in a solution that has a pH
of between 11 and 12.

Water is a neutral molecule in which two atoms of hydrogen combine with one
atom of oxygen (H2O). There is a limited amount of dissociation (separation of
positive and negative charged ions in solution) into hydrogen ions and
hydroxyl ions.

Water represents a neutral substance as it contains both acid H+ and alkali OH-
in equal amounts.

This can be noted in the form of an equilibrium:

H2O H+ + OH-

The relationship between these elements forms the basis of a scale of acidity.

All acids have one common property: that is the presence in aqueous solution
of the hydrogen ion, whereas, alkali has hydroxyl ions.

The opposite of acid is alkali or basic, which means that acids are neutralised by
alkalis

The method of defining acidity is by means of a term called pH, which indicates
the amount of hydrogen activity. It is measured on a scale of 0-14.

DIS1-30815
Types of Corrosion 8-11 Copyright © TWI Ltd
 Figure 8.9 The pH scale.

DIS1-30815
Types of Corrosion 8-12 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Trethewey K R, Chamberlain J, ‘Corrosion for Students of Science and

Engineering’, Longman Scientific & Technical, 1988,
ISBN 13: 9780582450899.

DIS1-30815
Types of Corrosion 8-13 Copyright © TWI Ltd
 27/08/2015

Types of Corrosion

 Corrosion cells using the process outlined in

section 7, can be set up by many different
means, but they all operate because there is
some dissimilarity between the anode and the
cathode:
CSWIP 3.1U Course
Types of Corrosion  Dissimilar metals.
Section 8  Dissimilar phases in the metal grains.
 Dissimilar energy levels.
 Dissimilar ion concentrations.
 Dissimilar oxygen concentrations.

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Dissimilar Metal Corrosion Dissimilar Metal Corrosion

Dissimilar metal corrosion (galvanic corrosion): It is possible to determine which of the two metals will
be the cathode and which the anode by reference to the
 It is found that when dissimilar metals are placed in the Galvanic Series.
same fluid (electrolyte) a potential difference (voltage)
exists between them. The rule is that metals found lower in the series are
anodic to any metal above them. For example, zinc is
 This can be demonstrated easily by placing two rods of lower in the series than mild steel; therefore, if zinc is
different metals in water and connecting a voltmeter connected to mild steel and immersed in seawater, zinc
between them. The voltmeter measures a voltage and will be the anode and corrode and mild steel will be the
current flows from the anode to the cathode via the cathode and not corrode.
outside connection.
If, on the other hand, mild steel, in the form of a ship’s
 The cell acts as a very basic, low powered battery and hull is connected to manganese bronze, the ship’s
in battery terms the anode is the negative and the propeller, the mild steel now becomes the anode and
cathode the positive. Electrons flow from the negative corrodes and the propeller is the cathode, which does
terminal to the positive terminal in the external circuit. not corrode.
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Concentration Cell or Crevice

Galvanic Series
Corrosion
This type of corrosion is associated with crevices in
the order of 20-100μm (mircometre) wide and
commonly involves chloride ions in the electrolyte.
Initially corrosion occurs uniformly over the
surface of the exposed metal producing positive
metal ions and negative hydroxyl ions.
This process consumes the dissolved oxygen.
This results in the diffusion of more oxygen from
the atmosphere wherever the electrolyte is in
contact with the air.
The oxygen in the bulk of the electrolyte will be
replaced more easily than that trapped in the
crevice.
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Concentration Cell or Crevice Concentration Cell or Crevice

Corrosion Corrosion
 This creates a low oxygen situation within the  The increase of hydrogen ion concentration
crevice which impedes the cathodic process, accelerates the metal dissolution process.
reducing hydroxyl ion production.
 Likewise there is an increase in chloride
 Excess positive ions accumulate in the crevice concentration within the crevice.
which causes negative ions to diffuse there from
the bulk of the electrolyte.  This initiates an autocatalytic process which
means once started it is self-sustaining.
 This is an attempt to maintain minimum potential
energy overall.  The electrolyte in an active crevice becomes
very acidic, due to the increase in hydrogen
 The metal ions, water molecules and chloride all possibly as a result of the electrolysis.
react in complicated chemical reactions forming
complex ions.  The metal within the crevice will corrode while
that outside will be catholically protected.
 These, it is thought, react with the water in an
electrolysis reaction resulting in corrosive products.
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Concentration Cell or Crevice Corrosion Pitting Corrosion

Pitting is localised corrosion that selectively

attacks areas of a metal surface

 The corrosion process initiates uniformly on the

surface of the steel under the water. This
consumes oxygen by the normal cathode reaction
in what is a neutral solution at this stage.
 This causes an oxygen gradient to form within the
water drop. It is obvious that the wetted area
around the water/air interface has more oxygen
Initial conditions Final conditions
General corrosion Corrosion is accelerated in crevice diffusion from the air than the centre of the drop.
 This concentration gradient anodically polarises
Concentration cell or crevice corrosion
the central region, which dissolves.

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Pitting Corrosion Pitting Corrosion

 The hydroxyl ions generated in the centre of the drop at

the cathode diffuse inwards and react with iron ions
diffusing outwards, causing the deposition of insoluble
corrosion product around the depression, or pit.

 This further retards the diffusion of oxygen, accelerates

the anodic process in the centre of the drop and causes
the reaction to be autocatalytic. This type of corrosion
would be possible in
 As the process continues, the corrosion products the splash zone of a
accumulate over the pit and its immediate
structure, if it were not
surroundings, forming a scab and isolating the
environment within the pit from the bulk electrolyte. It
protected with a
is thought that the autocatalytic process is assisted by coating such as paint
an increased concentration of chloride ions within the or Monel.
pit.

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Inter-granular Corrosion Inter-granular Corrosion

Intergranular corrosion occurs between the grain These intrusions are of two types:
boundaries in a material because of intrusions in
1. Intermetallics (intermediate constituents)
these regions. This is, primarily, because grain
Compounds that are formed from metal atoms and having
boundaries are the preferred sites for the identifiable chemical formulae can be either anodic or
precipitation and segregation processes, which occur cathodic to the parent metal.
in many alloys. 2. Compounds
These are formed between metals and non-metallic
elements, such as; hydrogen, carbon, silicon, nitrogen and
oxygen.
Iron carbide and manganese sulphide, which are both
important constituents of steel, are both cathodic to ferrite
(iron).

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Inter-granular Corrosion Grain Boundary Corrosion

In principle, any metal that has inter-metallics or The driving force behind grain boundary corrosion is
compounds at grain boundaries will be susceptible to the area of higher energy found at the grain
intergranular corrosion. Plain carbon steel is a two boundary itself. These higher energy regions
phase metal and some grains are cathodic, while become the anodic sites, while the bulk of the grain
others are anodic and intergranular corrosion itself becomes the cathode. This results in the loss
initiates. of material in the anodic reaction at the grain
boundaries themselves, in the form of a line.

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Grain Boundary Corrosion Stress Corrosion Cracking

 Weld decay, or preferential corrosion, is an Stress Corrosion Cracking (SCC) is the combined
example of this type of decay. In this case the action of stress and a corrosive environment,
boundary is the fusion boundary that forms along which leads to the formation of a crack, which
the toe of the weld and is a region of higher would not have developed by the action of the
energy. This region becomes the anode and stress or environment alone.
corrosion sets in, often giving quite significant
visual indications of its presence. The conditions necessary for SCC to occur are:

 Tensile stress.
 Suitable environment.
Weld decay or  Sensitive metal.
preferential  Appropriate temperature and pH range.
corrosion

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Stress Corrosion Cracking Stress Corrosion Cracking

Some examples of alloy system and Depending on the alloy system and corrodent combination,
environment combinations which cause SCC are the cracking can be intergranular or trans granular. SCC
cracks have the appearance of a brittle mechanical
given in this table. fracture, which is readily observed in this scanning
electron microscope picture.
Material Environment Problems with pipes and tubes
are common because of the
Brass Ammonia
hoop residual stresses that are
the result of the fabrication
Chloride-containing
Austenitic stainless steels process. Stress-relieving heat
solutions
treatments are a vital part of
the QC for these components
High strength steels Hydrogen because of this.

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Corrosion Fatigue Corrosion Fatigue

There are many similarities between corrosion Summarising the stages in the development
fatigue and stress corrosion cracking, but the of a fatigue crack yields:
most significant difference is that corrosion
fatigue is under dynamic stress, whereas SCC  Firstly the formation of slip bands.
is under static stress.  Next very small cracks form in the order of
10nm long.
Fatigue affects all metals causing failure at  Then the extension of this crack along
stress levels well below the UTS. favourable paths.
 Finally, 0.1-1mm crack propagation; in a
In aqueous environments it is frequently found direction at right angles to the maximum
that a metal’s fatigue resistance is reduced, or principal stress that leads to failure.
even that it no longer has a fatigue limit.

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Corrosion Fatigue Erosion Corrosion

Corrosion fatigue can occur in any of the three This is a self-explanatory name for a form of deterioration
states indicated by the Pourbaix diagram. It that results from a metal being attacked because of the
relative motion between an electrolyte and a metal
can also occur at stress levels much lower than surface, which accelerates the rate of corrosion. Examples
those for SCC. It is also true that, while SCC of this type of corrosion are attributable to mechanical
growth rates are independent of the stress effects, such as, wear, abrasion and scouring.
intensity factor during much of the crack growth,
fatigue crack growth is always effected by it. Soft metals, such as copper, brass, pure aluminium and
lead are particularly vulnerable.

It is thought that the use of cathodic protection Two main forms of erosion corrosion are:
systems that place the metal in the immune
state and over time cause calcareous deposits to 1. Corrosion associated with laminar (fluid) flow.
form, tend to inhibit crack growth, ensuring that 2. Damage caused by impingement in turbulent
conditions.
the structures are resistant to corrosion fatigue.

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Laminar Flow Turbulent Flow

The increased flow replenishes aggressive ions, such as chloride Taking the case of turbulent flow, however, the
and sulphide, this has a detrimental effect and corrosion rates situation is much more straightforward. The fluid
increase.
molecules now impinge directly on the metal causing
If the flow contains any solid particles, protective layers may be
scoured away causing excessive corrosion.
wear. This obviously increases the corrosion rate.
In pipes it is possible for the deposit of silt to be curtailed, thus
preventing the formation of any differential-aeration cells in the This effect can easily occur inside a pipe because
crevices beneath. turbulence can be caused by sudden changes in bore
A beneficial effect is that more oxygen is carried to the area, diameter, or direction (ie pipe bends), a badly fitted
which minimises the formation of differential-aeration cells. joint or gasket, circumferential welds or silt deposits.
Another beneficial effect is when a steady supply of inhibitor is
concentrated within the flow, as in a pipeline for example.
These circumstances make the effects of laminar flow
unpredictable.

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Turbulent Flow Fretting Corrosion

Fretting corrosion describes corrosion occurring at contact

areas between materials under load; subjected to vibration
and slip. In appearance, it shows pits and grooves in the
metal surrounded by corrosion products.
It has been observed in a number of different components
in machinery and in bolted parts. In essence this is a form
of erosion corrosion.
Fretting corrosion is very detrimental due to the
destruction of metallic components and the production of
oxide debris. This leads to loss of tolerance and may result
in fatigue fracture due to the excessive strain caused by
the extra movement and the pits acting as stress raisers.
A classic case on land of fretting occurs at bolted tie plates
on railway tracks.

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Fretting Corrosion Fretting Corrosion

The basic requirements for the occurrence of fretting Fretting corrosion occurs where surfaces that are
corrosion are: in contact, move slightly. This may occur
 The interface must be under load.
between risers and clamps.
 Vibration or repeated relative motion between the
interfaces must be sufficient to produce slip or
deformation on the surfaces.
 The load and relative motion of the interface must be
sufficient to produce slip or deformation on the
surfaces.

This type of corrosion could occur in the metal adjacent to

clamps and collars of risers, conductors and caissons if
there is the slightest movement underneath them.

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Biological Corrosion Biological Corrosion

Biological corrosion is also referred to as microbiologically- Corrosion by marine biological action can be
induced corrosion (MIC), emphasising the effect of living initiated in various ways, by:
organisms (referred to as bacteria).

Sulphate-reducing bacteria (SRB) is one of many types of  The production of corrosive substances like
bacteria, which can be found in drilling and pumping
hydrogen sulphide or ammonia, which result in
machinery, storage tanks, pipelines for water injection, oil
recovery and multiple production. direct chemical attack on the metal.
 Producing or actually being a catalyst in the
SRB metabolise sulphates and produce sulphuric acids or corrosive action.
H2S, thus introducing hydrogen sulphide into the system,
resulting in pitting or sulphite stress corrosion cracking  The reaction of Sulphate-Reducing Bacteria
(SSCC). (SRB) under anaerobic (no oxygen present)
conditions.

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Other Factors Affecting Corrosion

Biological Corrosion
Rates
 The most important of these are the bacteria, The corrosion rate is predictable within certain
Sporovibrio Desulfuricans. These thrive in the parameters and corrosion engineers work this
reduced oxygen conditions created under heavy out when designing a protection system.
accumulations of marine growth, under thick
deposits of corrosion products, or under mud.
 There are indications that, because oxygen is There are however, environmental factors that
unable to diffuse through the heavy marine affect the overall corrosion reaction.
growth, the effect of this organism is to take the
place of oxygen in the usual cathodic reaction.
 By the formation of concentration cells around and Specifically these are:
under the organisms.  Temperature.
 Fretting corrosion may also occur due to the  Water flow rate.
continued movement of hard shelled creatures on
 The pH of the water.
the structure’s surface.

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Other Factors Affecting Corrosion Other Factors Affecting Corrosion

Rates Rates
Temperature The corrosion rate of metals is directly affected by the pH
Most chemical reactions are speeded up by an increase in value of the electrolyte. Steel, for example, corrodes least
temperature. Hot risers, exhaust and cooling-water dumps when in a solution that has a pH of between 11-12.
are all sites that can and do corrode more quickly than
other sections of offshore structures. Therefore, these Water represents a neutral substance as it contains both
components being more susceptible must be inspected acid H+ and alkali OH- in equal amounts.
more regularly. The relationship between these elements forms the basis
of a scale of acidity.
Water flow rate
In general, if the water flow is increased then the rate at All acids have one common property: that is the presence
which the metal is removed is also increased. If there is in aqueous solution of the hydrogen ion, whereas, alkali
impingement of the flow on the metal or aeration takes has hydroxyl ions.
place near the surface, then a very much larger rate of
The opposite of acid is alkali or basic, which means that
metal removal is experienced locally. The pitting of ship’s
acids are neutralised by alkalis.
propellers and pump and dredger impellers are general
examples of this.

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6
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Other Factors Affecting Corrosion

Rates
The method of defining acidity is by means of a
term called pH, which indicates the amount of
hydrogen activity. It is measured on a scale of
0-14.

Any Questions?

The pH scale

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7
 Section 9

Corrosion Protection
9 Corrosion Protection
There are numerous methods for preventing corrosion including, coatings,
inhibitors (controlling the electrolyte), selective design, anodic protection and
cathodic protection.

Before considering these methods, a brief examination of the way in which the
corrosion process is influenced by the two main variables; the electrode
potential and the pH of the electrolyte, will assist in understanding the
various protection methods.

This data is often presented in a diagrammatic form known as a Pourbaix

Diagram. These diagrams are obtained from laboratory tests carried out under
controlled conditions of constant temperature and no flow.

14p

Figure 9.1 Pourbaix diagram for iron in water.

DIS1-30815
Corrosion Protection 9-1 Copyright © TWI Ltd
 It can be seen from the Pourbaix diagram, that there are three distinct possible
states of corrosion, depending on electrode potentials (measured against a
standard hydrogen reference electrode) and pH values:

1 Corrosion: At intermediate electrode potentials and over a wide range of

pH values corrosion takes place and metal is removed

2 Passivity: At higher electrode potentials and over a wide range of pH

values, there is a passivity region. This is the region in which a corrosion
product film is formed, that in most cases is an oxide film. It is worth
indicating that the diagram only indicates an oxide film is formed: it does
not mean that the oxide film gives protection. The properties of the film
must be known in order to determine this.

3 Immunity: At more negative electrode potentials and over almost the

whole of the pH range, the rate of corrosion is so low that the metal is said
to be immune

9.1 Cathodic protection

The three stages indicated in the Pourbaix diagram show it is possible to
determine basic strategies for preventing corrosion. It can be achieved by
lowering the electrode potential down to the zone of immunity, or raising the
potential up to the region of passivity:

 Making the electrode potential more positive will produce passivation at

point C.
 Making the electrode potential more negative will produce immunity at point
B.
 Making the electrolyte more basic will produce passivation at point D.

Altering the electrode potential to produce passivation or immunity by the

methods of CP is the most useful technique for offshore structures.

In designing a CP system, the system designer starts by determining an

acceptable corrosion rate and this information is inputted into a graph to
determine a value for current density. This level of current density will ensure
the required corrosion rate to be achieved.

The electrode potential to achieve this current is approximately -800mV. Now, it

may seem that potentials more negative than -800mV (against Ag/AgCl) would
produce even less metal loss.

There are two reasons why it is not prudent to use very much more negative
potentials:

1 At potentials much more negative than –1100mV (Ag/AgCl), the possibility

of hydrogen evolution exists and this can cause hydrogen embrittlement.

2 Large currents are associated with more negative potentials that produce
high local concentrations of hydroxyl ions that often damage barrier
coating, such as paint if it is present.

These points are more likely to occur with an ICCP system but are still quite
valid, making the choice of between -800 and -1100mV (against Ag/AgCl) a
valid design parameter in all cases for offshore platforms.

DIS1-30815
Corrosion Protection 9-2 Copyright © TWI Ltd
9.2 Sacrificial anode method
With this method of corrosion prevention the entire structure is made into the
cathode in a massive corrosion cell (see corrosion circuit diagram Section 7,
Figure 7.7). The structure will therefore not corrode but at the expense of the
anode, which is sacrificed providing the electron flow and gives the process its
name (Figure 9.2).

Figure 9.2 Sacrificial anode cathodic protection.

The anode must be selected from the appropriate galvanic series. The most
appropriate metals are zinc, aluminium and magnesium. Aluminium is often
selected for uncoated structures because of its higher current output and zinc
for coated ones (BP North Sea). Magnesium would provide the highest current
but may not last long enough to provide adequate protection for the design
lifetime of the structure.

In practice a balance between the level of protection and the length of time the
structure will need protecting will be needed. It should be mentioned that the
anodes will not be pure zinc or aluminium but these will be their main
constituent

The anode material used should be zinc or aluminium, as magnesium would

react too quickly.

A natural phenomenon does occur which assists in the protection of structures.

Calcium, magnesium other metal ions are present in sufficient quantities in
seawater to react with hydroxyl ions produced by the negative potential of the
cathodic steel surface. The reaction produces insoluble calcium and magnesium
salts, known as calcareous deposits. These form a strongly adherent film that
reduces the current requirement and may reduce environment-sensitive
cracking.

This method of corrosion protection is almost as straightforward as that. The

main question is: How much anode material will be required?

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Corrosion Protection 9-3 Copyright © TWI Ltd
 This question has two parts:

1 How large a surface area must the anodes protect?

2 How long will the protection last?

To answer the question, an example will be given:

An uncoated steel offshore drilling platform has a sacrificial anode CP system

installed and designed to last for 10 years. What anode material should be used
and how many anodes are required?

 Total wetted surface area of structure = 2500m2.

 Minimum number of anodes required = 2510.

 Anode material should be zinc or aluminium, as magnesium would react too

quickly.

Table 9.1 summarises advantages and disadvantages of sacrificial anode

systems.

Table 9.1 Advantages and disadvantages of sacrificial anodes.

Advantages Disadvantages
No external electric power required and
Current output decreases with time
therefore no danger to divers
Adds considerable weight and drag to the
No danger of overprotection
structure
No running costs Initial costs are comparatively high
Active from day of immersion Comparatively difficult to increase
A well proven and reliable method protection by retro-fitting anodes

9.3 Impressed current method

An impressed current cathodic protection (ICCP) system works on the same
principle as the sacrificial system in that the structure is made to be the
cathode. However, in the case of the ICCP system, the necessary potential and
current flow is provided by a DC generator rather than by a galvanic coupling.

This system can be made to be self-adjusting by incorporating reference

electrodes into the circuit that measure potential. The potential can vary
depending on the circumstances; if the structure has a coating initially that in
subsequent service becomes damaged, this will increase the exposed surface
area needing to be protected.

The control unit can deal with this by increasing the current density. If on the
other hand there were a reduction in the surface area; as for instance a
calcareous deposit building up, there would be less surface area exposed and
the current requirement would be less.

In both cases, the reference electrode provides the means of monitoring the
potential, which varies proportionally according to the current. Figure 9.3 shows
the system.

The anode material is selected from the top of the Galvanic Series, not the
bottom. Materials such as titanium, platinised niobium and lead/silver alloys
are used. Table 9.2 lists some properties of impressed current anode materials.
The anode and supply cables are insulated from the structure to prevent any of
the problems associated with over-protection.

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Corrosion Protection 9-4 Copyright © TWI Ltd
 Figure 9.3 Impressed current system.

Table 9.2 Some impressed current anode materials and their properties
(from brand).

Material Consumption Kg/yr- Recommended uses

Platinum Marine environments and high

8 x 10-6
Platinised metals purity liquids
Potable waters and soil or
High silicon iron 0.25-1.0
carbonaceous backfill
Marine environments and
Steel 6.8-9.1
carbonaceous backfill
Marine environments and
Iron Approximately 9.5
carbonaceous backfill
Marine environments and
Cast iron 4.5-6.8
carbonaceous backfill
Lead-platinum 0.09 Marine environments
Lead-silver 0.09 Marine environments
Marine environments, potable
Graphite 0.1-1.0
water and carbonaceous backfill

(Materials in bold are those commonly used in the North Sea).

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Corrosion Protection 9-5 Copyright © TWI Ltd
9.4 Practical considerations for installing ICCP systems
Anodes made from materials, such as listed in Table 9.2, are capable of
supplying high current densities and it would be possible to protect a structure
with a few large anodes supplied with a high current. However, in practice,
anodes are usually distributed at regular intervals over the whole structure.

This is because:

 The high current density that would be present in the immediate vicinity of
a single anode could damage paint surfaces and possibly cause
embrittlement, as previously discussed. The use of more anodes reduces
the current density for each one and reduces the probability of this type of
damage.

 Offshore structures have a reasonably complicated geometry that makes it

difficult for corrosion engineers to predict the total distribution potentials.
Therefore, it is prudent to use more anodes, each one protecting a smaller
area, thus minimising the areas at risk of inadequate protection.

 When designing the system if the corrosion engineers have any doubts
about protecting any particular area of the structure, sacrificial anodes may
be installed to work in conjunction with the ICCP system.

The ICCP system installed on the Claymore platform was designed to provide
160mA/m2 using 55 platinum-iridium anodes and 12 reference electrodes. The
Murchison platform uses 100 anodes and 50 reference electrodes. In general in
the North Sea the most common anode materials are platinum sheathed
titanium and lead/silver alloys.

It is vitally important that the power supply is connected with correct polarity.
The negative terminal must be connected to the structure and the
positive terminal must be connected to the anode (conventional electrical
circuit notation). Should these connections be reversed the structure would
corrode catastrophically.

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Corrosion Protection 9-6 Copyright © TWI Ltd
 Figure 9.4 Impressed current CP distribution of anodes and dielectric shield.

9.4.1 The ICCP anodes

The actual distribution of the anodes on any structure may be either:

 Platform based.
 Remote from the structure.

Platform based
Numerous anodes are attached to the structure at intervals around it in similar
fashion to sacrificial anodes but ensuring that they are insulated from the
structure. Figures 9.4 and 9.5 refer.

Two problems are associated with this method.

One is the possibility of shadow areas where inadequate protection is

provided. This can be solved by the use of sacrificial anodes complementing the
ICCP system as indicated earlier.

The second problem is the possibility of current flowing directly from the anode
to the adjacent structure. This could cause embrittlement as discussed earlier
and to avoid this dielectric shields are used to insulate the structure
electrically. Also, the current is limited by design, because each anode is
positioned to provide adequate protection for the local area only. This limits the
possibility of embrittlement and coating damage.

There is also a diver safety consideration in that these anodes are at about 80V
potential with some 1000A current. If divers are used adjacent to any of the
anodes, they should be isolated from the system. Normally only the anodes in
the immediate work area are switched off.

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Corrosion Protection 9-7 Copyright © TWI Ltd
 Figure 9.5 A platform based ICCP illustrating the various components.

Remote from the structure

A number of anodes may be placed on the seabed at a designated distance
from the structure.

This method avoids the possibility of current flowing directly from the anode to
the adjacent structure but because there are fewer anodes the current density
is higher. Therefore, the possibility of coating damage and embrittlement still
exists.

Design considerations generally favour more anodes distributed around the

structure.

There is a safety issue with divers but as the anodes are some distance away
from the structure, it may be possible to ensure safety by imposing an
exclusion zone around the anode (Figure 9.6).

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Corrosion Protection 9-8 Copyright © TWI Ltd
 Reference electrode

Figure 9.6 An ICCP system with anodes remote (removed) from the structure.

Table 9.3 Advantages and disadvantages of impressed current CP.

Advantages Disadvantages
The output is variable to react to Only works when switched on
changing current density requirements no power means no protection
Anodes smaller so less weight There are ongoing running costs
Too great a current density could lead to
Can be remotely monitored overprotection – continuous monitoring
required
Overprotection could cause hydrogen
Fewer required
embrittlement and coating damage
Safety zone required around any areas
where diving is undertaken
Possibility of shadow areas occurring
requires continuous monitoring

Can increase certain types of marine growth

- still under research

Cables are vulnerable especially in the splash

zone

9.4.2 Reference or control electrodes

As discussed earlier, these electrodes are commonly zinc, silver/silver-chloride
(Ag/AgCl), saturated calomel electrode (SCE) or copper/copper-sulphate (CSE).
CSE is favoured on reinforced concrete structures.

Reference or control electrodes are vital components of any ICCP system. They
determine the current required from the power source, without them, the
system cannot provide a quantifiable degree of protection (Figure 9.7).

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Corrosion Protection 9-9 Copyright © TWI Ltd
 Figure 9.7 Zinc reference electrode installed on an offshore structure.

9.5 Using coatings to protect the structure

Coatings form a barrier between the electrolyte and the surface of the
structure. They may be paints, organic films, varnishes, metal coatings or
enamels and even sheathing. It is surprising how effective coatings can be
when consideration is given to the thickness of a typical paint coat. This may be
only in the order of 25-100 microns thickness for some applications.

Paints
When paint is applied to a metal surface, it presents a barrier to air, moisture
and ions aggressive to the metal. However, paint cannot provide a complete
barrier to oxygen or water. In time these will penetrate through to the surface
of the metal. Any paint system used underwater must have a strong bond onto
the metal surface and therefore high quality metal surface preparation is
required, such as SA3.

The bonding between successive coats must also be strong and the topcoats
must provide as impervious a barrier to the electrolyte as is possible. This last
is achieved by ensuring the constituents making up the topcoats have very low
water absorption and transmission coefficients.

Coal tar epoxides

Extensively used on offshore structures. They consist of coal tar and epoxide
resin for the binder. These coatings are highly impermeable to water and
resistant to attack by most chemicals and hydroxyl ions (that are produced by
the cathodic reaction).

Zinc coatings
Utilise a combination of zinc dust and complex silicates with a solvent-based
self-curing binder, give good protection to steel surfaces. These coatings are
frequently over-painted by another system and are used on components such
as ladders in a marine environment.

DIS1-30815
Corrosion Protection 9-10 Copyright © TWI Ltd
 Concrete
Used to provide a protective coating to pipelines where it provides a passive
environment for the steel pipe as well as adding weight.

Metallic coatings
Such as galvanising, using zinc, impose a continuous barrier between the metal
surface being protected and the surrounding environment. These coatings may
be applied in a number of ways.

Electroplating
Uses a bath of salts as an electrolyte. The component and rods of the plating
metal are immersed in the electrolyte and a potential is applied between the
component and the rods. The component becomes the cathode and the rods the
anode, so metal ions of the plating material deposit from the solution onto the
component.

Hot dipping
Involves the component being immersed in a bath of molten coating metal.
Galvanising is accomplished by this method (Figure 9.8).

Figure 9.8 Galvanising.

Spray coats
Use a specialised torch that is fed with wires of the coating metal that are
melted and blown out by it. The molten metal is expressed in the form of
droplets travelling at 100-150m/s that flatten and adhere on impact with the
component.

Cladding
Uses metal skins laminated onto the component. The skin can be applied by:

 Rolling.
 Explosive welding.
 Buttering (building up a welded coat on the surface to be protected).
 Sheathing.

Aluminium roll-bonded to duralumin is marketed as Alcad.

Some offshore risers are sheathed with Monel (cupronickel alloy),

Figure 9.9.

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Corrosion Protection 9-11 Copyright © TWI Ltd
 Diffusion
Requires the component to be heated to just below the melting point of the
coating metal in the presence of the coating in powder form and in an inert
atmosphere. The component is allowed to baste for several hours and the
coating diffuses into the surface of the component.

Monel cladding
Used on some offshore risers; it is a Cupronickel alloy sheathing.

Figure 9.9 Monel cladding on an offshore riser.

9.6 Use of Inhibitors (controlling the electrolyte)

Remember, the Pourbaix diagram indicates three methods for preventing
corrosion:

1 Making the electrode more positive.

2 Making the electrode more negative.
3 Changing the electrolyte pH.

This section will outline methods for changing the electrolyte. The properties of
the electrolyte that can be affected by using inhibitors are:

 The conductivity of the electrolyte.

 The pH of the electrolyte.
 The interaction of the electrolyte with the metal surface, attacking or
strengthening passive films.

As an example of how this can be achieved, consider steel in seawater. If

distilled water is substituted for the seawater the conductivity and pH of the
electrolyte is reduced and a passive film will form on the surface of the steel.

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Corrosion Protection 9-12 Copyright © TWI Ltd
9.7 Corrosion protection by design
This aspect of corrosion protection has been indicated earlier in section 8. The
methods used to protect structures from corrosion can be summarised this
way:

 Avoid all unnecessary bimetallic corrosion cells, avoiding galvanic or

dissimilar metal corrosion.
 Avoid differential-aeration cells (crevices, debris traps and inadequate
drainage, etc,).
 Avoid stray currents from electrical machinery or conductors. These can be
quite powerful and make parts of the structure anodic with obvious results.
 Choose the material with the best properties for the environment.

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Corrosion Protection 9-13 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Trethewey K R, Chamberlain J, ‘Corrosion for Students of Science and

Engineering’, Longman Scientific & Technical, 1988,
ISBN 13: 9780582450899.

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 27/08/2015

Corrosion Protection

 There are numerous methods for preventing corrosion

including, coatings, inhibitors (controlling the electrolyte),
selective design, anodic protection and cathodic protection.

 Before considering these methods, a brief examination of the

way in which the corrosion process is influenced by the two
CSWIP 3.1U Course main variables; the electrode potential and the pH of the
Corrosion Protection electrolyte, will assist in understanding the various protection
methods.
Section 9
 This data is often presented in a diagrammatic form known as
Pourbaix diagrams. These diagrams are obtained from
laboratory tests carried out under controlled conditions of
constant temperature and no flow.

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Pourbaix Diagram Corrosion Protection

Altering the electrical potential In designing a cathodic protection system the system designer
starts by determining an acceptable corrosion rate, this
to produce passivation or information is put into a graph to determine a value for current
immunity by the method of density. This level of current density will ensure the required
cathodic protection is the most corrosion rate is achieved.
useful technique for offshore
structures. The electric potential to achieve this current is approx. -800mV.
Now it may seem that potentials more negative than -800mV
If the electrode (The (against Ag/AgCl) would produce even less metal loss. There
-640mV Structure) potential is made are two reasons why it is not prudent to use very much more
negative potentials.
more negative by the 1. At potentials much more negative than -1100mV the
application of free electrons possibility of hydrogen embrittlement exists.
14
pH then the structure will become 2. Large currents are associated with more negative potentials
immune to corrosion. that produce high local concentrations of hydroxyl ions that
often damage paint coatings.

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Sacrificial Anodes Sacrificial Anodes

This method of CP is fairly straightforward. We

are creating a massive galvanic corrosion cell in
our favour by attaching less noble materials to
the structure thereby making it the cathode.

The materials most commonly used for anodes

are:

 Aluminium (uncoated structures).

 Zinc (coated structures).
 Magnesium (rarer because they may not last long
enough for the design life of the structure).

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1
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Sacrificial Anodes Sacrificial Anodes

A natural phenomenon does occur which assists Advantages and disadvantages of sacrificial
in the protection of structures. Calcium, anode system
magnesium other metal ions are present in Advantages Disadvantages
sufficient quantities in seawater to react with No external electric power
Current output decreases
hydroxyl ions produced by the negative potential required and therefore no
with time
danger to divers
of the cathodic steel surface.
Adds considerable weight
No danger of overprotection
The reaction produces insoluble calcium and and drag to the structure
magnesium salts, know as calcareous deposits. No running costs
Initial costs are
These form a strongly adherent film that reduces comparatively high
Active from day of
the current requirement and may reduce immersion
Comparatively difficult to
environment-sensitive cracking. increase protection by retro-
A well proven and reliable
fitting anodes
method

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Impressed Current Cathodic

Protection
An impressed current cathodic protection (ICCP)
system works on the same principal as the
sacrificial anode system in that the structure is
made to be the cathode.

However, in the case of the ICCP system the

necessary potential and current flow is provided
by a DC generator rather than by galvanic
coupling.

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ICCP System ICCP System

This system can be made to be self-adjusting by

incorporating reference electrodes into the circuit that
measure the potential.
The potential can vary depending on the circumstances, if
the structure has a coating that in service became
damaged, this would increase the exposed surface area
needing to be protected.
The control unit can deal with this by increasing the
current density. If, on the other hand there was a
reduction in the surface area; ie a calcareous deposit
build-up, there would be less current required.
In both cases the reference electrode provides the means
of monitoring the potential, which varies proportionately
according to the current.

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ICCP Practical Considerations ICCP Practical Considerations

Anode material: These anodes are capable of supplying high current densities
and it would be possible to protect a structure with a few large
With the sacrificial system the anodes were anodes supplied with a high current. However, in practice
being eaten away, in the case of the ICCP smaller anodes are usually distributed at regular intervals over
system the surface area is very important and so the whole structure.
the anode must not corrode, so the anode
This is because:
material is selected from the top of the galvanic
series not the bottom. The high current density that would be present in the
immediate vicinity of a single large anode could damage paint
surfaces and possibly cause embrittlement. The use of many,
 Titanium. smaller anodes reduces the current density for each one and
 Platinised niobium. reduces the probability of this type of damage.
 Lead/silver alloys.

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ICCP Practical Considerations ICCP Practical Considerations

Offshore structures have a reasonably complicated The ICCP system installed on the Claymore
geometry that makes it difficult for corrosion platform was designed to provide 160mA/m2,
engineers to predict the total distribution potentials. using 55 platinum-iridium anodes and 12
It is, therefore, prudent to use more anodes, each reference electrodes.
one protecting a smaller area, thereby minimising
the areas at risk of inadequate protection. The Murchison platform uses 100 anodes and 50
reference electrodes.
When designing the system if the corrosion
engineers have any doubts about protecting any In general, in the North Sea, the most common
particular area of the structure, sacrificial anodes anode materials are platinum sheathed titanium
may be installed to work in conjunction with the and lead/silver alloys.
ICCP system.

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ICCP Practical Considerations Platform Based ICCP

Polarity The ICCP anodes can be distributed in either of

It is vitally important that the power supply is the following ways:
connected with the correct polarity.  Platform based.
 Remote from the structure.
The negative terminal must be connected to the
structure and the positive terminal must be
connected to the anode. Platform based – In this method numerous
anodes are attached to the structure at intervals
Should these connections be reversed the around it in a similar fashion to sacrificial anodes
structure would corrode catastrophically. but ensuring that they are insulated from the
structure.

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Platform Based ICCP Platform Based ICCP

Two problems are associated with this method. One is the

possibility of areas of inadequate protection known as shadow
areas. This can be solved by the use of sacrificial anodes
supplementing the ICCP system as previously mentioned.

The second problem is that of current flowing directly from the

anode to the adjacent structure. This could cause embrittlement
and coating damage and to avoid this dielectric shields are
deployed to insulate the structure electrically.

Also current is limited by design, each anode being positioned

to provide protection for the local area only, which also limits
the possibility of coating damage and embrittlement.

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Platform Based ICCP System Remote From The Structure

 There is a diver Remote from the structure a number of anodes

safety may be placed on the seabed at a designated
consideration in distance from the structure.
that
 these anodes have This method avoids the possibility of current
an output of flowing directly from anode to structure but,
between: there being fewer anodes, the current density is
 30-80 volts DC and higher and therefore there is still a possibility of
300-1000 amps. coating damage and embrittlement.

If divers are deployed adjacent to any of these anodes Therefore design considerations generally favour
they should be isolated from the system. There is no more anodes distributed around the structure.
need to switch off the whole system.

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System Remote From The Structure System Remote From The Structure
Reference electrode

Reference
Electrode

A diver safety issue still exists but, as the anodes are some
distance away from the structure, it is possible to ensure safety by
imposing an exclusion zone around the anode.

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4
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ICCP System Reference Electrodes

Advantages and disadvantages of ICCP Systems  Reference electrodes are

Advantages Disadvantages
The output is variable to react to changing Only works when switched on
commonly:
current density requirements no power means no protection  Zinc.
Anodes smaller so less weight There are ongoing running costs
Too great a current density could lead to
 Silver/silver-chloride (Ag/AgCl).
Can be remotely monitored overprotection – continuous monitoring  Copper/copper-sulphate (CSE).
required
Overprotection could cause hydrogen
Fewer required
embrittlement and coating damage
Safety zone required around any areas
These are vital components of any
where diving is undertaken ICCP system. They determine the
Possibility of shadow areas occurring
current required from the power
requires continuous monitoring
Can increase certain types of marine
source. Without these the system
growth - still under research can’t provide a quantifiable degree
Cables are vulnerable especially in the of protection.
splash zone

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Protection Using Coatings Protection Using Coatings

Coatings form a barrier between the electrolyte and the
Coal tar epoxides are used extensively on offshore
surface of the structure. They may be paints, organic
structures. They consist of coal tar and epoxide resin for
films, varnishes, metal coatings, enamels or sheathing. It
the binder. These coatings are highly impermeable to
is surprising how effective coatings can be when
water and resistant to attack by most chemicals and
consideration is given to the thickness of a typical paint
hydroxyl ions (that are produced by the cathodic reaction).
coat. This may be only in the order of 25-100 microns
thickness for some applications. Zinc coatings utilising a combination of zinc dust and
complex silicates with a solvent-based self-curing binder,
Paint is applied to a metal surface where it presents a give good protection to steel surfaces. These coatings are
barrier to air, moisture and ions aggressive to the metal. frequently over-painted by another system and are used
However, paint cannot provide a complete barrier to on components such as ladders in a marine environment.
oxygen or water. In time, these will penetrate through to
the surface of the metal. Any paint system used Concrete is used to provide a protective coating to
underwater must have a strong bond onto the metal pipelines where it provides a passive environment for the
surface and therefore, high quality metal surface steel pipe as well as adding weight.
preparation is required, such as SA3.

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Protection Using Coatings Protection Using Coatings

Metallic coatings such as galvanising, using zinc, impose a Spray coats use a specialised torch that is fed with
continuous barrier between the metal surface being wires of the coating metal that are melted and
protected and the surrounding environment. These
coatings may be applied in a number of ways.
blown out by it. The molten metal is expressed in
the form of droplets travelling at 100-150m/s that
Electroplating uses a bath of salts as an electrolyte. The flatten and adhere on impact with the component.
component and rods of the plating metal are immersed in
the electrolyte and a potential is applied between the
component and the rods. The component becomes the Cladding uses metal skins laminated onto the
cathode and the rods the anode, so metal ions of the component. The skin can be applied by:
plating material deposit from the solution onto the
component.  Rolling.
 Explosive welding.
Hot dipping involves the component being immersed in a
 Buttering (building up a welded coat on the surface to be
bath of molten coating metal. Galvanising is accomplished
protected).
by this method.
 Sheathing.

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5
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Protection Using Coatings Protection Using Coatings

Diffusion requires the component to be heated

to just below the melting point of the coating
metal in the presence of the coating in powder Monel sheathing
and in an inert atmosphere. The component is
allowed to baste for several hours and the
coating diffuses into the surface of the
component.

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Inhibitors: Controlling the Electrolyte Inhibitors: Controlling the Electrolyte

The pourbaix diagram indicates three methods As an example of how this can be achieved,
for preventing corrosion: consider steel in seawater. If distilled water is
substituted for the seawater the conductivity and
1. Making the electrode more positive.
2. Making the electrode more negative. pH of the electrolyte is reduced and a passive
3. Changing the electrolyte pH. film will form on the surface of the steel.

Properties of the electrolyte that can be affected

by using inhibitors are:
 The conductivity of the electrolyte.
 The pH of the electrolyte.
 The interaction of the electrolyte with the metal
surface, attacking or strengthening passive films.

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Summary

The methods employed to protect structures

from corrosion can be summarised thus:
Avoid:
 All unnecessary bimetallic corrosion cells, avoiding
galvanic or dissimilar metal corrosion.
 Differential-aeration cells: Crevices, debris traps, Any Questions?
inadequate drainage.
 Stray currents from electrical machinery or conductors.
These can be quite powerful and make parts of the
structure anodic with obvious results.

Choose:
 The material with the best properties for the
environment.

Copyright © TWI Ltd Copyright © TWI Ltd

6
 Section 10

Corrosion Protection Monitoring

10 Corrosion Protection Monitoring
It has been indicated several times during this discussion on corrosion that
there are variables presented in-service that cannot be adequately predicted.
Therefore a monitoring regime is necessary to ensure that the designed
corrosion protection system is operating to its design specifications and that
there are no in-service effects interfering with this.

Monitoring methods commonly used for the topside included such as weight loss
coupons and electrical resistance (ER) probes for the topside, while visual
inspection and CP survey, ultrasonic thickness surveys, ROV inspection and
intelligence pigging are widely used for the subsea. Chemical analyses and solid
particle detection of samples collected from the pipeline at the topsides will
provide corrosion information to assess both topsides and subsea pipelines.

The amount of current from sacrificial anodes or from an impressed current

system required for protection varies:

 From metal to metal.

 With the geometry of the structure.
 With differences in sea water environment (temperature, pH value, etc).
 With any other factors that affects the resistance of the circuit.

Since the amount of current required for the protection of any structure cannot
be accurately predicted or distributed evenly through the structure, the method
of checking for adequate protection is to measure the potential and current
density around the structure at various places.

10.1 Inspection requirements

Monitoring or inspection requirements for corrosion protection systems are
therefore as follows:

 Visual inspections of both sacrificial and impressed current anodes for

depletion.
 Visual inspection of the electrical connections of the sacrificial system to see
that it is intact and of the impressed current system to ensure that there are
no breaks in the insulation of the supply cables or anode connections.
 Potential measurements on the structure to confirm that it is still the
cathode by confirming the readings obtained are within protection
parameters. (-800mV to -1100mV generally).
 Current density measurements to confirm that the impressed current
system is providing adequate protection.
 Visual and ultrasonic inspection for corrosion damage including pitting and
loss of wall thickness.

The specific visual inspection requirements are detailed in section 15 Visual

inspection. The ultrasonic requirements are covered in section 14.

The potential measurements usually referred to as cathode potential (CP)

readings are obtained by:

 Taking contact readings with a CP meter:

- By hand employing a diver with a hand-held instrument.
- By mounting a contact probe on an ROV.
 Taking proximity readings with a proximity probe mounted on an ROV.
 Monitoring proximity readings via remotely mounted permanent sensors
with readout in a surface control room.

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Corrosion Protection Monitoring 10-1 Copyright © TWI Ltd
 Current density measurements and monitoring are obtained by:

 Taking current density readings normally with an ROV mounted sensor and
usually for a specific requirement. This method is not used for regular
inspections.
 Monitoring potential and current through remotely mounted electrodes
incorporated into the impressed current system.

10.2 Cathode potential measurement

The cathode potential is measured by using a reference electrode incorporated
into an instrument that has a readout calibrated in mV. As stated in the
previous chapters these electrodes are commonly:

 High purity zinc.

 Silver/silver-chloride (Ag/AgCl).
 Copper/copper-sulphate (CSE) (this is more favoured for concrete
structures).

10.3 High purity zinc electrodes (ZRE)

High purity zinc (99.9% pure) is most commonly used with remotely mounted
monitoring systems as shown in section 9, figure 9.6. The site for mounting the
electrode is selected because it is either a representative site, it is an area of
marginal protection or it is an area of high stress and it is installed as part of
the impressed current system. The electrode is connected to a meter in the
surface control room.

Figure 10.1 High purity zinc electrode (ZRE).

10.4 CP readings utilising silver/silver-chloride (Ag/AgCl) electrodes

The most common reference electrode used in offshore corrosion monitoring is
silver/silver chloride. This electrode is used extensively for both contact and
proximity applications whether diver or ROV deployed.

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Corrosion Protection Monitoring 10-2 Copyright © TWI Ltd
 Ag/AgCl electrodes, most frequently referred to as half-cells (because they form
a complete cell when the meter is connected to the cathode) are utilised in
several contact CP probes, including the Bathycorrometer and the Morgan
Berkeley Rustreader, the former being by far the most common world-wide.

They are deployed as either hand held or ROV probes. ROVs are more
commonly used for proximity measurements. The probe contact tip is placed on
the cathode and the meter gives the readout in mV of the electrical potential
between it and the half-cell. Figure 10.2 refers to a hand-held meter. Figure
10.3 illustrates an ROV contact probe and figure 10.4 shows the proximity
method.

When taking proximity CP readings it is vital that a good electrical connection is

made between the structure and the positive terminal of the surface control
room installed meter, a maximum of 100mm is maintained between the probe
and the structure, as indicated in Figure 10.4.

Semi-permeable High resistance

membrane voltmeter

Silver/silver-
chloride
(Ag/AgCl) half
cell

Figure 10.2 Diver-held CP meter (Bathycorrometer or similar).

High resistance
voltmeter
Semi-permeable
membrane

Cables incorporated into

umbilical and connected
to a voltmeter in surface
control room

Silver/silver-chloride
(Ag/AgCl) half cell

Figure 10.3 Contact CP reading taken by an ROV.

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 Figure 10.4 Proximity CP measurement.

10.5 Current density measurements

Current density may be measured using a specialised probe mounted on an
ROV. This type of inspection would be undertaken for a specific purpose such as
investigating a particular area of the structure that was suspected of being
under-protected or following up a visual inspection that had identified more
corrosion than was anticipated. Specific procedures will be provided for this
type of survey.

As stated earlier, impressed current systems have reference electrodes installed

to monitor current flow and potential. Figure 9.7 in section 9 shows a ZRE
monitoring potential and Figure 10.5 illustrates a monitored anode.

A monitored anode is a sacrificial anode that is isolated electrically from the

structure and is connected via an insulated cable to the surface control room.
Thus the current can be constantly monitored.

Figure 10.5 Monitored anode.

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10.6 Calibration procedures for hand-held CP meters
It is necessary to check the calibration of CP meters to ensure that the readings
obtained are accurate and comparable with other and previous readings. A
standard method of checking calibration has been adopted in the offshore
industry for this purpose. This procedure is detailed here.

10.7 Necessary equipment

Three K-series electrodes complete with electrical connectors, or three screw-on
K-series cells for hand-held CP meters (these are available for the
Bathycorrometer and can be provided with screw in electrical connectors, which
should be specified). The electric connector is provided so that the cells can be
proven as described below.

 High impedance (10 Ω voltmeter.

 Zinc (99.9% pure) block with clamp and electrical connector.
 Plastic bucket filled with fresh seawater (not from fire main which could
contain inhibitors).
 Log sheets.

10.8 Procedure
The first part of the procedure proves that the K-series cells are chemically
saturated and sufficiently stable enough to be used as reference cells.

10.8.1 Proving the K-series cells

There are different types of cells available. One type is specifically designed for
use with a Bathycorrometer. This type has a solid polymer body protecting the
K-series cell. The procedure outlined below also applies to this type of cell;
however, it is not possible to visually confirm they are fully saturated with
solution. They are sealed and to confirm they are saturated it is necessary to
unscrew a sealing cap to gain access to the solution reservoir.

 Visually inspect the electrodes to ensure they are undamaged and full of
solution. The solution is potassium chloride (KCl) and if the solution is
saturated or supersaturated solid crystals may be seen in the phial.
(Commonly the phials are glass or clear plastic).
 Label the electrodes and their wires 1, 2 and 3.
 Soak the electrodes in the bucket for 24 hours, being careful to immerse
each one only as far as the filling hole in the phial.
 While the electrodes continue to soak connect electrode 1 to the negative
terminal of the voltmeter and electrode, 2 to the positive terminal and
record the reading.
 Repeat the test with each permutation of electrodes.

1 and 3
2 and 3

Acceptable readings between any pair of electrodes is 0 ± 2MV.

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 Figure 10.6 K-series reference cells.

 If all the readings are within this range any electrode may be used.
 If one reading is out of this range the electrode not in that pair is the one to
use.
 If one reading is in range either of the electrodes in that pair can be used.
 If all of the readings are out of range either replace all the K-series cells or
flush out the phials with pure distilled water, obtain a new saturated
solution of potassium chloride, refill the phials and re-test.

On completion of the entire procedure rinse the electrodes in fresh water.

The second part of the procedure confirms the calibration of the CP meter.

10.9 Calibration of the meter

The calibration procedure for a contact CP meter is basically the same whether
it is diver hand-held or ROV deployed.

10.9.1 Calibration of a Bathycorrometer

The calibration checking procedure is slightly different if the meter being
calibrated is a Bathycorrometer with the specifically designed screw-on cells. In
this case the following procedure applies.

As the electronic components in the Bathycorrometer can affect calibration it is

necessary to use a BCM Checker (supplied separately by the manufacturer) to
check there are no electrical faults in the unit before continuing with this
procedure. Separate instructions are supplied with this equipment so will not be
detailed here.

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 Once satisfied that the electrical components are functioning correctly:

 Fully charge the CP meter batteries and soak in fresh seawater (not drawn
from the fire main).
 Remove the contact probe tip and white Delrin probe cone from the meter.
 Screw the K-series reference electrode onto the Bathycorrometer in place of
the Delrin probe and probe cone tip.
 Immerse the meter in the bucket at least far enough to submerge the semi-
permeable membrane. Gently shake the unit to remove any air bubbles
lodged in the holes. (The meter display may be left out of the water to
assist taking readings.)
 Allow time for the meter to stabilise (approximately 10 minutes).
 The voltage potential between the reference electrode and the meter’s own
Ag/AgCl cell is read off the meter display directly.
 Record the reading on the log.

Acceptable readings are between +42mV +/-5mV. It should be noted that

values will vary dependent upon salinity and temperature of the seawater, see
Figure 10.7.

The calibration of other types of contact CP meters is by comparison.

1 2 3

Soak for 30 minutes

Fully charge CP meter Unscrew tip from meter
In fresh seawater

4 5 6

+42mV +/- 5mV

Screw in K-Series electrode Immerse meter for 10 minutes Record reading in log

Figure 10.7 Calibration of a Bathycorrometer.

Note. If the reference electrodes used are calomel cells (as opposed to K-series)
then the expected reading will be 0 to -0mV.

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10.10 Operating procedures
To ensure that accuracy is maintained and that repeatable results are obtained,
CP monitoring methods should follow a procedure as follows.

 Ensure any self-contained meters are fully charged and maintain a battery-
charging log. (Usual requirements for charging batteries for battery-
operated equipment are 15 hours from fully discharged. This will give 60
hours of continuous use).
 Ensure the probe tip for contact meters is sharp (hand-held meters are
usually supplied with spare tips).
 Soak meters and half-cells for a minimum of 30 minutes before use. (This
allows time for ion penetration through the semi-permeable membranes.)
 Confirm the calibration of the system in use according to the appropriate
calibration procedure. Record the results on the appropriate log sheet.
 Record meter serial number and any other specified details on the
appropriate log sheet.
 Take a reference reading on zinc at the inspection site prior to starting the
survey.
 For each contact readings ensure there is correct metal-to-metal contact
between the probe tip and the cathode surface.
 With proximity probe surveys ensure there is a solid electrical connection to
the structure connected to the positive terminal of the surface instrument.
 For proximity probe readings ensure the standoff between the probe and the
cathode is correct, no more than 100mm away.
 During the course of the survey ensure that each reading is correctly
recorded on the appropriate log.
 On completion of the survey take another reference reading on zinc.
 Recover the equipment, wash in fresh water, dry and store. Charge any
battery-operated equipment as necessary and complete the battery-
charging log.

Notes:
 Morgan-Berkley meters can be left soaking in a solution of silver chloride,
on trickle charge continuously if required.
 If a large number of readings are being taken it is prudent to take check
readings periodically during the survey.

10.11 Normal cathode potential readings against Ag/AgCl

The following are the normal range of readings expected during a survey of a
steel structure.

Table 10.1 Expected CP readings.

Material CP Reading
Over-protected structure More negative than -1100mV
Zinc -1000 to -1050mV
Protected steel -800 to -1100mV
Under protected steel -640 to -800mV
Unprotected steel -450 to -640mV
Monel -50 to -150mV

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Corrosion Protection Monitoring 10-8 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Trethewey K R, Chamberlain J, ‘Corrosion for Students of Science and

Engineering’, Longman Scientific & Technical, 1988,
ISBN 13: 9780582450899.

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 27/08/2015

Monitoring Corrosion Protection

A monitoring regime is necessary to ensure the

corrosion protection system is operating to its design
specifications and that there are no in-service effects
interfering with it.
CSWIP 3.1U Course The amount of current, from sacrificial anodes or from
an impressed current system, required for protection
Corrosion Protection Monitoring varies:
Section 10  From metal to metal.
 With the geometry of the structure.
 With differences in sea water environment
(temperature, pH value).
 With any other factors that affect the resistance of
the circuit.

Copyright © TWI Ltd Copyright © TWI Ltd

Monitoring Corrosion Protection Inspection Requirements

Since the amount of current required for the  Monitoring or inspection requirements for
protection of any structure cannot be corrosion protection systems are as follows:
accurately predicted, the method of checking
is to measure the potential and current  Visual inspection of anodes for wear.
density around the structure at various
places.  Visual inspection of electrical connections.
 Potential measurements to ensure the
structure is still the cathode (-800mV to -
1100mV).
 Current density measurements.
 Visual and ultrasonic inspection for
corrosion damage.

Copyright © TWI Ltd Copyright © TWI Ltd

Inspection Requirements Cathode Potential Measurement

 CP readings are obtained by: The cathode potential is measured by using a

 A diver taking contact readings with a hand-held CP reference electrode incorporated into an instrument
meter or by mounting a contact probe on an ROV. that has a readout calibrated in
 Taking proximity readings with an ROV mounted mV (1 volt = 1000mV).
probe.
 Monitoring proximity readings via remotely mounted
permanent sensors with a readout in a surface These electrodes are commonly:
control room.
 High purity zinc.
 Current density measurements and monitoring
 Silver/silver-chloride (Ag/AgCl).
are obtained by:
 Taking current density readings with an ROV
 Copper/copper-sulphate (CSE)
mounted sensor. (this is more favoured for concrete structures).
 Monitoring remotely mounted electrodes
incorporated into the impressed current system.

Copyright © TWI Ltd Copyright © TWI Ltd

1
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CP Readings Utilising Ag/AgCl

Remote Monitoring
Electrodes
High purity zinc electrode (ZRE) The most common reference electrode used
It may be installed as part of the offshore is silver/silverchloride. This electrode is
impressed current system. used extensively for both contact and proximity
 High purity zinc (99.9% pure) is applications whether diver or ROV deployed.
most commonly used.
 The site for mounting the
Ag/AgCl electrodes, most frequently referred to as
electrode is selected because half-cells (because they form a cell when the meter
it is either a representative is connected to the cathode) are utilised in several
site, an area of marginal contact CP probes, including the Bathycorrometer,
protection or an area of high
stress.
that are hand held or ROV mounted probes.
 The electrode is connected to a
voltmeter in the surface control The probe tip is placed on the cathode (metal to
room. metal) and the meter gives the readout in mV of
the electrical potential between it and the half-cell.
Copyright © TWI Ltd Copyright © TWI Ltd

Bathycorrometer

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ROV Contact Meter

- 0.850

Cables incorporated into

the umbilical and
connected to a volt meter
in the control room

Copyright © TWI Ltd Copyright © TWI Ltd

2
 27/08/2015

Proximity CP Probe Current Density Measurements

Similar to reference
electrodes, but can be used to
Monitored anode monitor the efficiency of any
CP systems ie sacrificial
anodes.

A monitored anode is a
sacrificial anode that is
isolated electrically from the
structure and is connected to
an ammeter in the surface
control room.

When taking proximity CP readings it is vital that a sound electrical Current density may also be
connection is made between the structure and the positive terminal measured using a specialised
of the surface control room installed meter. probe mounted on an ROV.

Copyright © TWI Ltd Copyright © TWI Ltd

Calibration of Hand-held CP Meters Necessary Equipment

It is necessary to confirm calibration of CP Three k-series electrodes complete with

meters to ensure that the readings obtained are electrical connectors.
accurate and comparable with other and High impedance (10MΩ) voltmeter.
previous readings. Zinc (99.9% pure) block with clamp and
electrical connector.
A standard method has been adopted in the
offshore industry for this purpose. Plastic bucket filled with fresh seawater: Not
from fire main which could contain inhibitors.
The first part of the procedure proves that the k- Log sheets.
series cells are chemically saturated and
sufficiently stable enough to be used as
reference cells.

Copyright © TWI Ltd Copyright © TWI Ltd

Proving K-Series Cells Proving K-Series Reference Cells

 Visually inspect the electrodes to ensure they are undamaged

and full of solution. The solution is potassium chloride (KCl)
and if the solution is saturated or supersaturated, solid crystals
may be seen in the phial.

 Label the electrodes 1, 2 and 3.

 Soak the electrodes in the bucket for 24 hours, being careful
to immerse each one only as far as the filling hole in the phial.
 While the electrodes continue to soak, connect electrode
 1 to the negative terminal of the voltmeter and electrode,
 2 to the positive terminal and record the reading.

 Repeat the test with each permutation of electrodes.

Acceptable readings between any pair of electrodes is +/- 2mV

Copyright © TWI Ltd Copyright © TWI Ltd

3
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Proving K-Series Reference Cells Calibration of Bathycorrometer

If all readings are:

 Within this range any electrode may be used.
 Out of this range, either replace all the k-series cells or flush
out the phials with pure distilled water, obtain a new
saturated solution of potassium chloride, refill the phials and
re-test.

If one reading is:

 Out of this range the electrode not in that pair is the one to
use.
 In range either of the electrodes in that pair may be used.

 On completion of the procedure rinse the electrodes in fresh

water.

Copyright © TWI Ltd Copyright © TWI Ltd

Operating Procedures: Prior to

Operating Procedures: During Use
Deployment
 Ensure meters are fully charged. Take a reference reading on zinc prior to the survey.
 Maintain a battery-charging log. Ensure correct metal-to-metal contact between the
probe tip and the cathode surface.
 Ensure the probe tip is sharp.
With proximity probe surveys, ensure there is a solid
 Soak for a minimum of 30 minutes before electrical connection to the structure connected to the
use. positive terminal of the surface instrument.
 Confirm the calibration of the system. For proximity probe readings ensure the standoff
 Record the results on the appropriate log between the probe and the cathode is correct.
sheet. Ensure that each reading is correctly recorded in the
 Record meter serial number on the appropriate log.
appropriate log sheet. On completion of the survey take a reference reading
on zinc.

Copyright © TWI Ltd Copyright © TWI Ltd

Operating Procedures: After Use Normal CP Readings Against Ag/AgCl

Recover the equipment

 Wash in fresh water.

 Dry and store. Material CP Reading
 Charge any battery-operated equipment as Over-protected
More negative than -1100mV
necessary. structure
 Complete the battery-charging log. Zinc -1000mV to -1050mV
Protected steel -800mV to -1100mV
Note: If a large number of readings are being Under protected steel -640mV to -800mV
taken it is prudent to take check readings Unprotected steel -450mV to -640mV
periodically during the survey. Monel -50mV to -150mV
Magnesium (H1 alloy) -1400mV to -1600mV

Copyright © TWI Ltd Copyright © TWI Ltd

4
 Section 11

Welding and Welding Defects

11 Welding and Welding Defects
11.1 Joining metal components
In considering joining, forming or shaping metal components, there are four
ways in which they can be formed into shape: machining - when the material is
cut away; casting - where the metal is moulded to shape in the first place;
forging - where the material is worked into shape and fabricating - where the
component is built up a bit at a time from different parts.

The components inspected underwater are almost all formed by fabrication.

This being the case, a closer look at this process is in order.

Fabrication can be accomplished by mechanical fastenings, for example bolting

or riveting components together; by welding, where parts are joined together
by metallurgical bond; by brazing, where a metal of a different composition
from the pieces to be joined is melted between them to solidify and thus make
a bond (a stronger version of soldering) or by adhesive bonding, where parts
are glued together. The most important technique for consideration here is
welding.

11.2 Fabricating offshore structures

Steel fabricated structures are used extensively offshore as has been indicated
during previous sections. In fabricating the structures, the designers choose to
use welding as the prime means of joining the various parts together. However,
it is extremely difficult indeed to guarantee that any particular weld is free from
all faults and, because of this, welds are constantly inspected to ensure they
are not about to fail.

The knowledge of how the welding was achieved in the first place is of great
assistance when inspecting welded joints because; all techniques have certain
faults that are common to that technique. It is therefore important to have
some knowledge of the main type of welding.

11.3 Welding processes

Currently there are more than thirty-five different welding processes used in
industry. These different welding processes can be classified into seven major
groups. All processes within each group have similar characteristics and
therefore, similar effects on the parent metals.

The seven groups are:

1 Solid phase welding.

2 Thermo-chemical welding.
3 Electric-resistance welding.
4 Unshielded arc welding.
5 Radiant energy welding.
6 Flux-shielded arc welding.
7 Gas-shielded arc welding.

From the point of view of offshore structures and underwater inspection, the
following welding processes are the most widely used, either in the construction
of the structure itself or in the manufacture of the major components.

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Welding and Welding Defects 11-1 Copyright © TWI Ltd
11.4 Flux-shielded arc welding
This is the most widely used of all the welding processes. An arc is formed
between a consumable electrode and the work; the heat thus formed melts and
fuses the joint together. The electrode provides the filler metal and the flux is
used to prevent contamination.

For example:
Manual metal arc (MMA) welding
The most widely used technique. Heat to melt the work piece is supplied by an
electric arc; the electrode is covered by flux and melts down forming small
drops, which are transferred to the weld pool; the flux forms molten slag that
protects the weld together with protective gases formed at the same time.

Figure 11.1 Manual metal arc (MMA) welding.

a Colet or twist type. b Tongs type with spring-loaded jaws.

Figure 11.2 MMA electrode holders.

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Welding and Welding Defects 11-2 Copyright © TWI Ltd
 Common faults associated with MMA are:

 Overlap.
 Porosity.
 Slag inclusion.
 Excessive spatter.
 Lack of fusion.
 Crater cracks.
 Arc strike.
 Incomplete penetration.
 Undercut.
 Excessive penetration.

11.5 Metal inert or metal active gas welding (MIG/MAG) welding

A semi-automatic process, the welding electrode, that is also the weld filler
metal, is in the form of a continuous wire fed from a reel. This is usually
mounted on the main welding unit, but for industrial applications may form part
of the hand gun assembly.

The gun assembly itself consists of a gas shroud through which the shield gas,
either active or inert, is fed to protect the molten weld pool; an electrical pick-
up, through which the electrode wire is fed and at the same time energised and
a trigger, which when operated, controls the gas and wire feed.

Trigger

Gas shroud

Figure 11.3 MIG/MAG welding gun.

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 Figure 11.4 MIG/MAG welding.

Figure 11.5 An industrial MIG/MAG welding unit showing the wire reel
assembly.

Gases used for MIG/MAG welding will vary, typically they are:

 100% CO2.
 Argon/CO2 mixes.

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 Common faults associated with MIG/MAG are:

 Porosity.
 Excessive spatter.
 Lack of fusion.
 Incomplete penetration.
 Excessive penetration.
 Cracking.
 Arc strike.
 Undercut.

11.6 Tungsten Inert Gas (TIG) welding

Developed in the United States during WW2 and using a non-consumable
tungsten electrode, mounted inside a gas shroud in a similar way to the
MIG/MAG gun, in this method the electrode forms no part of the weld itself, but
is there to provide the arc. A filler rod of a suitable metal is used and there are
both manual and automatic versions of this equipment.

It produces particularly high quality welds, not only in steel, but is used for
joining aluminium and other alloys. It is a slow process and requires a high
standard of operator skill. Root welds in high quality, high pressure pipe-work
are often carried out using this system.

Figure 11.6 Tungsten inert gas welding.

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 Like MIG/MAG, TIG can also be used with different shielding gases:

Argon
For stainless steel, high carbon steel, aluminium and magnesium.

Helium/argon mixes
For carbon steel, stainless steel, aluminium, copper and magnesium.

Variations
Include adding nitrogen to helium/argon mixes for welding copper and adding
hydrogen to helium/argon mixes for welding austenitic stainless steels.

Common faults associated with TIG welding are:

 Excessive penetration.
 Arc strikes.
 Burn through.
 Incomplete penetration.
 Tungsten inclusion.
 Porosity.
 Undercut.
 Oxide inclusions.

11.7 Submerged arc welding (SAW)

Probably the second most common welding type seen offshore but used in the
manufacture of steel components used in the fabrication of offshore platforms
rather than the welded joints, ie nodal welds themselves.

Developed in the Soviet Union during WW2, this is a fully automatic welding
system. It is particularly useful for welding thick steel sections and used
extensively where long continuous weld runs are to be made. The following
photographs and drawings best illustrate the equipment and process.

Figure 11.7 Submerged arc welding (SAW) process.

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 The flux is in the form of powder or granules continually fed over the work area
and the electric arc is formed underneath so is totally submerged, giving the
process its name. Because of this, personnel do not need eye protection as the
arc is not visible.

The process uses amperages in the range of 100-2000amps, giving very high
current density to the electrode wire, which produces the deep penetration and
weld dilution needed for thick section steel.

Figure 11.8 Examples of submerged arc welding equipment: Units designed

with tractor carriages to enable them to follow tracks or negotiate curves.

SAW tandem arc with two wires

Figure 11.9 Submerged arc welding.

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 Figure 11.10 Specialised SAW being used to make circular welds on storage
tanks.

11.8 Types of welded joint

There are approximately 110 different welded joint variations; the majority of
which are not seen in the construction of offshore structures. So it is necessary
to have knowledge of only five types of joint.

11.8.1 The butt joint - (not to be confused with a butt weld!)

The two components that make up this joint are fitted together end to end at
an angle of between 135-180°. This joint is used to join pipe sections end to
end, welding plates together and numerous other applications (Figure 11.11).

Figure 11.11 Butt joint.

11.8.2 T Joint
The two components are fitted together at an angle of 5-90°. This configuration
is found on offshore jackets at nodes and in numerous other areas (Figure
11.12).

Figure 11.12 T Joint.

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11.8.3 Lap joint
The two components are fitted one on top of the other. The angle between
them is 0-5° (Figure 11.13).

Figure 11.13 Lap joint.

11.8.4 Corner joint

The two components are connected at the ends to make a joint at an angle
between 30-135° (Figure 11.14).

Figure 11.14 Corner joint.

11.8.5 Cruciform joint

Joint made by welding two components to a third at right angles, on the same
axis, on opposite sides of the third component to form the shape of a cross
(Figure 11.15).

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 Parentplate
Parent plate11

Parent
Parent plate
plate 2 2

Parent plate 3

Figure 11.15 Cruciform joint.

11.9 Types of weld

The two types of weld most frequently inspected on offshore structures are the
butt weld and the fillet weld.

A butt weld is defined as:

A tension resisting weld in which the bulk of the weld metal is

contained within the planes or thickness of the joined parent metals.

A fillet weld is defined as:

The bulk of a fillet weld is contained outside the parent metal planes or
thickness.

The fillet weld has less strength than the butt weld (Figure 11.16).

a Single V butt.

b Fillet welded butt joint.

Figure 11.16 Types of weld.

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 As fillet welds are not used for structural joints that must withstand high
stresses, the butt weld will be the type of weld most frequently inspected
offshore. All nodes, including any safety critical nodes on the structure, will be
constructed using butt welds.

11.10 Welding metallurgy

The welding processes outlined in previous paragraphs and the types of joint
and types of weld specified above, are all designed for the purpose of fixing
components together safely for the entire duration of the design life of the
structure. In order that this prime aim may be achieved, the mechanism by
which welding takes place must be understood.

The prime factor in welding is temperature. The various welding processes,

types of weld and types of joint are all designed so that the heat generated
during the welding process can be dissipated uniformly as the molten metal
cools after the weld metal is deposited. Figure 11.17 shows how this occurs.

Figure 11.17 Temperature variations in a butt weld.

At point 1 within the molten weld pool, the temperature will be above the
melting point of the filler rod metal. The welding current and technique of the
welder determine this temperature.

The main heat flow away from the weld pool will be along the parent plate.
Between points 1 and 2, the temperature must raise above the parent metal
melting temperature so that fusion (ie melting the parent plate and mixing with
the weld pool metal), occurs.

This region (between points 1 and 2), is known as the fusion zone and can be
readily seen if a sample of the weld is sectioned, polished and etched.

The temperature then reduces from point 2-3, which is a region of the parent
metal that has sufficient heat input to cause grain structure modification, known
as the heat affected zone (HAZ).

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 HAZ
One of the means of making a material softer (often called annealing) is to heat
it up and allow it to cool slowly. A common example of this is copper pipes for
domestic water systems that are softened in this way in order to bend them to
required shapes.

To achieve this softening effect a material has to be heated above its re-
crystallisation temperature (Trecry). Above this temperature, grains in the
material will reform and grow.

In Figure 11.17 this temperature is reached at point 3, so that the material

between point 2 and 3 that has been raised above the re-crystallisation
temperature will be liable to a change of properties. This region can also be
seen on a polished and etched sample of the weld.

The temperature continues to fall between points 3 and 4, which is ambient

temperature. Figure 11.17 only shows what is happening along the line AA; but
this happens throughout the section. This leads to the different regions of the
weld, as shown in Figure 11.18 and is a graphic indication of the way
temperature gradients have to be managed in any weld.

Fusion zones

Figure 11.18 Weld regions or zones.

This temperature management is as important for cooling as it is for heat

energy input into the weld. The cooling rate must be as controlled as the heat
flow during the actual welding. In general, fast cooling rates (often referred to
as quenching), make the material harder. In steel, this comes about by the
formation of a structure known as Martensite. Martensitic steel has a grain
structure arranged in a regular lattice, which makes the steel hard and less
tough (ie less able to withstand crack propagation).

Note: If the cooling rate is not properly controlled and the material is allowed
to quench, it has the opposite effect to annealing outlined above.

11.11 Further considerations for weld control

While heat input and cooling rate control may be of paramount importance to
the finished quality of a weld, there are several other factors that must also be
considered.

Defects, such as porosity, often arise in welds due to gas penetrating the weld
pool protection. Gases that are likely to be present in the weld are hydrogen,
oxygen and nitrogen. These are derived from the atmosphere, water,
hydrocarbons (usually in the form of grease and oil) and other oxides present in
the vicinity of the weld because of a lack of care in preparation, not ensuring
that the weld area is clean and dry.

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 These products get into the arc and provide a supply of gas that can be
dissolved in the liquid metal of the weld pool. On cooling, the solubility of the
dissolved gas in metal reduces and the gas comes out of solution to form
bubbles trapped in the weld metal; or sometimes, the gas diffuses into the
parent metal. Hydrogen diffusing into the HAZ will cause hydrogen
embrittlement, which may lead to cracking.

The different temperatures in the regions around the weld will cause differential
expansion. On cooling, if cracking does not immediately occur in the weld or in
the HAZ, the material is put under a permanent stress, unless a stress relieving
procedure is specified. This state of stress is referred to as residual stress.

Normal working stress is imposed on top of and in addition to this residual

stress, giving an in-service stress that is higher than the normal design working
stress. The effect of residual stresses will be, at the very least, a reduction in
the fatigue life of the joint. At the moment, there is no way that these residual
stresses can be measured during the course of a routine inspection. (Alternating
current field measurement (ACFM) may be developed for this purpose).

Atomic hydrogen Hydrogen

(H) diffusion

Molecular
hydrogen
(H2)

Steel in expanded condition Steel under contraction

above 300°C below 300°C

Figure 11.19 Hydrogen embrittlement.

11.12 Welding terms

There are a number of standard, defined, welding terms and also symbols that
are used internationally to define different parts or elements of welds. These
terms and symbols are defined in several international standards.

 BS EN 24063: 1992, ISO 4063: 1990: Welding, brazing, soldering and

braze welding of metals. Nomenclature of processes and reference numbers
for symbolic representation on drawings.
 BS EN 22553: 1995: Welded, brazed and soldered joints. Symbolic
representation on drawings.

 BS EN ISO 5817: 2003: Welding. Fusion-welded joints in steel, nickel,

titanium and their alloys (beam welding excluded). Quality levels for
imperfections.

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  BS EN 13622: 2002: Gas welding equipment. Terminology. Terms used for
gas welding equipment.

 BS 499-1: 1991: Welding terms and symbols. Glossary for welding,

brazing and thermal cutting (This standard has the status of being current,
partially replaced by BS EN 13622: 2002).

A list of extracts from these standards that may apply to in-service inspection is
compiled below.

11.13 Plate preparation terms

Double V butt weld
A butt weld in which the prepared faces will form two opposing V’s in section,
welded from both sides.

Included angle of a butt weld

The angle between the prepared faces.

Included angle of a fillet weld

The angle between the parent plates.

Parent plate
The metals that are to be joined by the weld.

Prepared angle, weld prep

The angle of bevel between the prepared face and the perpendicular.

Prepared face
The bevelled portion of the parent plate prior to welding.

Root gap
Separation between the parent plates to be joined.

Root face
The un-bevelled portion of the parent plate adjacent to the root gap.

Single bevel butt weld

A butt weld that has only one prepared face, welded from one side only.

Single V butt weld

A butt weld in which the prepared faces will form a V in section, welded from
one side only (Figure 11.20).

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 Prepared angle
Included angle and included angle
Prepared angle

Prepared face
Root face

Root Gap
Single V Butt Weld Single Bevel Butt Weld

Double V Butt Weld

Figure 11.20 Standard weld terms for plate preparation.

11.14 Terms defining weld features

Cap, face of the weld
Visible face of the completed weld.

Excess weld metal

Weld metal lying outside the line joining the weld toes.

Toe of the weld

Junction between the cap and the parent plate.

Root
Point where the back of the weld intersects the back face of the parent plate.

Weld zone
Area containing the weld and both HAZs.

Heat affected zone (HAZ)

Part of the parent plate that has been affected by heat from the welding
process but which has not melted.

Throat thickness
Total thickness of the weld metal.

Effective throat thickness (design throat thickness)

Weld thickness for design purposes, usually a line between both toes and the
root.

Weld width
Shortest distance between the toes of the weld.

Toe blend
Transition between the weld material and the parent plate.

Leg (of a fillet weld)

Distance from the root of the weld to the toe of the weld.

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 Figure 11.21 Weld feature terminology on a butt weld.

Figure 11.22 Weld feature terminology on a fillet weld.

11.15 Welding process terminology

Filler rod
Filler metal for a weld in the form of a rod 440mm long used in MMA welding.

Filler bead
When the weld is made up of more than one pass of a filler rod the successive
passes are called filler beads.

Run or pass
Weld metal laid down in a single pass from a filler rod.

Weldment
An alternative term to describe the weld zone.

Fusion zone
The edge of the parent plate along the prepared face and the root face along
which the weld metal fuses with the parent plate.

Root bead
Weld bead laid into the root that protrudes beyond the back wall of the parent
plate.

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 Run out length
The specified maximum lengths of weld run for a particular rod type.

Figure 11.23 Weld process terminology.

Welded nodes and nozzles

Underwater in-service inspection of offshore oil platforms is almost exclusively
on pipe work and will involve inspection of pipe joints. These will be nodes or
nozzles.

Node
A T or cruciform joint between two pipes that only has preparation on the minor
member, a single bevel weld. The minor tubular is called the brace and it is this
member that has the preparation. The major tubular is known as the chord. In
joints where both members are the same size the through tubular is the chord.

Nozzle
Both tubular members have preparation, which also means that the chord has a
hole to match the brace. This is a full penetration butt welded joint. This type of
joint is commonly found in pipelines and where fluid flow is required.

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 Figure 11.24 Nodes and nozzles.

11.16 Weld defect terminology

An inspector must be capable of not only recognising a fault in a weld, but
subsequently, be able to describe it accurately. In common with welding
terminology, this aspect of welding also has internationally agreed and defined
terms.

In this case the International Institute of Welding (IIW) and BS EN ISO 5817:
2007 apply. In the same way that welding terms are defined in this standard,
weld defect terminology is also defined.

The different types of defect are listed in six categories.

1 Cracks.
2 Cavities.
3 Solid inclusions.
4 Lack of fusion and penetration.
5 Imperfect shape.
6 Miscellaneous.

(Mnemonic - CCSLIM).
Internal weld defects are broadly sub-divided into:

Planar defects
Have a large surface area but small volume, such as cracks and laminations and
are essentially two dimensional.

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 Volumetric defects
Inclusions, porosity and other internal flaws, that have a large volume
compared to surface area, are in this category. They are three dimensional and
will also include undercut and lack of penetration. This category of defect is
caused during fabrication, not in-service; while planar defects may be caused
by in-service deterioration.

Only a certain number of these standard terms apply to defects that may be
found on the surface of the weld accessible to the underwater inspector but
knowledge of a representative sample of standard terms from all categories will
assist any inspector when reporting findings and conversing with engineers,
welders or weld inspectors.

11.17 Cracks
These are linear discontinuities produced by fracture, cracks may be:

 Longitudinal.
 Transverse.
 Crater.
 Centreline.
 Toe.
 HAZ.

Figure 11.25 Crack-like indications.

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 Figure 11.26 HAZ cracking.

11.18 Cavities
A number of flaws are covered by this category.

Porosity: Linear or cluster

Gas pores that may be located in different locations.

Elongated cavities
A string of gas pores parallel to the weld axis.

Shrinkage cavity
A cavity caused by shrinkage of the weld metal while it is in a plastic state.

Crater
A depression caused by shrinkage at the end of a run if the heat is removed
quickly.

Crater pipe
A hole in the centre of a crater, caused by shrinkage.

Figure 11.27 Cavities.

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11.19 Solid inclusions
Volumetric defects caused by solids trapped in the weld pool before it solidifies.

11.20 Lack of fusion and penetration

Lack of fusion
The weld metal has not bonded.

Lack of sidewall fusion

No union between the weld metal and the parent plate.

Lack of root fusion

No bonding at the root of the weld joint.

Incomplete root penetration

No weld metal extending into the root of the weld.

Figure 11.28 Slag inclusions.

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 Figure 11.29 Lack of fusion and penetration.

11.21 Imperfect shapes

Excess weld metal
Weld metal lying outside the plane joining the toes.

Excess penetration
Excess weld metal protruding through the root.

Root concavity
A shallow groove in the root.

Incompletely filled groove

A groove caused by insufficient weld metal being laid onto the cap.

Undercut
A groove in the toe of the weld where the parent plate is gouged due to the
welding current.

Overlap
Weld metal spilled over from the cap onto the parent plate outside the line of
the toe that has not fused with the parent metal.

Burn through
This is a collapse in the weld pool caused by excessive penetration resulting in a
hole in the weld.

Unequal leg length

Not a standard term but internationally understood, describing different leg
lengths on a fillet weld, usually a T joint.

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 Poor restart or poor stop/start
Not standard terms but internationally understood, an irregular start or pick-up
after one bead is ended or interrupted and the next arc strike is imperfectly
aligned with the previous bead.

Misalignment
(Not a standard term but internationally understood), poor fit-up resulting in
the parent plates being out of alignment either laterally or angularly.

Figure 11.30 Imperfect shape.

11.22 Miscellaneous
Stray flash or arc strike
Burn marks on the parent metal caused by striking arcs with the welding rod off
the line of the weld; can sometimes be caused by arcing of the weld supply
cable if the insulation is damaged.

Excessive dressing
Grinding away too much weld metal and leaving the weld below the level of the
surface of the parent plate.

Grinding mark
Grooves or marks on the parent plate caused by poorly controlled grinding or
surfacing tools.

Tool mark
Marks indented into the parent plate caused by chipping hammers or similar
hand tools.

Hammer mark
Obvious damage caused by a hammer blow.

Torn surface
Surface irregularity caused by breaking off temporary attachments, colloquially
known, though not always accurately, as dog scars see below.

Surface pitting
Small depressions on the weld or parent plate.

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Welding and Welding Defects 11-23 Copyright © TWI Ltd
 Spatter
Spots of weld metal thrown out from the weld pool and attaching themselves to
the parent plate.

Dog scar - colloquial term, see above

A welding scar left over after removal of a dog, (a temporary metal fixing used
to stabilise the parent plates during the welding process).

Figure 11.31 Miscellaneous defects.

11.23 Reporting defects in welds

As diver inspectors are concerned with in-service inspections, volumetric
defects will seldom be identified, as they are usually caused during fabrication.
Planar defects may be observed as these could be caused by stress or fatigue
failure leading to crack-like features becoming evident.

This type of discontinuity will be of most concern in the toe of the weld, which is
also the zone where it is most likely to be found. This is because at this point
there is a region that has been heated and melted causing grain structure
changes as outlined earlier.

Also in this area, the geometry of the weld changes, which may create a notch
effect; that is an area where stress is increased above the average for the rest
of the component.

Any defects identified must be reported by recording at least:

Type of defect
Describe the defect with correct terminology.

Location
State the global location, ie what component is damaged, where on the
component the damage is (state the clock and or tape position relative to a
known datum), give the relative location, ie is it on the HAZ, in the toe, on the
weld cap or in the parent plate.

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 Dimensions
State the start position and give length. If the defect is a crack-like feature
state whether it is continuous or branching, the orientation and if it is
measurable give width and depth.

Description
Describe the feature, if it is a crack, is it branching, if so state the orientation of
the branches.

11.23.1 Dimensional checking weld parameters

During fabrication the weld dimensions are checked and verified against the
weld design specifications to ensure that the welding is completed to the
required quality to meet design parameters; ensuring that it is fit for purpose.

Welding inspectors will confirm that the welds meet these requirements and for
the visual elements of the inspection requirements there are a number of
measuring gauges, templates and devices employed. These instruments are
available for in-service inspections and a review of a selection will be of interest
for underwater applications.

The welding Institute measuring gauge

A gauge specially designed to accurately measure weld reinforcement height,
leg length, throat thickness and depth of lack of fill.

Figure 11.32 Welding Institute Gauge measuring various weld parameters.

Welding Institute leg length gauge

Specially designed for measuring T joint leg length as indicated in Figure 11.33.

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 Figure 11.33 Welding Institute leg length gauge.

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Welding and Welding Defects 11-26 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Gourd L M, ‘Principles of Welding Technology’, Hodder Arnold, 1980,

ISBN 13: 9780713134025.

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Joining Metal Components

Metal components can be formed or shaped in

four ways:

1. Machining.
CSWIP 3.1U Course 2. Casting.
3. Forging.
4. Fabrication.
Welding and Welding Defects
Section 11 The components inspected underwater are
almost all formed by fabrication and the most
important technique is welding, so a closer look
at this process is in order.

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Welding Processes Welding Processes

It is extremely difficult indeed to guarantee that any There are 35 welding processes classified into
particular weld is free from all faults and, because of seven groups:
this, welds are constantly inspected to ensure they
are not about to fail. 1. Solid phase welding.
2. Thermo-chemical welding.
The knowledge of how the welding was achieved in 3. Electric-resistance welding.
the first place is of great assistance when inspecting 4. Unshielded arc welding.
welded joints because all techniques have certain 5. Radiant energy welding.
faults that are common to that technique. It is, 6. Flux-shielded arc welding.
therefore, important to have some knowledge of the 7. Gas-shielded arc welding.
main types of welding.

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Flux Shielded Arc Welding Manual Metal Arc (MMA)

Flux shielded arc welding is the most widely used Manual metal arc (MMA) process was first
process. developed in the late 19th century using bare
wire consumables.
An arc is struck between a consumable electrode and
the work.  MMA is a simple process in terms of
equipment.
This generates heat and melts the joint and the  The process can by used with AC, DC+ or DC-
electrode which provides the filler metal. current.
 The process is a manual process and demands
The consumable electrode is covered with a flux that high skill from the welder.
melts and provides slag and a gas shield which  The process is widely used throughout the
protects the weld pool from contamination. welding industry both for shop and site
working conditions.

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Manual Metal Arc (MMA) Manual Metal Arc (MMA)

Common faults
 Overlap.
 Porosity.
 Slag inclusions.
 Excessive spatter.
 Lack of fusion.
 Crater cracks.
 Arc strike.
 Incomplete
Collet or Tongs type penetration.
twist type with spring-  Excessive
loaded jaws
penetration.
 Undercut.

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MIG/MAG Welding MIG/MAG Welding

Metal inert gas or metal active gas, MIG/MAG

welding process was initially developed in the USA in
the late 1940’s for the welding of aluminium alloys.
Trigger
The latest EN Welding Standards now refer to the
Gas shroud
process by the American term GMAW (Gas Metal Arc
Common faults:
Welding):
 Porosity.
 The process uses a continuously-fed wire electrode.  Excessive spatter.
 The weld pool is protected by a separately supplied  Lack of fusion.
shielding gas.  Incomplete
 The process is classified as a semi-automatic welding penetration.
process but may be fully automated.  Excessive penetration.
Typical gases used are:
 The wire electrode can be either bare, solid wire or flux- 100% CO2 or Argon/CO2 mixes.  Cracking.
cored hollow wire.  Arc strike.
 Undercut.

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Tungsten Inert Gas (TIG) Welding Tungsten Inert Gas (TIG) Welding

Tungsten Inert Gas (TIG) welding process was Common faults:

first developed in the US during the 2nd world  Excessive
war for the welding of aluminum alloys. penetration.
 Arc strikes.
 Burn through.
The process:
 Porosity.
 Tungsten inclusion.
 Uses a non-consumable tungsten electrode.  Incomplete
 Requires a high level of welder skill. penetration.
 Produces very high quality welds.  Undercut.
Typical gases used are:  Oxide inclusions.
 Is considered as a slow process compared to Argon and helium/argon mixes.
other arc welding processes.

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Submerged Arc Welding (SAW) Submerged Arc Welding (SAW)

 Submerged arc welding was developed in the

Soviet Union during the second world war for the
welding of thick section steel.
 The process is normally mechanised.
 It uses amps in the range of 100 to over 2000,
which gives a very high current density in the
wire; producing deep penetration and high
dilution welds.
 A powdered flux is supplied separately via a flux
hopper.
 The arc is not visible as it is submerged beneath
the flux layer and no eye protection is required.

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Submerged Arc Welding (SAW) Types of Welded Joint

There are approximately 110 different welded

joint variations; however the vast majority of
these are not seen in the construction of
offshore structures.

Therefore it is necessary to have a knowledge of

only five.

SAW tandem arc

with two wires

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Types of Welded Joint Types of Welded Joint

Butt joint - not to be confused with a butt weld T joint

This is where two parent plates are fitted  Two parent plates are fitted together at an
together at an angle of between 135-180°. angle of between 5-90° such as the joint
between two tubular members in a node.
This is used offshore for circumferential and Parent plate 2
seam welds.

Parent plate 1 Parent plate 2 Parent plate 1

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Types of Welded Joint Types of Welded Joint

Lap joint Corner joint

 Two parent plates are fitted one on top of the  Two parent plates make a connection at the
other the angle between them is 0-5°. edges to make a joint at an angle of between
30-135°.
Parent plate 1
Parent plate 1

Parent plate 2

Parent plate 2

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Types of Welded Joint Types of Weld

Cruciform joint The two types of weld most frequently used on

 A joint at which two flat plates or two bars are offshore structures are:
welded to another flat plate at right angles
and on the same axis.  Butt weld.
Parent plate 1  Fillet weld.

Parent plate 2

Parent plate 3

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Butt Weld Fillet Weld

A tension resisting weld in which the bulk of the The bulk of a fillet weld is contained outside the
weld metal is contained within the planes or parent metal planes or thickness.
thickness of the joined parent metals.

Weld metal
Therefore fillet welds tend to have less strength.

Weld metal

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Fillet Weld Welding Metallurgy

As fillet welds are not used for structural joints The welding processes outlined in previous
that must withstand high stresses, the butt weld paragraphs, and the types of joint and types of
will be the type of weld most frequently weld specified, are all designed for the purpose
inspected offshore. of fixing components together safely for the
entire design life of the structure.
All nodes, including safety critical nodes on the
structure, will be constructed using butt welds. In order that this prime aim may be achieved,
the mechanism by which welding takes place
must be understood. The prime factor in welding
is temperature.

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Welding Metallurgy Temperature Variation in a Butt Weld

 The various welding processes, types of weld

and types of joint are all designed so that the
heat generated during the welding process can
be dissipated uniformly as the molten metal
cools after the weld metal is deposited.

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Heat Affected Zone (HAZ) Considerations in Weld Control

One of the means of making a material softer  This temperature management is as important
(often called annealing) is to heat it up and allow it for cooling as it is for heat input into the weld.
to cool slowly. To achieve this softening effect a The cooling rate must be as controlled as the
material has to be heated above its re- heat flow during the actual welding. In
crystallisation temperature (Trecry). general, fast cooling rates (quenching) make
In the previous diagram, this temperature is the material harder and less able to withstand
reached at point 3 so that the material between crack propagation.
point 2 and 3 that has been raised above the re-
crystallisation temperature will be liable to a  While the heat input and cooling rate control
change of properties. may be of paramount importance to the
Above this temperature the grains in the material finished quality of a weld, there are several
will reform and grow. This is known as the heat other factors that must also be considered.
affected zone (HAZ).
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Considerations in Weld Control Considerations in Weld Control

 Defects, such as porosity, often arise in welds These products get into the arc and provide a supply of
due to gas penetrating the weld pool gas that can be dissolved in the liquid metal of the weld
pool. On cooling the solubility of the dissolved gas reduces
protection. Gases that are likely to be present and the gas comes out of solution to form bubbles trapped
in a weld are hydrogen, oxygen and nitrogen. in the weld metal or sometimes the gas diffuses into the
These are derived from the atmosphere, parent metal. Hydrogen diffusing into the HAZ will cause
water, hydrocarbons (grease or oil) and other hydrogen embrittlement which may lead to cracking.
oxides present in the weld vicinity, due to a
lack of care in preparation.

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Considerations in Weld Control Considerations in Weld Control

 The different temperatures in the regions  The effect of residual stresses will be, at the
around the weld will cause differential very least, a reduction in the fatigue life of the
expansion. On cooling, if cracking does not joint. At the moment, there is no way that
immediately occur in the weld or in the HAZ, these residual stresses can be measured
the material is put under a permanent stress, during the course of a routine inspection.
unless a stress relieving procedure is (Alternating current field measurement
specified. (ACFM) may be developed for this purpose).

 This state of stress is referred to as residual

stress. Normal working stress is imposed on
top of and in addition to this residual stress,
giving an in-service stress that is higher than
the normal design working stress.
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Welding Terms and Definitions Plate Preparation Terms

BS EN 13622:2002 defines all the standard Prepared angle

terms used to describe a weld. There are 25 Included
angle Prepared angle
and included angle

terms that apply to this course.

Prepared face
Root face
These terms may be grouped into categories:
Root gap
Single V butt Single bevel butt
weld weld
 Plate edge preparation.
 Weld features.
 Welding terminology.

Double V butt weld

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Terms Defining Weld Features Terms Defining Weld Features

Butt weld Fillet weld

Toe of the weld
Weld zone

Weld width Included angle

Excess weld Cap or face

Weld cap of the weld
metal
Toe of the weld Toe of the weld Leg length
(Toe blend)

Effective
throat
Effective throat Throat
thickness
thickness thickness

Throat thickness

Root
Root

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Welding Terms Welding Terms

 Filler rod: The filler metal for the weld in the  Root bead: Weld bead protruding beyond the
form of a rod 440mm long used in MMA back wall of the parent plates.
welding.
 Fusion zone: The point at which parent plate
 Filler beads: When the weld is made up of melts and mixes with weld metal.
more than one pass of a filler rod the
successive passes are called filler beads.  Run out length: The specified run length for a
given welding rod.
 Run or pass: Weld metal deposited in a single
pass of a filler rod.

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Weld Process Terminology Welded Nodes and Nozzles

Offshore structures are constructed mainly of

Fusion zones tubular members. Connections of these
members may be either nodes or nozzles.

(HAZ)

Filler beads
Root bead or
Weld beads making up
penetration bead
the bulk of the weld

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Welding Defect Terminology Internal Weld Defects

An inspector must be capable of not only Planar defects:

recognising a fault in a weld but be able to describe  These have a large surface area but small volume
it accurately. In common with welding terminology such as cracks and laminations. They are
this has internationally agreed and defined terms. essentially 2-dimensional.
BS EN ISO 5817:2003 Defines six categories of Volumetric defects:
weld defects:  Inclusions, porosity and other internal flaws that
Cracks.

have a large volume compared to surface area
Cavities.

are in this category. They are 3-dimensional and
Solid inclusions.

will also include undercut and lack of penetration.
Lack of fusion and penetration.

This category of defect is caused during
Imperfect shape.

fabrication, not in-service; while planar defects
Miscellaneous.

may be caused by in-service deterioration.
 C.C.S.L.I.M.
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Cracks Cracks

Cracks are linear discontinuities produced by

fracture, they may be:

 Longitudinal.
 Transverse.
 Crater.
 Centreline.
 Toe (Fusion zone).
 HAZ.

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Cracks Cracks

Toe cracking
Transverse crack Longitudinal crack

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Cavities Cavities

Cluster Causes:
Gas pore porosity
 Loss of gas shield.
 Damp electrodes.
 Contamination.
 Arc length too large. Porosity
Blow hole  Damaged electrode
Herringbone porosity flux.
 Moisture on parent
plate.
 Welding current too
low.
Root piping Gas pore <1.5mm Root piping
Blow hole >1.6mm

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Cavities Cavities

Porosity
Cluster porosity Herringbone porosity

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Cavities Cavities

 Crater pipe is a shrinkage defect and not a gas

Weld crater defect, it has the appearance of a gas pore in
the weld crater.

Crater cracks Causes:

(Star cracks)  Too fast a cooling
rate.
 Deoxidization
reactions and liquid
to solid volume
Crater pipe change.
Crater pipe  Contamination.

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Solid Inclusions Solid Inclusions

Volumetric defects caused by non-metallic

inclusions which are trapped in the weld pool
before it solidifies.
Causes:
 Slag originates from
welding flux.
 MAG and TIG welding
process produce silica
Lack of sidewall
Slag fusion with inclusions.
inclusions associated slag  Slag is caused by
inadequate cleaning.
 Other inclusions include
tungsten and copper
inclusions from the TIG and
Lack of inter-run fusion with MAG welding processes.
slag

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Solid Inclusions Lack of Fusion

Typical causes of lack of fusion:

 Welding current too low.

 Bevel angle too steep.
 Root face too large (single-sided weld).
 Root gap too small (single-sided weld).
 Incorrect electrode angle.
 Linear misalignment.
 Welding speed too high.
Interpass slag inclusions Elongated slag lines  Flooding the joint with too much weld
metal (blocking out).

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Lack of Fusion Lack of Fusion

Causes: Lack of sidewall fusion

 Poor welder skill.
 Incorrect electrode
manipulation.
Incomplete filled groove and  Arc blow.
Lack of sidewall fusion
 Incorrect welding
current/voltage.
1
 Incorrect travel
speed. Lack of root penetration
2  Incorrect inter-run
cleaning.
1. Lack of sidewall fusion.
2. Lack of inter-run fusion.

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Lack of Fusion Imperfect Shape

Incompletely filled groove Poor cap profile

Excess
penetration
Poor cap profile and excessive
cap reinforcement may lead to
stress concentration points at
the weld toes and will also
contribute to overall poor toe
Lack of sidewall fusion and incompletely filled groove blend. Excessive cap height

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Imperfect Shape Imperfect Shape

Undercut
An irregular groove at the toe of a weld run in
the parent metal.
Causes:
 Excessive amps/volts.
 Excessive travel speed.
 Incorrect electrode
angle.
 Excessive weaving.
 Incorrect welding
technique.
 Electrode too large.
Excessive cap reinforcement Incompletely filled groove  Arc length too high.

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Imperfect Shape Imperfect Shape

Poor cap profile and poor toe blend

Intermittent cap undercut

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Imperfect Shape Imperfect Shape

Overlap Burn through:

An imperfection at the toe or root of a weld caused  This is a collapse in the weld pool caused by
by metal flowing on to the surface of the parent excessive penetration resulting in a hole in the
metal without fusing to it. weld.
Causes:
 Contamination.
 Slow travel speed.
 Incorrect welding
technique.
 Current too low.

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Imperfect Shape Imperfect Shape

A variation of leg lengths on a fillet weld

Linear misalignment
is measured from the
lowest plate to the
highest point.
Plate/pipe linear misalignment
(Hi-Lo)

Angular
misalignment is
measured in degrees
Note: Unequal leg lengths on a fillet weld may be specified
as part of the design, in which case it will not be considered
as a defect. Angular misalignment

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Imperfect Shape Miscellaneous

Arc strike or stray arc:

 Accidental striking of an arc on to base material.
 Loss of welding cable insulation.
 Poor connection of current return cable.

Spatter:
Poor stop/starts  Excessive current or voltage.

Slag:
 Poor workmanship (inadequate cleaning).

Grinding mark/mechanical damage:

 Torn surface.

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Miscellaneous Miscellaneous

Causes:
Causes:  Excessive current.
 Accidental striking of  Damp electrodes.
the arc onto the  Contamination.
parent material.  Incorrect wire feed
 Faulty electrode speed when
holder. welding with MAG
 Poor cable insulation. welding process.
 Poor return lead  Arc blow.
clamping.

Arc strike
Spatter

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Miscellaneous Miscellaneous

Mechanical damage can be defined as any surface Chipping marks

material damage caused during the manufacturing
process.

 Grinding.
 Hammering.
 Chiselling.
 Chipping.
 Breaking off welded attachments (torn surfaces).
 Using needle guns to compress weld capping runs.
Mechanical damage/grinding mark

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Reporting Defects in Welds Reporting Defects in Welds

 As diver inspectors are concerned with in-  This type of discontinuity will be of most
service inspections, volumetric defects will concern in the toe of the weld, which is also
seldom be identified, as they are usually the zone where it is most likely to be found.
caused during fabrication. Planar defects may This is because at this point there is a region
be observed as these could be caused by that has been heated and melted causing
stress or fatigue failure leading to crack-like grain structure changes as outlined earlier.
features becoming evident. Also, in this area the geometry of the weld
changes, which may create a notch effect;
that is an area where stress is increased above
the average for the rest of the component.

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Reporting Defects in Welds Reporting Defects in Welds

Any defects identified must be reported by Any defects identified must be reported by
recording at least: recording at least:

Type of defect Dimensions

 Describe the defect with correct terminology.  State the start position and length. If the
defect is a crack-like feature state whether it
Location is continuous or branching and if it is
measurable give width and depth.
 State the clock and or tape position relative to
a known datum, the relative location, is it in
the HAZ, the toe, the weld cap or the parent Description
plate.  If it is a crack, is it branching, if so state the
orientation of the branches.
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Dimensional Checking Weld Dimensional Checking Weld

Parameters Parameters
During fabrication the weld dimensions are checked  A gauge specially designed to accurately
and verified against the weld design specifications to measure weld reinforcement height, leg
ensure that the welding is completed to the required length, throat thickness and depth of lack of
quality to meet design parameters ensuring that it is fill.
fit for purpose.

Welding inspectors will confirm that the welds meet

these requirements and for the visual elements of
the inspection requirements there are a number of
measuring gauges, templates and devices employed.
These instruments are available for in-service
inspections and a review of a selection will be of
interest for underwater applications.
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Dimensional Checking Weld

Parameters
Welding institute leg length gauge.
 Specially designed for measuring T joint leg
length.

Any Questions?

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14
Section 12

Photography
12 Photography
The word photography comes from the Greek: Photo = Light and Graphos =
Drawing.

Photography is a technique for recording a scene in reality onto photographic

paper, film or digital medium to produce an image. It can trace its origin back
to the late eighteenth century.

Photography as a recording method for engineering inspections has proved to

be very useful for the following reasons:

 It is an objective recording method, ie photography records an image of the

subject that the camera is pointing at. It does not make assumptions nor
assign meanings to what it records. A description or a sketch of a subject is
dependent upon the interpretation of the image by the inspector and as
such, is subjective.

 It produces a permanent record of the subject. This is normally achieved by

storing the photograph using a digital media file. Where these files are
stored depends upon the type of computer system used but is commonly a
hard-drive and an optical drive backup such as CD-ROM. Of course, we can
also incorporate digital images very easily into word-processed reports for
electronic transmission or for hardcopy printing.

 Photographic images are typically high quality. The minimum requirement of

six mega-pixels for an underwater inspection photograph is easily
exceeded by using a modern digital camera. Whilst it is true that an image
taken with a typical digital camera would have a significantly higher
resolution than an image captured from a typical video camera (using a
picture grabber), modern high-definition video cameras can easily produce
acceptable images grabbed from the video signal.

 We can use the optical properties of the camera system to magnify the
subject. This can be useful as there is minimal loss of image quality if we
use the camera lens to perform this magnification as opposed to digitally
zooming in on a detail using a computer programme to process the image.

 Modern digital camera systems are relatively inexpensive to purchase. Even

if you add the cost of an underwater housing, strobe and ancillary
equipment, then underwater camera systems are a cost-effective alternative
to high-resolution video systems.

 There is a vast array of photographic equipment available for underwater

use. This means that we can choose the best camera, lens, housing,
strobe(s) and other ancillaries for the task at hand. This gives the
underwater digital camera a great degree of adaptability.

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  Accurate measurements may be taken from photographs. This technique is
known as photogrammetry and may be done with either a single
photograph or by taking two photographs using a technique known as
stereo photography.

 Unlike wet-film photography, digital photography does not require any time
consuming chemical development process – the image is immediately
available and may be reviewed by the photographer prior to moving away
from the inspection site.

 Digital post processing is available using readily available computer

software.

Unfortunately, digital photography does have some disadvantages:

 It is a fallacy to believe that because modern digital cameras are automatic

then good photographs are simply a case of pointing the camera in the right
direction and pressing the trigger. This is not true. The photographer must
have an understanding of the techniques used to frame and focus the image
and – most importantly – control the lighting of the image. Photography is a
skill that has to be learned.

 Underwater photography can be time-consuming if compared to

videography. However, with a high level of skill, the photographer can
progress a photo inspection fairly rapidly and has the ability to review each
and every photograph taken before moving on and leaving the inspection
site.

12.1 Light and photography

Although digital cameras record the light image electronically, a brief look at
the way traditional film records light will assist greatly in gaining a better
understanding of how the camera actually captures the image.

Traditional film is made of celluloid that has an emulsion coating containing

silver halide salts, which react when exposed to light. When the film is
processed, the silver halides are converted chemically to dyes to produce either
colour negatives or colour positives, (slides). The slides can be viewed, once
processed while the negatives must be further processed to produce colour
prints.

Digital cameras, of course, react to light electronically and the image is

recorded immediately the exposure is taken however, the image will be sharp
or muddy; crisp or blurred; good or bad, depending on how the shutter speed
and aperture controls are selected by the circuitry. In this regard the digital
camera is the same as the traditional camera, the quantity and quality of light
falling onto the recording medium must be correctly controlled. In other words
the exposure must be correct.

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 The basis for monitoring and controlling the amount of light entering the
traditional camera was the film speed. The film manufacturers grading the silver
halides used on the film stock determined this, the finer the grains the slower
the film eg more light was needed, or the larger the grains, the faster the film
eg less light was needed. The way that the film manufacturers marked the film
speed was by evolving a numbering system that indicated the film speed by
increasing numbers.

Traditionally, there were two numbering systems; one in the US called the ASA
system (American Standards Association), the other was in Europe called the
DIN system (Deutsches Institut fŭr Normung). Both were an increasing number
series where the higher the number the faster the film reaction to light.

Thirty years ago the International Organisation for Standardisation (ISO)

combined the two systems to one international standard. The chart in Figure
12.1 shows a typical series.

Figure 12.1 ISO Chart.

As can be seen by consulting the chart, the ISO system merely grouped the
other two systems together. The chart groups the films into slow, medium and
fast and as the ISO number increases the film speed; its speed of reaction to
light also increases. There is another, more fundamental but less obvious
relationship to film speed and light.

This relationship is more readily demonstrated by considering the ASA system.

Taking 100 ASA as an example; film of this rating will react twice as quickly
to a given quantity of light than 50 ASA but only half as quick as 200 ASA.
This relationship between adjacent film speeds is the same for the DIN system
and therefore for the ISO system. Thus ISO 400/27 will react twice as quickly
to light as ISO 200/24 but only half as quickly as ISO 800/30. This relationship
is known as being a one stop difference, which will be fully explained when
considering the lens system of a camera.

It is worth noting that the ISO setting can be altered frame by frame with most
digital cameras unlike film cameras. With film cameras the film stock dictates
the setting and once loaded cannot be changed without changing the film.

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12.2 The camera
In its simplest form, the camera is no more than a lightproof box with a hole in
it; the box containing the film or charged coupled device, CCD. Indeed that is
all the original cameras were. However, modern cameras have evolved and, as
the film stock became standardised, so cameras developed standard calibrated
controls to meter the amount of light entering to expose the film.

Thus the ISO number for a film has a tangible and fundamental impact on the
conventional camera. Modern digital cameras only differ from conventional
cameras in the recording medium and therefore they have either the same or
comparable controls.

12.2.1 Lens aperture

All cameras have a lens system containing three elements, including the most
fundamental; a diaphragm - called the aperture - that has a certain diameter,
which can be adjusted by the aperture control. The aperture control is
calibrated to integrate with the ISO numbering system although the units are
different.

Aperture controls are calibrated with f-numbers and the size of the aperture is
called the f-stop. In this system f stands for factor because that is what it is.
(Lens focal length ratio factor)

focal length
f
aperture diameter

Altering the aperture control by one stop will either double or halve the quantity
of light entering the camera. The chart in Figure 12.2 demonstrates this.

f stop 2.8 4 5.6 8 11 16 22

Light units 32 16 8 4 2 1 ½

Figure 12.2 Relationship between f stops and metered light.

The light units are purely arbitrary and are only used to illustrate the point

The aperture control steps are called stops because each step down stops a
metered amount of light from entering the camera.

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12.2.2 Shutter speed
The second lens control to be considered is the shutter speed control. The
shutter can be thought of as a curtain between the recording medium eg CCD
or film and the lens and hence the light. This is more straightforward than the
aperture control as it is quite obvious that if the speed, measured in seconds
and parts of seconds, of opening of the shutter is either halved or doubled the
quantity of light entering the camera will also be halved or double. The chart in
Figure 12.3 shows a standard set of shutter speeds.

Seconds 1 1/2 1/4 1/8 1/16 1/30 1/60 1/125 1/250

Light units 32 16 8 4 2 1 1/2 1/4 1/8

Figure 12.3 Shutter speeds.

12.2.3 The relationship between aperture and shutter speed

There is a fixed relationship between film speeds; apertures and shutter speeds
because they are all calibrated to the ISO film speed even though different units
are used.

Changing the lens settings by one stop will either halve or double the quantity
of light getting into the camera and changing the film speed by one does
exactly the same. The chart in Figure 12.4 shows the relationship between
aperture and shutter speed.

Figure 12.4 Relationship between aperture and shutter speed.

In the chart, the example shows that if f8 at 1/16th second allows the correct
amount of light (1 unit) to enter the camera, then f5.6 at 1/30th second and
f11 at 1/8th second would also allow the same amount of light in.

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12.3 How digital cameras compare with conventional cameras
The difference between a digital and a conventional camera is the digital
camera does not contain film. The light entering the camera does not fall on
light sensitive silver halides but instead onto a light sensitive computer chip.

This chip may be either a CMOS or a CCD sensor; both will have numerous
closely packed pixels as mentioned previously. The pixels react to the amount
of light falling on it in the same way as the grains of silver halide react in
conventional film.

The difference is that the reaction produces a varying electrical charge that is
transmitted to the built in microprocessor that then transforms this signal into
digital bits, which are stored onto the memory card. Of course, this circuitry is a
permanent part of the camera and cannot be changed, unlike traditional film
that can be taken out to put in a faster or slower film to compensate for
changing light conditions.

Camera manufacturers understand that it may be necessary to alter the light

sensitivity for different circumstances and provide an option to change the
equivalent ISO number of the pixels. This option may be either an option on the
camera’s computer menu or a physical control and, as previously mentioned,
can be altered frame by frame if circumstances dictate. Either way the camera’s
sensitivity to light, how quickly it reacts, is actually adjusted electronically, but
the ISO number system is maintained because it is so universally understood.

12.4 Bracketing – getting the exposure right

In most circumstances digital cameras will be self-selecting for exposure,
automatically selecting a correct exposure value. It may be that occasionally a
difficult lighting situation presents itself and the camera selection does not give
an acceptable result. In this case the camera can be made to bracket the
exposure. This entails stopping up and down by one stop on the aperture or
shutter speed.

Aperture 2.8 4 5.6 8 11 16 22

Shot 2 should be Shot 3 should be

slightly overexposed slightly underexposed

Shutter speed 1/8 1/16 1/30 1/60 1/125 1/250 1/500

Figure 12.5 Bracketing the exposure.

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12.5 Focusing
The final control on a lens system is the focus control. In digital cameras
focusing is automatic with a manual override facility. If the digital camera is a
single lens reflex (SLR) type, manual focus is straightforward as the image is
viewed through the lens and it can be seen in the viewfinder when sharp focus
is obtained.

If the subject is being viewed on the camera’s mini-screen, it may be difficult to

actually judge when sharp focus is obtained. In the underwater environment
setting the focus control to a pre-determined distance and then using either a
measured prod or string and magnet to actually measure the camera standoff
can overcome this.

1/3rd Visibility

Graduated rod or measured cord with magnet

As a general rule-of-thumb, underwater never take

photographs at a standoff greater than 1/3rd visibility

Figure 12.6 Camera stand-off distance.

There are other factors that must be considered when focusing the camera:

 Focal length of the lens.

 Depth of field.
 Framing the image or subject in the centre of shot.
 If actually measuring the physical distance using one of the methods above,
allowance must be made for refraction (an underwater metre is longer than
one in air! If setting the focus on the camera to 1m the measuring rod or
string must be 1.3m long).

All of these factors can affect whether the final recorded image is of acceptable
quality.

12.6 The lens focal length

The focal length of the lens determines the field of view. The actual focal length
of the lens is the distance from the film plane in the camera to the optical
centre of the lens when focused on infinity (∞). The larger this measurement,
the narrower the field of view and the greater the magnification. Long lenses
are telephoto, while short lenses are wide angle (optical element close to the
film plane).

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 Figure 12.7 Focal Length.

Underwater, where it is necessary to get close to the subject because light

levels are always low and contrast is always poor due to the environment, wide-
angle lenses are normally used, for example, a so-called fisheye lens is 15mm.

Lens focal length

28mm 50mm 135mm

Film plane
73° 45° 20°

Angle of view

Figure 12.8 Focal length/field of view.

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12.7 Depth of field
On a photograph, the depth of field is the amount of foreground in front of a
subject and the depth of background behind the subject that appears to be in
sharp focus.

Figure 12.9 Depth of field.

It may be desirable to alter the depth of field to achieve different photographic

results. For instance, the seahorse in figure 12.10 falls within the depth of field
shown and so will be in focus. If however, we wanted to include the starfish in
the foreground and have that in focus also, then we need to increase the depth
of field.

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 Lens Focus point

Film plane or
image
sensor Depth of field

Figure 12.10 Depth of field.

Imagine the shaded area in figure 12.11, which represents the depth of field, is
a tube of toothpaste, if we squeeze the lines together the paste would squash
up the tube, thereby increasing the width and this would also be the result in
our photograph example and so would increase the depth of field.

Figure 12.11 Increasing the depth of field.

So, how do we squeeze the lines together? We can do this in three different
ways:

Firstly, we could simply move the camera further away from the subject as
shown in figure 12.12. This has the effect of squeezing the lines together.
However, this is not the best solution for underwater photography, the visibility
is usually poor and it is therefore, better to be closer to the subject.

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 Figure 12.12 Increasing camera to subject distance.

Secondly, we could shorten the focal length as shown in figure 12.13. This too
squeezes the lines together and increases the DoF and, as mentioned, a short
focal length is desirable for underwater use as it allows us to get close without
losing our field of view.

Figure 12.13 Altering the focal length.

Finally, we can control the depth of field by changing our f-stop setting as
shown in figure 12.14. As normally we would have a short focal length lens
fitted then this would be the preferred method of further increasing the DoF
underwater.

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 f 2.8 Large

f 22 Small

Figure 12.14 Large f-stop number = small aperture = wide DoF.

12.8 Framing the subject

At first glance this would appear to be the easiest part of using the camera,
having set all the controls, just point in the right direction and shoot.
Unfortunately, this is not the case unless extreme close-up photography is
considered, where special close-up lenses or dioptres are used in conjunction
with specially designed close-up frames. In all other cases, it is the
photographers own responsibility to ensure that the actual required subject is in
the centre of the photograph.

Normally, with digital cameras, the subject is framed in the mini-screen built
into the camera. When this is the case, there is no problem with framing. But
underwater, the mini-screen may be difficult to see because of the effects of
refraction, reflection and parallax.

The effects of refraction and reflection are that the image on the mini-screen is
inside a housing in air. The image path from the air-filled housing to the outside
water must be both refracted and reflected, which will sometimes make it
impossible to see, especially when it is considered that the light path must then
go through another water to air interface to get to a diver’s eyes.

The effects of parallax are less obvious, but still may affect framing the subject.
The problem exists when a diver is trying to see the mini-screen image at the
same time as observing the subject himself. In this situation his eyes are not in
the same plane as the lens producing the image on the mini-screen. This can be
compared to the situation when using a hat-mounted video camera. The
camera on the hat is on a different plane to the diver’s eyes and neither is
seeing the same image.

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 Diver’s eye line
compared with the
camera view line

Figure 12.15 Parallax error.

12.9 Light and underwater photography

As photography is all about recording light and, in our case for underwater use,
it is necessary to examine how light is affected by water. As light entering the
water passes through an air/water interface the light wave will be subjected to
the Laws of Light and some will be reflected while the remainder is refracted as
it passes through the interface. As the light wave penetrates the water it is
subjected to absorption (attenuation), which is the dissipation of the light wave
energy as it passes through the water.

12.9.1 Colour absorption

Looking at the loss of colour firstly, it will be seen that in open sea conditions
the red light is absorbed first; followed by orange with indigo and violet
penetrating much deeper (Figure 12.16).

Colour Absorption
Water
Depth in
metres

10

15

20

25

30

35

Figure 12.16 Colour absorption in sea water.

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12.9.2 Loss of light intensity
The loss of light intensity is due to:

Reflection
Due to reflection at the surface of the ocean, only those light rays meeting the
interface at or very close to the vertical will penetrate into the water. This
means that only a limited amount of the visible light ever penetrates into the
sea at all. If the surface of the sea is rough this will exacerbate the problem by
providing many more reflecting surfaces at numerous different angles of
incidence (Figure 12.17).

Figure 12.17 Loss of light intensity with increasing water depth.

Attenuation
The light that does penetrate the interface will be attenuated as it penetrates
deeper into the water. The rate of attenuation increases with increasing water
depth (Figure 12.17).

Scatter
As the light passes through the water it meets suspended particles in the water
and is scattered and reflected by these particles (Figure 12.17).

These effects on the passage of light in water will also occur horizontally, which
is where they will impact most strongly on the underwater inspector trying to
record images for an inspection report.

The effects, outlined here, can largely be overcome by introducing artificial light
into the scene, normally in the form of electronic strobes for photography and
floodlights for video.

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12.10 Artificial light for underwater photography
The most common form of lighting used for underwater photography is the
electronic strobe flashlight. This is commonly referred to as the strobe light and
involves the use of LED’s.

12.10.1 Electronic strobe lighting

This form of flash photography uses an electronic capacitor discharge device to
produce an intense, short duration (in the order of 1/1000th sec) white light.
This type of flash recycles quickly and has a very long life, being capable of
taking tens of thousands of exposures.

Modern cameras of the type likely to be used for underwater inspection

photography will use dedicated or compatible strobes synchronised so that the
camera exposure value is automatically matched to the strobe output and
duration.

12.10.2 Strobe placement

The placement of the strobe relative to the subject is quite important if an
acceptable image is to be recorded. The preferred orientation is at as large an
angle to the centreline of the lens of the camera as possible and within a third
of the visibility distance to the subject (Figure 12.18).

Figure 12.18 Strobe placement to avoid backscatter.

12.11 Close-up weld mosaic photography

Diver inspectors will often be required to take close up photographs to record
the results of a CVI or an MPI. For this type of photography there are a number
of factors to bear in mind.

 Subject area will be quite small, so framing will be more critical.

 Depth of field will be minimal, so focus will also be more critical.
 There will be a minimum standoff distance determined by the camera lens.
 Some form of prods may be required to maintain the correct standoff.
 A scale will be required in each frame.
 If a weld is photographed, a mosaic of the entire weld may be required.

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Photography 12-15 Copyright © TWI Ltd
 Whereas it is usual practice to frame the stand-off photograph to encompass
the whole subject, it is common to frame the close-up shot to cover just
200mm in the long aspect (landscape) of the frame. This is so that the
photograph will show a level of detail sufficient to detect typical weld defects -
any further away than 200mm and such detail would be lost.

Note: The photographer should always check with the client’s procedure for the
task, as it will detail the specific requirements for framing the photographs.

In the case of a subject that is longer than 200mm (eg a weld) then a series of
overlapping photographs are taken to form a photo-mosaic. Again, the
photographer should refer to the client’s procedure for the task, but it is
common practice to overlap adjacent shots in the photo-mosaic by 30% to
50%. Eg For a series of photo-mosaic photographs framed for 200mm each, a
50% overlap would produce a series of pictures taken every 100mm along the
weld.

Whilst the subject should be in the middle-third of the frame, the tape must
also be visible, as should be any clock-markers or other markings relevant to
the inspection. To assist in framing close-up photographs, it may be beneficial
to fit stand-off prods to the front of the camera. Once set, the prods provide a
quick way of framing multiple shots with repeatable stand-off and frame size.

A final but important point to note when taking close-up photographs of welds
is that the joint-angle should be bisected – see below.

Figure 12.19 Bisecting the joint angle achieves even lighting and reduced
shadow.

Weld mosaic photographs require a minimum of 50% overlap for each frame.

Figure 12.20 6 Shot mosaic with 50% overlap.

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Photography 12-16 Copyright © TWI Ltd
 Figure 12.21 Close-up weld photograph showing identification tags.

An identification board containing specified information is normally required. A

typical example is shown in Figure 12.22.

Figure 12.22 Identification board on a spool-piece (North Sea).

12.12 Specific applications for offshore photography

Two specialised applications for underwater photography that are used
periodically in offshore inspection are; taking photographs to record indications
of defects identified during MPI and stereo-photography that is subsequently
analysed by computer programmemes, called photogrammetry.

DIS1-30815
Photography 12-17 Copyright © TWI Ltd
12.12.1 MPI photography
One of the drawbacks with MPI is that it lacks an intrinsic recording method. As
a result photography is sometimes used to record indications found using MPI
techniques. If the MPI has been performed using a daylight-ink then the
photograph may be taken using normal close-up technique.

If the MPI has been performed using UV-ink then a slightly different technique
must be employed – notably the photographer cannot use flash or strobe as the
indication would not be visible. The photograph must be illuminated using the
UV lamp only.

UV photography is best achieved by mounting the camera on a small tripod.

The camera flash and strobe unit should be turned off and the indication clearly
illuminated by the UV lamp. The camera may take an acceptable photograph of
the indication using an underwater macro mode setting, albeit with a long
exposure. If the camera over-exposes the indication then it may be necessary
to select a manual mode and stop-down the exposure by overriding the
automatic setting and selecting a smaller aperture (higher f-number).

Figure 12.23 MPI indications test photograph.

12.12.2 Stereo-photography and photogrammetry

The technique of taking measurements from a photograph is known as
photogrammetry. It is useful for assessing damage surveys, marine growth
surveys, anode wastage measurements, scour surveys and corrosion pitting
assessments, etc.

There are two methods that we could use for photogrammetric measurements.
Both assume that the photographs used are rectilinear ie that the images are
not significantly distorted by the lens. This is true so long as we do not use an
extremely wide-angle lens. In practice we can assume that the photograph is
rectilinear as long as we do not use a fisheye lens.

The Reseau plate comprises a set of graticules or cross-hairs overlaying the

photograph. The graticules enable the dimensions of features within the image
to be scaled with reference to the tape.

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Photography 12-18 Copyright © TWI Ltd
 Figure 12.24 An example of a Reseau plate photograph.

The other method that we can use involves taking two pictures of the subject
from slightly different positions and combining them using a computer
programme. This is called stereo photography. Figure 12.25 shows two cameras
set up for stereo photography.

Note: The calibration block.

The two cameras are mounted together, often with a set of prods to aid
framing. The shutter releases are synchronised so that both cameras take
pictures at the same moment. The picture files are then both downloaded to a
computer programme that combines them to form a digital model of the scene.

The computer may display the digital model as a pseudo-3-dimensional image

or as an anaglyph. A pseudo-3D (false-3D) image appears on the computer
display as if it were rendered by a CAD programme. The image may be rotated
and viewed from different angles in the same way that you would view a CAD
drawing. However, since the computer display screen is flat then the image is
also actually flat.

Figure 12.25 Stereo-cameras set to take photogrammetric pairs of photos.

DIS1-30815
Photography 12-19 Copyright © TWI Ltd
 The photogrammetric element of stereo photography may be achieved by
calibrating the computer programme with a linear scale in all three axes.
Modern computer programmes may be calibrated by mouse-clicking on known
lengths within the digital model and inputting the length-values.

This is most readily achieved by framing a calibration block of known

dimensions within the scene when the stereo photograph is taken. Modern
stereo photography programmes are relatively inexpensive and readily available
and this technique is likely to become more prevalent in the future.

12.13 Picture grabbers

Picture-grabbing is a technique whereby we can grab a still image from a video
signal. The video camera is simply held steady whilst the grab is made. In the
past it was the case that picture-grabs have not produced such high-quality
images as those made by stills cameras.

However, modern high-definition video systems are capable of producing

images of a quality that is perfectly acceptable for most applications. Indeed,
picture grabbers are replacing stills cameras for many inspection applications
and it would seem likely that they may completely replace the need for stills
cameras in the future.

12.14 Specific requirements for inspection photographs

When photographs record inspection findings, each frame must include:

 A scale.
 The subject in frame centre.
 Identification of the component.

In addition the following points should be addressed:

 Ensure there are no items hanging in the frame; umbilicals, hog-off lines
etc.
 Make sure there are no exhaust bubbles in shot.
 Avoid camera shake; be comfortable before taking the shot.
 Position lighting to avoid backscatter and reflection off the subject if shiny.
 Always be prepared to bracket the exposure.

With digital cameras, it may well be possible to take photographs using hat
lights or ROV vehicle lights, but remember that underwater photography is
always in a low contrast situation. This means that artificial light is even more
crucial to obtaining good, acceptable results than normal.

12.15 Recording photographs and care of equipment

It is most important that a photo-log be maintained during any photographic
survey and Figure 12.26 illustrates a typical example.

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Photography 12-20 Copyright © TWI Ltd
 Figure 12.26 Typical photo log sheet.

12.16 Procedure for close-up mosaic photography of a weld

Surface checks
 Check the appropriate permits have been obtained.
 Check camera and housing is set up properly and in accordance with the
client’s requirements.
 Take test shots.
 Ensure all idents with spares are available.
 The diver should ensure he has any rigging equipment which might be
needed while photographing the underside of the weld.
 Switch off the camera and strobe before deploying.
 Deploy the camera to the worksite.

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 In-water
 The diver should have confirmed he is on the correct weld already, but if
not, then now is the time to do it.
 Confirm that the weld has been adequately cleaned, typically to SA2½ a
minimum of 75mm either side of the weld; measured from the weld toes
and including the weld cap itself.
 Set up the tape and idents. It is usually best if the tape is one side of the
weld and the idents, ie o’clock and any others specified by the client, are
placed on the opposite side. Care should be taken to ensure that the HAZ is
not obscured. A distance of about 15-20mm from each weld toe will usually
suffice.
 Switch on the camera and strobe and check their settings and confirm them
with the surface controller.
 Wait until the strobe is charged and ready to fire, there will be an indicator
light fitted for this purpose.
 Tell the surface controller the start exposure number.
 The first photograph should span the 12 o’clock position ie the 12 o’clock
ident should be in the centre of the picture. Obviously, the client’s
requirements will dictate specifics.
 Each successive photograph should overlap the previous one by between
30-50%. Once again the client may have specific requirements but 30-50%
overlap is a typical working parameter.
 Tell the surface controller as each shot is taken and check each shot using
the inbuilt viewing screen. Continue taking shots after confirming each
photograph is in focus, overlap is correct and the weld, idents and tape are
all in the correct position.
 On completion tell the controller the end exposure number.
 De-rig the worksite if no other work is to be carried out.
 Switch off the camera and strobe.
 Send or bring the camera to the surface.

On surface
 Carefully wash off camera housing and strobe in freshwater.
 Dry thoroughly and remove the recording media from the camera.
 Download and check the photographs, make back-ups on a CD.
 Check all equipment is free of damage and serviceable.
 Put any batteries on charge, complete charging and equipment logs.
 Close down permits if required.

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Photography 12-22 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

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 27/08/2015

Photography

The word photography comes from the Greek:

 Photo = Light. Graphos = Drawing.

 Photography is a technique for recording a

CSWIP 3.1U Course scene in reality onto photographic paper, film
Photography or digital medium to produce an image. It can
trace its origin back to the late eighteenth
Section 12 century.

 Photography as a recording method for

engineering inspections has proved to be very
useful for the following reasons:

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Photography Photography

 Photography is an objective recording method,  Photographic images are typically high quality.
ie photography records an image of the The minimum requirement of six mega-pixels
subject that the camera is pointing at. It does for an underwater inspection photograph is
not make assumptions nor assign meanings to easily exceeded by using a modern digital
what it records. camera.

 Photography produces a permanent record.  We can use the optical properties of the
This is normally achieved by storing the camera system to magnify the subject.
photograph using a digital media file.
 Modern digital camera systems are relatively
inexpensive to purchase.

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Photography Photography

 There is a vast array of photographic  Unlike wet-film photography, digital

equipment available for underwater use. This photography does not require any time
gives us a great degree of adaptability. consuming chemical development process –
the image is immediately available and may
 Accurate measurements may be taken from be reviewed prior to moving away from the
photographs. This is known as inspection site.
photogrammetry and may be done with a
single photograph or by taking two  Digital post processing is available using
photographs known as stereo photography. readily available computer software.

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Photography Photography

Unfortunately, digital photography does have  Underwater photography can be time-

some disadvantages: consuming if compared to videography.
However, with a high level of skill, the
 It is a fallacy to believe that because modern photographer can progress a photo inspection
digital cameras are automatic then good fairly rapidly and has the ability to review each
photographs are simply a case of pointing the photograph before leaving the inspection site.
camera in the right direction and pressing the
trigger. This is not true. The photographer must
have an understanding of the techniques used
to frame and focus the image and – most
importantly – control the lighting of the image.
Photography is a skill that has to be learned.

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Light and Photography Light and Photography

 Although digital cameras record the light image  Digital cameras, react to light electronically
electronically, a brief look at the way traditional film and the image is recorded immediately the
records light will assist greatly in gaining a better exposure is taken however, the image will be
understanding of how the camera actually captures good or bad, depending on how the shutter
the image. speed and aperture controls are selected by
 Traditional film is made of celluloid that has an the circuitry. In this regard the digital camera
emulsion coating containing silver halide salts, is the same as the traditional camera, the
which react when exposed to light. When the film is quantity and quality of light falling onto the
processed, the silver halides are converted recording medium must be correctly
chemically to dyes to produce either colour controlled. In other words the exposure must
negatives or colour positives, (slides). The slides be correct.
can be viewed, once processed while the negatives
must be further processed to produce colour prints.
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Light and Photography Light and Photography

 The basis for monitoring and controlling the  Traditionally, there were two numbering systems; one in
amount of light entering the traditional the US called the ASA system (American Standards
Association), the other was in Europe called the DIN
camera was the film speed. The film system (Deutsches Institut fŭr Normung). Both were an
manufacturers grading the silver halides used increasing number series where the higher the number
on the film stock determined this, the finer the the faster the film reaction to light. Thirty years ago the
grains the slower the film eg more light was International Organisation for Standardisation (ISO)
needed, or the larger the grains, the faster the combined the two systems to one international standard.
film eg less light was needed. The way that
the film manufacturers marked the film speed
was by evolving a numbering system that
indicated the film speed by increasing
numbers.

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Light and Photography Light and Photography

 The chart groups the films into slow, medium This relationship is known as being a one stop
and fast. Also, it is obvious that as the ISO difference, which will be fully explained when
number increases the film speed; its speed of considering the lens system of a camera. It is
reaction to light also increases. There is worth noting that the ISO setting can be altered
another, more fundamental but less obvious frame by frame with most digital cameras if
relationship to film speed and light. required. With film cameras the film stock
dictates the setting and once loaded cannot be
 Taking 100 ASA as an example; film of this changed without changing the film.
rating will react twice as quickly to a given
quantity of light than 50 ASA but only half as
quick as 200 ASA.

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The Camera The Camera

 In its simplest form, the camera is no more

than a lightproof box with a hole in it; the box  Thus the ISO number for a film has a tangible
containing the film or charged coupled device and fundamental impact on the conventional
(CCD). Indeed that is all the original cameras camera. Modern digital cameras only differ
were. However, modern cameras have evolved from conventional cameras in the recording
and, as the film stock became standardised, medium and therefore they have either the
so cameras developed standard calibrated same or comparable controls.
controls to meter the amount of light entering
to expose the film.

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The Camera The Camera

Lens aperture  Altering the aperture control by one stop will either
All cameras have a lens system containing three double or halve the quantity of light entering the
elements, including the most fundamental; a camera.
diaphragm - called the aperture - that has a certain
diameter, which can be adjusted by the aperture
control. The aperture control is calibrated to integrate
with the ISO numbering system although the units “f” Stop 2.8 4 5.6 8 11 16 22
are different. Light units 32 16 8 4 2 1 ½

Aperture controls are calibrated with f-numbers and  The light units are purely arbitrary and are only
the size of the aperture is called the f-stop. In this used to illustrate the point
system f stands for factor because that is what it is.  The aperture control steps are called stops because
(Lens focal length ratio factor). each step down stops a metered amount of light
from entering the camera.
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The Camera The Camera

Shutter speed  There is a fixed relationship between film speeds;

The second lens control to be considered is the shutter speed apertures and shutter speeds because they are all
control. The shutter can be thought of as a kind of curtain calibrated to the ISO film speed even though different
between the recording medium eg CCD or film and the lens units are used. Changing the lens settings by one stop
and hence the light. This is more straightforward than the will either halve or double the quantity of light getting
aperture control as it is quite obvious that if the speed, into the camera and changing the film speed by one
measured in seconds and parts of seconds, of opening of the does exactly the same.
shutter is either halved or doubled, the quantity of light
entering the camera will also be halved or double.

Seconds 1 1/2 1/4 1/8 1/16 1/30 1/60 1/125 1/250

Light units 32 16 8 4 2 1 1/2 1/4 1/8

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The Camera The Camera

Camera manufacturers understand that it may Bracketing – Getting the exposure right.
be necessary to alter the light sensitivity for  In most circumstances digital cameras will be self-
selecting for exposure, automatically selecting a correct
different circumstances and provide an option to
exposure value. It may be that occasionally a difficult
change the equivalent ISO number of the pixels. lighting situation presents itself and the camera
This option may be either an option on the selection does not give an acceptable result. In this case
camera’s computer menu or a physical control the camera can be made to bracket the exposure. This
entails stopping up and down by one stop on the
and, as previously mentioned, can be altered
aperture or shutter speed.
frame by frame if circumstances dictate.
Aperture 2.8 4 5.6 8 11 16 22
Either way the camera’s sensitivity to light, how
quickly it reacts, is actually adjusted Shot 2 should be Shot 3 should be
electronically, but the ISO number system is slightly over-exposed slightly under-exposed

maintained because it is so universally Shutter speed 1/8 1/16 1/30 1/60 1/125 1/250 1/500
understood.
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The Camera The Camera

Focusing the camera There are other factors that must be considered when
In digital cameras focusing is automatic with a manual focusing the camera:
override option. Underwater it may be difficult to see
whether the focus is correct. Setting the focus control to a  The focal length of the lens.
pre-determined distance and then using a measured prod  The depth of field.
to actually measure the standoff distance can overcome
 Framing the subject in the centre of the shot.
this.
 If actually measuring the distance the physical
distance using a prod, allowance must be made for
refraction (an underwater metre is longer than one
in air! If setting the focus on the camera to 1m the
measuring rod must be 1.3m long).
As a general rule-of-thumb, underwater never
take photographs at a standoff greater than
 All of these factors can affect the quality of the final
1/3rd visibility distance from the subject recorded image.

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The Lens Focal Length Depth of Field

 Focal length is the distance between the optical  Depth of field - is the distance in front of
centre of the lens when set to infinity (∞) and the (foreground) and behind (background) the
film plane. point of focus (the subject), which appears in
focus.
28mm 50mm 135mm Subject

Focus distance

Film plane o o o
73 45 20

Film plane/Image sensor Depth of field

 Field of view is determined by the lens focal length

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Depth of Field Depth of Field

Depth Distance to subject

Field
Focus
point

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Depth of Field Depth of Field

Focal length Aperture or f-stop

f 2.8

f 22

D.o.F
Summary

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Framing The Subject Framing The Subject

 At first glance this would appear to be the  Normally, with digital cameras, the subject is
easiest part of using the camera, having set all framed in the mini-screen built into the
the controls, just point in the right direction camera. When this is the case, there is no
and shoot. Unfortunately, this is not the case problem with framing. But underwater, the
unless extreme close-up photography is mini-screen may be difficult to see because of
considered, where special close-up lenses or the effects of refraction, reflection and
dioptres are used in conjunction with specially parallax.
designed close-up frames. In all other cases, it
is the photographers own responsibility to
ensure that the actual required subject is in
the centre of the photograph.

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Framing The Subject Framing The Subject

 The effects of refraction and reflection are that  The effects of parallax are less obvious, but
the image on the mini-screen is inside a still may affect framing the subject. The
housing in air. The image path from the air- problem exists when a diver is trying to see
filled housing to the outside water must be the mini-screen image at the same time as
both refracted and reflected, which will observing the subject himself. In this situation
sometimes make it impossible to see, his eyes are not in the same plane as the lens
especially when it is considered that the light producing the image on the mini-screen. This
path must then go through another water to can be compared to the situation when using a
air interface to get to a diver’s eyes. hat-mounted video camera. The camera on
the hat is on a different plane to the diver’s
eyes and neither is seeing the same image.

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Framing The Subject Photography

Use the built in mini-screen as far as possible Colour absorption

to avoid parallax. Water
D Depth
Parallax error also affects helmet mounted e 5 metres
CCTV cameras. p
t 10 metres
h 15 metres

s 20 metres
c
25 metres
a
l 30 metres
e
Diver’s eye-line 35 metres
compared to the
camera view Light is attenuated underwater with a resultant loss of colour
with depth

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Artificial Light for Underwater

Photography
Photography
Loss of light The most common form of lighting used for underwater
intensity due to: photography is electronic strobe flashlight; commonly this is
referred to as a strobe light. Now, LED’s are very common.
Reflection.
Attenuation. Electronic strobe lighting
Scatter. This form of flash photography uses an electronic capacitor
discharge device to produce an intense, short duration (in
the order of 1/1000th sec) white light. This type of flash
recycles quickly and has a very long life, being capable of
taking tens of thousands of exposures.

Modern cameras of the type likely to be used for underwater

inspection photography will use dedicated or compatible
strobes synchronised so that the camera exposure value is
automatically matched to the strobe output and duration.

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Artificial Light for Underwater

Backscatter
Photography
Light placement
 The placement of the strobe relative to the subject is quite
important if an acceptable image is to be recorded. The
preferred orientation is at as large an angle to the
centreline of the lens of the camera as possible and within
a third the visibility distance to the subject.

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Cleaning Standard Cleaning Standard

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7
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Photography Photography

Close-up weld mosaic photography

Diver inspectors will often be required to take
close-up photos to record the results of CVI or MPI.
The following factors should be noted:
 Subject area is small so framing is critical.
 Depth of field is minimal so focus is critical.
 Lens determines the camera standoff.
 Standoff distance may require using prods.
 A scale is required in each frame.
 Overlap each frame so that the resulting photographs
can be stitched together after processing.
 Ensure a minimum of 30-50% overlap for each frame.

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Photography Photography

 Whilst the subject should be in the middle-third of the

frame, the tape must also be visible, as should be any
clock-markers or other markings relevant to the
inspection.
 To assist in framing close-up photographs, it may be
beneficial to fit stand-off prods to the front of the
camera.
 A final but important point to note when taking close-up
photographs of welds is that the joint-angle should be
bisected.

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Specific Photography Applications Specific Photography Applications

 Two specialised applications for underwater One of the drawbacks with MPI is that it lacks an
photography that are used periodically in intrinsic recording method. As a result photography
offshore inspection are; taking photographs to is sometimes used to record indications found using
record indications of defects identified during MPI techniques. If the MPI has been performed
MPI and stereo-photography that is using a daylight-ink then the photograph may be
subsequently analysed by computer taken using normal close-up technique.
programmes, called photogrammetry.
If the MPI has been performed using UV-ink then a
slightly different technique must be employed –
notably the photographer cannot use flash or strobe
as the indication would not be visible. The
photograph must be illuminated using the UV lamp
only.
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Specific Photography Applications MPI Photography

UV photography is best achieved by mounting the

camera on a small tripod. The camera flash and
strobe unit should be turned off and the indication
clearly illuminated by the UV lamp. The camera
may take an acceptable photograph of the
indication using an underwater macro mode
setting, albeit with a long exposure.

If the camera over-exposes the indication then it

may be necessary to select a manual mode and
stop-down the exposure by overriding the
automatic setting and selecting a smaller aperture Black light MPI photograph
(higher f-number).
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Stereo Photography Stereo Photography

Stereo photography and photogrammetry  There are two methods that we could use for
 The technique of taking measurements from a photogrammetric measurements. Both assume
photograph is known as photogrammetry. It is that the photographs used are rectilinear ie
useful for assessing damage surveys, marine that the images are not significantly distorted
growth surveys, anode wastage by the lens. This is true so long as we do not
measurements, scour surveys and corrosion use an extremely wide-angle lens. In practice
pitting assessments, etc. we can assume that the photograph is
rectilinear as long as we do not use a fisheye
lens.

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Stereo Photography Stereo Photography

 The Reseau plate comprises a set of graticules  The other method that we can use for
or cross-hairs overlaying the photograph. The photogrammetry involves taking two pictures
graticules enable the dimensions of features of the subject from slightly different positions
within the image to be scaled with reference and combining them using a computer
to the tape. programme. This is called stereo photography.

 The two cameras are mounted together, often

with a set of prods to aid framing. The shutter
releases are synchronised so that both
cameras take pictures at the same moment.
The picture files are then both downloaded to
a computer programme that combines them to
form a digital model of the scene.
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Stereo Photography Stereo Photography

 The computer may display the digital model as a pseudo-3-  The photogrammetric element of stereo
dimensional image or as an anaglyph. A pseudo-3D (false- photography may be achieved by calibrating
3D) image appears on the computer display as if it were
rendered by a CAD programme. the computer programme with a linear scale in
 The image may be rotated and viewed from different angles
all three axes.
in the same way that you would view a CAD drawing.
However, since the computer display screen is flat then the
 Modern computer programmes may be
image is also actually flat.
calibrated by mouse-clicking on known lengths
within the digital model and inputting the
length-values. This is most readily achieved by
framing a calibration block of known
dimensions within the scene when the stereo
photograph is taken.

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Stereo Photography Picture Grabbers

 Modern stereo photography programmes are  Picture-grabbing is a technique whereby we can grab
relatively inexpensive and readily available a still image from a video signal. The video camera is
and this technique is likely to become more simply held steady whilst the grab is made. In the
prevalent in the future. past it was the case that picture-grabs have not
produced such high-quality images as those made by
stills cameras.
 However, modern high-definition video systems are
capable of producing images of a quality that is
perfectly acceptable for most applications. Indeed,
picture grabbers are replacing stills cameras for
many inspection applications and it would seem
likely that they may completely replace the need for
stills cameras in the future.
Copyright © TWI Ltd Copyright © TWI Ltd

Requirements for Inspection

Photography
Photographs
Each photograph recording inspection findings must
include:
 A scale.
 The subject in frame centre. Photo log sheet
 Identification of the component.
In addition: It is most important
 Ensure there are no items hanging in the frame, umbilicals that a photo-log be
hog-off lines etc. maintained during
 Make sure there are no exhaust bubbles in shot. any photographic
 Avoid camera shake; be comfortable before taking the survey.
shot.
 Position lighting to avoid backscatter.
 Always be prepared to bracket the exposure.

Copyright © TWI Ltd Copyright © TWI Ltd

10
 27/08/2015

Care of Equipment Care of Equipment

Before use: After use:

 Charge batteries and fill out any charging log
sheets.  Wash off in fresh water.
 Dry thoroughly.
 Pre-set camera as required (f stop, shutter
 Download the images to the computer Back-up on a
speed and focus).
CD.
 Ensure a photo log sheet is ready for use.  Check for damage.
 Choose suitable lens for task to be  Recharge batteries as appropriate, complete
undertaken. charging logs.
 Set up data chamber (day, date etc).
 Inspect, lubricate and secure all seals.
 Take test shots of ID board with colour
reference etc and log them.
Copyright © TWI Ltd Copyright © TWI Ltd

Any Questions?

Copyright © TWI Ltd

11
 Section 13

The Use of Video in Offshore Inspection

13 The Use of Video in Offshore Inspection
13.1 Introduction
Video is used almost universally in diving for monitoring the diver and the task.
All video should be recorded as a matter of procedure; however, it does not
necessarily follow that just because a diver is equipped with a video camera
that the signal from the camera is actually being recorded. Sometimes it is
simply being monitored by the supervisor.

Since it is fundamental that inspections are recorded, we will not discuss non-
recorded video here. In the following section it may be safely assumed that the
terms video and videography refer to video data that is being recorded.

13.2 Advantages of video

The advantages of using video include the following:

 Real-time transmission of images from the diver to the surface.

 Useful for monitoring diver safety and wellbeing.
 If the ship has sufficient bandwidth available then it is possible to stream
the video signal using an internet connection to a remote location, eg to the
client’s headquarters.
 Instant playback is possible.
 Video generates a permanent record of the inspection.
 The video stream may be overlaid with telemetry data from sensors relating
to the inspection.
 Effectiveness and efficiency may be improved by topside specialists assisting
the diver-inspector with site identification, expert opinion and comments.
 Often the video is accompanied by an audio commentary given by either the
diver-inspector or the inspection controller.
 Remotely controlled cameras may be deployed in hazardous areas where it
may be imprudent to send a diver.
 High definition video is available which records excellent quality images.

13.3 Disadvantages of video

The disadvantages of using video include the following:

 Commonly, only a two-dimensional image is recorded. However, 3-D

camera systems (both single camera and twin camera) are becoming more
readily available and are likely to become more widely used for underwater
inspections in the future.
 Image quality is not as high as a contemporary digital stills camera.
However, modern camera and transmission systems may produce
acceptably good quality video and stills images that meet all the client’s
requirements.
 Analogue recording and copying of video data degrades image quality.
However, digital recording systems record and copy with a high degree of
fidelity.

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The Use of Video in Offshore Inspection 13-1 Copyright © TWI Ltd
13.4 Videography systems
The basic components of a video system are:

 Underwater video camera. There are different types of camera – see section
13.5.
 Underwater light. This may be separate or combined with the video camera,
eg a halo camera has a coaxial light around the periphery of the lens.
 Umbilical. Usually the light conductor and the video conductor are contained
within the same element of the diver’s umbilical.
 Power supply/video controller. This usually supplies the camera with a
constant voltage supply and a variable supply for the light.
 Video decoder/video recorder. The decoder will be part of the monitor, the
computer or the video recorder.
 Video monitor. This will usually be a computer monitor or a TV monitor.

Ancillary video equipment may include:

 Video typewriter/overlay writer.

 Picture-grabber and printer.

13.5 Video cameras

There are three types of camera commonly used for underwater applications:

13.5.1 Charged Coupled Device (CCD) Camera

The term charge coupled device refers to the type of sensor used to capture the
image. It is the same type as is commonly used in digital stills cameras. The
CCD is the most widely used type of video camera.

CCD cameras have the following features:

 Colour.
 Good resolution.
 Compact and lightweight.
 Solid state - ie they have no moving parts and are robust.
 Not damaged by bright lights or by magnetic fields.
 Excellent depth of field.

13.5.2 Silicon Intensified Target (SIT) Camera

The SIT camera is an image intensifier – ie a low-light camera. They are
typically used for ROV navigation and long range viewing. They are particularly
useful in turbid water when the intensity of the video lights may be turned
down in order to reduce backscatter.

SIT cameras have the following features:

 Monochrome – ie not colour.

 Lower resolution than CCD cameras. Therefore not used for detailed
inspection work.
 May be damaged by bright light and care must be taken to protect the
sensor.
 Generally larger and more bulky than CCD cameras.
 Often deployed by ROV.

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The Use of Video in Offshore Inspection 13-2 Copyright © TWI Ltd
13.5.3 Near-SIT CCD cameras
Although these CCD cameras do not have such good low-light performance as a
SIT camera, they do have very good low-light characteristics.

They have the following advantages over SIT cameras:

 Higher resolution than a SIT camera.

 Lighter and more compact than a SIT camera.
 More robust than a SIT camera.

13.6 Video transmission standards

The image is encoded by the camera to form the video signal. During this
process, the image is broken down into a number of lines, each containing a
number of pixels. The number of lines comprising the video image is a good
indicator of the picture quality – the more lines there are the higher the quality.

There are two basic types of video encoding; analogue and digital:

13.6.1 Analogue encoding standards

There are three standards for analogue video:

National television system committee (NTSC). This is an American

standard that encodes at 510 lines.

Phase alternating line (PAL). This is a European standard that encodes at

625 lines – ie a slightly higher quality than NTSC.

Sequential colour with (avec) memory (SECAM). This is a French standard

that encodes at 625 lines. Because of the difficulty of editing SECAM signals,
many users have migrated to either NTSC or PAL.

13.6.2 Digital encoding standards

High definition (HD) is defined as any video equipment that enables greater
than 625 lines to be encoded. At the time of writing, the highest quality system
in common use encodes at 1080 lines.

The HD encoding standards are linked to either the NTSC or PAL formats. The
fundamental difference between the two is the frame rate.

Another variation with digital systems is in the way that the image is built up on
the monitor screen. It may be refreshed by alternately renewing all the odd
lines and then all the even lines. This system is called interlacing.

The alternative system renews lines in order, starting with the first one and
finishing with line number 1080. This is called progressive scan and gives a
sharper image than an interlaced system.

It is critical that the decoding standard agrees with the encoding standard of
the camera.

DIS1-30815
The Use of Video in Offshore Inspection 13-3 Copyright © TWI Ltd
13.7 Video recording and storage
If the video signal is recorded using an analogue system then the quality is
degraded. Video home system (VHS) records at 200 lines and super-VHS
encodes at 400 lines.

Digital recording maintains image quality and it is common to record the video
data directly onto a computer hard-disc drive or onto a digital versatile disc
(DVD).

Management of the digital video files is often achieved using video database
software.

A video log sheet is used to index comments against the video counter during
the inspection. It can then be used as an index for the video file.

The format of the video log sheet is provided by the client. An example is
shown in figure 13.1

Figure 13.1 An example video log sheet.

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The Use of Video in Offshore Inspection 13-4 Copyright © TWI Ltd
13.8 Ancillary video equipment
A video typewriter is used to create title pages on the video. These are
typically the header and footer on the video recordings.

A video overlay is used to create a transparency over the video. This is

typically used to create overlays of telemetry data from sensors relevant to the
inspection, eg depth, heading, CP value, etc.

A picture-grabber is a software tool used to grab a still image from the video
stream. Although the image quality is not as high as a contemporary digital
stills camera image, the picture is usually adequate for most client’s
requirements. Indeed, modern picture-grabbers are of sufficient quality that
they are beginning to negate the need for digital stills imaging.

A printer is often used to generate hard-copies of picture-grabbed images.

13.9 Deployment of underwater video

13.9.1 Diver helmet-mounted video
This is commonly referred to as a helmet-mounted television camera (HMTV).

HMTV is a standard method of deploying video for offshore divers, whether

engaged in inspection work or not.

Specific advantages of HMTV include:

 Constantly active – useful for monitoring diver safety and operational

effectiveness and efficiency.
 Shows the diver’s point-of-view – ie where the diver is looking.
 The diver-inspector can respond to topside camera commands.
 The diver-inspector has both hands free.
 The light and the camera are both fixed to the helmet and thus always
aligned.
 The diver-inspector or the inspection controller can give an oral
commentary.
 The diver has eyes on the task and so can give the best description of
features that are seen and can respond to questions from the inspection
controller.

Specific disadvantages of HMTV include:

The helmet may not be manipulated into small spaces.

Parallax error must be managed by the diver.

Parallax error arises from the offset between the diver’s line of sight and the
axis of the video camera. Because of the parallel axes, the fields of view of the
diver and the camera are not coincident. Only in the area where the fields
overlap do both the diver and the camera see the subject.

The closer the diver is to the subject, the greater the parallax error.

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The Use of Video in Offshore Inspection 13-5 Copyright © TWI Ltd
 Figure 13.2 Parallax error

13.9.2 Hand-held video

The use of hand-held video cameras is less common than HMTV.

The specific advantages of hand-held video cameras include:

 The camera may be manipulated into small spaces.

 The diver inspector can respond to topside camera commands.
 The diver-inspector or the inspection controller can give an oral
commentary.
 The diver has eyes on the task and so can give the best description of
features that are seen and can respond to questions from the inspection
controller.

The specific disadvantages of hand-held video cameras include:

 Not constantly active but is switched on only during inspection work.

 Parallax error.
 The light and the camera are no longer mounted on the same platform and
may not be aligned.
 The diver has only one hand free.

13.9.3 ROV-mounted video

ROV deployed video is universally used for inspection and survey work.

The specific advantages of ROV-mounted cameras include:

 Exceptional endurance. The vehicle is not limited by decompression or

fatigue.
 May be safely deployed in hazardous environments. For example high water
currents.
 The vehicle is usually equipped with powerful lights giving excellent
illumination.
 The vehicle may carry multiple cameras – the best can be chosen for any
given conditions.
 Multiple sensors may provide detailed telemetry. For example depth,
heading, CP, etc.

DIS1-30815
The Use of Video in Offshore Inspection 13-6 Copyright © TWI Ltd
 The specific disadvantages of ROV-mounted cameras include:

 Vehicle access may be restricted, eg by penetration into the structure or

debris.
 Poor stability when operating in shallow water in rough sea conditions.
 ROVs are less versatile than divers.
 No human eyes on the subject – the best view is limited by the quality of
the video system.

13.9.4 Fixed video mounting

The specific advantages of fixed video mounted cameras include:

 Constantly active.
 Safely deployed in hazardous environments.
 May have remote pan, tilt and zoom controls.
 Excellent stability.
 Ideal for continuous monitoring. For example Lay-barge stinger monitoring.

The specific disadvantages of fixed video mounted cameras include:

 Camera is limited to one specific task.

 Camera requires regular cleaning.
 No human eyes on the subject - the view is limited by the quality of the
video system.

13.10 Preparation for deployment of underwater video

The preparations for the dive will include:

 Inspection of the video equipment for damage.

 Lubricate any seals according to the manufacturer’s recommendations.
 Test any residual current devices or other safety features.
 Function test the video system.
 Centre the camera and light on their brackets.
 Prepare the video log sheets.
 Prepare the recorder.

13.11 Practical techniques for underwater video inspection

13.11.1 Standard camera commands
The diver-inspector must be able to respond to basic camera direction
commands:

Figure 13.3 Direction command: Pan left, pan right.

This means that the camera is to look left or right.

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The Use of Video in Offshore Inspection 13-7 Copyright © TWI Ltd
 Figure 13.4 Direction command: Tilt up, tilt down.

This means that the camera is to look up or down.

Figure 13.5 Direction command: Move left, move right, move forwards (in) and
move backwards (out).

This means that the camera is to bodily move in a given direction.

Figure 13.6 Direction command: Rotate clockwise, rotate anticlockwise.

This means that the camera is to rotate about its view-axis.

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The Use of Video in Offshore Inspection 13-8 Copyright © TWI Ltd
13.12 Video commentary
13.12.1 Fluency
The ability to give a fluent commentary is a skill that requires practice but there
are four basic points to keep in mind:

1 Keep a steady rhythm to your speech.

2 Keep to a reasonable speed. Most people tend to talk quickly when giving a
commentary, so concentrate on slowing yourself down.
3 Keep an even volume. The client will not want to continually adjust the
playback volume when listening to your commentary.
4 Keep a natural pitch to your voice. People naturally change the intonation of
their voice at the end of a sentence. This is done sub-consciously but serves
an important function – it lets the receiving person know that the packet of
information we have been communicating has come to an end. It can be
very confusing to listen to a person who does not change pitch in this way
at the end of a sentence.

The best way to achieve fluency when giving a commentary is to have in mind
what you are going to say before you start to say it!

13.12.2 Introduction
There are four elements to a video introduction:

1 Who? Who am I? Who is the contractor? Who is the client?

For example ‘This is diver-inspector Jon Smith. The contractor is JS Diving.
The client is TWI Oil...’.

2 What? What am I doing?

For example ‘Performing a general visual inspection...’.

3 Where? Where am I?
For example ‘On the Charlie-1 leg of North Cormorant platform between zero
and minus 20m elevation…’.

4 When? Time and date.

For example ‘The date is the 1st of January, 2013. The time is 16:30 hours’.

13.12.3 Termination
Do not forget to let the viewer know when the inspection has terminated.
For example ‘This concludes my inspection. The time is 17:30 hours’.

13.13 Video pointer

The diver-inspector uses a video pointer during inspections in order to provide a
scale and a colour reference for the viewer. It is also more precise when
pointing at features than using a finger.

An adequate video pointer may be fabricated from a rod wrapped with tape.
The pointer should have a series of gradations on it – typically every 10mm. It
should also have the primary colours near the tip.

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The Use of Video in Offshore Inspection 13-9 Copyright © TWI Ltd
 Figure 13.7 A typical video pointer.

Note: The tip should not be sharpened!

13.14 Post-inspection
Post inspection actions should include the following:

 Gently rinse the camera in fresh water. Be careful not to force water past
any seals.
 Clean the camera lens with lens tissue and fit the lens cap.
 Inspect the equipment for damage.
 Store the equipment in a dry, well ventilated place.
 Complete the video log.
 Finalise the DVD and label the disc.
 Back up data on hard drive.

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The Use of Video in Offshore Inspection 13-10 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990. ISBN 13:
9780419135401.

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The Use of Video in Offshore Inspection 13-11 Copyright © TWI Ltd
 27/08/2015

The Use of Video in Underwater

Inspection
 Video is used almost universally in diving for
monitoring the diver and the task. All video should
be recorded as a matter of procedure; however, it
does not necessarily follow that just because a
diver is equipped with a video camera that the
CSWIP 3.1U Course signal from the camera is actually being recorded.
The Use of Video in Offshore Inspection Sometimes it is simply being monitored by the
Section 13 supervisor.

Copyright © TWI Ltd Copyright © TWI Ltd

The Use of Video in Underwater

Advantages of Videography
Inspection
 Real-time transmission of images from the
 Since it is fundamental that inspections are diver to the surface.
recorded, we will not discuss non-recorded  Useful for monitoring diver safety and
video here. In the following chapter it may be wellbeing.
safely assumed that the terms video and  If sufficient bandwidth is available, it is
videography refer to video data that is being possible to stream the video, using the
recorded. internet, to a remote location, eg to the
client’s HQ.
 Instant playback is possible.
 Video generates a permanent record of the
inspection.

Copyright © TWI Ltd Copyright © TWI Ltd

Advantages of Videography Disadvantages of Videography

 The video stream may be overlaid with  Commonly, only a two-dimensional image is
telemetry data. (CP etc.) recorded. However, 3-D camera systems (both
 Efficiency may be improved by topside single and twin camera) are now readily
specialists assisting the diver with site available and are likely to become more widely
identification, expert opinion and comments. used in the future.
 Often the video is accompanied by an audio
commentary given by either the diver-  Image quality is not as high as a
inspector or the inspection controller. contemporary digital stills camera. However,
 Remotely controlled cameras may be deployed modern camera and transmission systems
in hazardous areas. may produce acceptably good quality video
 HD video is available which records excellent and stills images that meet all the client’s
quality images. requirements.

Copyright © TWI Ltd Copyright © TWI Ltd

1
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Disadvantages of Videography
Videography Systems

 Analogue recording and copying of video data The basic components of a video system are:
degrades image quality. However, digital  Underwater video camera. There are several
recording systems record and copy with a high types of camera.
degree of fidelity.  Underwater light. This may be separate or
combined with the camera.
 Umbilical. Usually the light conductor and the
video conductor are contained within the same
element of the diver’s umbilical.
 Power supply/video controller. This usually
supplies the camera with a constant voltage
supply and a variable supply for the light.

Copyright © TWI Ltd Copyright © TWI Ltd

Video Cameras
Videography Systems

The basic components of a video system are: There are three types of camera commonly used
underwater:
 Video decoder/video recorder. The decoder will be
part of the monitor, the computer or the video 1. Charged coupled device (CCD) camera
recorder.
The term charge coupled device refers to the
 Video monitor. This will usually be a computer type of sensor used to capture the image. It is
monitor or a TV monitor. the same type as is commonly used in digital
stills cameras. The CCD is the most widely used
Ancillary video equipment may include: type of video camera.

 Video typewriter/overlay writer.

 Picture-grabber and printer.
Copyright © TWI Ltd Copyright © TWI Ltd

Video Cameras Video Cameras

CCD cameras have the following features: Silicon intensified target (SIT) camera.
The SIT camera is an image intensifier – ie a low-light camera.
They are typically used for ROV navigation and long range
 Colour. viewing. They are particularly useful in turbid water when the
 Good resolution. intensity of the video lights may be turned down in order to
reduce backscatter.
 Compact and lightweight. SIT Cameras have the following features:
 Solid state - ie they have no moving parts and Monochrome – ie not colour.
are robust.  Lower resolution than CCD cameras. Therefore not used for
 Not damaged by bright lights or by magnetic detailed inspection work.
fields.  May be damaged by bright light and care must be taken to
protect the sensor.
 Excellent depth of field.
 Generally larger and more bulky than CCD cameras.
 Often deployed by ROV.

Copyright © TWI Ltd Copyright © TWI Ltd

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Video Cameras
Video Transmission Standards

3. Near-SIT CCD cameras  The image is encoded by the camera to form

Although these CCD cameras do not have quite the video signal. During this process, the
such good low-light performance as a SIT camera, image is broken down into a number of lines,
they do have very good low-light characteristics. each containing a number of pixels. The
number of lines comprising the video image is
a good indicator of the picture quality – the
They have the following advantages over SIT more lines there are then the higher the
cameras: quality.

 Higher resolution than a SIT camera.  There are two basic types of video encoding;
 Lighter and more compact than a SIT camera. analogue and digital.
 More robust than a SIT camera.

Copyright © TWI Ltd Copyright © TWI Ltd

Video Transmission Standards

Video Transmission Standards

1. Analogue encoding standards 2. Digital encoding standard

There are three standards for analogue video: High definition (HD) is defined as any video
equipment that enables greater than 625 lines to
National television system committee (NTSC). This is an be encoded. At present, the highest quality
American standard that encodes at 510 lines.
system in common use encodes at 1080 lines.
Phase alternating line (PAL). This is a European standard
that encodes at 625 lines – ie a slightly higher quality The HD encoding standards are linked to either
than NTSC. the NTSC or PAL formats. The difference
between the two is the frame rate.
Sequential colour with (avec) memory (SECAM). This is a
French standard that encodes at 625 lines. Because of the
difficulty of editing SECAM signals, many users have
migrated to either NTSC or PAL.

Copyright © TWI Ltd Copyright © TWI Ltd

Video Transmission Standards

Video Recording and Storage

Another variation with digital systems is in the way  If the video signal is recorded using an analogue system
that the image is built up on the monitor screen. It then the quality is degraded. Video home system (VHS)
may be refreshed by alternately renewing all the records at 200 lines and super-VHS encodes at 400
odd lines and then all the even lines. This system is lines.
called interlacing.
 Digital recording maintains image quality and it is
common to record the video data directly onto a
The alternative system renews lines in order, computer hard-disc drive or onto a (DVD).
starting with the first one and finishing with line  Management of the digital video files is often achieved
number 1080. This is called progressive scan and using video database software.
gives a sharper image than an interlaced system.
 A video log sheet is used to index comments against the
video counter during the inspection. It can then be used
It is critical that the decoding agrees with the as an index for the video file.
encoding of the camera.  The format of the video log sheet is provided by the
client.

Copyright © TWI Ltd Copyright © TWI Ltd

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Video Log Sheet Ancillary Video Equipment

 A video typewriter is used to create title pages

on the video. These are, typically, the header
and footer on the video recordings.

 A video overlay is used to create a

transparency over the video. This is used to
create overlays of telemetry data from sensors
relevant to the inspection, eg depth, heading,
CP value, etc.

Copyright © TWI Ltd Copyright © TWI Ltd

Ancillary Video Equipment Deployment of Underwater Video

 A picture-grabber is a software tool used to Diver helmet-mounted video (HMTV)

grab a still image from the video stream. HMTV is a standard method of deploying video
Although the image quality is not as high as a for offshore divers, whether engaged in
contemporary digital stills camera image, the inspection work or not.
picture is usually adequate for most client’s Specific advantages of HMTV include:
requirements. Indeed, modern picture-
grabbers are of sufficient quality that they are Constantly active – for monitoring diver safety
beginning to negate the need for digital stills and operational efficiency.
imaging.  Shows the diver’s point-of-view – ie where the
diver is looking.
 A printer is often used to generate hard-copies  The diver can respond to topside camera
of picture-grabbed images. commands.
 The diver has both hands free.
Copyright © TWI Ltd Copyright © TWI Ltd

Deployment of Underwater Video Deployment of Underwater Video

 The light and camera are both fixed to the Diver helmet-mounted video (HMTV)
helmet and so always aligned. Specific disadvantages of HMTV include:
 The diver or the inspection controller can give  The helmet may not be manipulated into small
an oral commentary. spaces.
 The diver has eyes on the task and so can give  Parallax error must be managed by the diver.
the best description of features that are seen  Parallax error arises from the offset between the
and can respond to questions from topside. diver’s line of sight and the axis of the video camera.
 Because of the parallel axes, the fields of view of the
diver and the camera are not coincident. Only in the
area where the fields overlap do both the diver and
the camera see the subject.
 The closer the diver is to the subject, the greater the
parallax error.
Copyright © TWI Ltd Copyright © TWI Ltd

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Helmet-Mounted Video Hand-Held Video

The specific advantages of hand-held video

cameras include:
 The camera may be manipulated into small
spaces.
 The diver inspector can respond to topside
camera commands.
 The diver or the inspection controller can give
an oral commentary.
 The diver has eyes on the task and so can give
the best description of features that are seen
Parallax error
and can respond to questions from the
inspection controller.

Copyright © TWI Ltd Copyright © TWI Ltd

Hand-Held Video ROV-Mounted Video

The specific disadvantages of hand-held video The specific advantages of ROV-mounted

cameras include: cameras include:
 High endurance. The vehicle is not limited by
 Not constantly active, is switched on only decompression or fatigue.
during inspection work.  May be safely deployed in hazardous
 Parallax error. environments.
 The light and the camera are no longer  Equipped with powerful lights giving excellent
mounted on the same platform and may not illumination.
be aligned.  Carries multiple cameras – the best can be
 The diver has only one hand free. chosen for any conditions.
 Multiple sensors may provide detailed
telemetry eg depth, CP, etc.

Copyright © TWI Ltd Copyright © TWI Ltd

ROV-Mounted Video Fixed Video Mounting

The specific disadvantages of ROV-mounted The specific advantages of fixed video mounted
cameras include: cameras include:

 Access may be restricted, eg by structure  Constantly active.

geometry or debris.  Safely deployed in hazardous environments.
 Poor stability when operating in shallow water  May have remote pan, tilt and zoom controls.
in rough conditions.  Excellent stability.
 ROVs are less versatile than divers.  Ideal for continuous monitoring eg lay-barge
 No human eyes on the subject – the view is stinger monitoring.
limited by the quality of the video system.

Copyright © TWI Ltd Copyright © TWI Ltd

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Fixed Video Mounting Preparation for Deployment

The specific disadvantages of fixed video The preparations for the dive will include:
mounted cameras include:
 Inspection of the video equipment for damage.
 Camera is limited to one specific task.  Lubricate any seals according to the
 Camera requires regular cleaning. manufacturer’s recommendations.
 No human eyes on the subject - the view is  Test any residual current devices or other
limited by the quality of the video system. safety features.
 Function test the video system.
 Centre the camera and light on their brackets.
 Prepare the video log sheets.
 Prepare the recorder.

Copyright © TWI Ltd Copyright © TWI Ltd

Practical Techniques Video Commentary

 Standard camera commands Fluency:

The ability to give a fluent commentary is a skill that
requires practice but there are four basic points to keep in
mind:
1. Keep a steady rhythm to your speech.
Pan left or Move left or
right right, in or 2. Keep to a reasonable speed. Most people tend to talk quickly
out when giving a commentary, so concentrate on slowing
yourself down.
3. Keep an even volume. The client will not want to continually
adjust the playback volume when listening to your
commentary.
Tilt up or down Rotate 4. Keep a natural pitch to your voice.
clockwise or The best way to achieve fluency when giving a commentary is to
anticlockwise
have in mind what you are going to say before you start to say it!

Copyright © TWI Ltd Copyright © TWI Ltd

Video Commentary Introduction Video Commentary Introduction

There are four elements to a video introduction:  When? Time and date.
Who? Who am I? Who is the contractor? Who is  Eg ‘The date is the 1st of January, 2013. The
the client? time is 16:30 hours’.
 Eg ‘This is diver-inspector Jon Smith. The
contractor is JS Diving. The client is TWI  Remember to let the viewer know the
Oil...’. inspection has ended.
 What? What am I doing?
 Eg ‘Performing a general visual inspection...’.
 Where? Where am I?
 Eg ‘On the Charlie-1 leg of North Cormorant
platform between zero and minus 20m
elevation…’.
Copyright © TWI Ltd Copyright © TWI Ltd

6
 27/08/2015

Video Pointer Post-Inspection

 The diver-inspector uses a video pointer during Post inspection actions should include the
inspections in order to provide a scale and a colour following:
reference for the viewer. It is also more precise when
pointing at features than using a finger.  Gently rinse the camera in fresh water. Be
careful not to force water past any seals.
 An adequate video pointer may be fabricated from a rod  Clean the camera lens with lens tissue and fit
wrapped with tape. The pointer should have a series of the lens cap.
gradations on it – typically every 10mm. It should also
have the primary colours near the tip.  Inspect the equipment for damage.
 Store the equipment in a dry, well ventilated
place.
 Complete the video log.
 Finalise the DVD and label the disc.
The tip should not be sharpened!
 Back up data on hard drive.
Copyright © TWI Ltd Copyright © TWI Ltd

Any Questions?

Copyright © TWI Ltd

7
Section 14

Ultrasonics
14 Ultrasonics
14.1 Physics of Ultrasound
Sound is made when something vibrates. You can twang a ruler on a table or
flick a stretched elastic band to verify this. The stretched surface of the rubber
band or the ruler vibrates and sets up a series of vibrations, sound waves, in
the air.

As the surface of the band or ruler pushes into the air, the air molecules are
forced together and a region of high pressure forms; compression. As the
surface moves back, the air molecules move apart, forming a low pressure area
or rarefaction. As the surface vibrates, alternate compressions and rarefactions
are set up in the air and travel out from the surface to form a sound wave.

The air molecules don’t move with the wave - they vibrate to and fro in time
with the vibrating surface. If we plot the displacement of the particle against
time it will produce a sine wave as shown below.

Wavelength
Figure 14.1 Sine Wave showing amplitude and wavelength.

The sound wave so produced travels through the air at a speed of about
332m/sec, at 0°C, at sea level. We hear the sound when it hits a membrane in
our ear and cause it to vibrate.

Sound will travel through any medium that has molecules to move, but it
travels faster in more elastic materials because the vibrations are passed on
more quickly. Sound travels faster in water or metal than it does in air, as
liquids and solids are more elastic than air.

The speed of sound increases with the stiffness, (elasticity) and decreases with
density, in fact, it’s actually the square root of the stiffness divided by the
density.

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14.2 Frequency
As sound is a series of vibrations, one way of measuring it is to count the
number of vibrations per second - the frequency. Frequency is measured in
Hertz (Hz). One vibration in one second is one Hertz. Two vibrations in one
second is two Hertz. Ten vibrations in one second is 10 Hertz and 1000
vibrations in one second is 1000 Hertz or one kilohertz (kHz). One million
vibrations in a second is one Megahertz (MHz).

The higher the frequency - the higher the note sounds - the higher the pitch. If
you twang the ruler or the rubber band hard, the noise is louder, it has greater
amplitude, but the note remains the same. If, however, you shorten the ruler or
tighten the rubber band, they vibrate more quickly and the note given out is
higher, the frequency is greater. To raise the pitch of their instrument, guitar
players move their fingers down the frets, thus shortening the string and
making it vibrate more quickly.

We can only hear sounds between certain frequencies - more than 16Hz and
less than 20,000 Hz. If you were able to move your arm up and down 20 times
a second, it would sound like a very low hum. You cannot move your arm this
fast, so you cannot hear the vibrations in the air caused by your moving arm. A
dog whistle vibrating at 25,000 Hz cannot be heard by humans, but it can be
heard by the sensitive ears of a dog.

FREQUENCY SPECTRUM

Speed of sound in air = 332m/sec

Sound Waves
Frequency Electro-magnetic Waves
(Travelling at the speed of
(kHz) (Travelling at the speed of light)
sound)
Infrasound
10-3
Less than 16Hz
10-2
10-1
Human hearing range 1
(16 to 20,000Hz) 10 VLF
102 LF
Ultrasound – frequencies
above 20,000Hz up to 103 MF
Radio frequencies
10MHz 104 HF
106 UHF
107 SHF

Figure 14.2 The frequency spectrum.

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 It rarely occurs to us that there is a whole world of sound that we cannot hear.
Some animals can hear sounds at higher frequencies - bats can hear sound at
100,000 Hz - while others, like snakes, have worse hearing than we have.

A sound with frequencies above the upper range of human hearing is called
ultrasound. Sound below about 16Hz is called infrasound. Therefore the
definition of ultrasound is; sound with a frequency greater than 20kHz.

However, there is an advantage for the lower frequencies. The lower the
frequency, the more penetrating a sound wave is - that is why foghorns give
out very low notes and why the low throbbing notes from your neighbour’s
stereo set come through the wall rather than the high notes. Elephants and
hippos can communicate over distances of up to 30 kilometres using
infrasound, while whales can communicate through water across an ocean!

For most practical ultrasonic testing, the frequency range used is between 0.5-
6MHz; the lower frequencies between 0.5-1.5MHz are used for materials with
large grain structures, such as concrete or cast iron. Frequencies from 2-6MHz
are used for testing materials with fine grain structures including steel.

14.3 Velocity
So far, only the effects of the wave passing one point in the material have been
considered. However, the wave itself is passing along and through the material.
Like a surface wave on water, the water at any point goes up and down, but as
well as this the wave travels forward.

Ultrasonic waves travel through a material at the speed of sound for a given
type of wave for a given material. That is, the speed of sound is different for
different types of wave and the speed of travel is different in different
materials.

14.4 Types of ultrasonic waves

Sound waves propagate through a material (liquid, solid or gas) by causing the
atoms to oscillate as the wave front passes through it. There are two types of
wave that propagate through solid material and three types that travel along
the surface skin of the material.

The three types of surface wave have no application underwater and will not be
discussed further. The two types of wave that propagate through a solid are
discussed below.

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14.4.1 Longitudinal/Compression waves
This type of wave is denoted by the symbol L, with the addition of the letter V -
VL indicates the velocity of propagation of Longitudinal/Compression waves.
With this type of wave propagation, the direction of oscillation of the atoms is
the same as the wave propagation, see Figure 14.3.

Figure 14.3 Longitudinal/Compression wave.

14.4.2 Transverse/Shear waves

This type of wave is denoted by the symbol T, with the addition of the letter V –
VT indicates the velocity of propagation of these waves. With this type of wave
propagation, the direction of oscillation of the atoms is at 90° to the wave
propagation, see Figure 14.4.

Figure 14.4 Transverse/Shear wave.

Table 14.1 Properties of ultrasonic waves.

Type of wave Symbol Atomic motion Used for
Longitudinal VL Longitudinal Thickness and
or In the direction of the wave lamination
Compression propagation measurements

Transverse VT Transverse Defect sizing and

or At 90° to the wave weld inspection
Shear propagation

In order that ultrasonic sound waves can be used to measure depths and sizes
within any material, it is a fundamental principle that the velocity of the sound
wave remains constant for different samples of the same material. This is in
fact the case; and furthermore, the ultrasonic wave obeys the Laws of Light and
we can, therefore, predict its behaviour.

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 A summary of wave velocities of the various waves discussed here for a
selection of materials is shown in Table 14.2.

Table 14.2 Acoustic velocities in various materials.

Material Acoustic velocity (m/sec)
VL VT
Aluminium 6350 3100
Concrete 4600
Naval Brass 4430 2120
Copper 4660 2260
Air 332
Structural steel 5940 3250
Water 1490
Perspex 2730

Note. Blank spaces in VT column because Shear waves cannot be produced in a

liquid, gas or in polymers (such as concrete or Perspex).

14.5 Wavelength
A wave in the sea is a vibration of energy. As the wave passes a fixed point it
produces a constant rise and fall of energy. A complete vibration is a change in
energy from maximum to minimum and back to maximum.

A wavelength is the distance a stress wave moves forward during one complete
cycle. It varies with the speed of sound and with the frequency. We can work
out the wavelength if we know the speed and frequency of a sound wave.
Wavelength is the velocity in metres per second divided by the frequency.
Wavelength is represented by the Greek letter lambda ().

If we want to know the wavelength of a 2MHz compression wave travelling

through steel, we can again use the formula, as we know the compressional
speed of sound in steel, 5,940m/sec.

5,940,000
= = 2.97mm
2,000,000

If we want to know the wavelength of a shear wave of 2MHz in steel we can use
the formula again, but this time we use the shear speed of sound in steel which
is 3,250m/sec.

An easy way to remember how this formula works is to split it down within a
triangle - with the velocity, wavelength and frequency at the corners.

The velocity must be placed at the top (Note: how it forms a diamond shape)
and the wavelength and frequency at either of the bottom two corners.

λ 

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 If we want to work out wavelength we cover the wavelength symbol - this
leaves the V over . If we need to find the velocity, cover the V which gives us 
x . Covering the frequency () will leave V over .

V V
=
f
λ  f

V
V= xf

λ  f

V V
f=


λ  f

Figure 14.3 Velocity, frequency and wavelength relationships.

The wavelength of an ultrasonic wave is important because the shorter the

wavelength, the smaller the flaws that can be discovered. Defects of a diameter
of less than half a wavelength may not show on the CRT. On the other hand,
the shorter the wavelength the less the ultrasound will penetrate the test
material.

14.6 Further effects of ultrasonic properties in materials

As the ultrasonic signal passes through a material, a pressure or stress front
will be initiated in the material, which will present resistance to the passage of
the sound wave energy. The amount of resistance will depend on the properties
of the material. This is a useful parameter of a material and must be
determined if the pressure or stress magnitude of the ultrasonic wave is to be
determined.

14.7 Acoustic impedance (Z)

Acoustic impedance (Z) is a measure of the resistance a material presents to
sound waves travelling through it. It is a function of the density (ρ) of the
material and the velocity (v) of the sound wave. Acoustic impedance is the
material property that causes the ultrasound to travel at different speeds in
different materials.

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14.8 Attenuation
Acoustic attenuation is a phenomenon of decreasing sound pressure as a sound
wave travels through a material. It arises from absorption and scatter.

Absorption represents the transformation of ultrasonic energy into other types

of energy eg thermal.

Scatter is the random reflection of energy caused by grain structure and/or by

small discontinuities in the beam path. Scatter occurs as sound energy is
reflected from grains in the test material. In general, the larger the grains, the
more scatter occurs.

14.9 The direction of propagation of an ultrasonic wave

So far it has been established that the ultrasonic wave travels at a known speed
in a straight line and that it obeys the Laws of Light. In order to predict the
direction that the wave travels as it passes through an interface into a different
material, it is necessary to determine what happens when a wave meets an
interface.

An interface is any boundary between two materials of differing properties (eg

perspex/steel, water/air etc.). It will include the outside edges of a component
and indeed, the back surface is referred to as the back wall. Similarly, the
surfaces of a crack or porosity bubble are boundaries also.

At these interfaces, in accordance with the laws of light, the direction of travel
of the wave after meeting the interface will be determined by the Law of
Reflection and The Law of Refraction.

14.9.1 The Law of Reflection

This states that the angle the reflected wave makes with the normal angle to
the interface from which the wave is being reflected is the same as the angle
that the incident wave makes with the same normal angle.

Figure 14.4 Law of Reflection.

When the angle of incidence is 0°, the reflected angle is also 0°, so the wave is
reflected back along the incident direction. The wave is travelling in the same
material; therefore, there will be no change in wavelength of the signal or the
mode of travel of the wave. This is the ideal condition for thickness
measurements using ultrasonic compression waves.

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14.9.2 Law of Refraction (Snell’s Law)
At an interface, part of the ultrasonic wave is reflected and the rest will pass
into the second material. The path in the second material will still be a straight
line, but the direction of this wave will not be continuous with the direction of
the incident wave as it will have been turned through an angle that can be
determined by Snell’s Law.

Figure 14.5 Law of Refraction (Snell’s Law).

14.10 Generating ultrasound

Sound is created when something vibrates. It is a stress wave of mechanical
energy. The Piezo-electric effect changes mechanical energy into electrical
energy. It is reversible, so electrical energy - a voltage - can be changed into
mechanical energy or sound, which is the reverse Piezo-electric effect. The first
people to observe the Piezo-electric effect were the Curie brothers who
observed it in quartz crystals.

14.10.1 Piezo-electric crystals

Jaques and Pierre Curie used quartz for their first experiments. Nowadays
polarised ceramics are used instead of quartz crystals.

It was later discovered that by varying the thickness of crystals and subjecting
them to a voltage, they could be made to vibrate at different frequencies.
Frequency depends on the thickness of the Piezo-electric crystal.

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 Figure 14.6 The Piezo-electric effect.

In practice, the transducer is mounted in a probe assembly for protection and

to enable electrical connections to be made. The crystal is fitted with silver foil
electrodes to apply the voltage across the crystal if acting as a transmitter, or
to take the voltage signal from it if acting as a receiver. The crystal is attached
to the base by the mounting, which acts not only as a fixing, but also as a
backing to the crystal.

A pulse of ultrasound from a Piezo-electric crystal has a length or width of

several vibrations or wavelengths. When you strike a bell it continues to ring for
several seconds as the metal continues to vibrate. The vibrations get steadily
weaker and the sound dies away. If you put your hand on the bell you stop the
vibrations and the sound dies away more quickly - you dampen the sound.

A Piezo-electric crystal continues to vibrate after it is hit by an electrical charge.

This affects sensitivity, as the longer the pulse length, the worse the resolution.
In most probes a slug of tungsten loaded Araldite is placed behind the crystal to
cut down the ringing time.

14.11 Types of transducers (probes)

A transducer is any device that transforms energy from one form into another.

There are a number of different types of probes, some designed for specific
tasks. However, in our sphere of NDT we need only be familiar with the main
types.

 Single crystal.
 Twin crystal.
 Compression or zero degree.
 Angle.

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14.11.1 Single crystal probes
These probes are designed to utilise a single Piezo-electric crystal that both
transmits and receives the ultrasonic signal. The crystal must transmit the
signal, stop ringing, ring down to rest, pick up any reflected signal, ring up to
produce electric energy to pass to the receiver amplifier.

Figure 14.7 Single crystal probe.

14.11.2 Twin crystal probes

This type of probe has separate transducer crystals for transmission and
reception. The two crystals are mounted in the same housing but are carefully
isolated from each other both electrically and acoustically.

The material properties of the crystals are quite different from those of the
single crystal probe because the two crystals are not required to ring down to
receive. One is constantly transmitting while the other is constantly receiving.
The electric isolation is achieved by provision of two co-axial connectors, one
for transmit and one for receive, while the acoustic barrier is generally a thin
layer of cork.

A twin crystal probe is designed to minimise the problem of dead zone. A twin
crystal probe has two crystals mounted on Perspex shoes angled inwards
slightly to focus at a set distance in the test material. Were the crystals not
angled, the pulse would be reflected straight back into the transmitting crystal.

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 Figure 14.8 Twin crystal probe.

14.11.3 Advantages/disadvantages in probe selection.

Ultrasonic probes are selected depending on their output characteristics.

Single crystal probes

Advantages
 Good power output.
 Greater penetration.

Disadvantages
 Poor near zone resolution.
 Cannot measure thin plate.

Twin crystal probes

Advantages
 Good near zone resolution.
 Can measure thin plate - because dead zone is contained within the shoe.
 Can be focused.

Disadvantages

 Less power output.

 Less penetration.

14.11.4 Compression or zero degree probes

This type of probe transmits longitudinal/compression wave that are introduced
into the test piece at a zero degree angle. Therefore, there is no refraction and
the signal passes directly through the specimen at a zero angle. This type of
probe may be single or twin crystal. It is the type of probe fitted to a digital
thickness meter.

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14.11.5 Angle probes
These probes produce an ultrasonic beam that is introduced into the material at
an angle to the interface and not perpendicular. The angle is determined to
either match the weld angle of preparation or to introduce the beam at an angle
best suited to reflect from internal defects. These types of probe are used for
weld inspection tasks, see Figure 14.9.

Figure 14.9 Angle probe.

14.12 Couplant
Ultrasonic testing cannot be carried out in air without the use of a suitable
coupling agent between the probe and the test surface. This is because the
mechanical pulses cannot travel across the small air gap that exists between
the two surfaces, because of the mismatch in acoustic impedance between the
shoe of the transducer and the air. For underwater inspection the seawater acts
as a couplant and aids the passage of ultrasound into the material.

Probe

Couplant

Figure 14.10 Couplant.

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14.13 The sound beam
The spread of sound waves from a Piezo-electric crystal has been likened to the
beam of a torch, an elongated cone. Just as the intensity of light from a torch
diminishes with distance, so sound pulses get weaker the further they travel
from the crystal.

An acoustic sound wave has also previously been described as being a single
sinusoidal wave propagating through a material. These analogies do not
however present a totally true picture.

The sound produced from an ultrasonic crystal does not originate from a single
point but rather it is derived from many points along the surface of the
Piezo-electric crystal. This results in a sound field with many waves interacting
or interfering with each other.

Far zone

Dead zone Near zone

Figure 14.11 Ultrasonic Sound Beam

Figure 14.11 Ultrasonic sound beam.

14.13.1 Near Zone (or Near Field)

A Piezo-electric crystal is made up of millions of molecules. Each of these
vibrates when the crystal is hit by an electric charge and they send out shock
waves. The shock waves jostle each other.

Exponential decay
Intensity

Distance
Figure 14.12 Variations in sound intensity.

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 After a time, the shock waves, or pulses, even out to form a continuous front.
The area between the crystal and the point where the wave front evens out is
what we call the near zone. Inside the near zone signals from a reflector bear
no accurate relation to the size of the reflector, as the sound vibrations are
going in all directions. This affects the accuracy of flaw sizing of small reflectors
inside the near zone.

Near Zone

Figure 14.13 Regions of a sound beam.

The near zone of a crystal varies with the material being tested, but it can be
worked out by a formula:

Near Zone NZ = D2
4λ
14.14 Principles of ultrasonic testing
There are two basic principles of ultrasonic testing.

The first is based on the detection of a decrease in energy of the ultrasonic

beam due to absorption by the flaw.

Figure 14.14 Through Transmission Technique.

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 The second principle is based on the reflection of energy from a flaw or
interface. This is the method used in digital thickness meters and in A-scan
inspections. It is the basis of the majority of ultrasonic test systems and is
commonly referred to as the pulse echo technique.

Figure 14.15 Pulse Echo Technique.

Ultrasonic inspections are largely performed by the Pulse Echo Technique in

which a single probe is used to both transmit and receive ultrasound. In
addition to the fact that access is required from one surface only, further
advantages of this technique are that it gives an indication of the type of defect,
its size and its exact location within the item being tested.

The major disadvantage is that pulse echo inspection is reliant upon the defects
having the correct orientation relative to the beam in order to generate a
returning signal to the probe and is not, therefore, considered fail safe. If the
sound pulse hits the flaw at an angle other than 90°, much of the energy will be
reflected away and not return to the probe with the result that the flaw will not
show up on the screen.

Figure 14.16 Pulse echo testing with a zero degrees compression wave probe.

Figure 14.17 Pulse echo testing with an angle shear wave probe.

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14.15 Ultrasonic test systems
An ultrasonic test system should be able to measure either the amplitude of the
signal if a through transmission test is used, or the time required for the
ultrasonic signal to travel between specific interfaces if the Pulse Echo
Technique is employed.

A versatile test system in fact measures both the amplitude and the time
simultaneously. For thickness measurement, the main use of ultrasonic testing
is the measurement of the time the signal takes to travel between specific
interfaces, and the instrument is referred to as either a Digital Thickness Meter
or an A-scan flaw detector.

14.15.1 Digital Thickness Meters

Digital Thickness Meters measure the thickness of material using longitudinal
waves propagated by a compression probe and transmitted into the material
under test at the normal angle and are commonly used underwater for
thickness checks.

Figure 14.18 Cygnus 1 Digital Thickness Meter.

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 Figure 14.19 Simplified Block Diagram of Digital Thickness Meter.

A DTM is only designed to give a single readout for each application of the
probe and, as such, can only give a readout from the major reflector, which is
its main limitation. This is different from an A-scan instrument, which is
designed to display multiple reflections simultaneously.

Figure 14.20 Possible problems associated with a DTM.

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14.15.2 Advantages of DTMs
 Quick and easy to use.
 Divers and ROVs can use them.
 Only a small amount of training is necessary to use one.
 Only isolated cleaning is required.
 The Cygnus DTM will take readings through firmly adhered paint.

14.15.3 Procedure for Using a Cygnus DTM

 Check the unit for damage.
 Check correct probe fitted, diaphragm in place and undamaged.
 Fully charge battery and complete charging log.
 Switch on and carry out calibration check using appropriate block V1/V2.
 Deploy unit to worksite.
 Diver carries out pre-cal check - results recorded on data sheet.
 Carry out the survey to the client’s requirements.
 At the end of the survey, or in accordance with the client’s instructions
during the inspection, carry out a post-cal check - results to be recorded on
the data sheet.
 Return the unit to the surface and wash in fresh water.
 Inspect the unit for damage, if any found, mark unit as appropriate and
remove from service.
 Put the battery on charge and complete the charging log.
 Store in a secure, dry environment.

Figure 8.21 Baugh and Weedon PA 1011 control panel.

The ultrasonic flaw detector, the UT set, sends a voltage down a coaxial cable
to the probe. The piezo-electric crystal in the probe is hit by the voltage and
vibrates. The vibration creates an ultrasonic pulse, which enters the test
material. The pulse travels through the material until it strikes a reflector and is
reflected back to the probe.

It re-enters the probe, hits the crystal and vibrates it, causing it to generate a
voltage. The voltage causes a current which travels back to the flaw detector
along the cable. The set displays the time the pulse has taken through the test
material and back and the strength of the pulse as a signal on the CRT screen.

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 This is basically how a UT set works. It transmits energy into material via a
probe and measures the time in microseconds that the sound pulse takes to
return to the probe. The controls on the UT set are almost entirely concerned
with presenting a display on the CRT screen for the operator to interpret.

The cathode ray tube is a device for measuring very small periods of time. The
CRT displays electrical pulses on a screen in a linear time/distance relationship.
That is, the longer the distance on the screen time base (the x-axis), the longer
the time that has been measured.

On the y-axis (vertical) the amplitude of the returning signal is indicated, the
higher the amplitude of this signal, the greater the strength of the reflected
signal. Of course, in the case of thickness measurements this will be the back
wall echo.

14.15.4 Calibration and reference blocks

A-scan type instruments all require calibration before being used for testing.
Normally calibration will require the use of a calibration block, however a
reference block may be used if this is either specified or agreed by the client.

Reference blocks
These are manufactured for the client to agreed specifications and surface finish
and are used solely for a particular job and are not intended for any other
purpose. This is usually because they are intended for use on specialised steels
that would have different velocities.

Calibration blocks
There are several different calibration blocks available for ultrasonic testing.

The two most popular are the V1 and V2 calibration blocks. A calibration block
is manufactured to standard specifications and to international standards. It is
produced from specified material and is machined to close tolerances and laid
down standard of surface finish.

All the dimensions on the block are also specified and it is used to calibrate
ultrasonic flaw detectors in general. V1 blocks are used on the surface to
calibrate A-scan units and V2 blocks are used subsea by divers for calibration
checks.

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 Figure 8.22 IIW V1 calibration block.

Figure 8.23 IIW V1 block. Figure 8.24 IIW V2 block.

A step wedge as shown in Figure 8.27 it is a type of calibration block used for
thickness measurement.

W
Wh
Couplant 

Figure 8.25 Step wedge, dimensions in mm.

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14.15.5 Pre-calibration checks
For portable sets check the power supply is fully charged. Switch on the set and
allow it to warm up for 15 minutes, or the manufacturer’s recommended time.
This allows the CRT and other circuits to reach operating temperature and
stabilise.

CRT display
Adjust the focus and brilliance of the spot on the CRT screen. The spot will
normally not be visible, but will, however, appear as a line across the screen.
Use the delay control to adjust the time base to display the initial pulse (the
first transmitted pulse) on the screen.

Position the course range control to the required range. (This may be between
10mm and 1m depending on the actual instrument). Select and connect the
required probe. (For thickness measurement and lamination testing, this will be
a zero degrees or normal angle compression probe – either single or twin
crystal).

Time base linearity

Place the probe onto a calibration block and use the course and fine range (time
base) controls to display four back wall echoes on the screen. Adjust the back
wall echoes (BWE) so that they are equally spaced along the x-axis, then adjust
the second and fourth so that they are on the fifth and tenth divisions on the
graticule. Provided this is achieved the time base is linear.

Linearity of amplification
Set the reject or suppression control to off or zero. Place the probe over the
1.5mm diameter hole on a V1 calibration block. Adjust the gain controls to
display the height of the reflected signal to 80% full screen height (FSH).

Note: The gain settings.

 Use the fine (2dB or 1dB) gain control to increase the signal by 2dB.

This represents the difference in height between 80-100% that is a ratio of 4:5,
which will increase the signal on the screen by one quarter of its displayed
height and the signal should be at full screen height.

 Readjust the fine gain control to attenuate the signal back to 80% full
screen height.
 Attenuate the signal by 6dB.

This represents a decrease of a half and the signal therefore should reduce to
40% of full screen height.

 Now attenuate a further 12dB.

This represents a decrease of one quarter of the displayed signal and the height
should then be 10% of the full screen height.

If the gain adjustment does not produce these results the amplifier is not linear
and the instrument must be recalibrated internally, which involves stripping it
down to readjust internal trim settings.

DIS1-30815
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14.15.6 Calibration procedure for 100mm thickness.
Place the probe onto the face of a suitable calibration or reference block that is
25mm thick ensuring there is adequate coupling.

A number of back wall echoes should be displayed on the CRT screen. Adjust
the gain settings as necessary to display the second echo signal amplitude to
75% full screen height.

Adjust the fine range and delay controls to alter the screen display so that four
back wall echoes are shown, all equally spaced across the x-axis.

As the screen on the CRT has a graticule that is divided into 10 equal segments,
the four echoes are adjusted to 2.5, 5, 7.5, and 10 divisions along the x-axis.

Figure 8.26 A-scan flaw detector calibrated for 100mm.

14.15.7 Setting sensitivity

On completion of calibration it is necessary to adjust the sensitivity to conform
to the requirements for identifying and sizing the smallest specified flaw. This
may be, for example a flaw that is 2 or 3 in size, although this would be a very
sensitive setting as the minimum flaw size that may be achieved is of the order
of 1 to 2λ.

Whatever the required sensitivity is should be detailed in the workbook or the

procedure agreed with the contracting party.

The procedure will require that the gains be adjusted so that a reflected signal
from the smallest identifiable defect is displayed discretely on the screen and is
not lost in background clutter.

The general practise for scanning with compression probes for laminations is to
adjust the gain setting so that the first back wall echo from the parent plate is
displayed at full screen height.

DIS1-30815
Ultrasonics 14-22 Copyright © TWI Ltd
 An alternative method is to adjust to full screen height the first back wall echo
from a specified (say 1.5mm diameter) hole drilled horizontally into a reference
block at the maximum range for the test.

Figure 8.24 shows a diagram of the International Institute of Welding V1

calibration block. This block is an industry standard calibration block and
contains a 1.5mm diameter hole.

Setting resolution
The final process in the calibration procedure is setting resolution. This is
adjusting the gain controls so that the CRT display is capable of displaying
several reflected signals from the smallest detectable flaws at the maximum
range of the test. This may be accomplished using an agreed and specified
reference block.

The common method is to display the three signals reflected from the step, cut
out and back wall on the V1 block, Figure 8.29.

Figure 8.27 A method for setting resolution.

14.15.8 The 6dB drop method for plotting laminations

It is common for steel plate to be tested for laminations before structural
welding is undertaken. A normal angle probe will be used and the entire test
surface will be scanned, initially fairly quickly to get an impression of how the
reflected signals are displayed on the CRT display. This is followed by a detailed
scan based on a grid search to fully scan the entire test area.

If any laminations are discovered the initial reaction is to make temporary

marks with a paint-stick or chinagraph pencil on the surface of the parent plate
to give a first impression of the size and shape of the defect. This is followed by
a careful scan, employing the 6dB drop method, to accurately size the flaw and
determine its shape.

The 6dB drop method explained

The probe is manipulated over the defect area until a maximum amplitude
signal is displayed on the CRT. The height of this signal is noted or, if it is very
strong, the gains are adjusted to display this echo at full screen height.

The probe is then manipulated around the defect area until the signal displayed
on the screen reduces in amplitude.

DIS1-30815
Ultrasonics 14-23 Copyright © TWI Ltd
 The probe is manoeuvred to a position where the initial echo signal is reduced
to half the maximum that was obtained. This represents a 6dB reduction in
signal strength, which gives this method its name. The reason for this reduction
at this point is that now only half the flaw echo signal is being reflected from
the flaw while the remainder of that signal is now not reflected but continues on
its transmission path through the material.

The centre point where the probe is now positioned is marked accurately on the
surface of the test piece and this procedure is continued until the entire outline
of the lamination is plotted on the surface of the plate.

The outline shape is recorded and measured as necessary, Figure 8.30.

25mm

60mm

First flaw BWE

maximised to full
screen height

First flaw BWE reduced

to ½ screen height

BWE from parent plate

Figure 8.28 The 6dB drop method.

14.15.9 The use of angle or shear wave probes

Angle probes as their name indicates propagate the ultrasonic signal into the
material at an angle. As mentioned earlier the actual angle will be determined
by the expected orientation of the flaws that are being scanned for.

Most frequently these will lie on the fusion boundary of a weld and therefore the
probe angle will be the same as the angle of preparation. This will ensure that
any signal reflected from a flaw will be returned along the same path as the
transmission signal because the angle of reflection will equal the incident angle.

There are standard probes available for 45o and 60o as these angles are
common preparation angles in structural welds. Other angles may be calculated
by trigonometry and the application of Snell’s Law, Figure 8.7 refers.

DIS1-30815
Ultrasonics 14-24 Copyright © TWI Ltd
 Figure 8.29 Angle probe scanning the preparation face of a weld.

The probe is traversed along the weld until the signal from the flaw is
maximised. Further traversing of the probe will drop the signal height on the
CRT in the same manner as for a 6dB drop but with angle probes the signal is
reduced to one tenth using a 20dB drop. This point can be plotted and the
defect size obtained by numerous such manipulations of the probe.

14.15.10 Lamination plotting

Should there be any laminations in the scanning area the signal will be reflected
from these and not the flaw. To avoid this, the entire area is scanned with a
compression probe prior to using the angle probe. The area may be marked out
in a grid pattern to ensure 100% coverage, see Figure 8.32.

Figure 8.30 Scanning for laminations.

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14.15.11 Advantages and disadvantages of A-scan ultrasonics
Advantages:
 Can find and size subsurface defects.
 Possible to record the results via a computer system, photograph or video
the CRT screen.
 Accurate thickness readings can be carried out on thinner material than with
DTMs.
 Areas of pitting on the back wall can be assessed.
 Very adaptable, various probes can be fitted ie shear and compression
probes as well as twin and single crystal probes.
 More than one person can view the results either by looking at the same
unit or by setting up repeaters.
 Real-time results, no waiting for films to be developed.

Disadvantages:
 Requires highly trained operators to set up and interpret the results.
 Relies on accurate calibration.
 High level of cleaning required SA2.5 or SA3.
 Special arrangements have to be made for recording the results.

14.15.12 Care and maintenance of equipment

Care of UT equipment follows the same pattern as all equipment deployed
underwater.

 Clean all terminations, plugs, leads and controls.

 Charge all batteries in accordance with manufacture’s recommendations
using the correct type of charger.
 Do not overcharge (with some batteries this may evolve hydrogen gas and
cause an explosive hazard).
 Store equipment in a dry place.
 Be aware of the danger of electric shock from some components.
 Never operate equipment that is thought to be damaged – get it repaired.

DIS1-30815
Ultrasonics 14-26 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990. ISBN 13:
9780419135401.

DIS1-30815
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Physics of Ultrasound

 Sound is made when something vibrates. You can

twang a ruler on a table or flick a stretched elastic
band to verify this. The stretched surface of the
rubber band or the ruler vibrates and sets up a
CSWIP 3.1U Course series of vibrations, sound waves, in the air.

Ultrasonic Inspection
 As the surface of the band or ruler pushes into the
Section 14 air, the air molecules are forced together and a
region of high pressure forms; compression. As the
surface moves back, the air molecules move apart,
forming a low pressure area or rarefaction.

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Physics of Ultrasound Physics of Ultrasound

 As the surface vibrates, alternate If we plot the displacement of the particle against
compressions and rarefactions are set up in time it will produce a sine wave.
the air and travel out from the surface to form
a sound wave. The air molecules don’t move
with the wave - they vibrate to and fro in time
with the vibrating surface.

Wavelength

The sound wave, so produced, travels through the air

at sea level, at a speed of about 332m/sec at 0°C. We
hear the sound when it hits a membrane in our ear
and cause it to vibrate.
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Physics of Ultrasound Frequency

Sound will travel through any medium that has As sound is a series of vibrations, one way of
molecules to move, but it travels faster in more measuring it is to count the number of vibrations per
elastic materials because the vibrations are passed second - the frequency.
on more quickly.
 Frequency is measured in Hertz (Hz).
Sound travels faster in water or metal than it does  One vibration in one second is 1Hz.
in air, as liquids and solids are more elastic than air.  Two vibrations in one second is 2Hz.
 Ten vibrations in one second is 10Hz.
The speed of sound increases with the stiffness,  One thousand in one second is 1000Hz or
(elasticity) and decreases with density, in fact, it’s 1 kilohertz (kHz).
actually the square root of the stiffness divided by  One million vibrations in a second is 1Megahertz
the density. (MHz).
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Frequency Frequency

The higher the frequency - the higher the note  We can only hear sounds between certain
sounds - the higher the pitch. If you twang the frequencies - more than 16Hz and less than
ruler or the rubber band hard, the noise is 20,000Hz. If you were able to move your arm
louder, it has greater amplitude, but the note up and down 20 times a second, it would
remains the same. sound like a very low hum. You cannot move
If, however, you shorten the ruler or tighten the your arm this fast, so you cannot hear the
rubber band, they vibrate more quickly and the vibrations in the air caused by your moving
note given out is higher, the frequency is arm.
greater.
To raise the pitch of their instrument, guitar  A dog whistle vibrating at 25,000Hz cannot be
players move their fingers down the frets, thus heard by humans, but it can be heard by the
shortening the string and making it vibrate more sensitive ears of a dog.
quickly.
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Frequency Frequency Spectrum

 It rarely occurs to us that there is a whole Speed of sound in air = 332m/sec

world of sound that we cannot hear. Some Sound Waves

(Travelling at the speed of
Frequenc
y
Electro-magnetic Waves
(Travelling at the speed of
animals can hear sounds at higher frequencies sound) (kHz) light)

- bats can hear sound at 100,000Hz - while Infrasound Less than 16Hz 10-3
10-2
others, like snakes, have worse hearing than 10-1
we have. Human hearing range
1

(16 to 20,000Hz) 10 VLF

102 LF
103 MF
104 Radio frequencies HF
Ultrasound – frequencies
above 20,000Hz up to
106 UHF
10MHz
107 SHF

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Ultrasonic Inspection Ultrasonic Inspection

 A sound with frequencies above the upper  Elephants and hippos can communicate over
range of human hearing is called ultrasound. distances of up to 30 kilometres using
Sound below about 16 Hz is called infrasound. infrasound, while whales can communicate
Therefore the definition of ultrasound is; through water across an ocean!
sound with a frequency greater than 20kHz.

 However, there is an advantage for the lower

frequencies. The lower the frequency, the
more penetrating a sound wave is - that is
why foghorns give out very low notes and why
the low throbbing notes from your neighbours
stereo set come through the wall rather than
the high notes.
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Frequency Revision

 For most practical ultrasonic testing the The number of atomic particle oscillations it
frequency range used is from 0.5-6MHz. causes per second determines the frequency
of a signal.
 Lower frequencies, between 0.5-1.5MHz are
used for testing materials with very large grain The basic unit of frequency is the Hertz,
structures, such as concrete or cast iron abbreviated to Hz.
(castings).
One Hertz is one complete cycle of an event
 Frequencies from 2-6MHz are used for testing per second.
materials with fine grain structures including
steels.

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Velocity Types of Ultrasonic Wave

So far only the effects of the wave passing one Sound waves propagate through a material
point in the material have been considered. (liquid, solid or gas) by causing atoms to
However, the wave itself is travelling along oscillate as the wave front passes through it.
through the material. Like a surface wave on
water, the water at any point goes up and down, There are two types of wave that propagate
but as well as this the wave travels forwards. through the solid material and three types that
propagate as surface waves along the surface
Ultrasonic waves travel through a solid at the skin of the material.
speed of sound for a given type of wave in a
given material. That is, the speed of travel of a The three types of surface wave have no
sound wave is different for different types of application underwater and will not be discussed
wave and the speed of travel is different in further.
different materials.
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Waves That Propagate Through Solids Longitudinal or Compression Wave

Longitudinal/compression waves

This type of wave is denoted by the symbol L

V is used to denote velocity.

Therefore, VL denotes the velocity of propagation

of longitudinal or compression waves.

With this type of wave the direction of oscillation

of the atoms is the same as the direction of the Particle movement of a longitudinal or compression
wave
wave propagation.

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Transverse/Shear Waves Shear or Transverse Waves

Transverse/Shear waves are denoted by the

symbol T.

Therefore, VT denotes the velocity of

propagation of these waves.

The direction of oscillation of the atoms in

this mode of travel is at right angles to the
direction of motion of the propagating wave.

Particle movement of a transverse wave

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Properties of Ultrasonic Waves Velocity of Ultrasonic Waves

In order that ultrasonic waves can be used to

Type of measure depths and sizes within any material, it
Symbol Atomic motion Used for is a fundamental principle that the velocity of
wave
Thickness the sound wave remains constant for different
Longitudinal
Longitudinal
In the direction of
and samples of the same material.
or VL Lamination
the wave
compression measurement
propagation This is in fact the case and furthermore, the
s
ultrasonic wave obeys the laws of light and we
Transverse Transverse Defect sizing can, therefore, predict its behaviour.
or VT At 90° to the wave and weld
shear propagation inspection

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Summary of Velocities Ultrasonic Wavelength

A wave in the sea is a vibration of energy. As the

Acoustic velocity (m/sec) wave passes a fixed point it produces a constant
Material
VL VT rise and fall of energy. A complete vibration is a
Aluminium 6350 3100
change in energy from maximum to minimum
Concrete 4600
and back to maximum.
Naval brass 4430 2120 The Wavelength indicates how far the ultrasonic
Copper 4660 2260 stress wave moves forward during one complete
Air 332 stress cycle.
Structural steel 5940 3250 The method of calculation is:
Water 1490
Perspex 2730 1430

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Ultrasonic Wavelength Ultrasonic Wavelength

If we want to know the wavelength of a 2MHz compression The velocity must be placed at the top (note: how it forms a
wave travelling through steel, we can again use the diamond shape) and the wavelength and frequency at either of
formula, as we know the compressional speed of sound in the bottom two corners.
steel, 5,940m/sec.

λ =
5,940,000
= 2.97mm
v
λ f
2,000,000

If we want to know the wavelength of a shear wave of

2MHz in steel we can use the formula again, but this time The wavelength of an ultrasonic wave is important because the
we use the shear speed of sound in steel which is shorter the wavelength, the smaller the flaws that can be
3,250m/sec. discovered. Defects of a diameter of less than half a wavelength
may not show on the CRT. On the other hand, the shorter the
An easy way to remember how this formula works is to wavelength the less the ultrasound will penetrate the test
split it down within a triangle - with the velocity, material.
wavelength and frequency at the corners.

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Further Effects of Further Effects of

Ultrasonic Properties in Materials Ultrasonic Properties in Materials
As the ultrasonic signal passes through a material, a Acoustic attenuation:
pressure or stress front will be initiated in the material,  A further effect on the sound wave is the reduction in
which will present resistance to the passage of the sound energy of the wave as it passes through a material.
wave energy. The amount of resistance will depend on the The strength of the ultrasonic signal will be reduced
properties of the material. This is a useful parameter of a and eventually will be so low that it cannot be
material and must be determined if the pressure or stress detected.
magnitude of the ultrasonic wave is to be determined.
Acoustic impedance (Z):
 Is the resistance to the passage of the ultrasound
and is the reason that ultrasound travels at different
speeds in different materials.

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The direction of Propagation The direction of Propagation

of an Ultrasonic Wave of an Ultrasonic Wave
 So far it has been established that the  An interface will include the outside edges of a
ultrasonic wave travels at a known speed in a component and indeed, the back surface is
straight line and that it obeys the laws of light. referred to as the back wall. Similarly, the
In order to predict the direction that the wave surfaces of a crack or porosity bubble are
travels as it passes through an interface into a boundaries also.
different material, it is necessary to determine
what happens when a wave meets an  At these interfaces, in accordance with the
interface. laws of light, the direction of travel of the
wave after meeting the interface will be
 An interface is any boundary between two determined by the law of reflection and the
materials of differing properties (eg law of refraction.
perspex/steel, water/air etc.)

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Ultrasonic Wavelength Law of Reflection

 The angle the reflected wave makes with the

normal angle to the interface from which the wave
is being reflected is the same as the angle that the
incident wave makes with the same normal angle.

The angle of
incidence is equal
to the angle of
reflection

When the angle of incidence is 0° the reflected angle is also 0° this

is the ideal condition for thickness measurements using
Compression wave passing from Perspex into steel
compression waves.

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Law of Refraction Generating Ultrasound

At an interface, part of the ultrasonic wave is reflected and the  Sound is created when something vibrates. It
rest will pass into the second material. The path in the second is a stress wave of mechanical energy. The
material will still be a straight line, but the direction of this wave
will not be continuous with the direction of the incident wave, as
Piezo-electric effect changes mechanical
it will have been turned through an angle that can be determined energy into electrical energy. It is reversible,
by Snell’s Law. so electrical energy - a voltage - can be
changed into mechanical energy or sound,
which is the reverse Piezo-electric effect.
 The first people to observe the Piezo-electric
effect were the Curie brothers who observed it
Material 2 is in quartz crystals. Jaques and Pierre Curie
denser than
material 1 used quartz for their first experiments.
Nowadays polarised ceramics are used instead
of quartz crystals.
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Generating Ultrasound Piezo Electric Crystal

 It was later discovered that by varying the

thickness of crystals and subjecting them to a
voltage, they could be made to vibrate at
different frequencies. Frequency depends on
the thickness of the Piezo-electric crystal.

When switch is closed the When the crystal is squeezed

potential across the crystal by the returning sound an
will cause it to change electrical potential will be
shape (vibrate). produced.

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Piezo Electric Crystal Piezo Electric Crystal

The transducer is mounted in a probe assembly  A pulse of ultrasound from a Piezo-electric

for protection and to enable electrical crystal has a length or width of several
connections to be made. vibrations or wavelengths. When you strike a
bell it continues to ring for several seconds as
The crystal is fitted with silver foil electrodes to the metal continues to vibrate. The vibrations
apply the voltage across the crystal if acting as a get steadily weaker and the sound dies away.
transmitter, or to take the voltage signal from it If you put your hand on the bell you stop the
if acting as a receiver. vibrations and the sound dies away more
quickly - you dampen the sound.
The crystal is attached to the base by the
mounting, which acts not only as a fixing, but
also as a backing to the crystal.

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Piezo Electric Crystal Revision

 A Piezo-electric crystal continues to vibrate  A transducer is any device that transforms energy
after it is hit by an electrical charge. This from one form into another.
affects sensitivity, as the longer the pulse
length, the worse the resolution. In most
probes a slug of tungsten loaded Araldite is  In the case of the ultrasonic transducer, it transforms
placed behind the crystal to cut down the high frequency electrical signals to the same high
ringing time. frequency mechanical signals and vice versa.

 This is the Piezo electric effect.

 The Piezo electric transducer crystal is made from a

range of synthetic crystalline and ceramic materials.

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Types of Transducers (Probes) Single Crystal Probes

All probes are designed to transmit an ultrasonic  These probes are designed to utilise a single Piezo
signal into a test specimen with maximum electric crystal that both transmits and receives the
efficiency. The configuration of any probe is ultrasonic signal.
dependant on the actual task it is designed for.  The acoustic characteristics of this transducer are
quite specific and the selected crystals possess
There are, broadly speaking, four types of particular characteristics.
probe:  The crystal must transmit the signal, stop ringing,
ring down to rest, pick up any reflected signal, ring
1. Single crystal probes. up to produce electrical energy that is passed on to
2. Twin crystal probes.
3. Compression or zero degree probes. the receiver amplifier.
4. Angle probes.  So the natural frequency of the crystal needs to be
very widely separated from the ultrasonic frequency
being used for the test.
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Single Crystal Probe Twin Crystal Probes

 This type of transducer has separate crystals

for transmission and reception. The two
crystals are mounted in the same housing but
are carefully isolated from each other
electrically and acoustically.

 The material properties of the crystals are

quite different from those of the single crystal
probe because the two crystals are not
required to ring down in order to receive.

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Twin Crystal Probes Twin Crystal Probe

 One is constantly transmitting while the other

is constantly receiving. The electrical isolation
is achieved by providing two co-axial
connectors, one to transmit and one to
receive.

 The acoustic barrier is generally a thin layer of

cork.

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Advantages/Disadvantages Compression or Zero Degree Probes

Single crystal probes Twin crystal probes This type of probe transmits longitudinal waves
that are transmitted into the test specimen at a
Advantages: Advantages:
zero angle.
 Good power output.  Good near zone resolution.
 Greater penetration.  Initial pulse and dead zone
are contained in the shoe. Therefore there is no refraction and the signal
 Can be focused to any passes directly through the specimen at a zero
depth. angle.
 Can measure thin plate.

 Disadvantages: This type of probe may be single or twin crystal.

Disadvantages:
 Poor near zone resolution.  Less power output.
 Cannot measure thin plate.  Less penetration. And this is the type of probe that is fitted to a
digital thickness meter.

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Angle Probes Single Crystal Angle Probe

Angle probes produce an ultrasonic beam that

is introduced into the material at an angle to
the interface, and not perpendicular.

The angle is determined to either match the

weld angle of preparation or to introduce the
beam at an angle most suited to reflect from
internal defects.

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Couplant The Ultrasonic Beam

 Ultrasonic testing cannot be carried out in air  The spread of sound waves from a Piezo-
without the use of a suitable coupling agent electric crystal has been likened to the beam
between the probe and the test surface. of a torch, an elongated cone. Just as the
 This is because the small mechanical pulses cannot intensity of light from a torch diminishes with
travel across the small air gap that exists between distance, so sound pulses get weaker the
the two surfaces, because of the mismatch in further they travel from the crystal.
acoustic impedance between the shoe of the
transducer and the air.  An acoustic sound wave has also previously
Probe
Probe been described as being a single sinusoidal
Couplant wave propagating through a material. These
analogies do not however present a totally
true picture.

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The Ultrasonic Beam The Ultrasonic Beam

 The sound produced from an ultrasonic crystal

does not originate from a single point but
rather it is derived from many points along the Far Zone
surface of the Piezo-electric crystal. This Here the beam diverges
at a predictable rate, it
results in a sound field with many waves decays exponentially and
interacting or interfering with each other. can be used to accurately
size small defects

Dead Zone Near Zone

An area where Area of fluctuating intensities
nothing useful can can be used for thickness
be obtained readings and sizing large
reflectors such as laminations

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The Ultrasonic Beam The Ultrasonic Beam

Near Zone (or Near Field)

A Piezo-electric crystal is made up of millions of molecules.
Each of these vibrates when the crystal is hit by an electric
charge and they send out shock waves. The shock waves
jostle each other.
The near zone of a crystal varies with the
After a time, the shock waves, or pulses, even out to form
material being tested, but it can be worked out
a continuous front. The area between the crystal and the
point where the wave front evens out is what we call the by a formula:
near zone.
Inside the Near Zone signals from a reflector bear no D2
accurate relation to the size of the reflector, as the sound NZ =
4λ
vibrations are going in all directions.
This affects the accuracy of flaw sizing of small reflectors
inside the Near Zone.

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Principles of Ultrasonic Testing Principles of Ultrasonic Testing

There are two basic principles of ultrasonic The Pulse/Echo Technique is based on the reflection of
testing. energy from a flaw or interface.

The through transmission technique - is based

This is the method used in A-scan ultrasonic inspection and
on the detection of a decrease in energy of the digital thickness meters and is the basis of a majority of
ultrasonic beam due to absorption by the flaw. ultrasonic test system.

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Principles of Ultrasonic Testing Principles of Ultrasonic Testing

Ultrasonic inspections are largely performed by the Pulse

Echo Technique in which a single probe is used to both
transmit and receive ultrasound. In addition to the fact
that access is required from one surface only, further
advantages of this technique are that it gives an indication
of the type of defect, its size and its exact location within
the item being tested. Pulse Echo testing with a zero degrees compression wave
probe.
The major disadvantage is that Pulse Echo Inspection is
reliant upon the defects having the correct orientation
relative to the beam in order to generate a returning signal
to the probe and is not, therefore, considered fail safe. If
the sound pulse hits the flaw at an angle other than 90°,
much of the energy will be reflected away and not return
to the probe with the result that the flaw will not show up Pulse Echo testing with an angle shear wave probe.
on the screen.

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10
 27/08/2015

Ultrasonic Test Systems Digital Thickness Meters

 An ultrasonic test system should be able to measure  Digital thickness meters measure the thickness of
either the amplitude of the signal if a through material using longitudinal waves propagated by a
transmission test is used, or the time required for compression probe and transmitted into the
the ultrasonic signal to travel between specific material under test at the normal angle and are
interfaces if the Pulse Echo Technique is employed. commonly used underwater for thickness checks.

 A versatile test system in fact measures both the

amplitude and the time simultaneously. For
thickness measurement, the main use of ultrasonic
testing is the measurement of the time the signal
takes to travel between specific interfaces, and the
instrument is referred to as either a Digital
Thickness Meter or an A-scan flaw detector.
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Digital Thickness Meter Digital Thickness Meter

A DTM is only designed to give a single readout

for each application of the probe and, as such,
can only give a readout from the major reflector,
which is its main limitation.

This is different from an A-scan instrument,

which is designed to display multiple reflections
simultaneously.

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Possible Causes of
Care and Maintenance of Equipment
False Readings With DTMs
 Clean all terminations, plugs, leads and
controls.
 Wash off all housings with fresh water.
 Charge all batteries as per manufacturer’s
recommendations.
 Do not overcharge (some batteries may
evolve Hydrogen gas and cause an explosive
hazard).
9  Store equipment in a dry place.
 Be aware of the danger of electric shock from
some components.
 Never operate equipment that is damaged –
get it repaired.

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11
 27/08/2015

Pulse Echo System with A-Scan

An A-scan flaw detector
Display

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6 6

Calibration and Thickness

Calibration and Reference Blocks
Measurement
A-scan types of ultrasonic instruments all require  Normally calibration will require the use of a
calibration before they can be used for testing. calibration block, however a reference block may
be used if this is either specified or agreed by
 Modern instruments incorporate computer chips the contracting party.
that assist in this process but a standard
calibration block will still be required.  Reference Block:
 A reference block is produced to agreed measurements
and specifications. It is manufactured to the same
 Digital thickness meters are pre-calibrated for tolerances and surface finish as a calibration block and is
use with low carbon steel but the calibration used in the same way. The difference between this and a
must be verified before each use. This will calibration block is that the reference block is intended
require the use of a calibration block or a to be used only for the specified task and is not intended
to be used for general work on any other components.
specified reference block or step wedge.

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6 7

Calibration and Reference Blocks V1 Calibration Block

Calibration Block
There are several different calibration blocks
available for ultrasonic testing.
The two most popular are the V1 and V2
calibration blocks.
 A calibration block is manufactured to standard
specifications and to international standards. It is
produced from specified material and is machined to
close tolerances and laid down standard of surface
finish.
 All the dimensions on the block are also specified and it
is used to calibrate ultrasonic flaw detectors in general.

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7 7

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A-Scan Calibration Procedure for Scanning

 Ensure the A-scan is fully charged.

 Calibrate the set.
 Record the serial number of the instrument.
 Record the serial number of the
calibration/reference block.
 Clean to SA2.5 or SA3.
 Complete a visual inspection of the test
surface.
 Scan and plot any laminations or thin sections.
This picture shows the unit set up for a linearity check, peaks  Report and record results.
at 2.5, 5, 7.5 and 10 representing 25, 50, 75 and 100mm.
The unit is now calibrated for measuring up to 100mm.

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7 7

Thickness Plotting The 6 dB Drop Method

25mm

60mm

First flaw BWE

maximised to full
screen height

First flaw BWE reduced

to ½ screen height

BWE from parent plate

If any laminations are discovered, the initial reaction is to

make temporary marks with a paint stick or chinagraph to
gain a first impression of the size and shape of the defect.
This is followed by a careful scan employing the 6dB drop
method to accurately size the flaw and determine the shape.

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7 7

Any Questions?

Copyright © TWI Ltd

13
 Section 15

Inspection Methods Available to Inspect

Underwater Structures
15 Inspection Methods Available to Inspect Underwater Structures
15.1 Visual inspection
Visual inspection is the most commonly used method of inspection. This is
because:

 It is cheap and readily available.

 Many features of engineering importance can be seen on the surface of the
material or structure.
 It generally precedes any non-destructive testing (NDT) that is to be carried
out on the component.

However, visual inspection cannot detect sub-surface features such as lack of

sidewall fusion within a weld – hence, often the requirement for NDT.

We have two approaches to conducting underwater visual inspections:

15.1.1 General visual inspection (GVI)

The aim of the GVI is to provide the client with a general impression of the
underwater state of the structure. The area inspected is often not cleaned as an
assessment of marine growth is commonly conducted as part of the GVI.

The datum for the GVI is given to the diver-inspector by the inspection
controller (by reference to the client’s procedure) and will comprise of a 12
o’clock datum for clock positioning and a reference datum for distance
positioning. The diver-inspector may then report the location of features
according to their clock position and an estimation of the distance from a
datum.

For example: ‘At the 6 o’clock position, 3m from leg Charlie, there is an area of
impact damage. It is approximately 1m long by ½m wide and is 20mm deep’.

Features that are likely to be reported during a GVI include:

 Structural features, eg welds, pad eyes, intersecting members, etc.

 Imperfections, eg impact damage, debris, tool marks, etc.
 Marine growth – type, thickness and extent.
 Paint coatings – type, condition and cover.
 Corrosion damage, etc.

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Inspection Methods Available to Inspect Structures 15-1 Copyright © TWI Ltd
 Figure 15.1 Diver conducting a GVI with helmet-mounted video and video
pointer.

15.1.2 Close visual inspection (CVI)

The aim of the CVI is to provide the client with an accurate report of detailed
features such as those seen on a weld. The area is cleaned prior to conducting a
CVI – the specific cleaning requirements will be detailed in the client’s
procedure, but for weld inspection these are generally; clean to a matt, bare-
metal surface finish (SA2½) the weld cap and to 75mm either side of the toes
of the weld.

Again, the datum for the CVI is given to the diver-inspector by the inspection
controller (by reference to the client’s procedure). It is often marked by three
punch-marks near the weld. The diver-inspector may then report the location of
features according to their start and stop positions on a tape laid with zero at
the datum. As such, distances are measured to the nearest millimetre (ie to +/-
½ mm accuracy).

For example: ‘From 1330mm to 1345mm we have porosity on the cap of the
weld’.

DIS1-30815
Inspection Methods Available to Inspect Structures 15-2 Copyright © TWI Ltd
 Features that are likely to be reported during a CVI include:

 Normal features, eg stop/start, other intersecting welds, etc.

 All imperfections, whether specified in the client’s CNC or not.
 Corrosion damage, etc.

Figure 15.2 A weld set for a CVI.

It is important to note that visual inspection does not generate a record of the
inspection – a separate recording technique must be employed. This could
simply be the diver’s memory or notes and sketches on a scratch-board. Most
often, videography and photography are used.

15.2 Videography
Videography is often used in conjunction with visual techniques to record their
results.

Generally, diver-inspectors may use either:

 Helmet-mounted cameras (HMTV).

 Hand-held cameras.

Offshore, IMCA guidance recommends that all divers wear HMTV cameras as a
matter of safety. Cameras may also be mounted on ROVs or have fixed
installations on the structure.

The video recording is accompanied by an oral commentary that may be

performed either by the diver-inspector or by the inspection controller. Still
images may be taken from a video stream by picture-grabbing the image.

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Inspection Methods Available to Inspect Structures 15-3 Copyright © TWI Ltd
15.3 Photography
There are two commonly used photographic techniques for underwater
inspection:

15.3.1 Close-up photography

Close-up photography is when the camera lens is within 1 metre from the
subject.

The optics of the camera can be used to magnify the subject to elicit greater
detail. Linear subjects, such as welds, will require multiple overlapping
photographs (30 to 50% overlap) to be taken to form what is termed a photo-
mosaic.

15.3.2 Stand-off photography

Stand-off photography is when the camera lens is more than 1 metre from the
subject.

This technique is used to capture images of large features such as flanges,

anodes, etc.

For reasons of picture quality, the maximum stand-off from the subject to the
camera lens is limited to one-third of the underwater visibility.

15.3.3 Other photographic techniques

If we combine two images within a computer programme, we may produce a
three-dimensional representation of the subject. This method is known as
stereo-photography.

Photogrammetry is a technique for taking measurements from photographs.

This may be achieved using a Reseau plate or by stereo-photographic analysis.

15.4 Ultrasonic techniques

Ultrasonic NDT utilises the behaviour of high-frequency sound waves. It is
particularly suitable for investigating two-dimensional (planar) features because
they have relatively flat surfaces. These surfaces will readily reflect the
ultrasonic beam and will be easily detected.

There are several ultrasonic techniques that may be used underwater:

15.4.1 Digital Thickness Meter (DTM)

The DTM is also known as the ultrasonic thickness meter (UT meter).

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Inspection Methods Available to Inspect Structures 15-4 Copyright © TWI Ltd
 Figure 15.3 A Cygnus 1 underwater Digital Thickness Meter.

The DTM filters out all minor signals and displays only the distance to the major
reflector on the digital display, making it relatively easy and quick to use. It is
commonly employed for wall thickness measurement and basic lamination
checking.

Figure 15.4. A Cygnus 1 DTM being used.

DIS1-30815
Inspection Methods Available to Inspect Structures 15-5 Copyright © TWI Ltd
15.4.2 A-scan flaw detector
The A-scan flaw detector displays major and minor signals on a graphical
screen. Interpreting the display requires skill and can be time-consuming.

Typically used for thickness measurement, lamination checking and for

ultrasonic weld inspections. A frequency of 5MHz is commonly used (A-scan &
DTM).

Figure 15.5 The A-scan requires skilled interpretation.

15.5 Flooded member detection (FMD)

It is important to gauge whether the inside of a structural member is wet or
dry. If the member is wet, then further investigation as to the cause of the
flooding will be required.

Both diver-inspectors and ROVs can test for flooded members by using
ultrasonic FMD probes. The equipment makes use of the fact that an ultrasonic
beam is blocked by a gas – ie a dry space. A low frequency ultrasound of
0.5MHz is used.

An ROV may also be fitted with radiographic FMD equipment. This fires gamma
radiation, using caesium 137, through the structural member and meters the
amount of radiation that passes through the enclosed space – a high reading
would indicate a dry void.

Note: Radiographic FMD is too hazardous for diver use.

15.6 Crack detection techniques

There are three crack detection techniques that can be used underwater:

DIS1-30815
Inspection Methods Available to Inspect Structures 15-6 Copyright © TWI Ltd
15.6.1 Magnetic particle inspection (MPI)
MPI is a technique that involves magnetising the part that is to be inspected,
applying a detection ink and visually inspecting for indications that betray the
presence of surface imperfections. As such, it can only be performed on
ferromagnetic metals such as steel but not on non-ferromagnetic materials.
Also, the surface requires cleaning to a matt, bare-metal (SA2½) finish.

Underwater MPI is used to detect the presence of surface-breaking cracks but

will not give any indication of their depth.

15.6.2 Electromagnetic detection techniques

A major advantage of electromagnetic crack detection techniques is that, unlike
MPI, they do not require the removal of protective paint coatings. Also, they
can be used to detect cracks in non-ferromagnetic materials.

However, a disadvantage of electromagnetic detection techniques is that they

do not give visual indications of the locations of defects and they require skilled
operators to interpret the data.

There are two electromagnetic techniques that are used underwater:

Eddy current technique

Eddy current works by inducing a circular (eddy) current to flow in the surface
of the material. Any surface discontinuity will upset the symmetry of the eddy
current and be detected by the equipment.

Eddy current technique will detect the presence of a surface-breaking crack, but
will not give any indication of its depth.

Alternating current field measurement (ACFM)

ACFM is a development of the eddy current technique but works in two
perpendicular planes. Thus, it has all the same features as eddy current
technique, but it can also indicate the depth of the cracks that it finds.

15.6.3 Radiography
Although X-ray radiation is used for topside inspections, the equipment is too
complex to use underwater - gamma radiation is employed instead. Gamma
radiation is produced by the radioactive decay of radioisotopes such as iridium-
192.

Gamma radiography may detect planar defects within the material if the
radiation pathway is carefully aligned to the same expected orientation as the
defect. Volumetric defects are more easily detected by radiography as the user
does not need to align the radiation pathway.

As well as being an inspection technique, radiography also produces a

permanent recording – the radiograph. Measurements of the sizes of features
and defects within the material may be made from the radiograph, as can the
thickness of the material itself.

The main drawback in using radiographic inspection is that it is a potentially

hazardous technique. The costs of the equipment and safe management make
it relatively expensive.

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Inspection Methods Available to Inspect Structures 15-7 Copyright © TWI Ltd
 Figure 15.6 A radiographer unlocking the safety on the gamma source.

15.7 Taking measurements underwater

Many topside measuring tools may be used successfully underwater with little
or no modification.

15.7.1 Linear measurements

Vernier calipers may be used for measurements of typically up to 150mm. It
can measure both linear features and internal/external diameters.

Figure 15.7 A Vernier caliper used to measure weld width.

Weld gauges may be used to measure small scale linear features such as
undercut, excess weld material and fillet leg-length. They may also be used to
measure angles of up to 90 degrees.

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Inspection Methods Available to Inspect Structures 15-8 Copyright © TWI Ltd
 Figure 15.8 A weld gauge.

A weld gauge is a practical and cost-effective small-scale measuring tool.

The Linear-Angular Measurement (LAM) gauge may be used to measure

many small scale linear and angular features as per the weld gauge. The LAM
gauge is superior to the weld gauge in that it may be used on a curved surface
and is particularly accurate when measuring the depths of remedial grinds.

Figure 15.9 A LAM gauge.

A LAM gauge is a more expensive precision measuring tool.

Rulers or tape-measures are simple and practical small-to-medium scale

measuring tools. Magnetic tape-measures are especially useful in underwater
close visual inspection.

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Inspection Methods Available to Inspect Structures 15-9 Copyright © TWI Ltd
 For large scale linear measurements we may use an open reel tape-measure,
position referencing using underwater transponders or narrow-beam
ranging sonar that may be mounted on an ROV or hand-held by the diver-
inspector.

15.7.2 Circular measurements

Internal and external diameters may be measured using calipers and gauges.
Gauges possess a calibrated scale and are direct measurement tools. Calipers
are generally used as comparators and are an indirect measuring tool (an
exception being the Vernier caliper – Vernier referring to the type of scale
used).

15.7.3 Angular measurements

Angles may be measured using a protractor, weld gauge or LAM gauge.

Angles of large components such as mooring chains may be measured using an

inclinometer (pendulum gauge).

15.7.4 Special tools and jigs

Peculiar measurements may require the fabrication of a special tool or jig. For
example, an ovality jig is used for measuring circular distortion in structural
members.

15.7.5 Photogrammetry
Measurements may be obtained by computer analysis of photographs in a
technique termed photogrammetry. This is an especially useful method of
measuring marine growth, anode wastage, damage, etc.

15.8 Crack depth measurement

The depth of cracks may be gauged using one of three techniques:

15.8.1 Alternating current field measurement (ACFM)

ACFM can both detect the presence of a surface-breaking crack and give an
indication of its depth

15.8.2 Alternating current potential drop (ACPD)

Any conducting material will possess an electrical resistance. ACPD equates a
calibrated measurement of electrical resistance (measured in terms of a voltage
or potential drop) to the distance travelled by the electrical current.

The ACPD probe is calibrated and then placed so as to span the known crack
position. The potential drop measured by the equipment indicates the depth of
the crack. Thus, ACPD will not locate a crack in a material but it will indicate the
depth of a crack whose position is known.

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Inspection Methods Available to Inspect Structures 15-10 Copyright © TWI Ltd
 Figure 15.10 ACPD probe is placed across a crack.

15.8.3 Investigation by hand grinding

The depth of a crack may be discerned by a careful sequence of shallow grinds
using a hand grinder.

15.9 Cathodic Potential (CP) measurement

CP is a measurement of the electrical potential (voltage) at a certain location on
a structure. This is achieved by connecting a voltmeter between the structure
and a reference electrode (reference cell) – most commonly silver/silver
chloride. Both the voltmeter and the reference cell are built into the tool, called
a Bathycorrometer.

The CP value can imply the state of the corrosion protection being offered to the
structure.

Figure 15.11 A diver using a Bathycorrometer.

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Inspection Methods Available to Inspect Structures 15-11 Copyright © TWI Ltd
15.10 Recording shapes and surface profiles
On the microscopic-to-small scale, surface profiles may be recorded using a
casting material. These are pressed onto the surface of the material and
allowed to take up the shape of any defects present. The resulting cast is
removed and then analysed.

A carefully taken cast may be put into an electron microscope to reveal detail of
grain-sized features.

Small-to-medium scale surface features may be recorded by taking a series of

spot measurements with a Vernier caliper along an area of impact damage or
by using a profile gauge – see Figure 15.12.

Figure 15.12 A profile gauge.

Used to build up a series of cross-sections and the results traced onto a dive
slate or scratch-board.

Small-to-large scale surface profiles may be recorded using stereo-

photography.

Large scale surface profiles may be recorded using a taut-wire survey. In this
technique, a wire is anchored at either end and stretched tight along eg a
damaged brace. Regular measurements are made along the wire to create a
profile of the deformation. If performed twice along perpendicular planes then a
three-dimensional representation of the deformation may be gained.

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Inspection Methods Available to Inspect Structures 15-12 Copyright © TWI Ltd
 Bibliography
ASME Boiler and Pressure Vessel Code: ‘Non-destructive Examination – Section
V, ASME International, Washington, D.C (2011), https://www.asme.org.

Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

BS EN 13018: 2001 ‘Non-destructive testing - Visual inspection - General

principles’, https://www.bsigroup.co.uk.

Davey V S, Forli O, Raine A, Whillock R T, ‘Non-Destructive Examination of

Underwater Welded Steel Structures’, IIS/IIW Document V-1097-97, Abington
Publishing, 1999.

Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office

Books), 1988, ISBN 13: 9780114129118.

Standards for reference

BS EN ISO 9934-1 Non Destructive Testing - Magnetic particle testing
- Part 1: General Principle.
BS EN ISO 9934-2 Non Destructive Testing - Magnetic particle testing
- Part 2: Characterisation of products.
BS EN ISO 9934-3 Non Destructive Testing - Magnetic particle testing
- Part 3: Equipment.
BS EN 3059 Non Destructive Testing - Penetrant testing and
magnetic particle testing - Viewing conditions.
BS EN ISO 17638 Non-destructive testing of welds - Magnetic particle
testing.
BS EN ISO 23278 Non-destructive testing of welds - Magnetic particle
testing of welds. Acceptance levels.
BS EN 12084 Non-destructive testing - Eddy current testing -
General principles and guidelines.
BS EN 1711 Non-destructive examination of welds - Eddy current
examination of welds by complex plane analysis.
BS EN 1435 Non-destructive testing of welds - Radiographic testing
of welded joints.
BS EN 462-1 Non-destructive testing - Image quality of radiographs
- Part 1: Image quality indicators (wire type) -
Determination of image quality value.

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Inspection Methods Available to Inspect Structures 15-13 Copyright © TWI Ltd
 27/08/2015

Inspection Methods

Visual inspection is the most commonly used

method of inspection.

CSWIP 3.1U Course This is because:

 It is cheap and readily available.
Inspection Methods Available  Many features of engineering importance can be
To Assess Underwater Structures seen on the surface of the material or structure.
Section15  It generally precedes any non-destructive testing
(NDT) that is to be carried out on the component.

However, visual inspection cannot detect sub-

surface features such as lack of sidewall fusion
within a weld – hence, often the need for NDT.
Copyright © TWI Ltd Copyright © TWI Ltd

Inspection Methods Inspection Methods

We have two approaches to conducting The datum for the GVI is given to the diver by
underwater visual inspections: the inspection controller (by reference to the
client’s procedure) and will comprise of a 12
1. General visual inspection (GVI) o’clock datum for clock positioning and a
reference datum for distance positioning. The
diver-inspector may then report the location of
The aim of the GVI is to provide the client with a features according to their clock position and an
general impression of the underwater state of estimation of the distance from a datum.
the structure. The area inspected is often not
cleaned as an assessment of marine growth is
commonly conducted as part of the GVI.

Copyright © TWI Ltd Copyright © TWI Ltd

Inspection Methods Inspection Methods

For example: At the 6 o’clock position, 3m from leg Charlie, 2. Close Visual Inspection (CVI).
there is an area of impact damage. It is approximately 1m long
by ½m wide and is 20mm deep. The aim of the CVI is to provide the client with an accurate
report of detailed features such as those seen on a weld.
Features that are likely to be reported during a GVI include: The area is cleaned prior to conducting a CVI – the specific
cleaning requirements will be detailed in the client’s
 Structural features, eg welds, pad eyes, intersecting procedure, but for weld inspection these are generally;
members, etc. clean to SA2½ the weld cap and 75mm either side.
 Imperfections, eg impact damage, debris, tool marks, etc.
 Marine growth – type, thickness and extent. Again, the datum for the CVI is given to the diver by the
inspection controller (by reference to the client’s
 Paint coatings – type, condition and cover. procedure). It is often marked by three punch-marks near
 Corrosion damage, etc. the weld. The diver may then report the location of
features according to their start and stop positions on a
tape laid with zero at the datum.

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1
 27/08/2015

Inspection Methods Inspection Methods

Distances are measured to the nearest mm (ie

to +/- ½ mm accuracy).
For example: From 1330-1345mm we have
porosity on the cap of the weld.
Features that are likely to be reported during a
CVI include:

 Normal features, eg stop/start, other

intersecting welds, etc.
 All imperfections, whether specified in the
client’s CNC or not.
A weld set for a CVI
 Corrosion damage, etc.
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Inspection Methods Inspection Methods

Video. Photography.
Videography is often used in conjunction with visual There are two commonly used photographic
techniques to record their results.
techniques for underwater inspection:
Generally, diver-inspectors may use either:
 Helmet-mounted cameras (HMTV).
 Hand-held cameras.  Close-up photography
Offshore, IMCA guidance recommends that all divers wear  Close-up photography is when the camera lens is
HMTV cameras as a matter of safety. Cameras may also be within 1m from the subject.
mounted on ROVs or have fixed installations on the
structure. The optics of the camera can be used to magnify the
The video recording is accompanied by an oral commentary subject to elicit greater detail. Linear subjects, such
that may be performed either by the diver-inspector or by as welds, will require multiple overlapping
the inspection controller. Still images may be taken from a photographs (30 to 50% overlap) to be taken to form
video stream by picture-grabbing the image. what is termed a photo-mosaic.

Copyright © TWI Ltd Copyright © TWI Ltd

Inspection Methods Inspection Methods

Stand-off photography: Other photographic techniques:

Stand-off photography is when the camera lens is more

 If we combine two images within a computer
than 1m from the subject.
program, we may produce a three-dimensional
representation of the subject. This method is
This technique is used to capture images of large
known as stereo-photography.
features such as flanges, anodes, etc.

For reasons of picture quality, the maximum stand-off  Photogrammetry is a technique for taking
from the subject to the camera lens is limited to one measurements from photographs. This may be
third of the underwater visibility. achieved using a Reseau plate or by stereo-
photographic analysis.

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Inspection Methods Inspection Methods

Ultrasonic techniques.
The A-scan flaw detector displays major and minor signals
Ultrasonic NDT utilises the behaviour of high-frequency sound on a graphical screen. Interpreting the display requires
waves. It is particularly suitable for investigating two-dimensional skill and can be time-consuming.
(planar) features because they have relatively flat surfaces.
The A-scan is typically used for thickness measurement,
These surfaces will readily reflect the ultrasonic beam and will be
lamination checking and for ultrasonic weld inspections. A
more easily detected.
frequency of 5MHz is commonly used (A-scan & DTM).
There are several ultrasonic techniques that may be used
underwater:
 Digital Thickness Meter (DTM)
 The DTM filters out all minor signals and displays only the
distance to the major reflector on the digital display, making it
relatively easy and quick to use.

The DTM is commonly employed for wall thickness measurement

and basic lamination checking. The A-scan requires skilled interpretation.

Copyright © TWI Ltd Copyright © TWI Ltd

Inspection Methods Inspection Methods

Flooded member detection (FMD) Crack detection techniques

It is important to gauge whether the inside of a structural There are three crack detection techniques that can be
member is wet or dry. If the member is wet, then further used underwater:
investigation as to the cause of the flooding will be required.
Both diver-inspectors and ROVs can test for flooded 1. Magnetic particle inspection (MPI)
members by using ultrasonic FMD probes. The equipment MPI is a technique that involves magnetising the part that
makes use of the fact that an ultrasonic beam is blocked by is to be inspected, applying a detection ink and visually
a gas – ie a dry space. A low frequency ultrasound of inspecting for indications that betray the presence of
0.5MHz is used. surface imperfections. As such it can only be performed on
ferromagnetic metals such as steel, but not on non-
An ROV may also be fitted with radiographic FMD ferromagnetic materials. Also, the surface requires
equipment. This fires gamma radiation, using caesium 137, cleaning to a matt, bare-metal (SA2½) finish.
through the structural member and meters the amount of
radiation that passes through the enclosed space – a high Underwater MPI is used to detect the presence of surface-
reading would indicate a dry void. breaking cracks but will not give any indication of their
Radiographic FMD is too hazardous for diver use. depth.

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Inspection Methods Inspection Methods

2. Electromagnetic detection techniques There are two electromagnetic techniques that are used
A major advantage of electromagnetic crack detection underwater:
techniques is that, unlike MPI, they do not require the
removal of protective paint coatings. Also, they can be 1. Eddy current technique
used to detect cracks in non-ferromagnetic materials. Eddy current works by inducing a circular (eddy) current to
flow in the surface of the material. Any surface
However, a disadvantage of electromagnetic detection discontinuity will upset the symmetry of the eddy current
techniques is that they do not give visual indications of the and be detected by the equipment. Eddy current will
locations of defects and they require skilled operators to detect the presence of a surface-breaking crack, but will
interpret the data. not give any indication of its depth.
2. Alternating current field measurement (ACFM)
ACFM is a development of the eddy current technique but
works in two perpendicular planes. Thus, it has all the
same features as eddy current technique, but it can also
indicate the depth of the cracks that it finds.

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3
 27/08/2015

Inspection Methods Inspection Methods

3. Radiography As well as being an inspection technique, radiography also

Although X-ray radiation is used for topside inspections, produces a permanent recording – the radiograph.
the equipment is too complex to use underwater - gamma
radiation is employed instead. Gamma radiation is Measurements of the sizes of features and defects within
produced by the radioactive decay of radioisotopes such as the material may be made from the radiograph, as can the
iridium-192. thickness of the material itself.

Gamma radiography may detect planar defects within the The main drawback in using radiographic inspection is that
material if the radiation pathway is carefully aligned to the it is a potentially hazardous technique. The costs of the
same expected orientation as the defect. Volumetric equipment and safe management make it relatively
defects are more easily detected by radiography as the expensive.
user does not need to align the radiation pathway.

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Inspection Methods Taking Measurements Underwater

Many topside measuring tools may be used

successfully underwater with little or no
modification.

Linear measurements

Vernier calipers may be

used for measurements of
typically up to 150mm.
The Vernier caliper can
measure both linear features
and internal/external diameters.

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Taking Measurements Underwater Taking Measurements Underwater

Weld gauges may be used to measure small The Linear-angular Measurement (LAM) gauge may
scale linear features such as undercut, excess be used to measure many small linear and angular
weld material and fillet leg-length. They may features as per the weld gauge. The LAM gauge is
also be used to measure angles of up to 90°. superior to the weld gauge in that it may be used on
a curved surface and is particularly accurate when
measuring the depths of remedial grinds.

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Taking Measurements Underwater Taking Measurements Underwater

 Rulers or tape-measures are simple and Circular measurements:

practical small-to-medium scale measuring
Internal and external diameters may be measured using
tools. Magnetic tape-measures are especially calipers and gauges. Gauges possess a calibrated scale
useful in underwater close visual inspection. and are direct measurement tools. calipers are generally
used as comparators and are an indirect measuring tool
(an exception being the Vernier caliper – Vernier referring
 For large scale linear measurements we may to the type of scale used.)
use an open reel tape-measure, position
referencing using underwater transponders or Angular measurements:
narrow-beam ranging sonar that may be
Angles may be measured using a protractor, weld gauge
mounted on an ROV or hand-held by the
or LAM gauge.
diver-inspector.
Angles of large components such as mooring chains may
be measured using an inclinometer (pendulum gauge).

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Taking Measurements Underwater Taking Measurements Underwater

Special tools and jigs: Crack depth measurement

Peculiar measurements may require the The depth of cracks may be gauged using one of
fabrication of a special tool or jig. For example, three techniques:
an ovality jig is used for measuring circular
distortion in structural members. Alternating current field measurement (ACFM)
ACFM can both detect the presence of a surface-
Photogrammetry: breaking crack and give an indication of its
Measurements may be obtained by computer depth.
analysis of photographs in a technique termed
photogrammetry. This is an especially useful
method of measuring marine growth, anode
wastage, damage, etc.

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Taking Measurements Underwater Taking Measurements Underwater

Alternating current potential drop (ACPD).

Any conducting material will possess an electrical

resistance. ACPD equates a calibrated measurement of
electrical resistance (measured in terms of a voltage or
potential drop) to the distance travelled by the electrical
current.

The ACPD probe is calibrated and then placed so as to

span the known crack position. The potential drop
measured by the equipment indicates the depth of the
crack. Thus, ACPD will not locate a crack in a material, but
it will indicate the depth of a crack whose position is
known.

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Taking Measurements Underwater Taking Measurements Underwater

Investigation by hand grinding: Recording shapes and surface profiles.

The depth of a crack may be discerned by a careful
sequence of shallow grinds using a hand grinder.
On the microscopic-to-small scale, surface
Cathodic potential (CP) measurement: profiles may be recorded using a casting
Cathodic potential is a measurement of the electrical material. These are pressed onto the surface of
potential (voltage) at a certain location on a structure. This the material and allowed to take up the shape of
is achieved by connecting a voltmeter between the any defects present. The resulting cast is
structure and a reference electrode (reference cell) – most
commonly silver/silver chloride. Both the voltmeter and
removed and then analysed.
the reference cell are built into the tool, called a
Bathycorrometer. A carefully taken cast may be put into an
electron microscope to reveal detail of grain-
The cathodic potential value can imply the state of the sized features.
corrosion protection being offered to the structure.

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Taking Measurements Underwater Taking Measurements Underwater

Small-to-medium scale surface features may be Small-to-large scale surface profiles may be
recorded by taking a series of spot recorded using stereo-photography.
measurements with a Vernier caliper along an
area of impact damage or by using a profile Large scale surface profiles may be recorded
gauge. using a taut-wire survey. In this technique, a
wire is anchored at either end and stretched
tight along eg a damaged brace.

Regular measurements are made along the wire

to create a profile of the deformation. If
performed twice along perpendicular planes then
a three-dimensional representation of the
deformation may be gained.

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Any Questions?

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6
 Section 16

Inspection Maintenance and Repair,

Quality Assurance and Quality Control,
Recording and Reporting
16 Inspection Maintenance and Repair, Quality Assurance and
Quality Control, Recording and Reporting
16.1 Legislation relating to inspection of offshore structures
In 1992 the UK Government brought into force The Offshore Installations
(Safety Case) Regulations SI 2885 (1992). These regulations expand the Health
and Safety at Work Act (1974) to offshore structures in the UK sector of the
North Sea. These regulations are very wide ranging in scope and are further
clarified and made more precise by amplifying Guidance Notes, Approved Codes
of Practice and further Statutory Instruments.

DCR SI 913 (1996) Design and construction regulations

PSR SI 825 (1996) Pipeline safety regulations

PFEER SI 743 (1995) Prevention of fire, explosion and emergency

response regulations

MAR SI 738 (1995) Management and administration regulations

PUWER SI 2932 (1998) Provision and use of work equipment regulations

There are numerous requirements laid down in this legislation, but the main
intent for all of it is to reduce any risk to be:

As low as reasonably practical (ALARP).

DCR SI 913 (1996) and TOISCR SI 2885 (1992) specify and rely on
verification NOT certification. The regulations also specify that there must be
an appointed Duty Holder who has the authority to carry out a self-
certification scheme. The Duty Holder will normally be the operating company
and is responsible for ensuring that the structure remains in a safe condition to
carry out its design purpose.

The regulations specify that verification must be obtained from an

Independent Verifying Body (IDVB).

There are four IDVB’s appointed by the regulations:

1 Lloyd’s Register of Shipping.

2 DNV GL.
3 Bureau Veritas.
4 American Bureau of Shipping.

There is no statutory requirement to inspect structures, however the Duty

Holder must satisfy the IDVB that a particular structure or component does not
require any inspection to ensure safety and obtain verification of this.

The IDVB does have the authority to stop all operations on any structure if it
considers that it is damaged or that major alterations or deterioration are likely
to impair the structure’s ability to perform its design task.

In practice the Duty Holder will invariably evolve a full inspection programme
that will ensure the safety of plant and personnel, which must be submitted to
the IDVB for verification.

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 The Duty Holder must appoint the IDVB at the design stage of the structure
development so that continuity of verification may be maintained. More details
on the above regulations are given in section 1.

16.2 Structural integrity management of ageing installations

Ageing and life extension were addressed explicitly for the first time in the 2005
revision of the Offshore Installations (Safety Case) Regulations. This requires
the submission of a Revised Safety Case to the HSE where material changes to
the previous Safety Case have occurred, including extension of use of the
installation beyond its original design life.

Within this context, the Duty Holder is expected to be able to demonstrate that
major hazards due to, or associated with, ageing have been identified, that they
are adequately controlled and that all relevant statutory provisions will be
complied with.

This means that deterioration and degradation must be integrated into an Asset
Integrity Management (AIM) system and associated plan. The purpose of the
AIM plan is to provide a link between the assessment process and the
inspection strategy on an on-going basis.

16.3 The importance of QA and QC

QA and QC are two sides of the same coin. All the Offshore Operators operate
Quality Management Systems which all rely on Quality Assurance procedures to
ensure that all management functions, including but not limited to, efficient
operations, safety, conformance to legislation and protection of assets are
completed effectively, without waste or duplication up to required standards.

Furthermore, all these functions must be applied the same way every time to a
measurable standard; the entire system must also be actively managed and
continuously improved. The Quality Control is applied to ensure that all the
processes associated with the Management System are in fact complied with
and executed correctly. The QC ensures that the processes meet the measured
standard and that this fact is recorded.

The QA for offshore structures starts with the written procedures and continues
with the inspections, audits and other documentation that is certified and
recorded throughout the life of the structure. The QC follows this same path,
producing the documentation that verifies that the various processes have all
been completed.

There are a number of different methods that any company can adopt to ensure
that Total Quality Management (TQM) is achieved, but commonly BS EN ISO
9002:2002 is adopted.

The importance of QA and QC to any large organisation is that it is fundamental

to the ability of a company to function. Without procedures that demand a
certain standard, that are controlled by verification to ensure correct
compliance, a large company would descend into anarchy and bankruptcy very
quickly in the modern business world.

As indicated in section 1 QA and QC begin with the conception of the structure

and will continue throughout all the stages of the structure’s life right through
to decommissioning.

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16.4 Databases and trend analysis
Modern QA systems make extensive use of databases and offshore inspection
reporting follows this trend. The major factor to emphasise with a database
system is the way information is stored.

The great advantage of a database is the accessibility to the information. Each

item of information will have a number of tags; the information can then be
called up from different points. Take as an example that an anomaly has been
reported on a horizontal brace on a jacket.

The anomaly can be called up or accessed by:

 The type of anomaly: - pitting, crack, impact damage, etc.

 Using member identification.
 Using platform identification, in this case all damage on the platform will
be listed with its location.
 Using platform identification and defect type, in this case all defects of
that type on the platform will be listed with its location.

Once the database is set up trend analysis is facilitated and all the data
required for any type of analysis is both more extensive and more easily
accessed.

16.5 The importance of documentation and record keeping

The fundamental importance of the QA system indicates how important
documentation and record keeping are, and of course, documentation and
record keeping are fundamental elements of any quality system. The various
inspection reports, damage registers, fabrication drawings, material
documentation and other records, become part of the QA, forming the archives
that prove structural integrity.

Additionally, the offshore operators will maintain records for:

 Engineering assessments and analysis.

 Recording defects and damage.
 Maintaining the damage register.
 Monitoring defects or damage that is not repaired.
 Modifying the existing IMR programme where necessary.
 Evolving future structural designs.
 Compliance with the statutory requirements and verification by the
IDVB.

16.6 Types of reporting systems

There are basically two types of reporting systems:

16.6.1 Full reporting

This system requires that every item or component inspected, that has any
blemishes, deterioration or damage, no matter how minor, is reported as being
defective.

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 This system:

 Generates large volumes of data that must be reviewed by responsible

engineers.
 Much of the data will be considered non-relevant after being reviewed.
 The review of the data requires a good deal of time.
 It is possible that serious defects could deteriorate further during the
time taken to review the inspection data.

This system is seldom used offshore as the anomaly based reporting system is
preferred.

16.6.2 Anomaly based reporting

This system requires that only items that are outside specified parameters be
reported. Any other, blemishes, deterioration or minor damage is accepted with
the component being considered as fit for purpose. This system still requires
that every item included in the inspection program is fully inspected, but only
items outside the specifications are reported as defective.

This has several ramifications:

 The Duty Holder must specify the parameters for all types of damage or
deterioration. The normal response to this requirement is that a Criteria
of Non-Conformance (CNC), is evolved by the Duty Holder.

 All inspection personnel must be fully qualified in the various inspection

methods and skilled enough to apply the parameters laid down in the
CNC.

 There is a high level of responsibility on all inspection staff to properly

identify any indication found during the inspection, whether it is to be
reported or not, that is whether it is an anomaly or not. If an inspector
misses a reportable defect, it will remain undetected until the next
scheduled inspection that includes that component.

 Every reportable defect will require some actions to be initiated in

accordance with the instructions given in the CNC. These actions may be
to repair, conduct further inspections or to monitor. In any event, there
will be more reports and records generated to prove that the reported
anomalies have been dealt with properly.

16.7 Reasons why inspection is required

There are a number of reasons why any structure must be inspected regularly.

The safety of personnel is of paramount importance and a regular Inspection,

Maintenance And Repair (IMR) programme will ensure that component or
structural failure is avoided, thus guaranteeing the safety of personnel.

Any structure will deteriorate in service and a properly applied IMR cycle will be
designed to detect degradation, deterioration, fabrication defects, installation
damage, design uncertainties or errors, environmental overload or accidental
events. Then, items that require repair, renewal or replacement can be
identified and addressed in a timely manner, allowing these actions to be
undertaken as part of a planned, controlled programme.

The operators of any offshore structure must comply with the requirements of
Government Legislation and Statutory Instruments.

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 Whether structures are insured against damage by outside agencies or the risk
is carried by the operator, an IMR programme will be required so that the risk
of catastrophic failure is minimised.

A properly implemented IMR programme will provide raw data that can be
entered into databases to allow computer-based trend analysis for engineering
applications.

Inspection data can be used to evolve improved designs for later generation
structures, or for changes in permanent loading or re-use of the structure.

16.8 Continuity of inspection

The life of a structure, after the concept is agreed, may be split into six stages:

1 Design.
2 Production of the raw materials.
3 Fabrication.
4 Launch and commissioning.
5 In service.
6 Decommissioning.

Structural inspection programmes are instigated immediately after the

conception of the structure and then run throughout its life, forming part of the
quality management approach to structural engineering.

Statutory regulations also require that the operators ensure the structures are
fit for design purpose and that verification of this is obtained from appointed
Independent Verification Bodies.

16.9 Design stage

QA and QC (QA/QC) are an integral part of the design stage for any structure.
All the designs, design calculations and drawings have to be prepared and
completed to specified procedures that include checks and internal verification,
to ensure compliance with the numerous standards that apply to the various
engineering disciplines and functions.

At this stage, the Duty Holder will nominate the Independent Verification Body
that will be responsible during the life of the structure for verifying that all the
statutory requirements are met. At the outset, the design drawings are
subjected to a form of inspection, validation and verification before they are
sent to the yard for fabrication.

Also, at the outset, a marking system will be evolved or adopted so that every
component can be identified, tracked, audited and inspected throughout its life
cycle. With topside items this may be a Tag System where unique identification
numbers are assigned to components such as pumps or generators. The jacket
of a structure will similarly have an identification system specific to the
requirements for the component parts.

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16.9.1 Structural marking systems
There are several marking systems used in the North Sea, but all are based on
a grid system, where the structure is considered in plan view to have x and y
co-ordinates and the various depth levels are the z component.

Examples of three systems are included here, but any personnel involved with
structural inspection must ensure that they understand the actual system used
on whatever structure they inspect.

16.9.2 Unique Identification System

In this system, which is an extension of the Tag System, each platform has a
unique 3-digit identification number. This is used as a prefix to a 6-digit number
made up of 3 pairs. The first pair is a 2-digit code number for the type of
component.

 Main legs 11
 Horizontal braces 12
 Main nodes 13

And so on for the various types of member making up the jacket. The next 2
digits indicate the level starting with 0 at the top of the jacket where the
module is located. Finally, the last 2 digits are the identification number of that
type of component on that level (figure 16.1).

Numbering starts here

Example -
013/12/03/04

Figure 16.1 Unique Identification System.

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16.9.3 The Alpha Numeric System
This system uses letters to denote different levels, starting with A at the top of
the jacket and working down the alphabet as the levels descend. This first letter
is followed by an alphanumeric pair that identify the x and y co-ordinates on the
level.

Figure 16.2 Alpha Numeric System.

16.10 The Box Matrix System

The box matrix system firstly denotes a letter for each type of member.

Member M
Node N
Riser R
Conductor C
Pile guides P
Anodes A

And so on for all types of members.

Then, the levels are denoted by letters, starting with A at the top of the jacket
and working down the alphabet as the depth levels increase. Finally, there are 2
digits that represent the x and y co-ordinates.

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 Figure 16.3 The Box Matrix Marking System.

16.11 Clock orientation and datum points

In conjunction with the platform marking system, datum points will be
nominated. Tubular members are always inspected clockwise and so the
clock orientation is nominated.

The 12 o’clock position is invariably the datum point and this may be marked
with up to three punch marks during the fabrication stage. Figure 16.4 shows
the common orientation for reading the clock.

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 12
9
3
9 6

12
6
12 3
3

9
12
3 3

6 12 9 6

Figure 16.4 Clock positions.

In order to determine the clock positions you must know which way to face,
whether up or down, into the node or platform North etc.

Datum Marking

Figure 16.5 Datum marks.

Datum marks can be three punch marks at the 12 o’clock position.

16.12 Safety Critical Elements (SCE)

Another design function is to calculate, identify and specify the numerous safety
critical elements (SCEs) that exist on the jacket, the platform modules and in all
the systems and sub-systems. A SCE is any part or parts of an offshore
installation the failure of which, would cause or substantially contribute to a
major incident, or a component the purpose of which is to prevent or limit the
effect of a major incident.

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 Examples of SCE are:

Systems
 Primary structure.
 Fire and water systems.
 Fire and gas detection systems.
 Hydrocarbon containment systems.

Subsystems
 Mooring system.
 Deluge system.
 Control panels and computer software controlling safety systems.

Equipment
 Mooring system main bearing.
 Fire pumps.
 Fire detection heads.
 Electrical equipment in hazardous areas.

The following advice on SCEs in fixed and mobile installations has been given by
Stacey and Sharp (OMAE2011-49090).

Fixed steel installations

Steel substructure, foundations, topsides, temporary refuge and escape routes,
and helideck.

Semi-submersible structures
Hull, mooring system, stability (ballast and control) systems, temporary refuge
and escape routes and the helideck.

Jack-up structures
Legs, rack and jacking system, foundations (spud cans etc.), deck, temporary
refuge and escape system.

16.13 Production of the raw materials

During the design stage of the structure’s development, decisions are made
regarding what materials are to be used in the fabrication of the structure.
These materials are chosen with careful attention as to their suitability for the
design task.

Mild steel, to the 50D specification with tight quality control over manufacture,
is a common choice for North Sea steel structures. Other materials are selected
for different design specifications and concrete for gravity structures is equally
carefully specified.

Taking steel as an example, the steel foundry producing the steel will provide
certified documentation with casting specification and material composition.
Plates are serial numbered and totally traceable. Other specified materials will
have similar documentation.

All these documents are verified and filed as part of the QC function. The
materials supplied to the fabrication yards are stored in controlled locations so
that it remains fit for purpose and traceable. As it is issued, certification goes
with it and is filed with the as built drawings and other documentation.

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 The raw materials may contain flaws or manufacturing defects in spite of the
best QA and QC arrangements.

This has two ramifications:

1 The QA systems employed to minimise defects will be aimed at 100%

effectiveness, however 98% may be realistically achievable. This would be
normally acceptable provided that these defects were with design limits.

2 The presence of manufacturing defects in the raw material would not

normally cause in-service failures, but there is a small risk that they might.
Within the realms of good engineering practice and design tolerances this is
acceptable.

16.14 Fabrication stage

The QA and QC continue throughout the fabrication stage of the structure
development. Welding procedures and parameters are all carefully applied and
certified. Concrete composition is monitored, confirmed and certified. All the
inspection documentation and certification is verified and filed with the
remaining documents, continuing to build up the QA database.

The types of flaws that may occur during fabrication were discussed in Section
11.

16.15 Launching and installation

The launching and installation stages in the structure life are again subjected to
tight QC and the relevant supervision, inspection, control and reporting
processes continue to be implemented and recorded. It is reasonably obvious
that launching a 15,000T jacket, swinging it from horizontal to vertical, then
sinking it upright in the correct position, at the correct orientation, must require
very thorough procedures and tight control.

It is at this stage that the most extensive underwater inspection will take place.
The inspection undertaken on the structure so far has been extensive and will
not be so comprehensive for the remainder of the life of the structure, although
it will continue un-interrupted.

16.15.1 Base line survey

The first Major In-Water Inspection will be totally comprehensive, including
the entire jacket and all underwater components, attachments and
appurtenances including, of course, the SCEs. The seabed surrounding the
structure will be inspected up to 30-50m from the base. A complete CP system
survey will also be completed, including CP readings and an anode count to
confirm the presence and the physical integrity of each anode. This survey is
usually referred to as the Base Line Survey.

The base line survey will confirm that:

 The structure is in the correct position.

 Whether any significant damage has occurred during launch and
installation.
 That any damage that is identified is accurately reported and recorded.
 That any significant defects are highlighted for immediate rectification.

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 The structure will only be declared fit for purpose when either there are no
reported significant defects or any reported significant defects have been
rectified and re-inspected and the results of the Base Line Survey are accepted
by the Duty Holder and verified by the appointed approved Verification Body.

The base line survey will be used to evolve or modify an on-going inspection
programme that will extend throughout the remaining life of the structure.

16.15.2 In service
The in-service IMR programme will ensure that adequate monitoring is
accomplished to satisfy the requirements of the Duty Holder, Safe Working
Practices and to achieve verification by the IDVB.

The usual programme for in-service inspections is based on a five-year cycle.

Some items are inspected each year while others are inspected less frequently
within the five-year cycle. Each year 20% of all SCE are inspected as a matter
of routine, so that over the five years all will be inspected once. There is always
the option to change the schedule should the need to do this be identified.

A typical annual inspection would include:

 CP survey.
 Inspection of all risers, conductors and caissons.
 CVI of 20% of selected representative welds.
 MPI or similar inspection system eg ACFM of 20% of selected
representative welds.
 Complete GVI of the entire structure.
 Debris and marine growth survey.
 Scour survey.

Items on the damage register may be monitored during the annual inspection.

The annual inspection programme reports are monitored and used to update
the damage register and to modify the IMR programme as necessary.

16.16 Damage survey

During the service life of a structure, damage may occur at any time due to
environmental forces, accidents, failure etc. It is a requirement that all damage
be reported and that any incidents are also reported. Unfortunately, not all
incidents are actually reported and damage underwater cannot be seen unless
there is an underwater inspection programme.

It has been determined that approximately 70% of damage found offshore is

primarily due to:

 Collision by shipping.
 Fatigue failure.
 Dropped objects.

Furthermore, the biggest majority of this type of damage was discovered by

routine inspections.

During any inspection, the basis of the inspection is to report anything that
appears to be not in the as made condition. Superimposed on top of this basic
rule of thumb, are any specific instructions contained in the Damage Survey
Workbook.

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16.17 How the Criteria of Non-Conformance System is applied
The Criteria for Non-Conformance is a set of parameters, issued by the Duty
Holder’s Engineering Department, that defines the limit of acceptance of any
damage or defect that may be identified during any inspection.

During any inspection if an item is identified as not being in the as made

condition, the CNC is referred to, to determine whether the anomaly is
reportable or can be considered as being within acceptable engineering
parameters. As an illustration, an extract of a typical CNC table is shown below.

Table 16.1 Extract of a typical CNC table.

Inspection Possible Anomaly Criteria of non-
field anomalies code conformance
Coating damage CD Any
General visual
Debris DB Metallic or hazardous
inspection
Physical damage PD Any
Greater than 2mm
Weld inspection Corrosion on welds CR
deep
Outside – 850 to
CP survey Cathodic Potential CP
–1100 mV
Anode survey Anode wastage AW Severe > 75%
Riser Leak LK Any

Table 16.2 Extract of a typical technical specification.

Key to Abbreviations
Abbreviation Meaning Abbreviation Meaning
AW Anode wastage LI Lack of integrity
CD Coating damage LK Leak
CP Cathodic potential readings PD Physical damage
CR Corrosion SD Seal displacement
DB Debris WT Wall thickness

Table 16.3.
Additional
Anomaly Actions to be taken
checks
AW Record anode identification and position CR,CP,DB,LI
Take additional CP measurements to
CP AW,CR,CB
establish extent
Measure corroded area and cover in area,
CR maximum and average depth and diameter of AW,CR,CP,DB
pile in the area
Record type, position and dimensions, include
DB AW,CD,CP,DB
a sketch
Record flange identification and location,
LK PD,LI,DB,CR,SD
sketch and estimate rate of loss of product
Record element and location, take additional
WT CR,PD,CP
WT readings to assess the extent of the area

In the event of finding an anomaly, the CNC will normally include either, a list
of follow-up actions that are authorised to be implemented without further
instructions, or to have a table of technical specifications that lays down these
follow up actions.

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 The method for applying the CNC is straightforward. When any item is identified
as being outside the CNC, the actions dictated by the technical specification are
implemented. The item, and all the data concerning it, is recorded and
submitted in the inspection report. Any authorised immediate follow up action is
implemented and the fact that this has been done, together with the results of
the actions taken, is recorded and reported (figure 16.5).

Figure 16.5 Flow chart for CNC actions.

In the flow chart, the reference to the job card applies to the actions taken by
the Duty Holder’s Engineering Department. A job card system, for initiating
work actions of any kind, is a common approach to control of resources,
personnel and finances throughout industry.

16.18 Documentation in an anomaly-based reporting system

The documentation involved with an anomaly-based system, is normally
standardised data report sheets and is commonly on a computer-based system.
With this method, the responsible engineers become familiar with one method
of presentation of information, which saves time when reviewing the inspection
data.

Using standard data sheets also ensures that all the required information is
included and does not rely on memory.

The data report sheets will be reviewed and subsequently form the basis for:

 Further follow-up or additional structural inspection programmes.

 Maintenance or repair projects.
 Any engineering analysis that is required by the Duty Holder.
 Reports submitted to the IDVB for verification.

On completion of any review action, the reports will be archived to become part
of the QA records for the structure.

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16.18.1 Work scopes and workbooks in an anomaly-based system
A central feature of an anomaly-based system will be a defined Scope of Work
for each inspection campaign, detailing the extent of the inspection, the
components or items to be inspected and the required tasks. The Scope of Work
will be contained in the Workbook issued at the start of the campaign. The
Workbook is a part of the contractual documentation.

The workbook will contain:

 Scope of work.
 List of inspection tasks.
 Procedures.
 CNC.
 All the required drawings.
 Blank log sheets.
 Blank data sheets.
 Anomaly report forms.
 Daily report forms.
 An extract of the damage register applying to the items in the Scope of
Work.

In some Operating Companies, the inspection tasks take the form of task code
listings, which contain three digit codes. Each code represents a group of tasks,
all associated with one activity. Then, within that group, different aspects or
applications of the activity can be specified (table 16.4).

Table 16.4 Example of task load listing.

Task
Task Specifications
Group
100 General swim round GVI report any gross damage
101 Specific swim round Anode inspection
200 Weld inspection Cleaning for inspection
201 Weld CVI CVI of specified welds
300 Marine growth survey General marine growth survey
301 Specific marine growth survey Report build-up of specified species

The Workbook will generally be in sections:

Work scope
This will define the extent of the inspection.

Technical specifications
Define design details, such as brace diameters, wall thicknesses and structural
marking system.

Field and platform data

These are structural drawings, field layout drawings and environmental details,
such as water depth, tides and currents.

CNC and anomaly reporting requirements

Acceptance criteria to be applied to any anomalies identified during the
inspection and reporting requirements.

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 Data sheets and log sheets
These contain master copies of standard report forms, to be copied as
necessary in a paper reporting system. In a computer reporting system, the
formats will be in the associated computer programme.

16.18.2 Damage register

The damage register forms a crucial part of the documentation. Any damage on
the structure is recorded on the register, along with the engineering action
taken to resolve the damage. Any new damage identified during inspection
activities will be added to the register.

16.18.3 Data sheets

Data sheets are all computer-generated and most commonly the inspection will
be recorded directly onto a computer. It is becoming more prevalent that the
data gathered during the inspection is input into a database, which is accessible
by any authorised party.

The advantage of having a database is the accessibility of the information that

lends itself very well to any engineering analysis that may be required. An
example of a typical data sheet is shown in figure 16.6.

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 Figure 16.6 Typical data sheet.

16.19 Verbal reporting

Verbal reports are normally concerned with diver intervention and frequently
follow a question and answer scenario, with the topside asking specific
questions for the diver inspector to respond to. The voice reproduction is via the
diver communication unit, which may leave something to be desired regarding
clarity of reproduction.

A radio procedure approach is adopted, with the diver being the control station.
Care must be exercised to speak plainly and the diver’s speech is sometimes
difficult to understand because of poor amplification.

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 When receiving verbal reports from saturation divers, the situation can be quite
difficult, as their speech has to go through a un-scrambler because of the
distortion to the voice pitch caused by the helium environment.

Always:

 Use correct terminology.

 Maintain a fluent flow of speech (decide what to say before you speak).

If a full verbal report is to be recorded onto a video, ensure that the

introduction includes answers to the questions:

 Who is speaking?
 Where is the inspection site?
 What actions are being completed?
 When is the verbal report being made?

16.20 Corrosion protection and coating inspection report requirements

The normal inspection reporting requirements for CP surveys are:

 Visual assessment of anode condition and any deposits, including their type
and extent.
 % of anode wastage (may be required to be measured).
 Sacrificial anode stub integrity.
 Impressed current cable and cable duct integrity, also electrode
connections.
 Marine growth build-up on any anodes.
 Metallic debris in contact with the structure, identify location, type and
quantity.
 CP readings at specified sites or spacing.
 Photographs of representative anodes.

Coating inspections will normally require the following:

 Note % presence of topcoat, primer and bare metal.

 Note any blistering and burst them, (if the client requests this), and try to
collect a sample of any deposit.
 Surface condition of the steel under any blister.
 Note % of any paint cracking.
 Note any paint sagging with the extent.
 Note any paint wrinkling with the extent.
 Note any flaking and note the extent.

16.21 Procedure for the close visual inspection of a weld

Verbal reports are commonly recorded onto data sheets during the actual
inspection.

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 A typical procedure for a weld CVI would include:

 Complete surface checks of all the equipment.

 Obtain the work permits or confirm they have been obtained.
 Locate the correct weld.
 GVI to assess any gross damage and the build-up of marine growth.
 Clean to SA2½ (or any other specified standard) 75mm either side of the
weld, plus as much as necessary for access to SA1.
 Establish the datum and a tape measure and then mark up the clock
positions.
 Measure and record the overall weld length (in accordance with the
established platform conventions or procedure).
 Complete the CVI.
 Record the results in real time on the data sheet.
 Record the CVI onto video/DVD/hard drive.
 Take still photographs as required.
 Recover all equipment.
 Wash all equipment in fresh water.
 Report any anomaly outside the CNC in accordance with the instructions
given (normally to the Offshore Client Representative).
 Take any follow up remedial actions authorised in the CNC (normally in
consultation with the Offshore Client Representative).
 Cancel work permits when clear of the work site.
 Incorporate the data sheet into the final report, along with any new
anomaly report sheets and any remedial actions taken.

An example of a typical CVI sheet is contained in figure 16.7.

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 Visual Inspection Report Sheet Report Sheet Number:
Client: Date: Sheet: of:
Dive spread: Diver: Dive No.:
Drawing sheet No.: CCTV Log No.: Photo Log No.:
Inspection Eng.: Cleaning Standard:
Equipment used: Component Ref:

Detail:












Signed:
Supervisor: Inspection Engineer: Client:

Figure 16.7 Typical CVI data sheet.

16.22 Summary of other recording methods used underwater

Because underwater inspection with intervention methods is so costly and
weather dependent, a good deal of effort is expended in ensuring that any data
gained during inspection activities is recorded. This can then be retrieved
whenever there is a future need to reassess an item. The methods for recording
can be very simple or complex as illustrated in the following notes.

Scratchboards
This may be a rigid white plastic board or a plastic sheet of paper with a pencil.
Can be used underwater by a diver to make a sketch or take notes when
nothing else is available.

Sketches
These can be useful if there is no other way of recording what the diver
inspector has identified or, incorporated into a report to show a point of detail.
Both scratchboards and sketches may be the only practical way of recording a
diver’s impressions in black water or bad visibility.

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 Photography
Good quality images of any anomalous items or points of interest may be
obtained by photography and prints included in any type of report.

Video/DVD/Hard drive
Is a prime method for recording all manner of inspection information and is
extensively used throughout the underwater commercial industry. The video
records real time images, which may subsequently be incorporated into a
report. Increasingly, good quality grabbed photographs are being used instead
of still-photographs and, though picture quality lags behind the best digital still
cameras, they are often adequate for most needs.

Radiography
A radiograph is a permanent record by itself and is easily incorporated into a
report.

Casts
These may be taken on occasion, but they are difficult to take and incorporate
into a report. There are other ways of recording the images set into casts and,
whenever possible, these are used in preference to taking a cast. When they
are taken, care must be exercised in storing them as they can be easily
damaged.

EMD, EMT and ACFM incorporating computer recording

These methods are all available with computer recording facilities and the
ability to have the programme print out the data. This is ideal for incorporation
into a formal report.

Sampling
This is normally a specialist form of recording of marine life for example. It does
have other uses, such as collecting gas escaping from the seabed for analysis.
Storage of samples is a difficulty that must be anticipated before the sampling
is undertaken.

16.23 Certification of personnel and equipment

Any Quality Management System (QMS) will include the requirement that
personnel and equipment are certified to a known standard. In the case of
personnel in the UK sector of the North Sea the Certification Scheme for
Personnel (CSWIP) Scheme for underwater inspectors has been adopted.

The scheme certifies four types of inspectors as detailed in the preface. The
more important proficiencies for 3.1U diver inspector level of certification is
outlined below

16.23.1 CSWIP grade 3.1U diver inspector

This diver inspector will be proficient in:

 Visual inspection.
 The use of video in hat mounted or hand held deployment mode.
 Taking still photographs.
 Taking CP readings using contact type equipment.
 Taking ultrasonic wall thickness readings using an ultrasonic digital
thickness meter.

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16.23.2 Equipment certification
Any equipment used during an inspection programme, must be certified so as
to conform to the QA requirements. This requirement is intended to ensure that
all equipment is, safe to operate, in good working order and is within the
required calibration specifications.

The requirements for legal compliance and the typical QA calibration

requirements are:

 Electrical equipment shall be tested six monthly.

 All electrical equipment must be tested for safety each time it is used; eg all
residual current devices (RCD) should be tested daily, any other type of
electrical equipment should be visually inspected and confirmed safe to use
before each use.

 All inspection equipment shall be calibration checked before and after each
use.

 A competent person shall calibrate all electrical equipment at intervals

prescribed by regulations or manufacturer’s recommendations and certify
this fact. This includes voltmeters and digital volt meters, which shall be
calibrated annually and tagged to confirm the date the next calibration is
due.

16.24 Inspection activities in an anomaly based system

Good planning and briefings are the difference between success and, at worst,
failure with any project, not least inspection campaigns. Good planning and
briefing leads to efficient, focused and effective inspection activities that are
themselves crucial in ensuring that the tasks are completed as quickly as
possible, without having to go back, that the correct items are inspected to the
correct standard; that the personnel involved are all focused on the job,
motivated and sufficiently well briefed and skilled to be able to recognise any
defects and apply the correct actions to them.

These activities are fundamental in any anomaly based reporting system, as it

is recognised that any items being inspected in real time will not be subjected
to any secondary inspection. Should anything be missed during the current
inspection, it will remain undiscovered until there is another scheduled
inspection of that component.

16.24.1 Real time data gathering

Real time data gathering means that the personnel on the job inspect and
assess the components under inspection. Any items, judged to be outside the
CNC, will be flagged up and recorded in real time, on video, by photography or
by electronic means. This data may then be re-evaluated, at a later date, for
second opinions or follow up analysis. The crucial point is that data is only
submitted if it is judged to be anomalous at the time of inspection.

As a comparison some forms of inspection lend themselves to retrospective

inspection. For example, Eddy Current inspection and ACFM inspection store
real time data on computer programmes. The original inspector will make on
the spot judgements, but all the data can subsequently be reviewed. The
computer will contain all of the gathered information, not a selection
determined by the original inspector.

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16.25 Decommissioning
When the fields become depleted, the various platforms and seabed
completions will be removed. The QA and QC will continue throughout this
process to ensure that everything actually is removed and to verify that the
ocean floor is left clear of any debris.

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Inspection Maintenance and Repair 16-23 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Galbraith D N, Wolfram W R, Leivestad S, ‘ISO floating and fixed Standards ISO

19902, ISO 19903 and ISO 19904’, Proc. Of the 2008 Offshore Technology
Conference, Paper OTC 19608, 2008, Houston.

Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office

Books), 1988, ISBN 13: 9780114129118.

Stacey A, Birkinshaw M, Sharp J V, May P, ’Structural integrity management

framework for fixed jacket structures’, Proc. 27th Int. Conf. on Offshore
Mechanics and Arctic Engineering, Paper OMAE2008-57413, 2008, Portugal.

Stacey A, ‘KP4: Ageing and life extension inspection programme for offshore
installations’, Proc. 30th Int. Conf. on Ocean, Offshore Mechanics and Arctic
Engineering, Paper OMAE2011-49089, 2011, The Netherlands.

Stacey A, Sharp J V, ‘Ageing and life extension considerations in the integrity

management of fixed and mobile offshore installations’, Proc. 30th Int. Conf. on
Ocean, Offshore Mechanics and Arctic Engineering, Paper OMAE2011-49090,
2011, The Netherlands.

Stacey A, Sharp J V, ‘Structural integrity management framework for mobile

installations’, Proc. 30th Int. Conf. on Ocean, Offshore Mechanics and Arctic
Engineering, Paper OMAE2011-49656, 2011, The Netherlands.

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 27/08/2015

Legislation Relating to Inspection

The Offshore Installations (Safety Case) Regulations SI 2885

(1992) expand the Health and Safety at Work Act (1974) to
offshore structures in the UK sector of the North Sea.

These regulations are very wide ranging in scope and are

CSWIP 3.1U Course further clarified and are made more precise by:
Inspection, Maintenance and Repair,
 Amplifying guidance notes.
QA/QC, Recording and Reporting  Approved Codes of Practice.
 Statutory Instruments.
Section 16
There are numerous requirements laid down in this legislation
but the main intent for all of it is to reduce any risk to be:

 As Low As Reasonably Practical (ALARP).

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Legislation Relating to Inspection Legislation Relating to Inspection

DCR SI 913 and TOISCR SI 2885 specify and rely on The regulations specify that verification must be
verification not certification: obtained from an Independent Verifying Body
(IDVB).
 The regulations also specify that there must be
an appointed Duty Holder who has the There are four IDVBs appointed by the
authority to carry out a self-certification regulations:
scheme.
 The Duty Holder will normally be a named  Lloyds Register of Shipping.
individual of the operating company.  DNV GL.
 The Duty Holder is responsible for ensuring that  Bureau Veritas.
the structure remains in a safe condition to
carry out its design purpose .  American Bureau of Shipping.

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Legislation Relating to Inspection Legislation Relating to Inspection

 There is no statutory requirement to inspect  In practice, the Duty Holder will, invariably,
structures, however, the Duty Holder must evolve a full inspection programme that will
satisfy the IDVB that a structure or ensure the safety of plant and personnel,
component does not require any inspection to which must be submitted to the IDVB for
ensure safety and obtain verification of this. verification.

 The IDVB does have the authority to stop all  The Duty Holder must appoint the IDVB at
operations on any structure if it considers that the design Stage of the structure
it is damaged or that major alterations or development so that continuity of
deterioration are likely to impair the verification may be maintained.
structure’s ability to perform its design task.

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Legislation Relating to Inspection Legislation Relating to Inspection

Structural integrity management of ageing Within this context, the Duty Holder has to
installations. demonstrate that major hazards due to, or
associated with, ageing have been identified,
Ageing and life extension were addressed explicitly for adequately controlled and that all relevant
the first time in the 2005 revision of the Offshore statutory provisions will be complied with.
Installation (Safety Case) Regulations. This requires
the submission of a revised safety case to the HSE This means that deterioration must be integrated
where material changes to the previous safety case into an Asset Integrity Management (AIM)
have occurred, including extension of use beyond system and associated plan. The purpose of the
original design life. AIM plan is to provide a link between the
assessment process and the inspection strategy
on an on-going basis.

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The Importance of QA and QC The Importance of QA and QC

All the offshore operators operate Quality Furthermore, all these functions must be applied
Management systems, which all rely on quality the same way every time to a measurable
assurance procedures to ensure that all standard; the entire system must also be
management functions including, efficient actively managed and continuously improved.
operations, safety, conformance to legislation
and protection of assets are completed The quality control is applied to ensure that all
effectively, without waste or duplication up to the processes associated with the management
the required standards. system are in fact complied with and executed
correctly. The QC ensures that the processes
meet the measured standard and that this fact is
recorded.

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The Importance of QA and QC Databases and Trend Analysis

 The QA for offshore structures starts with the  Modern QA systems make extensive use of
written procedures and continues with the databases and offshore inspection reporting
inspections, audits and other documentation follows this trend.
that is certified and recorded throughout the
life of the structure.  The major factor to emphasise with a
database system is the way the information is
 The QC follows this same path, producing the stored.
documentation that verifies that the various
processes have all been completed.  The great advantage of a database is the
accessibility to the information, each item will
have a number of Tags, the information can
then be called up from different points.

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Importance of
Databases and Trend Analysis
Documentation and Record Keeping
Take for example that an anomaly has been reported on a Documentation and record keeping are fundamental elements
horizontal brace on a Jacket. The anomaly can be called up of any quality system. The various inspection reports, damage
or accessed by: registers, fabrication drawings, documents and records become
part of the QA, forming the archives that prove structural
 The type of anomaly, pitting, crack, impact damage etc. integrity.
 The member identification.
 The platform identification, which will show all damage Additionally, the offshore operators will maintain records for:
on the platform listed with its location.  Engineering assessments and analysis.
 The platform identification and defect type will list all  Recording defects and damage.
defects of that type on the platform with its location.  Maintaining the damage register.
 Monitoring unrepaired defects or damage.
Once the database is set up, trend analysis is facilitated  Modifying the existing IMR programme where necessary.
and all the data required for any type of analysis is both  Evolving future structural designs.
more extensive and more easily accessed.  Compliance with statutory requirements and verification by
the IDVB.

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Types of Reporting Systems Types of Reporting Systems

There are basically two types of reporting systems. Anomaly based reporting (The preferred
system):
Full reporting:
 This system requires that every item or component
inspected that has any blemishes, deterioration or  This system requires that only items that are
damage no matter how minor, is reported as being outside specified parameters are reported. Any
defective. other blemishes, deterioration or minor
 This generates large volumes of data that must be damage is accepted with the component being
reviewed by responsible engineers.
 Much of the data will be considered non-relevant after
considered as fit for purpose.
being reviewed.  This system still requires that every item
 The review of the data requires a good deal of time and included in the inspection programme is fully
It is possible that serious defects could deteriorate inspected, but only items outside the
further during the time taken to review the data. specifications are reported as defective.
 This has several ramifications…

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Types of Reporting Systems Types of Reporting Systems

 The Duty Holder must specify the parameters  There is a high level of responsibility on all inspection
for all types of damage or deterioration. The staff to properly identify any indication found, whether
it is to be reported or not, (ie whether it is an anomaly
normal response to this requirement is that a or not). If any inspector misses any reportable defect, it
Criteria of Non-Conformance (CNC) is evolved will remain undetected until the next scheduled
by the Duty Holder. inspection that includes that component.

 All inspection personnel must be fully qualified  Every reportable defect will require some actions to be
initiated in accordance with the instructions given in the
in the various inspection methods and skilled CNC. These actions may be to repair, monitor or
enough to make value judgements on the job conduct further inspections. In any event there will be
site, in real time, to apply the parameters laid more reports and records generated to prove that the
down in the CNC. reported anomalies have been dealt with properly.

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Reasons Why Inspection Is Required Reasons Why Inspection Is Required

There are a number of reasons why any structure must be  Whether structures are insured against damage
inspected regularly.
by outside agencies or the risk is carried by the
The safety of personnel is of paramount importance and a Operator an IMR programme will be required so
regular IMR programme will ensure that component or structural that the risk of catastrophic failure is minimised.
failure is avoided, thereby guaranteeing the safety of personnel.
 A properly implemented IMR programme will
Any structure will deteriorate in service and a properly applied
IMR cycle will target and identify items that require repair,
provide raw data that can be entered into data
renewal or replacement in a timely manner, allowing these bases for computer analysis to complete a trend
actions to be undertaken as part of a planned, controlled analysis for engineering applications.
programme.
 Inspection data can be utilised to evolve
The operators of any offshore structure must comply with the
requirements of Government Legislation and Statutory
improved designs for later generation structures.
Instruments.

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Continuity of Inspection Design Stage

The life of a structure may be split into six Stages:  QA/QC is an integral part of the design Stage for any
structure. All the design calculations and drawings have
1. Design. to be prepared and completed to specified procedures,
2. Production of the raw materials. that include checks and internal verification, to ensure
3. Fabrication. compliance with the numerous standards.
4. Launch and commissioning.
5. In service.  At this Stage, the Duty Holder will nominate the IDVB
6. Decommissioning. that will be responsible during the life of the structure,
for verifying that all statutory requirements are met.
Structural inspection programmes are instigated immediately
after the conception of the structure, then run throughout its  At the outset then, the design drawings are subjected to
life, forming part of the quality management approach to a form of inspection, validation and verification before
structural engineering. they are sent to the yard for fabrication.
Statutory regulations also require that the operators ensure the
structures are fit for design purpose and that verification of this
is obtained from appointed IDVBs.

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Design Stage Structural Marking Systems

 At the outset, a marking system will be  There are several marking systems used in the
evolved so that every component can be North Sea, but all are based on a grid system,
identified, tracked, audited and inspected where the structure is considered in plan view
throughout its life cycle. to have x and y co-ordinates and the various
depth levels are the z component.
 With topside items this may be a Tag system,
where unique numbers are assigned to each  Examples of three systems are included here,
component. but any personnel involved with structural
inspection must ensure that they understand
 The Jacket structure will similarly have an the actual system used on whatever structure
identification system specific to the they inspect.
requirements for the component parts.

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Structural Marking Systems Unique Identification System

Unique identification system: Numbering starts here

In this system, an extension of the Tag system, each
platform has a unique 3-digit identification number. This is Example - 013/12/03/04
used as a prefix to a 6-digit number made up of 3 pairs.
Identifies the
The first pair is a 2-digit code number for the type of 013 structure
component.
Identifies the
12 component
 Main legs are 11.
 Horizontal braces are 12. 03 Denotes the level
 Main Nodes are 13.
Is the fourth
And so on for the various types of member making up the 04 Horizontal brace
on that level
Jacket. The next 2 digits indicate the level starting with 0
at the top of the Jacket where the module is located.
Finally, the last 2 digits are the identification number of
that type of component on that level.

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Alpha Numeric System The Box Matrix System

 This system uses This system firstly denotes a letter for each type of component.
letters to denote
different levels Member M
starting with A Diagonal
at the top of the F
member
Jacket working Node N
down the alphabet And so on for all types of components
Riser R
as the levels
descend. Conductor C
 This first letter is Pile Guides P
followed by an Anode A
alphanumeric pair
that identify the x Then, the levels are denoted by letters, starting with A at the
and y co-ordinates top of the Jacket and working down the alphabet as the levels
on the level. descend. Finally, there are 2 digits that represent the x and y
co-ordinates.

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The Box Matrix System Clock Orientation and Datum Points

 In conjunction with the

platform marking system,
datum points will be
nominated.
 Tubular members are
always inspected clockwise
and so the clock
orientation is nominated.
 12 o’clock is invariably the
datum point and this may
be marked with up to three
punch marks during the
fabrication Stage.

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Safety Critical Elements (SCE) Safety Critical Elements (SCE)

 Another design function is to calculate, Systems:

identify and specify the numerous safety  Primary structure.
 Fire and water systems.
critical elements (SCE) that exist on the  Fire and gas detection systems.
Jacket, the platform modules and in all the  Hydrocarbon containment systems.
systems and sub-systems. Sub-systems:
 Mooring system.
 A SCE is any part or parts of an offshore  Deluge system.
installation, the failure of which, would cause  Control panels.
Equipment:
or substantially contribute to a major incident
 Mooring system main bearing.
or, a component the purpose of which is to  Fire pumps.
prevent or limit the effect of a major incident.  Fire detection heads.
 Electrical equipment in hazardous areas.
 Examples of SCE’s are…

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Production of Raw Materials Fabrication Stage

 During the design Stage of the structure’s development,  The QA and QC continue throughout the
decisions are made regarding what materials are to be fabrication stage. Welding procedures and
used, these materials are chosen with careful attention
to their suitability for the design task.
parameters are all carefully applied and
certified. Concrete composition is monitored,
 Steel, for example, will have been provided with
certification regarding casting specification and material
confirmed and certified.
composition. Plates are serial numbered and totally
traceable.  All the inspection documentation and
 All these documents are verified and filed as part of the certification is verified and filed with the
QC function. The materials supplied to the fabrication remaining documents, continuing the build up
yards are stored in controlled locations so that it of the QA database.
remains fit for purpose and traceable.
 As it is issued, certification goes with it and it is filed
with the as-built drawings and other documentation.

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Launching and Installation Base Line Survey

The launching and installation Stages are again The first major in-water inspection (Base line survey) will be
subjected to tight QC and the relevant totally comprehensive, it will comprise of the entire Jacket and
all underwater components, including the SCE. The seabed will
supervision, inspection control and reporting be inspected up to 50m from the base.
processes continue to be implemented and A CP system survey will be completed including CP readings
recorded. and an anode count to confirm the presence and physical
integrity of each anode.
It is at this Stage that the most extensive
The base line survey will confirm that:
underwater inspection will take place.
 The structure is in the correct position.
The inspection undertaken on the structure so  Whether any significant damage has occurred during
far, has been extensive and will not be so installation.
comprehensive for the remainder of its life,  Any damage identified is accurately reported and recorded.
 Any significant defects are highlighted for immediate
although it will continue un-interrupted. rectification.

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Base Line Survey In Service

 The structure will only be declared fit for purpose The In-service IMR programme will ensure that adequate
when either there are no reported significant monitoring is accomplished to satisfy the requirements of:
defects or, any reported defects have been
 The Duty Holder.
rectified, re-inspected and the results accepted by  Safe Working Practices.
the Duty Holder and verified by the appointed  To achieve verification by the IDVB.
IDVB.
The usual programme for in-service inspections is based on
 The Base Line Survey will be used to evolve or a five-year cycle. Some items are inspected each year while
others are inspected less frequently within the five-year
modify an ongoing inspection programme that will cycle.
extend throughout the remaining life of the
structure. Each year 20% of all SCE’s are inspected as a matter of
routine so that over the five years all will be inspected once.
There is always the option to change the schedule should
the need to do this be identified.

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In Service Damage Survey

A typical annual inspection would include: During the service life of a structure, damage
may occur at any time due to environmental
 CP survey. forces, accidents, failure and numerous other
 Inspection of all riser, conductors and caissons.
 CVI of 20% of selected representative welds.
causes.
 MPI of 20% of selected representative welds.
 Complete GVI of the entire structure. It is a requirement that all damage be reported
 Debris and marine growth survey. and that any incidents are also reported.
 Scour survey.
 Items on the damage register may be monitored during
the annual inspection.
Unfortunately, not all incidents are actually
 The annual inspection programme reports are reported and damage underwater cannot be
monitored and used to update the damage register and seen unless there is an underwater inspection
to modify the IMR programme. programme.

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Damage Survey How the CNC System is Applied

It has been determined that approximately 70% of The Criteria of Non-Conformance is a set of
damage found offshore is primarily due to: parameters issued by the Duty Holder’s
 Collision by shipping. engineering department that defines the limit of
 Fatigue failure. acceptance of any damage or defect that may be
 Dropped objects. identified during any inspection.
Furthermore, the biggest majority of this type of damage
was discovered by routine inspections. During an inspection, if any item is identified as
not being in the as-built condition, the CNC is
During any inspection the basis of the inspection is to
report anything that appears to be not in the as made referred to, so as to determine whether the
condition. anomaly is reportable or can be considered as
being within the acceptable engineering
Superimposed on top of this basic rule of thumb are any parameters.
specific instructions contained in the Damage Survey
Workbook.
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Extract of a Typical Technical

Extract of a Typical CNC Table
Specification
Anomaly Actions To Be Taken Additional Checks
Possible
Inspection field Anomaly code Criteria of non-conformance
anomalies
AW Record anode identification and position CR, CP, DB, LI

Take additional CP measurements to establish

Coating damage CD Any CP AW, CR, DB
General visual extent
Debris DB Metallic or hazardous
Inspection
Physical damage PD Any Measure corroded area & cover in area,
CR maximum and average depth and diameter of AW, CR, CP, DB
pits in the area
Weld Corrosion on
CR Greater than 2mm deep
inspection welds Record type, position and dimensions, include
DB AW, CD, CP, DB
a sketch
Cathodic Outside – 850 to –1100mV
CP survey CP
potential Will vary from client to client Record flange identification and location,
LK PD, LI, DB, CR, SD
sketch and estimate rate of loss of product
Anode survey Anode waStage AW Severe > 75%
Record element and location, take additional
WT CR, PD, CP
WT readings to assess the extent of the area
Riser Leak LK Any

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How the CNC System is Applied Flowchart of CNC Actions

In the event of finding an anomaly, the CNC either

includes follow up actions that are authorised to be
implemented without further instructions or, has a table of
technical specifications that lays down these follow up
actions.

So, the method for applying CNC is straightforward.

When any item is identified as being outside the CNC, the

actions dictated by the technical specification are
implemented. The item and all data concerning it is
recorded and submitted in the inspection report.

Any authorised follow up action is implemented and the

fact that this has been done, together with the results of
the actions taken, is recorded and reported.

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Documentation in Documentation in
Anomaly Based Reporting System Anomaly Based Reporting System
The documentation involved with an anomaly- The data report sheets will be reviewed and
based system is normally in the form of subsequently form the basis for:
standardised data report sheets and is
commonly on a computer-based system.  Further follow-up or additional structural inspection
programmes.
With this method, the responsible engineers  Maintenance or repair projects.
 Any engineering analysis that is required by the Duty
become familiar with one method of presentation Holder.
of information, which saves time when reviewing  Reports submitted to the IDVB for verification.
the inspection data.
On completion of any review action the reports
Using standard data sheets also ensures that all will be archived to become part of the QA
the required information is included and does records for the structure.
not rely on memory.

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Work Scopes and Workbooks Work Scopes and Workbooks

A central feature of an anomaly based system The workbook will contain:

will be a defined Scope of Work for each
inspection campaign, detailing the extent of the  The scope of work.
inspection, the components or items to be  The list of inspection tasks.
inspected and the required tasks.  The procedures.
 The CNC.
The Scope of Work will be contained in the  All the required drawings.
workbook issued at the start of the campaign  An extract of the damage register applying to
and is part of the contractual documentation. the items in the scope of work.
 Blank log sheets.
 Blank data sheets.
 Anomaly report forms.
 Daily report forms.

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Work Scopes and Workbooks Work Scopes and Workbooks

The workbook will generally be in sections: CNC and anomaly reporting requirements -
Acceptance criteria to be applied to any
Work scope - Defines the extent of the anomalies identified during the inspection and
inspection. reporting requirements.

Technical specifications - Defines design details Data sheets and log sheets - Contains master
such as brace diameters, wall thicknesses, copies of standard report forms, to be copied as
structural marking system. necessary in a paper reporting system. In a
computer reporting system the formats will be in
Field and platform data - Structural drawings, the associated computer programme.
field layout drawings and environmental details
such as water depth, tides, currents.

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Damage Register/Data Sheets Typical Data Sheet

Damage register forms a crucial part of the

documentation. Any damage on the structure is recorded
on the register along with the engineering action taken to
resolve the damage. Any new damage identified during
inspection activities will be added to the register.

Data sheets are all computer generated and most

commonly, the inspection will be recorded directly onto a
computer. It is becoming more prevalent that data
gathered during the inspection is input into the database,
which is accessible by any authorised party, an advantage
which lends itself to any engineering analysis that may be
required.

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Verbal Reporting Verbal Reporting

Verbal reports are normally concerned with diver A radio procedure approach is adopted with the
intervention and frequently follow a question and diver being the control station. Care must be
answer scenario with the topside asking specific exercised to speak plainly and the diver’s speech
questions for the diver inspector to respond to. is sometimes difficult to understand because of
poor amplification.
The voice reproduction is via the diver
communication unit and may leave something to With saturation divers the situation can be quite
be desired regarding clarity of reproduction. difficult, as their speech has to go through an
un-scrambler because of the distortion to the
voice pitch caused by the helium environment.

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Verbal Reporting CP Reporting Requirements

The normal inspection reporting requirements for CP
Always: surveys:
 Use correct terminology.  Visual assessment of anode condition, and deposits
 Maintain a fluent flow of speech (decide what including their type and extent.
to say before you speak).  % of anode wastage (may be required to be
 RSVP - rhythm, speed, volume, pitch. measured).
 Sacrificial anode stub integrity.
If a full verbal report is to be recorded onto a  Impressed current cable and cable duct integrity
video ensure that the introduction includes and electrode connection.
answers to the questions:  Marine growth build up on any anodes.
 Who is speaking?  Metallic debris in contact with the structure identify
 Where is the inspection site? location, type and quantity.
 What actions are being completed?  CP readings at specified sites or spacing.
 When is the verbal report being made?  Photographs of representative anodes.
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Coatings Inspection Reporting Procedure for CVI of a Weld

Coating inspections will require that the following be • Locate the correct weld.
assessed: • GVI to assess any gross damage and build up of marine
 Note the % present of topcoat, primer and bare metal. growth.
 Note any blistering, burst them (if the client • Clean to SA2½ (or any other specified standard) 75mm
permits/requests) and try to collect a sample of any either side of the weld.
deposit. • Establish the datum - position a tape measure and mark
 Assess the surface condition of the steel under any up the clock positions.
blister. • Measure and record the overall weld length (in
 Note the % of any paint cracking. accordance with the established platform conventions or
 Note any paint sagging with the extent. procedure).
 Note any paint wrinkling with the extent. • Complete the CVI.
 Note any flaking and note the extent. • Record the results in real time on the data sheet.
• Record the CVI on video.
If inspecting Monel cladding be careful to inspect very • Take still photographs as required.
closely for any breaks or deformation however small in the
monel surface.

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Typical CVI Data Sheet Summary of Other Recording Methods

Recording methods can be very simple or

Visual Inspection Report Sheet Report Sheet Number:

Client: Date: Sheet: of:

Dive spread: Diver: Dive No.: complex and includes:

Drawing sheet No.: CTV Log No.: Photo Log No.:

Inspection Eng.: Cleaning Standard:

Equipment used: Component Ref: • Scratchboards.

Detail: • Sketches.
• Photography.
• Video.
• Radiography.
• Casts.
• EMD, EMT and ACFM with computer recording.
• Sampling.
Signed:

Supervisor: Inspection Engineer: Client:

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Certification of Personnel Certification of Equipment

CSWIP grade 3.1U Diver Inspector Equipment used during an inspection programme must be
certified so as to conform with legal and QA requirements.
This is to ensure that all equipment is safe to operate, in
This diver inspector will be proficient in: good working order and is within the calibration
specifications.
 Visual inspection.
 The use of CCTV in the hat mounted or hand  Electric equipment should be tested six monthly.
held deployment mode.  All electric equipment must be tested for safety each
time it is used eg RCD’s should be tested daily.
 Taking still photographs.  All inspection equipment should be calibrated before
 Taking CP readings. and after each use.
 Taking ultrasonic wall thickness readings using  A competent person shall calibrate all electric
an ultrasonic digital thickness meter. equipment at intervals prescribed by regulations or by
the manufacturer and certify this fact.

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Inspection Activities Inspection Activities

in an Anomaly Based System in an Anomaly Based System
Good planning and briefings are the difference between These activities are fundamental in any anomaly
success and failure with any project. based reporting system as it is recognised that
They lead to efficient, focused and effective inspection
any items inspected in real time will not be
activities that are themselves crucial in ensuring that the subjected to any secondary inspection.
tasks are completed as quickly as possible without having
to go back, that the correct items are inspected to the Should anything be missed during the current
correct standard, that personnel involved are focused, inspection it will remain undiscovered until the
motivated and sufficiently briefed and skilled and to be
able to recognise any defects and apply the correct actions
next scheduled inspection of that component.
to them.

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Real Time Data Gathering Decommissioning

Real time data gathering means that the personnel on the When the fields become depleted the various
job inspect and assess the components under inspection. platforms, pipelines and seabed completions will
Any items judged to be outside the CNC (Criteria of non-
be removed.
conformance) will be flagged up and recorded in real time
on video, by photography or by electronic means. The QA and QC will continue throughout this
process to ensure that everything is actually
This data may then be re-evaluated at a later date for removed and to verify that the ocean floor is left
second opinions or follow up analysis.
clear of any debris.
The crucial point is that data is only submitted if it is
judged to be anomalous at the moment in time when it is
inspected.

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Any Questions?

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12
 Section 17

Cleaning for Inspection and Profile Grinding

17 Cleaning for Inspection and Profile Grinding
17.1 Cleaning
There are two prime reasons for cleaning areas of structures. The first is to
prepare the area for CVI, MPI or other NDT. The second is to remove excessive
marine growth, debris or other fouling.

In either case only discrete areas of the structure will be cleaned, not the entire
jacket. A number of methods for cleaning exist and are listed in Table 17.1.

Table 17.1 Existing methods for cleaning.

Method Advantages Disadvantages ROV
Hand cleaning Inexpensive, easy to Slow, diver fatigue, poor No
(scrapers, etc.) deploy finish
Pneumatics More efficient Depth limits, control, exhaust Yes
Hydraulics More efficient Expensive, limited choice, Yes
bulky hose
HP water jet Fast, effective, least Hazardous, leaves reflective No
damaging to steel surface
HP water jet with Fast, matt finish Hazardous, may damage the No
grit entrainment surface of steel
Grit blasting Removes all growth, Hazardous, maintenance, Yes
(with water) matt finish backup team required
LP air grit Fast, matt finish May be depth limited, large Yes
compressor required
Cavitation jet Effective on hard growth, Will not remove soft growth Yes
safe, no grit
Inhibitors/ No diver intervention Only in splash zone, No
Henderson rings required environmental impact

17.1.1 HP water jets

HP water jets are widely used offshore as are the grit entrainment and LP grit
blasting systems. These methods of cleaning have potential to harm the
operator and, therefore, some safety considerations must be included with any
discussion as to their use. A Hughes standard system is shown in Figure 17.1.

Figure 17.1 Hughes HP water jet.

DIS1-30815
Cleaning for Inspection and Profile Grinding 17-1 Copyright © TWI Ltd
 Safety considerations are:
 Never block or wire the trigger open.
 When in use never point at anything other than the area to be cleaned.
 Keep clear of any retro-jets.
 Never get any part of the body in front of the jet.
 Ensure that all HP hoses, fittings and unions are in test, good condition and
are correctly fitted and tightened. Whip-checks should be fitted to all joints.
 If grit is used be aware of the grit entering the life-support system because
of circulation in the water.
 If grit penetrates the suit or gloves under pressure take medical advice
immediately.
 Treat the equipment with respect; it is capable of maiming or even killing if
not handled correctly.
 All HP water jet or grit guns must be properly designed for underwater use.

17.2 Diving Medical Advisory Committee (DMAC) advice

The Diving Medical Advisory Committee (DMAC) has published the following
advice on managing any accident that might occur while using this type of
equipment.

The wound caused may appear insignificant and give little indication of the
extent of the injury beneath and the damage to deeper tissue. Large quantities
of water may have punctured the skin, flesh or organs through a very small
hole that may not even bleed.

Initial mild damage to the wall of an organ may result in subsequent rupture,
particularly if infection has been introduced. The development of subsequent
infection is particularly important in abdominal injuries.

17.2.1 Management of any injury

The outcome depends upon the extent of the initial injury and the presence or
absence of infection. Even though the injury seems trivial on the surface and
the patient has no complaints, it is of great importance to arrange for medical
examination as quickly as possible.

DIS1-30815
Cleaning for Inspection and Profile Grinding 17-2 Copyright © TWI Ltd
 In a remote location, where surgical examination is not immediately possible,
first aid measures are confined to dressing the wound and observing the patient
closely for the development of further complaints over four or five days.

The development of fever and a rising pulse rate suggest the injury is serious,
together with the persistence or occurrence of pain. On evacuation, the diver
should carry the following card, which outlines the possible nature of the injury.

This person has been involved with high pressure water jetting up to
14,500psi (100MPa, 1000 bar, 1019Kg/cm) with a jet velocity of 900mph
(1440Km/hr.)

Please take this into account when making your diagnosis.

Unusual infections with micro-aerophilic organisms occurring at low

temperatures have been reported. These may be gram negative
pathogens such as those found in sewage. Bacterial swabs and blood
cultures may, therefore, be helpful.

17.3 Standard of surface finish

The standard of surface finish that is normally adopted in the North Sea was
originally a Swedish standard for specifying grit blast cleaning of steel prior to
the application of paint coatings.

This standard in now incorporated into BS EN ISO 8501-1:2001, BS 7079-A1:

1989 and BS EN ISO 8501-1: Supplement: 2001, BS 7079-A1: Supplement 1:
1996.

The specifications from these standards normally applied offshore are as

follows:

SA1: Light cleaning, removal of gross fouling (for general visual

inspection).

SA2: Cleaning to paint coat including removal of loose paint and

corrosion products.

SA2½: Very thorough blast cleaning with grit entrainment, resulting in a

dull matt metal finish. This is the most widely adopted cleaning
level applied offshore as it leaves a matt surface, sometimes
referred to as stippled, that does not unduly reflect light. It is an
excellent surface for all NDT.

SA3: Very thorough blast cleaning to bright shiny metal. This is good for
most inspection but will reflect light and is, therefore, not such a
good surface if video and photography are employed. Most
commonly used for A-scan ultrasonic inspections.

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Cleaning for Inspection and Profile Grinding 17-3 Copyright © TWI Ltd
17.3.1 Area to be cleaned
The size of the cleaned area must be large enough to ensure that there is a
valid inspection but small enough to ensure that time is not wasted in
unnecessary cleaning.

For CVI and MPI the area cleaned to SA2½ should include the weld cap and an
area 75mm either side of it, measured from the weld toe. Also, an area large
enough to allow access for the inspection equipment and the diver inspector
should be cleaned to SA1 either side of the weld, see Figure 17.2.

Figure 17.2 Cleaned area.

17.4 Profile grinding

A metal subjected to alternating stresses in the elastic range, can after a
certain number of cycles, develop surface cracking. If the metal continues to be
exposed to these cyclical stresses, it will ultimately fail. For welded structures,
their fatigue life is reduced over that of plain metal, firstly, due to the profile of
the weld and secondly by the presence of flaws within the weld and the heat
affected zone.

The angle between the weld reinforcement and the parent material
concentrates the stresses and the greater the angle the poorer the fatigue life
of the weld, with failure most likely to occur at the weld toe. Flaws within the
finished weld will also concentrate stresses, with planar flaws such as lack of
fusion having the greatest effect.

For welded joints, one of the commonest flaws is undercut in the toe of a weld.
As the weld profile may already be concentrating stresses, the presence of even
minute undercut in this area will increase this further.

A number of post weld methods such as grinding, hammer peening, shot

peening and TIG and plasma re-melting are available to improve the fatigue
strength of welded joints.

DIS1-30815
Cleaning for Inspection and Profile Grinding 17-4 Copyright © TWI Ltd
 Practically grinding equipment is commonly available on-site and therefore is a
readily applicable technique. Care should be taken however as heavy disc
grinding can result in score marks and it has been shown that fatigue cracks
can initiate from these if they are perpendicular to the direction of applied
stress.

Profile grinding may be required during the fabrication stage of the structure’s
life as a means of improving the profile of fabrication welds that may have
process faults, such as, excessive weld metal, undercut, poor restart, stray arc,
spatter or any other fabrication flaws.

If pressure vessels, such as caissons and conductors are constructed to PD

5500:2000, all welds should be dressed to comply with the requirements of the
standard. Profile grinding obviously has an established place in welding
fabrication.

Regarding the in-service stage for any structure, the need to employ profile
grinding may be dictated by:

 The need to establish whether or not any indications identified during MPI or
EMD investigations are actually cracks.
 The requirement to grind out any cracks that are actually confirmed during
inspection activities.
 The practice of removing identified notches or stress raisers discovered
during the normal IMR cycle.

When profile or remedial grinding is undertaken it will be authorised, either by

the CNC or by instructions from the Duty Holder’s Engineering Department, via
the Onsite Client Representative. The actual parameters for the grinding will be
given in a written instruction.

17.4.1 Remedial grinding

The most common application of remedial grinding is when crack-like features
are identified during either MPI or EMD activities undertaken as part of the
annual IMR program.

A common inclusion in a typical CNC is the instruction to grind out any

indications to a maximum depth of 2mm, in 0.5mm steps. There is normally a
requirement to re-inspect after each step to determine whether the indication
has been ground out or not.

The Onshore Engineering Department, in accordance with their requirements

and procedures, will initiate further follow up actions if the indication remains
after the full 2mm depth is reached (Figure 17.3).

DIS1-30815
Cleaning for Inspection and Profile Grinding 17-5 Copyright © TWI Ltd
 Figure 17.3 Remedial and profile grinding.

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Cleaning for Inspection and Profile Grinding 17-6 Copyright © TWI Ltd
 Bibliography
Bayliss M, Short D, Bax M, ‘Underwater Inspection’, CRC Press, 1990, ISBN 13:
9780419135401.

Lancaster J F, ‘Metallurgy of Welding’, Woodhead Publishing, 1999,

ISBN 13: 9781855734289.

Porter L K, ‘A Handbook for Underwater Inspectors’, HMSO (Stationery Office

Books), 1988, ISBN 13: 9780114129118.

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Cleaning for Inspection and Profile Grinding 17-7 Copyright © TWI Ltd
 27/08/2015

Cleaning for Inspection

There are two prime reasons for cleaning an

area of a structure.

 To prepare the area for CVI, MPI or other NDT.

CSWIP 3.1U Course  To remove excessive marine growth, debris or
Cleaning for Inspection and Profile Grinding other fouling.
Section 17
In either case only discrete areas of the
structure will be cleaned, not the entire jacket.

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Cleaning Methods

Method Advantages Disadvantages ROV

Hand Cleaning Cheap, easy to deploy Slow, poor finish, diver fatigue No
Pneumatics More efficient Depth limits, exhaust Yes
Hydraulics More efficient Expensive, limited choice, bulky hose Yes
Fast, effective, Hazardous,
HP water jet Yes
least damaging leaves reflective surface
HP Water with Hazardous,
Fast, matt finish No
grit entrainment may damage the surface
Removes all growth Hazardous,
Grit blasting Yes
matt finish maintenance backup team required
LP air grit Depth Limited,
Fast, matt finish Yes
(LP slurry) large compressor required
Effective on hard
Cavitation jet Will not remove soft marine growth Yes
growth, safe, no grit
Inhibitors/ Environmental impact
No diver intervention No
Henderson rings only in splash zone

Copyright © TWI Ltd Copyright © TWI Ltd

HP Water Jets

HP water jets are widely used offshore as are the grit

entrainment and LP grit blasting systems. These methods of
cleaning have potential to harm the operator and therefore
some safety points must be considered.

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 27/08/2015

Diving Medical Advisory Committee

Safety Considerations
(DMAC)
 Never block or wire the trigger open.
DMAC has published advice on managing any accident
 Never point at anything other than the area to be
cleaned. that might occur while using this type of equipment.
 Keep clear of any retro-jets. The wound caused may appear insignificant and give
 Never get any part of the body in front of the jet. little indication of the extent of the injury beneath and
 Ensure that all HP hoses, fittings and unions are in the damage to deeper tissue. Large quantities of
test, good condition and are correctly fitted and water may have punctured the skin, flesh and organs
tightened. through a very small hole that may not even bleed.
If grit is used be aware of the grit entering the life-
Initial mild damage to the wall of an organ may result


support system because of circulation in the water.

 If grit penetrates the suit or gloves under pressure
in subsequent rupture, particularly if infection has
take medical advice immediately. been introduced. The development of subsequent
 Treat the equipment with respect, it is capable of infection is particularly important in abdominal
maiming or even killing if not handled correctly. injuries.

Copyright © TWI Ltd Copyright © TWI Ltd

Management of Any Injury Management of Any Injury

 The outcome depends upon the extent of the  Where this is not immediately possible, first
initial injury and the presence or absence of aid measures are confined to dressing the
infection. wound and observing the patient closely for
the development of further complaints over
 Even though the injury seems trivial on the four or five days.
surface and the patient has no complaints, it is
of great importance to arrange for medical  The development of fever and a rising pulse
examination as quickly as possible. rate suggest the injury is serious together with
persistence or occurrence of pain.

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Management of Any Injury Standard of Surface Finish

On evacuation the diver should carry the following card SA1: Light cleaning, removal of gross fouling for GVI.
which outlines the possible nature of the injury.
SA2: Cleaning to paint coat including removal of loose paint and
corrosion products.
This person has been involved with high pressure water
jetting up to 14,500psi (1000bar) with a jet velocity of
900mph (1440kmh). SA2½: Thorough blast-cleaning with grit entrainment
resulting in dull matt metal finish. This is the most widely
Please take this into account when making your diagnosis. adopted cleaning level applied offshore as it leaves a
surface, sometimes referred to as stippled, that does not
Unusual infections with micro-aerophilic organisms occurring unduly reflect light. It is an excellent surface for all NDT.
at low temperatures have been reported. These may be
gram negative pathogens such as those found in sewage.
Bacterial swabs and blood cultures may therefore be helpful. SA3: Thorough blast cleaning to bright shiny metal. This is good
for most inspection but will reflect light and is therefore
not such a good surface if CCTV and photography are
employed.

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Cleaning Areas for Weld Inspection Profile Grinding

A metal subjected to alternating stresses in the elastic

range, can after a certain number of cycles, develop
surface cracking. If the metal continues to be exposed to
these cyclical stresses, it will ultimately fail. For welded
structures, their fatigue life is reduced over that of plain
metal, firstly, due to the profile of the weld and secondly
by the presence of flaws within the weld and the heat
affected zone.

The angle between the weld reinforcement and the parent

material concentrates the stresses and the greater the
angle the poorer the fatigue life of the weld, with failure
most likely to occur at the weld toe. Flaws within the
finished weld will also concentrate stresses, with planar
flaws such as lack of fusion having the greatest effect.

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Profile Grinding Profile Grinding

 For welded joints, one of the commonest flaws is undercut Profile grinding may be required during the
in the toe of a weld. As the weld profile may already be fabrication stage of the structure’s life as a means
concentrating stresses in this area, the presence of even
minute undercut in this area will increase this further.
of improving the profile of fabrication welds that
may have process faults, such as, excessive weld
 A number of post weld methods such as grinding, hammer
metal, undercut, poor restart, stray arc, spatter or
peening, shot peening and TIG and plasma re-melting are any other fabrication flaws.
available to improve the fatigue strength of welded joints.
If pressure vessels, such as caissons and
 Practically grinding equipment is commonly available on-
conductors are constructed to PD 5500:2000, all
site and therefore is a readily applicable technique. Care
should be taken however, as heavy disc grinding can result welds should be dressed to comply with the
in score marks and it has been shown that fatigue cracks requirements of the standard. Profile grinding
can initiate from these if they are perpendicular to the obviously has an established place in welding
direction of applied stress. fabrication.
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Profile Grinding Remedial Grinding

During the in-service stage of any structure the need to employ  The most common application of remedial grinding is
profile grinding may be dictated by: when crack-like features are identified during either MPI
or EMD activities undertaken as part of the annual IMR
 The need to establish whether or not any indications program.
identified during MPI or EMD investigations are actually
cracks.  A common inclusion in a typical CNC is the instruction to
 The requirement to grind out any cracks that are actually grind out any indications to a maximum depth of 2mm,
confirmed during inspection activities. in 0.5mm steps. There is normally a requirement to re-
 The practice of removing identified notches or stress raisers inspect after each step to determine whether the
discovered during the normal IMR cycle. indication has been ground out or not.
 When profile or remedial grinding is undertaken it will be
authorised, either by the CNC or by instructions from the  The Onshore Engineering Department, in accordance
Duty Holder’s Engineering Department, via the Onsite Client with their requirements and procedures, will initiate
Representative. The actual parameters for the grinding will further follow up actions if the indication remains after
be given in a written instruction. the full 2mm depth is reached.

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 27/08/2015

Grinding Indications Final Profiling

Crack

Weld

Pit gauge
Weld

Weld Area ground to maximum

agreed and profiled to relieve
Toe ground out to an stress
agreed maximum

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