ARPEL Guideline

Storage tanks inspection plans

July 2012
ARPEL Guideline: storage tanks inspection plans

ARPEL Guideline MP02-2012

July 2012

This document was devised within the context Carlos Hughes – STAATSOLIE
of the Mechanical Integrity and Risks Analysis Aubrey Nai Chung Tong – STAATSOLIE
Project, of the ARPEL Refining Committee. The David Jalma – WPC
ARPEL Refining Committee was in charge of Roberto Cuadros – YPFB (Vice-chairmanship)
the content and review of this document: Guillermo Achá Morales - YPFB

Mauro de Camilli - ANCAP The ARPEL Mechanical Integrity Project Team

Daniel Cebey - ANCAP was responsible for drawing up this
Rosario Martino- ANCAP document.
Nikolai Guchín – ANCAP
Jaime López – ECOPETROL ANCAP: Silvia Infanzón; ECOPETROL: Luis
Jairo Buitrago – ECOPETROL Eduardo Zabala; ENAP: Gustavo Jiménez;
Daniel Ramirez Livingston -ENAP ExxonMobil: Daniel Santamarina; IBP: Luiz
Oscar Gonzalez Contreras -ENAP Moschini; PCJ: Brian Case; ARPEL: Irene
Daniel Santamarina - ExxonMobil Alfaro.
Victor Casalotti - IAPG
Ernani Filgueiras – IBP In addition, we are most greatful to the
Andrea Reid- PCJ (Vice-chairmanship) following professionals for their invaluable
Othón Valverde Yáñez - PEMEX contribution:
Rodrigo Abramof – PETROBRAS (Vice-
chairmanship) Álvaro Montes de Oca, Juan Carlos Hernández,
Frederico Kremer – PETROBRAS Rodolfo Ibarrondo (ANCAP); Francisco Elicer
Angela Martins – PETROBRAS (ENAP); Hervandil Morosini Sant'Anna
Sergio Fontes – PETROBRAS (PETROBRAS).
Geraldo Marcio Diniz Santos - PETROBRAS
Marco Calvopiña – EPPETROECUADOR
Nelson Chulde - EPPETROECUADOR
Gerardo León Castillo - PETROPERU
Alfredo Coronel Escobar – PETROPERU
Margaret Ocando – PETROTRIN
Sergio Cavallín – PLUSPETROL
Julian Meiller – PLUSPETROL
Henry Arias Jiménez - RECOPE
Oscar Acuña Céspedes – RECOPE
Carlos Jiménez López – REPSOL
(Chairmanship)
Arsedio Carbajal González - REPSOL

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 ARPEL Guideline: storage tanks inspection plans

Copyright
The copyright of this document, whether in
print or electronically, is held by the Regional
Association of Oil, Gas and Biofuels Sector
Companies in Latin America and the Caribbean
(ARPEL). Any copy of this document must
include this copyright notice. When this
document is used in the future, the user shall
give full credit to ARPEL as the source of
information.

Disclaimer
Even though every effort has been made to
ensure the accuracy of the information
contained in this publication, neither ARPEL,
nor any of its members will assume liability for
any use made thereof. Any reference to
names or registered trademarks neither by
the authors, nor by ARPEL or any of its
member companies does not represent an
endorsement.

ARPEL Guideline Nº MP02-2012

ARPEL Guideline: storage tanks inspection plans

Content

1. Objective of the guideline ................................................................................................................1

2. Scope of the guideline ......................................................................................................................1
3. Related documents...........................................................................................................................1
4. Background .......................................................................................................................................1
5. Glossary 1
6. Reasons to establish a tank inspection plan.....................................................................................7
7. Causes to revise a tank inspection plan............................................................................................7
8. Development of a tank inspection plan ...........................................................................................8
8.1. Tank records ...........................................................................................................................9
8.2. Referential analysis of typical failure cases ............................................................................9
8.3. Tank failures record................................................................................................................9
8.4. Degradation mechanisms .....................................................................................................10
8.5. Failure scenarios ...................................................................................................................14
8.6. Inspection and testing methods ...........................................................................................15
8.6.1. Visual inspection......................................................................................................15
8.6.1.1. Thermal insulation ...................................................................................15
8.6.1.2. Protection paint .......................................................................................15
8.6.1.2.1. Blisters ....................................................................................15
8.6.1.2.2. Chalking ..................................................................................16
8.6.1.2.3. Abrasion/erosion....................................................................16
8.6.1.2.4. Crackings, wrinkles and corrosion points scattered across the
painted area. ..........................................................................16
8.6.1.3. Cathodic protection .................................................................................16
8.6.1.4. Electrical grounding .................................................................................16
8.6.1.5. Stairs and platforms .................................................................................16
8.6.1.6. Safety devices...........................................................................................17
8.6.1.7. Tank base inspection ................................................................................17
8.6.1.8. External inspection of shell and roof .......................................................17
8.6.1.9. Internal inspection ...................................................................................17
8.6.1.10. Roof evaluation ........................................................................................17
8.6.1.11. Auxiliary systems evaluation ....................................................................18
8.6.2. Inspection with hammer .........................................................................................18
8.6.3. Penetrating liquids...................................................................................................18
8.6.4. Magnetic particles ...................................................................................................18
8.6.5. Ultrasound ...............................................................................................................18
8.6.6. Radiography.............................................................................................................19
8.6.7. ACFM (Alternative Current Field Measurement) ....................................................19
8.6.8. Magnetic flux leakage (MFL and LFET) ....................................................................19
8.6.9. Thermography .........................................................................................................19
8.6.10. Acoustic emission ....................................................................................................19
8.6.11. Tightness..................................................................................................................20

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 ARPEL Guideline: storage tanks inspection plans

8.6.12. Resistance test .........................................................................................................20

8.6.13. Settlement evaluation .............................................................................................21
8.6.13.1. Shell settlement .......................................................................................21
8.6.13.2. Edge settlement .......................................................................................22
8.6.13.3. Other types of settlements ......................................................................22
8.6.13.4. Establishing acceptable settlement .........................................................22
8.6.13.5. Assessment to be carried out during the hydrostatic test.......................22
8.6.13.5.1. Initial study .............................................................................22
8.6.13.5.2. Study during hydrostatic test .................................................22
8.7. Summary of testing methods ...............................................................................................22
8.8. Effectiveness of testing methods .........................................................................................26
9. Analysis of inspection results .........................................................................................................27
9.1. Calculation of remaining life ................................................................................................27
9.2. Risk based inspection ...........................................................................................................28
9.2.1. Data collection .........................................................................................................29
9.2.2. Risk analysis .............................................................................................................29
9.2.3. Inspection plan ........................................................................................................30
9.2.4. Mitigation ................................................................................................................30
9.2.5. Re-assessment: ........................................................................................................30
9.3. Inspection frequency ............................................................................................................30
9.4. Advanced analysis methods and adaptation to use – approval criteria ..............................31
10. Qualification of inspection and testing personnel .........................................................................31
10.1. Introduction..........................................................................................................................31
10.2. Certification methodology ...................................................................................................32
10.3. Certified non-destructive testing methods ..........................................................................33
10.4. Certification levels ................................................................................................................33
10.5. Certification and qualification requirements .......................................................................33
10.6. Exams....................................................................................................................................34
10.7. Certificates ...........................................................................................................................35
11. Repairs, alterations and quality control .........................................................................................35
11.1. Materials...............................................................................................................................36
11.2. Replacement of components ...............................................................................................36
11.3. Welding ................................................................................................................................36
11.3.1. Welding procedure specification.............................................................................36
11.3.2. Welder qualification and identification...................................................................36
11.4. Quality control of repairs .....................................................................................................37
11.5. Hydrostatic testing ...............................................................................................................37
11.6. Records updating..................................................................................................................37
12. Asset management document .......................................................................................................37
13. Implications and legal matters on the inspection of equipment ...................................................39

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Tables

Table 8.4: degradation mechanisms ................................................................................................10

Table 8.7: summary of inspection and testing methods and techniques ........................................23
Table 8.8: example of inherent effectiveness of some inspection methods for the detection of
some typical discontinuities ............................................................................................27
Table 9.3.a: maximum inspection intervals for protection systems...................................................31
Table 9.3.b: maximum inspection intervals according to the procedure used ..................................31
Table 10.3: non-destructive testing methods ....................................................................................33
Table 10.5: training and experience typical requirements ................................................................34

Figures

Figure 5: settlement point................................................................................................................6

Figure 9.2: implementation process of the RBI methodology ..........................................................29
Figure 9.2.2 : risk criticality matrix ........................................................................................................29

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5. Glossary
1. Objective of the guideline
ABENDI
To show the guidelines to implement a
storage tank inspection plan, taking API 653 Brazilian Association for Non-destructive
standard as frame of reference, as well as the Testing and Inspection.
best practices developed within ARPEL´s
scope. ACFM

2. Scope of the guideline The ACFM technique, Alternating Current

Field Measurement, is a pretty highly used
It applies to above-ground tanks, hydrocarbon technique to detect fatigue and sub-surface
storage tanks, located in distribution plants, cracks. It may be particularly used in the
refineries, petrochemical companies, detection of cracks in the bottom of tanks.
distribution terminals and oil production
facilities. In general, these storage tanks are Alteration
manufactured according to API standards: 650
Any modification in a tank which changes its
Welded Steel Tanks for Oil Storage, 620, etc. It
shape or size.
covers the tank’s floor, shell, and roof, up to
the flange against its shell. It does not apply to Anchorage
its auxiliary input and output systems.
System by which a tank is fixed to its base or
3. Related documents foundation.

API 650, 353, 580, 650, 579, 575, 581, 620, Rings
ASTM D 86, D 610, D 659, D 661 and D 714,
API RP 651, NACE RP 01-93, ISO 9712. Concentric cylinders that make up the tank’s
layer.
4. Background
Anode
The development of this guideline is part of
the activities in charge of the Mechanical Metal surface from which current emerges to
Integrity and Risk Analysis Project of the go through a solution, in which the metal
ARPEL Refining Committee. This project was corrosion or dissolution is carried out.
created with the aim of improving the
management in the oil and gas industry in the Sacrificial anodes
mechanical integrity and risk analysis areas, as
Metal with a normal oxidation potential
they represent critical elements of the
higher than the one of the metallic structure
environmental, health and safety
to be protected, so that it is consumed with
management system in downstream.
protective current. It is used in cathodic
protection systems in which the metal that
acts as anode is sacrificed (disintegrated) in
favor of the one acting as cathode. In this type
of system, the anode material is consumed

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depending on the protection current demand Condensate

of the structure to be protected, the
electrolyte’s resistivity and the material used Hydrocarbon liquid separated from natural
as anode, during its discharge process. gas which is condensed due to changes in
temperature and pressure and remains liquid
Settlement in standard conditions.

Gravity natural accommodation of the tank to Operative conditions

its base or foundation.
Contained fluid, temperature, liquid level,
ASNT filling and emptying speed, and other
conditions given during the service. In general,
American Society for Non-destructive Testing. this concept is used related to the design
conditions, which limits should not be
Point of entry surpassed in operation.

Flanged hole situated in the first ring of the Impressed current

tank. Used by people to go into the tank.
Cathodic protection system that introduces
Piping direct current by means of a transformer in
the circuit which consists of the structure to
Set of components used to transmit, be protected and the anode bed. The current
distribute, mix, separate, discharge, measure, dispersion is carried out with the help of inert
control or purge. Pipes also include the anodes which characteristics and application
support elements, but do not include the depend on the electrolyte (scrap metal, ferro-
support structures, such as building posts, tilts silicon, titanium oxides, lead – silver, graphite,
and/or foundations. It is a pipe that transports etc.). The positive terminal of the source
fluids. should always be connected to the anode bed,
so as to force the protection current discharge
Cathode
for the structure.
The cathode is the metal surface in which the
Corrosion
current leaves the solution and returns to the
metal, there is no metal dissolution in the Electrochemical process by which the refined
cathode. metals tend to form thermodynamically stable
compounds (oxides, hydroxides, etc.) due to
Certification
the interaction with the environment.
It is a document which guarantees the
Grading
qualification. In general, it is a certificate
issued which is a proof that states that all the Done to prove that the personnel that is going
requirements established in a certification to be certified has the education level, the
system are met. experience, the formal training and the skills
to carry out a certain job. The proof is carried
out through examinations.

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Mass loss coupon Chalking

Metallic test tube (corrosion coupon) of Superficial degradation of the paint in a

known weight that is exposed to the corrosive uniform and progressive way due to the
environment to be analyzed and the weight exposure to the sun’s ultraviolet rays.
loss for a specific period is monitored, after
having previously eliminated the corrosion Shell
products through adequate methods.
The shell of an above-ground storage tank is
Degradation the vertical component containing the fluid
inside the tank. Moreover, it supports the roof
It is the reduction of the ability of a and other connections of the tank like the
component to achieve its purpose. This can be stairs, nozzles, and pipes.
caused by different deterioration mechanisms
(examples: slimming or loss of thickness, Failure scenario
cracking, fissures, etc.). Damage or
degradation can be used instead of It is the physical expression of the damage (for
deterioration. example: slimming of wall, rusting, cracking,
rupture).
Pipeline
Risk assessment or estimation
Transportation system through pipelines
which includes components such as valves, Process used to measure the level of risk on
flanges, cathodic protection, communication life, health, the environment or properties and
and/or data transmission lines, safety or relief includes a frequency analysis or failure
devices by which liquid hydrocarbons and probability for each threat, consequence
gases are transported; generally placed under analysis and its integration. In the risk
the surface (buried) in dry, wet soil or under assessment or evaluation, judgments and
water streams. In some areas they are placed values are part of the decision process,
in elevated structures to get over land explicitly or implicitly, including considerations
depressions. on the importance or seriousness of the
estimated risks, the social, physical,
Electrolyte environmental and economic consequences
linked with the purpose of identifying
Chemical substance, or a combination of alternatives for their mitigation or reliable
them, liquid or solid, which contains ions that management.
migrate under the action of an electric field.
Evaluation of the service ability
NDT
It is a methodology by which the defects
Non-destructive Testing. Test performed on within a structure are evaluated so as to
an object to verify its quality or status without determine the adaptation of the defective
causing any damage or making it useless. structure for the continuous service with no
imminent failure.

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Evaluation of similar services importance required for them to be evident.

Monitoring and inspection programs of the
It is the process through which corrosion threats and the consequences, and their
speed and inspection intervals are set for a mitigation actions can be defined from the RBI
candidate tank with corrosion speeds and the exercise.
history of a control tank in order to set the
next inspection date. ICP

Failure Individual Certification Programs.

Undesirable fault or imperfection in a External inspection

material. It is also defined as failure when an
equipment does not offer the service for It is a visual inspection carried out by an
which it was devised or built anymore. authorized inspector to evaluate all the
possible aspects of the tank without
HAZOP interrupting operations or withdrawing the
tank from operation.
HAZard and OPerability. The operating
functional analysis is a technique for Inspector
identifying operative risks based on the
premise that hazards, accidents or operating Competent person, with the necessary
problems occur as a consequence of a knowledge and ability to carry out the
deviation of the process variables from the mechanical integrity inspection of an
normal parameters of an operation in a equipment and prepare the respective report.
specific system and in a certain stage. When the equipment to be inspected is a
Therefore, even if it is applied in the design tank; different types of inspectors can be
stage, or in the operation stage, the distinguished: non-destructive testing ones,
procedure is to evaluate, in all the lines and in welding ones, quality control inspectors, etc.
all the systems, the consequences of possible
deviations in all the process units, whether if it Internal inspection
is continous or not. The technique is to
systematically analyze the causes and the It is a complete inspection by an authorized
consequences of some deviations of the inspector of all the tank’s accessible internal
process variables, expressed through “guiding surfaces.
words”.
ISO
RBI
International Organization for
Abbreviation for Risk Based Inspection. It is a Standardization.
methodology through which, from the risk
Product side
evaluation of a static equipment (pipe, tank,
vessel, furnace, boiler, others), threats and
Tank surface that is in contact with the liquid
failure modes that those equipment can have
product stored.
are established to define the methods and
techniques, with the frequencies and

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ARPEL Guideline: storage tanks inspection plans

Soil side is a quick and efficient way to detect a loss of

thickness.
Tank lower surface that is in contact with the
land. Tank floor

LFET Tank lower part, located horizontally, which

separates the fluid contained from the
Descriptive abbreviation for Low Frequency ground.
Electromagnetic Technique. The LFET
inspection is a quick and efficient way to Pitt (or pitting)
detect a loss of thickness.
Local metal loss, as a cavity or hole, which
Compound material diameter or deepness is lower than the
thickness of the sheet.
Material made up of two components to
obtain a combination of properties that is not Probability of failure
feasible to be obtained in the original
materials. These compounds can be selected Probability of a leak or failure in the system in
to obtain unusual combinations as for a certain period. It can also be defined as the
stiffness, strength, weight, performance under level of occurrence susceptibility of damage or
high temperatures, resistance to corrosion, loss of integrity by each possible threat in the
hardness or conductivity. system.

Degradation mechanisms Certification process

It is a process that leads to micro and/or It is a process in which a person proves his/her
macro changes in a material; which could be knowledge, experience and skills to carry out
dangerous for the material conditions or its a job. The verification is proved by a
mechanical properties. Damages are usually certificate that is obtained when someone has
incremental, cumulative, and, in some cases, gone through exams and other requirements
irretrievable. The most frequent damage from an association or organization that
mechanisms include: corrosion, stress monitors the application of standards for a
corrosion cracking, erosion, fatigue, fracture, certain type of industry.
etc.
Owner/operator
Non-destructive Testing Method (NDT)
It is the legal entity that is in charge of, or is
Discipline that applies a physical principle in responsible for the operation and
non-destructive testing (for example, maintenance of an existing storage tank.
ultrasonic testing).
Cathodic protection
MFL
Technique to control the corrosion of a metal
Descriptive abbreviation for: Magnetic Flux surface by turning it into the cathode of an
Leakage. The magnetic flux leakage inspection electrochemical cell.

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 ARPEL Guideline: storage tanks inspection plans

Hydrostatic testing c) inclusion or replacement of support plates

(or some of them) for the penetrations of
A test carried out with a liquid in which the existing shells; and
fluid static height is used to produce test
d) repairing defects, cracking or erosions,
loads.
with grinding or lowering and then
Breakover point welding.

Point in the bottom of a tank in which the Soil resistivity

settlement begins.
Degree of difficulty of electrons displacement
through the ground. It is the specific electrical
resistance of a surface and it is expressed in
Ohm-cm.

Risk

According to API RD 581, it is the combination

of the failure probability and the failures
consequence. This product is arithmetical
when the risk assessment methodology is
quantitative and can be a matrix combination
when the risk assessment is qualitative.

Figure 5: settlement point Sprinklers

Reconstruction Fire extinguishing device. Generally, they

activate when they detect the effects of a fire,
Any job necessary to set up a tank again that as for example a fire-related increase in
has been dismantled and moved to a new temperature, or the smoke caused by
place. combustion.

Repairing Auxiliary systems

Job necessary to keep or restore a tank in Additional systems, equipment or

good conditions for a safe operation. components to the main parts that are
Examples: necessary for the reliable and safe operation
of the tank such as: fire protection system,
a) removal and replacement of material (as dikes, etc.
the roof, shell, or bottom material,
including the welding metal) to keep the Corrosion rate
integrity of the tank;
Loss of estimated metal for a structure that is
b) leveling and/or lifting of the shell, the exposed to a corrosive environment for a
bottom or the roof of a tank; certain period (mm/year).

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External floating roof • to establish better conservation and

protection conditions against degradation
It is the highest roof of a tank and it is made mechanisms;
up of a double roof, single roof, or pontoon,
which depends on and is compatible with the • to keep a clear and useful record of the
liquid contained and is equipped with a tank, with concise and concrete, easily
closure seal or seals. accessible information;
• to establish maintenance plans to optimize
Internal floating roof costs and reduce maintenance times to
preserve the integrity of the tank during its
A roof in a fixed roof tank that depends on or life cycle;
is floating in the oil liquid contained and is
equipped with a closure seal or seals to close • to keep the availability and reliability of the
the space between the tank shell and the tank tank; and
roof. • to facilitate the fulfillment of obligatory
national legal requirements and act
NDT technique
according to international reference
standards.
Specific way of using a NDT method (for
example, immersion ultrasonic testing).
7. Causes to revise a tank
Residual or remaining life inspection plan
It is the space of time left until the end of the The tank inspection plan should be revised
useful life of a component or facility, which faced with the following circumstances:
ends when there is no more capacity left to
render the service under the acceptable • change of service;
technical, safety and financial standards.
• change of operative conditions;
6. Reasons to establish a tank • change of inspection or building rules;
inspection plan • near the end of the useful life;

The reasons to have a plan like this are: • change in regulations;

• after repairing or major changes;
• to reduce the probability of failures and
liberation of products stored; • presence of new or aggravated
deterioration mechanisms; or
• to unify inspection standards;
• after extreme events (earthquakes, fires,
• to know the degradation status compared tsunamis, etc.).
with the original manufacturing conditions;
• to estimate its remaining useful life;
• to define inspection frequencies;

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8. Development of a tank Before every inspection, and when an

inspection plan is developed, the following
inspection plan should be taken into account:
Failures in tanks can cause safety,
• analysis of the history of the equipment, at
environmental, as well as financial problems,
least the last three inspection reports, in
and besides, risk the supply infrastructure. A
order to verify design changes,
good inspection plan should combine
deterioration or defects that have
engineering techniques with knowledge of the
appeared, and critically analyze the
history of the tank.
inspection methods used;
The main objectives achieved in the storage • verify if the inspection recommendations
tank inspection are: and possible repairings were carried out
and if there are any pending;
• to detect and identify damages and
deterioration mechanisms; • consult operative records and verify the
things that come up that may interfere
• to determine the intensity of damages and with the remaining life of the tank, such as:
deterioration mechanisms; pressure rise, temperatures above the
• to determine the remaining life of the tank design, unexpected polluting fluids,
components; and vibrations, leaks and losses, as well as
unexpected efforts;
• to determine possible repairing and
changes. • reasearch the action of process fluids and
their pollutants to the materials involved,
Developing an inspection plan allows to taking operative conditions into account
reduce the probability of failure for a given (when the equipment operates with
component (not its consequence), which will several fluids and undefined conditions, for
reduce the risk to acceptable levels. example breathing tank, it is advisable that
an analysis is carried out for the worst
The magnitude of the reduction of the possible condition);
probability of failure is, in general, directly • verify the operation start date, out of
proportional to the inspection effort carried service periods, and start date of the last
out. The effectiveness of the inspection and operative period; and
the failure detection probability are directly
proportional to the quantity and quality of the • analyze thermal cycles involved, when
resources used in the inspection. applicable (thermal tensions).

Two types of inspection plans can be A tank inspection plan is established in an

identified: in service and out of service. In asset management document, which, at least
each case, the corresponding safety measures consists of:
should be taken, including the measures to
access limited areas in the case of an out of • tank records;
service inspection. • referential analysis of typical failure cases;
• tank failure records;

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ARPEL Guideline: storage tanks inspection plans

• degradation mechanisms; • updating calibration;

• failure scenario; • replacement of exterior and interior paint,
if there is, showing type and thickness; and
• inspection methods;
• treatment of the base floor of the tank and
• establishment of the inspection frequency;
characteristics of the concrete ring, if there
and
is one.
• conclusion and recommendation.
8.2. Referential analysis of typical
8.1. Tank records failure cases

Records should include: The failure analysis in tanks is particularly

important for new tanks or for those for which
• plans and technical data sheets, with the there is no record available. As a reference
following characteristics: see “Integrity Management Plan -
o process conditions (fluid, pressure, Hydrocarbon Storage Facilities” (Argentinean
temperature, etc.); Oil and Gas Institute- IAPG). Likewise, API 353
o dimensions and manufacturing aspects publication presents different quantitative
(type of roof and base, thickness of values for the analysis of typical failures.
components, internal accessories,
points of entry, floor and floor The most frequent failure causes in tanks, in
schemes, etc.); decreasing order, are:
o materials used in the construction,
• atmospheric discharges (a flash of
including accessories and welding; and
lightning);
o design code used.
• protective components used in tanks • high temperature maintenance repair;
(cathodic protection, atmospheric
• operational errors;
protection/lightning conductors, coverings
and compound material reinforcements, • bad operation of systems;
special protective coatings for roofs,
• sabotage;
corrosion protection system for the floor in
contact with soil such as: covering, • fissures and ruptures of the tank;
cathodic protection or sacrificial anodes;
• static electricity;
• operative record of the tank (services to
• natural disasters; and
which it was subject to);
• internal reactions.
• inspections record (dates, inspection types
and methods, tests and findings);
8.3. Tank failures record
• descriptive list of repairing and changes
carried out, and change control analysis, if A record of all the failures should be kept, that
there is one; includes – at least- the following information:
• roundness and vertical position • date of failure;
verification;

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 ARPEL Guideline: storage tanks inspection plans

• failure mechanism; • Internal corrosion as a result of water

wastes when water is used in the reception
• root cause;
operations for product displacement, or by
• repairs carried out and their quality condensation of the environment
control; humidity.
• real and potential consequences; and • Rapid corrosion of sharp points or edges,
as in those places the paint thickness is
• recommendations and corrective actions.
smaller than the recommended one.
8.4. Degradation mechanisms • Rapid corrosion in manufacture defects
that cause the accumulation of humidity
The degradation mechanisms that affect each and dust from the atmosphere.
component of the tank should be
documented. All the mechanisms indicated by • Erosion.
the experience should be included in the plan. • Material fatigue.
Some of the possible mechanisms are
mentioned below. • Differential settlements in the tank base.
• Fragile fracture by low temperature, etc.
• Localized corrosion.
• Generalized corrosion.
• External corrosion as a result of the sea
breeze or smog.
Table 8.4: degradation mechanisms1
TYPE OF DETERIORATION
DESCRIPTION
DAMAGE MECHANISM
Atmospheric corrosion It is a corrosion mechanism of electrochemical nature in which
the electrolyte is composed of a water film on the surface. The
severity of the corrosion process depends, especially, on the time
during which the water film remains on the metalic surface and
on the air pollution. It is the most common corrosion process in
storage tanks and usually occurs in roofs and shells.
Soil corrosion The land or soil, due to its variable humidity, salts and organic
matter decomposing, is the most complex electrolyte that can be
Uniform
found. A natural soil contains sand, clay, lime and humus. These
thickness loss
components can be combined in different propotions which give
rise to different levels of corrosive agressiveness. The corrosion
speed is connected with the soil resistivity, presence of oxygen,
humidity and microorganisms. Soil corrosion can occur in the
lower part of the bottom plates and in some cases it can also be
localized.
Abrasion wearing away Abrasion is the loss of thickness resulting from friction mechanical
action between two solid materials.

1
Data based on API 581 and the ARPEL Manual for pipelines integrity management

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TYPE OF DETERIORATION
DESCRIPTION
DAMAGE MECHANISM
Abrasion and erosion Erosion is the loss of thickness of a material through the impact of
wearing away a dynamic agent like water, wind, sand, etc.
Galvanic corrosion Galvanic corrosion is an electrochemical process in which a metal
corrodes preferentially when it is in electrical contact with a
different type of metal. The pair of metals in known as galvanic
pile. In tanks, this type of corrosion can occur in the fixing terminal
of the tank grounding cable.
Microbiological Microbiological corrosion is a corrosion process in which the metal
corrosion loss is caused by direct or indirect action of microorganisms.
Microorganisms that participate can be aerobic or anaerobic.
Most often, their metabolic processes change the chemical
characteristics of the electrolyte. This type of corrosion is more
frequent in the bottom of tanks, either in the parts that are in
contact with the floor or in the parts that are in contact with water
accumulated in the bottom.
Corrosion under Corrosion under insulations can occur under different
thermal insulation circumstances and can be caused mainly by water or fluids that
enter the insulations or assembly failures and in the external
metal jacket. Once pollution (overflows, vapor or liquid leaks,
Localized rainwater, etc.) enters the external protection, it comes into
thickness loss contact with the insulation (fibre glass, mineral wool, calcium
silicate), and reaches the equipment surface through the “wick
effect” in which corrosion may occur. It can also occur by the
combination of insulations that contain chloride wastes in contact
with the austenitic stainless steel surface, and especially of the
300 series, where there is humidity and a temperature higher than
140°F (60°C). In these cases it becomes apparent as crackings or
fissures.
Selective corrosion Type of attack in which one of the components of an alloy is
dissolved, generally the most electronegative one. Examples of
this type of corrosion are dezincification, dealumination and even
intergranular corrosion in stainless steels. This process is not very
frequent in storage tanks.
Erratic currents Deterioration due to erratic currents, particularly direct, that
corrosion escape from electricity systems near the tank and penetrate the
bottom metal plates corroding them where they come out to the
floor. This type of localized corrosion can lead to deep rusts in a
short space of time.

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TYPE OF DETERIORATION
DESCRIPTION
DAMAGE MECHANISM
Aeration and Environmental conditions in a crack may, with time, become more
differential corrosive than those existing in a clean and open surface.
concentration Corrosion intensification, also know as cracking, is generally
corrosion caused by one or more of the following factors:
a) Acidity changes in the crack or fissure.
b) Shortage of oxygen in the crack.
c) Development of different ions in the fissure.
In storage tanks, this type of corrosion can occur under tanks,
microorganisms, washers or screw heads, in parts that are in
contact with joints or in the interstices between the bottom and
the floor plates.
Pitting corrosion It is a process that occurs when a passivated metalic surface is
exposed to an aggressive environment. During pitting, the attack is
localized in isolated spots in passive metalic surfaces and
penetrates the metal forming, sometimes, microscopic tunnels. It
is a highly localized process that occurs in areas with a low
generalized corrosion and in which the anode process (reaction)
produces small localized perforations. It can be observed generally
in surfaces with a low or almost inexistent generalized corrosion. It
occurs as a local anode dissolution process in which the metal loss
is accelerated by the presence of a small anode and a much larger
cathode. Pitting is the most frequent type of corrosion in stainless
steels.
Exfoliation corrosion Exfoliation corrosion is a subsuperficial corrosion that starts on a
clean surface, but it spreads under it and it is different from the
pitting corrosion as the attack has a laminate appearance.
Complete layers of materials are corroded and the attack is
Loss of layers generally recognized by the flaky appearance, and sometimes with
thickness blisters, of the surface. At the end of the attack, the material looks
like a pack of cards in which some of them have been taken away.
This mechanism is pretty known in surfaces in contact with marine
atmospheres and in the upper internal part of tanks which vapors
contain sulfur derivatives.
Hydrogen Hydrogen embrittlement can be defined as the loss of resistance
embrittlement and ductility induced by hydrogen. Hydrogen embrittlement is
particularly devastating due to the nature of the originated failure.
That failure follows very small tensions (in comparison with the
Metallurgical
necessary ones when there is no hydrogen), it is pretty fragile and
changes
has such a variable “incubation” period that is almost
unpredictable. This type of damage occurs at temperatures higher
than 200°C, under pressures and with the presence of this
element. This process is not very frequent in storage tanks.

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TYPE OF DETERIORATION
DESCRIPTION
DAMAGE MECHANISM
Hydrogen blistering The damage mechanism is caused by hydrogen that penetrates
the material and is recombined in the most sensitive areas: matrix-
inclusion and matrix-carbides interfaces, grain cavities and limits;
causing an increase of the internal pressure and material
decohesion in these areas. This phenomenon is known as
blistering. This type of damage is most commonly found in low
mechanical resistance steels that work in environments that
promote a strong hydrogen intake to the material.
Oxidation Mechanism that is not very frequent in storage tanks.
High
Sulfidation Mechanism that is not very frequent in storage tanks.
temperature
Thermal fatigue Mechanism that is not very frequent in storage tanks.
Ductile fracture The ductile fracture of a metal occurs after an intense plastic
deformation and is characterized by a slow crack propagation. It
may occur in tanks as a consequence of border or shell
settlements.
Fatigue fracture It is a phenomenon by which the breakage of materials under
cyclic dynamic loadings (forces repeatedly applied to the material)
is produced faced with loadings which are below the static
loadings that would cause the breakage. Its main risk is that it may
occur at a smaller tension than the traction resistance or the
Mechanical
elastic limit for a static loading and appear without warning,
failures
causing catastrophic breakages.
Fragile fracture by low The fragile fracture occurs along cristalographic planes called
temperature fracture planes and has a fast crack propagation. Most of the
fragile fractures are transgranular, that is, they travels through the
grains of the material. But if the grain limits constitute a weak
area, there may be an intergranular propagation of the fracture.
Low temperatures and high deformations favor fragile fracture. In
tanks, they can occur more frequently due to pressure tests
carried out at temperatures under 20°C.
Stress corrosion It is a corrosion process that occurs when there is interrelation of
cracking, SCC two essential factors: the material surface exposed to the
corrosive environment should be under tensile stress and the
corrosive environment should be specifically a cause of the stress
Processes corrosion. Most of the alloys are liable to suffer this attack, but
assisted or fortunately, the number of combinations alloy-corrosive that
related cause this problem is relatively low. Tensile stress can be the result
among of applied loads, internal pressure in the system or residual
themselves stresses from previous welding or bending. For carbon steel, the
corrosive environments that can cause this corrosive phenomenon
are those that contain chlorides, caustic soda and sulphides at
high temperatures. If the temperature is under 50°C, stress
fracture corrosion rarely occurs.

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TYPE OF DETERIORATION
DESCRIPTION
DAMAGE MECHANISM
Fatigue corrosion Failures because of breakages due to the combined action of a
corrosive environment and mechanical tension cycles. This process
may occur in the bottom of tanks near the floating roof legs
support.
Erosion corrosion The joint effect of the corrosive and abrasive action of a fluid
moving at a high speed, which causes the constant destruction of
the corrosion products protective layers. The presence of
suspended particles speeds up the process.
Abrasion corrosion Corrosion in the interfase of two metalic surfaces in contact, sped
up by the relative sliding between them. It is considered that
corrosion plays one of the following roles: the friction heat rusts
the metal and then oxide wears away; or the mechanical removal
of the oxide protective particles, or the resulting corrosion
products. This erosion may occur in the internal shell surface in
floating roof tanks due to the constant sliding of the seal.

8.5. Failure scenarios

• Leaks at joints or damages in the sealing
For each pair “degradation mechanism- settlements.
component”, all the possible scenarios that • Vacuum deformation.
may lead to a failure in the tank should be
described. Some potential failure scenarios • Deformation due to excessive pressure,
etc.
are listed below:
A probability and a consequence are assigned
• Localized corrosion due to a failure of the
to each scenario and its risk is expressed.
paint because of an inadequate
preparation of the surface or bad paint
In order to express the risk, a risk matrix
application.
scheme is used, for example, a matrix like the
• Failure in the tank shell welding. one proposed by API 353 is suggested, with
probabilities from A to E (decreasing) and
• Floor failure because of external attack of
consequences from 1 to 5 (increasing).
the soil to the floor metal sheet.
• Floor failure because of internal attack of Consequences should be evaluated in 4
the fluid contained in the tank. Tank shell categories: fire risk, explosion risk, spill risk
failure due to internal corrosion. and economic risk (derived from the spill
remediation cost, repair cost, and the logistics
• Shell failure due to external corrosion. cost due to the non-availability of the tank).
• Roof failure due to internal and external
corrosion. When the initial risks of each scenario are
assigned in the matrix (without mitigative
• Equipment and accessories failure: stairs, measures) it will become evident which of
handrains, reception connections, dispatch them are not acceptable (those in the high,
and purges, vents. medium-high and medium risk zones). For

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these non-acceptable risks, preventive and/or The insulation lining joints that are defective
mitigative measures should be developed to or with cracking or fissures are areas that have
reduce risk to acceptable levels. a tendency to experience infiltrations. The
inspector should identify them even though
8.6. Inspection and testing methods there is no loose tape or area with
deformations. In tanks that have been out of
There are several inspection and testing operation for a long time, the entire insulation
methods. Each of them has its advantages and should be removed as, under these
disadvantages, so it is advisable that they are conditions, corrosion is intense.
used jointly. The main inspection and testing
methods applicable to storage tanks are: Regions under the platforms, as well as
regions close to connections and supports, if
8.6.1. Visual inspection any, tend to have insulation failures.

It consists of a detailed visual verification of It is recommended to remove parts of the

the tank’s surface and its auxiliary systems. thermal insulation to evaluate the conditions
The visual inspection can be carried out with of the shell plates, mainly in low pressure
the tank in normal operation conditions or on tanks that operate at low temperatures. For
the occasion of tank stoppage. The inspection those tanks, it is advisable o take a larger
with the tank in operation allows to identify sample or even the entire insulation as
leaks, freezing, overheating, etc. that would experience shows that there could be
not be usually detected with the tank out of humidity condensation between the tank wall
operation. and the insulation with a corrosive process in
localized areas.
The internal visual inspection, on the occasion
of tank stoppage, is of great importance for 8.6.1.2. Protection paint
the identification of internal damage
mechanisms in the roof, the shell and the The most common defects found in tank
bottom, which characteristics are not uniform protection paints are the following:
and that are difficult to detect through
external non-destructive testing. For the 8.6.1.2.1. Blisters
inspection to be objectively carried out, the
inspector should follow the inspection plan The main causes of paint blisters are:
and complete each of its steps.
− Humidity, oils, greases, or dirt during
The visual inspection should include, at least, application. It appears in the short term,
the following aspects: after the application.
− Tank operation, even during short periods
8.6.1.1. Thermal insulation at temperatures above the resistance limit
of the paint. It appears immediately after
If the tank is insulated, a visual inspection of
deviation.
the whole area should be carried out
identifying moisture infiltration places from − Incompatibility between coats that make
rain or sprinkler systems. up the paint scheme.

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− Inadequate intervals between the paint For all defects, the repairing requires the
coats, causing adhesion problems complete application of new paint. ASTM D
between coats. 610, D 659, D 661 and D 714 standards show
photographic examples that can be used as an
To identify the probable cause of blisters, aid in your evaluation.
some of them should be broken and the inner
part of it should be observed, verifying if there 8.6.1.3. Cathodic protection
is water or other liquid. In the case of
hydrogen blisters, the inner surface will The evaluation of the cathodic protection
always be clean and dry. system should be carried out according to API
RP 651 and NACE RP 01-93 recommended
The inspector should also verify if blisters are practice. Tank (exterior part) – soil potentials
limited to the finish coats or if they are also in should be measured, and the operation
the first coats. In the first case, the conditions of the cathodic protection rectifier
recomposition of the finish coats should be units should be evaluated as well as sacrificial
recommended and, in the second, the anodes and their connections. These
recomposition of the entire paint. inspections are carried out more frequently
than tank inspections. If applicable, dielectric
8.6.1.2.2. Chalking joints should also be inspected and the
possible interference with other protection
In order to take a decision, the intensity of the systems should be evaluated.
wearing away should be evaluated. For
example, redo the finish paint or specify a 8.6.1.4. Electrical grounding
more adequate scheme.
It is frequent that an intensive corrosion
8.6.1.2.3. Abrasion/erosion process occurs in the fixing terminal of the
tank grounding cable. The inspection hammer
Wearing away in localized areas, due to the should be used to verify the joint integrity.
action of solid particles moved by the wind or Besides, the grounding resistance should be
the friction between two solid parts. Floating measured, taking the precaution to previously
roof tanks are particularly sensitive to disconnect the impressed current for cathodic
abrasion in their intermediate rings. protection, if there is one. The grounding
integrity is important to avoid the effects of
8.6.1.2.4. Crackings, wrinkles and static electricity.
corrosion points scattered
across the painted area. 8.6.1.5. Stairs and platforms

The appearance of these defects suggests: The most common problem found in stairs
and platforms is corrosion due to the
• In recent paint: incorrect application; deterioration of the protection paint. Steps
and stair parapets should be attentively
• In relatively new paint: inadequate paint
verified as the safety of the staff that has
scheme;
access to the tank depends on their integrity.
• In old paint: conclusion of the useful life of For platforms, it should be verified if there are
the system. areas with signs of rainwater accumulation. In

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these regions, it is advisable to make a hole in • When the tank is opened, the existence of
the metal sheet to drain water, so it does not deposits, wastes and inlays should be
accumulate. verified; observing their kind, quantity and
location.
8.6.1.6. Safety devices
• The shell, bottom, weldings and
It should be verified: connections should be inspected as for
deformations, fissures, corrosion and
− the apparent condition and signs of leaks erosion, and damages due to cleaning or
or obstruction of components; and maintenance. In some cases, it may be
necessary to remove internal components
− for devices like safety or relief valves, or
of the tank.
vacuum and pressure valves, if the
adjustment pressure corresponds to the • The internal condition of connections as
one specified in the tank design. for corrosion and obstruction should be
evaluated.
8.6.1.7. Tank base inspection
• The integrity of the inner liner should be
The inspection of this component should verified, if there is one.
always be considered in the visual inspection • The location, securing and integrity of
planning. Some points should be verified more internal components should be evaluated,
carefully as the area is subject to localized if there are, such as: distributors, support
corrosive processes due to cracking. The points, coils, articulated drains, anti-
anchorage exposed area should be also rotational, etc. It should be verified that
verified and, with the help of an inspection there are no deformations in the floor due
hammer, the integrity of the equipment to a wrong support of the roof legs.
jetnuts should be evaluated. During this
inspection the aspects that may show • Places to be prepared for non-destructive
settlement problems of the base and, testing should be identified.
therefore, of the tank, should be carefully
verified. For more details see item 8.6.13 – 8.6.1.10. Roof evaluation
Settlement evaluation.
The structural condition and the support
system should be verified, as well as the
8.6.1.8. External inspection of shell and
pontoons tightness in the case of floating
roof roofs, the floating membrane, if there is one,
the roof drains, the perimeter seal system,
It consists of the detailed visual verification of
and the venting systems. Moreover, evidence
the external surface of the tank and its
of corrosion should be sought and it should be
systems, in order to detect any anomaly in
verified that its position is correct in relation
relation to its design, such as deformations,
to the shell. The mobility of the roof support
external corrosion, etc.
legs should also be verified.
8.6.1.9. Internal inspection

In an internal visual inspection of a tank, the

inspector should focus on the following:

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8.6.1.11. Auxiliary systems evaluation 1. Identifying an appropriate magnetic field

in the object to be tested.
The following systems should be visually
2. Applying magnetic particles to the surface
inspected: fire protection system, dams,
of the object to be tested.
pipings, flanges, steam system for coils,
electrical connections, pressure caps, double 3. Examining the surface of the object to be
bottom leak detection system, emergency tested to detect accumulation of particles
vents, etc. and evaluate the ability of the object to
remain in service.
8.6.2. Inspection with hammer
The method can detect any discontinuity in
It is used as a complement of the visual the surface and, under certain conditions,
inspection by means of a ball hammer hit on those completely located under the surface. It
those surfaces that are found suspicious by detects discontinuities such as: fissures,
the inspector, evaluating the sound and the inclusions, lack of penetration, lamination,
movement of components such as rivets, pores, etc.
studs, bolts and nuts.
It depends on the magnetic properties of the
In this way, debilitated areas, structural object to be tested and it is only suitable for
problems, detachments and loose metallic materials that can be considerably
components are detected in the tank itself, as magnetized. Non-ferromagnetic materials that
well as in auxiliary systems. can not be strongly magnetized can not be
inspected with this method. Some examples
8.6.3. Penetrating liquids are: aluminum, magnesium, brass, copper,
bronze, lead, titanium, and austenic stainless
Testing method based on the capillarity steel. In appropriate ferromagnetic materials,
phenomenon that, in other words, is the the magnetic particle inspection is highly
penetration power of a liquid in cavities or sensitive and shows the condition of the
narrow fissures due to the physical and surface to be tested quickly and noticeably. An
chemical characteristics, like surface tension. experienced inspector may –when the
characteristics of the area and the indication
The penetrating liquids testing is used to degree of the testing are examined- interpret
detect discontinuities which are open to the their causes and evaluate discontinuities.
surface in suspicious areas.
8.6.5. Ultrasound
8.6.4. Magnetic particles
It detects internal discontinuities in materials
It is a non-destructive method for detecting based on the acoustic waves reflection
discontinuities in ferromagnetic materials. phenomenon when obstacles for its diffusion
are found in the material.
It is based on the principle in which the
magnetic field lines in a ferromagnetic Ultrasound is also used to measure thickness
material are distorted by an interruption of and determine corrosion very easily and
the material continuity. It consists in the precisely. The traceability and identification of
following three steps: the measuring points are important for the

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calculation of the corrosion rate and the the scanning area is of around 95% of the
remaining life monitoring. Measuring points inspected area. It is always necessary to
should include different areas of the tank such perform at least one cross checking of the
as the water level area, the operative area, MFL results with ultrasound before trusting
the product level area, and the vapors area. the probability evaluation.
Thickness measuring is the most commonly
used method to establish the remaining life of MFL (magnetic flux leakage) and LFET (low
the tanks that have a uniform thickness loss. frequency electromagnetic technique),
characterize the corrosion degree and provide
There are several additional techniques that qualitative values according to the nominal
are an evolution of the basic principle, such as thickness, that is why for an accurate
TOFD and phased array, among others. verification, it is complemented with the
ultrasound technique which does provide a
8.6.6. Radiography quantitative value.

It is a method based on the change of There is a considerable variability in the

intensity of electromagnetic radiation (X or quality of these inspections. Experience shows
gamma rays), caused by the presence of that they can be very effective with
internal discontinuities, when the radiation appropriately trained and experienced
goes through the material and the image is operators using equipment with the
saved in a radiographic sensor, a film or a appropriate detection capacities.
digital sensor.
8.6.9. Thermography
Radiographic testing is used in tanks to detect
discontinuities in welded joints, especially in Method that detects the infrared radiation
their construction and repairing. emitted by surfaces and allows to observe
differential patterns of temperature
8.6.7. ACFM (Alternative Current Field distribution, in order to give rise to
Measurement) information regarding the operational
condition of a component, equipment or
This electromagnetic technique may be process.
particularly used in the detection of fissures in
the bottom of tanks. Thermographic inspection (thermography) is a
useful testing to detect piping obstruction,
8.6.8. Magnetic flux leakage (MFL and tank levels, insulation failures, gas leaks,
LFET) freezing in the base of cryogenic tanks, etc.

Allows for a quick scanning of big surfaces, 8.6.10. Acoustic emission

detecting corrosion in the external as well as
in the internal part of the tank floor sheet. It It is a non-destructive testing method that is
provides information about the location and based on the detection of sound waves
the relative severity. emitted by diffusion discontinuities in the
material. It is necessary to apply a load or
The probability of detecting isolated rusts is stress (usually pressure), to cause the
higher than in the case of ultrasonic testing, as diffusion of discontinuities.

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Acoustic emission testing results are not 8.6.11. Tightness

conventional. In fact, this method should not
be used to establish the type or size of This type of test is carried out for the
discontinuities of a structure; but to register detection and location of possible leaks. There
the evolution of discontinuities during the are different types:
application of tensions if loads are enough to
cause localized deformations, increase of a) with lime and penetrating hydrocarbon: it
discontinuities, friction or other physical is used for detecting leaks in the first
phenomenon. interior welding between the shell and the
floor of the tank.
The acoustic emission is applied when the
b) with vacuum compartment: generally
dynamic behavior of defects in complex
used for welding in internal roof and floor.
metallic parts or structures is intended to be
analyzed or studied, as well as to register their c) pneumatic test of floor: used for the floor
location. The acoustic emission testing allows test in its entirety. It is not recommended
for the location of the failure, picking it up as it requires a very precise control of the
with sensors installed in the structure or in the pressure to avoid floor deformations that
equipment to be monitored. The experience may damage welding.
in the interpretation of signals results is d) hydrostatic test: it is a test of the tank in
important, as this test also detects other types its entirety. It is usually carried out
of acoustic emissions from the environment together with the pressure test described
or auxiliary equipment. Likewise, it is later. In the case of fixed roof, cryogenic
necessary to complement this test with or low pressure tanks (API 620), the
ultrasound or other techniques, to measure hydropneumatic test can also be carried
discontinuities. In the specific case of tanks, out.
testing is used to detect defects or leaks in the
bottom of the tank. e) helium test: this test is used in cases of
highly dangerous products, like ammonia,
This method has the advantage of not having due to its high sensitivity.
the need to take the tank out of service; f) acoustic emission: acoustic emissions
however, a good correspondence with the generated by leaks are detected by
results obtained by other non-destructive correctly placed sensors. This method can
testing has not been proved yet. In addition, it also be used for the detection of
only detects diffusion discontinuities, which is discontinuities, as it was previously
a limitation for the objective of performing explained.
preventive inspections. Nevertheless,
developments for optimizing its use in the It is also recommended to carry out tightness
inspection of tanks are being carried out. tests to coils and articulated drain arms (or
drains).
It proved to be very useful as a support for
hydrostatic tests, in which it allows to detect 8.6.12. Resistance test
diffusion discontinuities and stop the test
before a rupture occurs. In the case of new tanks, or at the end of
inspection and maintenance services in which
repairs that could have affected the structure

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of the tank were recommended or performed, the adoption of level measures (using levels,
it is necessary to carry out resistance tests, theodolites, plumb lines, water level, or other
that could be carried out with water or appropriate elements) around the
another incompressible fluid that provides the circumference of the tank and in its entire
same pressure effect, as long as it is diameter.
compatible with the material of the tank. For
the specific case of low pressure tanks, 8.6.13.1. Shell settlement
resistance or hydropneumatic tests are usually
used. The settlement of a tank is the result of one or
a combination of the three types of
Because of the elevated tension levels in settlements detailed below:
resistance tests, it is necessary to take the
room temperature into account. Under these a) Uniform settlement: this type of
conditions, a fragile fracture usually occurs, settlement is generally predictable, with
which shows no deformation due to the quick enough accuracy, from soil testing. It can
crack propagation. This phenomenon is vary in magnitude, depending on the
favored by the thickness and the quality of the characteritics of the soil. The uniform
construction material of the tank. In general, settlement does not provoke tensions in
resistance test in tanks should be avoided the tank structure. However, pipings and
when the temperature is under 15°C. accessories should be taken into acount to
avoid problems caused by this type of
8.6.13. Settlement evaluation settlement.
b) Flat slope: this settlement makes the tank
The determination of the soil settlement
turn and causes its inclination. The
effects in storage tanks is a very common
inclination causes an increase in the liquid
practice to control the tank bottom
level and, therefore, an increase of the
settlement. In most cases, a monitoring
circumferential tension in the tank shell.
program is started during the construction
Besides, the excessive inclination may
and continues during the hydrostatic test and
cause the floating roof peripheral seals to
the operations. During service, settlement
join together and block its movement.
measurements should be carried out with a
programmed frequency. c) Differential settlement (out-of-plane): due
to the fact that a tank is a more flexible
If at any time, settlement is considered to be structure, it may settle in a configuration
excessive, the tank should be emptied and which is not plane, causing additional
releveled. tensions in the shell. Out-of-plane
settlements of the shell may cause
The methods used to correct tank settlement roundness problems in its upper part,
include techniques such as localized repairing which, depending on their extension, they
of bottom metal sheets, partial re-leveling of may interfere with the correct operation
the tank perimeter, among others. of the floating roof. This out-of-roundness
could also affect the internal structures of
The main types of settlement are related to the tank, such as roof support structures,
the tank shell and the floor metal sheets. columns, girders, etc.
These settlements can be registered through

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8.6.13.2. Edge settlement during the receipt hydrostatic test. The tank
settlement should be initially studied with the
The edge settlement occurs when the tank tank empty with an even number of
shell is strongly settled around the perimeter measuring points uniformly distributed
of the tank, which causes a deformation of the around the circumference. This analysis
bottom metal sheet, next to the shell-floor provides basic readings for the assessment of
joint. future settlements. When this initial study is
not available, the tank is supposed to be
8.6.13.3. Other types of settlements initially leveled.

Floor settlements next to the shell, as well as The tank settlement measurements should be
away from it, can be found in storage tanks. evaluated for acceptance according to API STD
For a detailed analysis of them please see API 653, Appendix B.
STD 653, Appendix B.
8.6.13.5.2. Study during hydrostatic test
8.6.13.4. Establishing acceptable
settlement Settlement shall be measured during filling
and when the water reaches 100% of test
For each of the aforementioned settlements, level. Excessive settlement, in accordance
an evaluation should be carried out, through with API STD 653, Appendix B, will be enough
level testings, of the critical nature of the cause to stop the test for foundation
settlement and its influence on the storage investigation and/or repair.
tank integrity. For details please see API STD
653, Appendix B. 8.7. Summary of testing methods

8.6.13.5. Assessment to be carried out Table 8.7 shows a summary of the techniques
during the hydrostatic test used in the research and detection of damage
mechanisms.
8.6.13.5.1. Initial study

In the case that a settlement is predicted, the

foundation settlement should be evaluated

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Table 8.7: summary of inspection and testing methods and techniques

Type of failure
mechanisms or
Method Advantages Limitations
discontinuities that can
be detected
Wearing away, corrosion, It can be carried out on-site Only what is accessible to
erosion, abrasion, and does not need special the inspector´s view is
fissures, deformations, equipment. It only requires detected. It requires a lot of
Visual
blisters, paint chalking, illumination and cleaning and experience.
inspection
general condition of can be easily registered
different components, through photographs.
etc.
Structural damages, It is simple and easy to It provides limited
debilitated areas, loose implement. Good information. Spoils paint. It
Inspection with
rivets, studs, bolts or nuts, complement to visual is not advisable with the
hammer
detachments. inspection. tank in service (do not
implement under pressure).
Discontinuities which are It is highly portable, does not It loses efficiency if
open to the surface, require electricity. cleanning is not
especially fissures and appropriate, if the
Penetrating
pores in welding. superficial preparation
liquids testing
closes discontinuities to the
surface or if they are full of
corrosion products.
Discontinuities which are It has a high sensitivity to It does not detect internal
superficial or very near detect superficial discontinuities. Can not be
the surface, especially discontinuities, even when used in non-ferromagnetic
fissures and pores in the superficial preparation materials.
Magnetic welding. can close them in the surface
particles testing or when they are full of
corrosion products. Better
resolution and sensitivity than
the penetrating liquids
testing.
Internal discontinuities, It is highly sensitive to It requires personnel with a
Ultrasound especially in welding. fissures, lack of fusion and lot of experience and who
lack of penetration. are highly reliable.

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 ARPEL Guideline: storage tanks inspection plans

Type of failure
mechanisms or
Method Advantages Limitations
discontinuities that can
be detected
Generalized corrosion. It is a highly portable and There can be false readings
Thickness verification in quick testing for which if an instrument that only
localized corrosion areas personnel can be relatively shows one number is used.
located by MFL (especially easily trained. If thickness The testing for the
Ultrasonic rusts in the lower part of measurements are carried out detection of remaining
thickness the floor). in the same areas, corrosion thickness in rusts requires
measurement speed can be calculated and an instrument with scan A
the remaining life can be screen, a specific training of
estimated. the operator and is limited
to small areas where the
scan is carried out.
Internal discontinuities, It allows to evaluate the The thickness to be x-rayed
especially in welding. radiographic recording after is limited depending on the
the execution, which can be radioactive source available.
reevaluated. Allows to test The detection of fissures
extensive areas in a relatively and lack of fusion is limited,
Radiography short time, so it is commonly depending on the
used in construction. orientation. It requires
radiological protection
special cares (exclusion area
for personnel foreign to the
execution).
Superficial discontinuities It does not require direct It requires highly qualified
or very near the surface. contact, it allows for less and experienced personnel.
demanding superficial
ACFM preparations than the
(Alternating penetrating liquids and the
Current Field magnetic particles testings
Measurement) and can be carried out
without removing paint. It can
be used in non-ferromagnetic
materials.

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ARPEL Guideline: storage tanks inspection plans

Type of failure
mechanisms or
Method Advantages Limitations
discontinuities that can
be detected
Thickness loss, especially It allows to detect thickness It requires to be
of the floor. losses in the interior as well as complemented by ultrasonic
in the exterior side of the tank testing to obtain
floor. It allows to identify the measurements of the floor
places in which there is rust in thickness, as results depend
Magnetic flux the lower part (not visible) of on the volume and on the
leakage MFL the floor, testing up to 95% of geometry of the missing
and LFET the floor surface in a relatively material. It has difficulties to
short time. MFL and LFET detect very localized
allow to obtain comparable thickness variations (for
data, which have been proved example, holes made by a
by later ultrasonic drill may not be detected).
measurements.
Insulation failures. It is relatively simple and It requires special
effective. Especially suitable instruments. Limited to
Thermography for qualitative determination surface. Influence of the
of areas with different surface emissivity which
temperatures. affects temperature values.
Active discontinuities It can be carried out in service Interference with noises
(which are increasing as trying to identify those tanks and difficult interpretation.
the equipment is in worst conditions in order to Its efficiency has not been
subjected to a growing be opened and to localize, proved yet. There are
Acoustic
charge). through acoustic emission, reports of cases in which
emission
the areas in which other NDT acoustic emission test
should be implemented to predictions have not been
evaluate discontinuities. confirmed when tanks were
opened.
Tightness Passing discontinuities in It is simple and effective as Sensitivity is reduced if the
testing, the base welding pass intermediate testing in the superficial finish of the
technique with between the tank shell manufacturing and repairing welding is not adequate.
lime and and floor. that involve welding between
penetrating the tank shell and floor.
hydrocarbon
Passing discontinuities in Simple and highly sensitive It requires a constant
Tightness the tank floor and shell testing. It allows to carry out control of vacuum
testing, weldings. the welding quality control as conditions in each location
technique with it is being done. of the compartment. It
vacuum requires special
compartment compartments to adapt to
areas which are not flat.
Tightness Passing discontinuities on It tests the entire floor at the It requires a very precise
testing: the floor. same time. control of pressure to avoid
pneumatic test floor deformations.
of the floor

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 ARPEL Guideline: storage tanks inspection plans

Type of failure
mechanisms or
Method Advantages Limitations
discontinuities that can
be detected
Tightness Passing discontinuities. A tightness test and a Relatively low sensitivity
testing, Especially used in resistance test which allow to (lower than the vacuum
hydrostatic test construction and welding evaluate the entire tank are compartment test).
technique repairs. carried out simmultaneously.
Tightness Passing discontinuities. Highly sensitive test. It requires special
testing, helium equipment and specific
technique personnel training.
Passing discontinuities. It is highly sensitive. It allows It requires the use of
Tightness
to detect leaks in hydrostatic specialized material and
testing, acoustic
tests as pressure rises, with personnel.
emission
the possibility to stop the test
technique
to avoid rupture.
Discontinuities that It allows to verify the tank´s It requires enough water
Resistance test: prevent the tank from ability to resist load. It allows and may take a considerable
hydrostatic supporting required loads. to carry out an overall test of time for water filling,
testing the tank. It lightens residual inspection and water
tensions. discharge.
Differential settlement. Relatively simple. It allows to It should be implemented
detect serious structural periodically in order to
Settlement problems which lead to detect deviations in
evaluation tensions or damage auxiliary advance, as settlement
systems. problems can be hard to
solve.

8.8. Effectiveness of testing methods

Each method has certain effectiveness and

therefore, a reduction of the failure
probability. The effectiveness depends on the
limitations of the method itself (inherent
effectiveness), but also on the area covered,
instruments, methods, personnel, frequency,
special analysis and conditions. In table 8.8
below, the effectiveness of several inspection
methods is compared.

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Table 8.8: example of inherent effectiveness of some inspection methods for the detection of some
typical discontinuities
Discontinuities
Inspection Localized Generalized Internal
method Superficial Internal planar Dimensional
thickness thickness volumetric Blisters
fissures discontinuities change
loss loss discontinuities
Visual inspection 1-3 3-5 2-4 5 5 1-3 1-3
Penetrating liquid 5 5 2-3 5 5 5 5
Magnetic
5 5 1-3 5 5 5 5
particles
Ultrasound 1-4 1 2-4 1-3 1-3 5 1-3
Radiography 1-3 1-3 5 1-3 4 3-5 5
MFL - LFET 1-2 3 5 5 5 5 5
Acoustic emission 5 5 2-4 4 5 5 4-5

1= highly effective; 2= moderately effective; 3= satisfactory; 4= not very effective, 5= ineffective (rarely used)

corrosion or environmental cracking) have a

In general, a combination of inspection and limited scope in the calculation of the
testing methods and techniques should be remaining life and require a special approach
used to get the desired effectiveness. API 581 oriented towards the control and monitoring
recommended practice provides some guide of the variables that may cause damage.
tables for assigning the inspection
effectiveness for the most frequent failure The remaining life of the damage mechanisms
mechanisms and inspection strategies in that lead to thickness loss can be calculated
storage tanks. based on the inspection results.

Based on the effectiveness and the inspection The objective of an inspection is to determine
frequency of each tank in particular, and the remaining life of the inspected
following the guidelines of that recommended component. For example, when the thickness
practice, it is possible to estimate the of the shell metal sheet is measured over
probability reductions for failure mechanisms time, its corrosion rate can be established in
identified as probable. mm/year, generalized as well as localized (as
pitts) corrosion, and its remaining life can be
9. Analysis of inspection results determined until the withdrawal thickness is
reached.
9.1. Calculation of remaining life
If the corrosion rate controls the tank’s life,
The remaining life is calculated in relation to the remaining life (RL) should be calculated in
the main damage mechanism and depending the following way:
on its respective failure characteristic.
R L = (TMEA- TREQ)/CORRR
Damage mechanisms with characteristics that
Where:
are not related to the age (for example, stress

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 ARPEL Guideline: storage tanks inspection plans

− RL is the remaining life, in years; to provide a greater coverage level to those

− TMEA = thickness measured during items that contain the highest risk, and an
inspection, in the section used to adequate attention to those with the lowest
determine the TREQ, in millimeters risk.
(inches); The method defines the risk of operating
− TREQ = minimum acceptable thickness in equipment as the combination of two
the tank’s section or area under analysis, separate terms: the probability of a failure to
in millimeters (inches); and occur in a certain period and the failure
− CORRR = corrosion rate that indicates the consequence. In mathematical terms, the risk
amount of metal removed as a result of can be expressed as:
corrosion, in mm/year or thousandth of Risk = Probability x Consequence
an inch/year.
The probability analysis is based on a generic
For new tanks or for those that changed their data bank of failure frequency, by type of
operation conditions, one of the following equipment, which is modified by factors that
methods can be used to determine the reflect the difference between the generic
estimated corrosion rate: and the particular item analyzed.
The corrosion rate is established through data The analysis of the consequence of fluid
gathered by the owner, or by tank users in the release is calculated by estimating the amount
same or similar operation conditions, available released; by the prediction of the way in
in specialized literature. If data for the same which the fluid affects the environment and
or similar operation conditions is not by the implementation of models that allow to
available, the corrosion rate may be estimated evaluate the effects on people, the
by the inspector’s experience and knowledge. environment or its economic impact.
If the probable corrosion rate cannot be The RBI provides a connection between the
established by the previous methods, damage mechanisms and the inspection
thickness measurement values can be activities that reduce the associated risks.
collected after about 1000 hours of operation. Even though the inspection does not directly
Other subsequent measurements will be reduce the risk, it is a risk management
carried out, at similar intervals, until the activity which involves a risk reduction.
corrosion rate can be established. However, it should be considered that the risk
cannot be reduced to zero only with
9.2. Risk based inspection inspection activities, there are factors that
The “risk based inspection” is a method that may produce a containment loss including,
uses risk as a basis to prioritize and manage but not limited to, the following:
the efforts of an inspection program.
• Human errors
In an operating plant, in general, a relatively
large percentage of the risk is associated with • Natural disasters
a small percentage of equipment items. • External events (collisions or hits with
The “risk based inspection” focuses the objects)
inspection and maintenance resources so as

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ARPEL Guideline: storage tanks inspection plans

• Side effects of other plants or units unacceptable risks, the inspection plan should
also include mitigation actions that should be
• Deliberate acts (sabotage)
applied to mitigate the risk to acceptable
• Design errors levels.

The main deliverable of the RBI methodology The process to implement the RBI
implementation is an inspection plan for the methodology is shown in figure 9.2 and each
different tank components. If there are step is shortly described below.

Risk evaluation process

Failure
consequence
Information
and data
Risk Inspection Mitigation
collection
classification plan (if there is)

Failure
probability

Re-evaluation

Figure 9.2: implementation process of the RBI methodology

failure. A criticality matrix of 5 x 5 is
9.2.1. Data collection
recommended, as shown in figure 9.2.2, or
the one already established in each business
Data collection sources should include, but are
unit. This guideline recommends the use of a
not limited to:
unified risk matrix for assets.
• Design and construction records.
• Inspection records.
• Process information.
• Change control records.
• Off-site information.
• Failures records.

RBI is a dynamic program and should include a

permanent information update; it is important
to have a database to keep this information. Figure 9.2.2 2: risk criticality matrix

9.2.2. Risk analysis The probability and consequence categories

are identified from the lowest to the highest
A risk matrix should be defined to establish 2
the probability and the consequences of the Reference: ARPEL reference manual for pipelines
integrity management

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 ARPEL Guideline: storage tanks inspection plans

and the combination of these two variables 9.2.5. Re-assessment:

defines risk. It is classified from very high or
unacceptable to low. Each company should It is important to keep the RBI study updated
define its acceptable level of risk. to make sure that the recent inspection,
process and maintenance activities are
9.2.3. Inspection plan included. Every inspection or process or
design change will produce a risk variation,
As part of the inspection plan, activities with which should involve its re-assessment.
the tank in service as well as activities with the
tank out of service should be included. This 9.3. Inspection frequency
plan should include:
The analysis result is introduced in a matrix of
• Inspection frequency (see item 9.3). five x five that classifies the equipment in
levels that go from low risk to high risk.
• Inspection technique.
• Inspection scope. To guarantee the physical integrity of storage
tanks, an external or an internal inspection is
9.2.4. Mitigation recommended taking -for frequency- the
results of the last inspections, the project’s
As it was previously mentioned, inspection requirements, the operational conditions, and
does not always involve a sufficient reduction accordance with the applicable legislation into
of risk or, in some cases, it is not a cost- account. In general, the external inspection
effective method, so additional risk mitigation should precede –or be carried out together
actions should be considered. These activities with- the internal inspection.
can belong to one or more of the following
groups: The interval from the start-up until the first
internal inspection should not be longer than
• To reduce the consequence severity. 10 years. Alternately, if a RBI study is carried
out in accordance with numeral 9.2 and the
• To reduce the failure probability.
tank has some of the following protection
• To improve the survival of people or systems, the initial interval should not exceed
facilities faced with a failure. what is established in table 9.3.a below:
• To mitigate the primary source of the
consequence.

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Table 9.3.a: maximum inspection intervals for protection systems

Tank safeguard Max. inspection interval
i) Original nominal thickness of the floor 5/16 in. or more 12 years
1
ii) Effective cathode protection of the floor soil side 12 years
2
iii) Floor covering product side 12 years
2
iv) Fibre glass reinforcement of the floor product side 13 years
v) Cathode protection and covering 14 years
vi) Cathodic protection and fibre glass reinforcement 15 years
3
vii) Leak protection barrier 20 years
3
viii) Leak protection barrier when a RBI study has been carried out 25 years
1. Effective cathode protection means in accordance with API 651.
2. Covering of product side means in accordance with API 652.
3. Protection barrier means a leak detection system in accordance with appendix I of API 650.

study described in 9.2, but cannot exceed the

The subsequent interval for internal
maximum interval described in table 9.3.b
inspection should be established according to
below:
the remaining life described in 9.1 and/or by a
RBI

Table 9.3.b: maximum inspection intervals according to the procedure used

Procedure used Maximum interval
i) Calculation of corrosion speed 20 years
ii) RBI study 25 years
iii) RBI study and leak prevention barrier (see note) 30 years
Note: protection barrier means a leak detection system in accordance with appendix I of API 650.

flexible and accepting the existance of

9.4. Advanced analysis methods and
damages faced with control conditions.
adaptation to use – approval
criteria 10. Qualification of inspection and
Equipment can have problems such as testing personnel
fissures, localized thickness loss, deformations
or others, during the operational period. 10.1. Introduction
There are advanced calculation techniques or
methods, such as those described in API 579 Different types of personnel certifications are
and BS 7910, with the purpose of defining the applicable to storage tanks. Certifications
need to repair or alter the frequency and/or more frequently used are:
the inspection methods.
• certification of inspectors in non-
In these cases, the approval criteria differ destructive testing;
from those used by the manufacturing and/or
inspection codes, being sometimes more • procedures and personnel certification for
carrying out welding;

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• welding inspectors’ certification; an independent organization, sectoral

certification, certification of very reduced
• paint inspectors’ certification; and
scope, etc., but all of them are due to the
• inspectors’ certification (ICP API 653). same philosophy, the need for a qualification
to carry out NDT, being certification a proof of
Procedures and personnel certification for this qualification.
carrying out welding is applicable on the
occasion of the construction of the tank or in Only the standardization of the different
repairs that imply welding. methodologies will guarantee the same level
of competence and quality throughout the
The paint inspectors’ certification is applicable countries. The first step for international
for quality control of use of new paint systems harmonization was the creation of a common
or in repairs. international standard for national standards
to be based on. With this purpose, the
The welding inspectors’ certification is international standardization ISO 9712 was
applicable for quality control of welding carried out "Non-destructive testing,
during the construction of the tank or for the qualification and certification of personnel".
control of repairs that imply welding.
The present trend of countries in Latin
The inspectors´ certification according to API America and the Caribbean is that, at least an
653 is applicable for inspection and repairs of important part of the certification is carried
storage tanks. out by independent organizations or by a third
party, recognized for their certification in
The non-destructive testing certification is accordance with internationally accepted
very important as personnel frequently carry standards. Examples of national standards: EN
out critical evaluations that may affect the 473 (Europe), IRAM-ISO 9712 (Argentina), and
safety and integrity of the storage tank. As NBR ISO 9712 (Brazil).
many non-destructive testing methods do not
generate permanent records of results, the The NDT certification methodology even more
certification presents objective evidence that known is the one of the American Society for
the level of knowledge and skills of the Non-Destructive Testing – ASNT, which
professional that carries out the inspection is establishes the minimum reccomended
appropriate. It also guarantees that the requirements in the document ASNT-SNT-TC-
inspection will be carried out with the same 1A. It is a first-person certification in which
quality level regardless of the person who professionals are not certified by independent
carries out the test. organizations.

10.2. Certification methodology It is advisable to use the existing certification

methodology of the country in which the
There are many organizations that produced storage tank is located for professionals'
standardization and minimum qualification and certification of non-
recommendations for qualification and destructive testing used in their construction
certification. At present, there are many or repair. In the case that there is no
methodologies, and different among methodology, the methodology presented by
themselves, like for example: certification by ASNT should be specified.

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10.3. Certified non-destructive testing He/She should also have some knowledge of
methods materials and products manufacturing. These
professionals are also responsible for training
For storage tanks, the certification for the level I and II professionals.
different non-destructive testing methods
listed below can be obtained. 10.5. Certification and qualification
requirements
Table 10.3: non-destructive testing methods
The candidate for qualification in non-
Acoustic emission AT
destructive testing should meet minimum
Electromagnetic test ET
Sealing test LT formal education, physical conditions
Penetrating liquids test PT (generally vision), training and experience
Magnetic particles test MT requirements prior to the qualification exam.
Radiographic test RT
Infrared test TT The procedures that guarantee that the NDT
Ultrasonic test UT personnel have the necessary qualifications
Visual test VT should include:

10.4. Certification levels • training to acquire the necessary

knowledge;
In general, the certification personnel are
• experience with supervision of more
certified in different levels for each NDT
experienced professionals;
method. The usual levels in almost all the
countries are the following: • physical condition;

Level I – inspector qualified to carry out very • qualification exams to prove competence;
specific and simple tests and calibrations, in and
which the approval or rejection criteria are • certification to show approval according to
determined by a procedure. The level I the certification system criteria.
inspector should act under the supervision of
an inspector of higher level. The level I The training level and the experience for
inspector cannot be responsible for choosing different specifications are similar for the
testing methods or the interpretation and different systems. Typical requirements are
evaluation of results. shown in the table below:

Level II – inspector with the ability to carry out

the calibration of instruments and the
inspection following procedures, to interpret,
evaluate and register the test results. This
level should know the applicable standards
and other applicable documents.

Level III - inspector with the ability to develop

techniques and procedures, interpret codes,
and specify non-destructive testing methods.

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Table 10.5: training and experience typical include calculations and questions on codes,
requirements norms, specifications and procedures.
Minimum
Hours Practical exam: it consists in the application of
working
Level
Test required in the method in specific pieces, recording and -
experience in the
method NDT training for level II - interpreting and assessing the
(2) method (months)
(3) results of the test.
Acoustic I 40 3
emission II 104 12 For level III:
Electromag I 40 3
netic II 104 12
Level III basic exam: multiple choice questions
I 16 1
on 3 aspects:
Penetrating
liquid II 40 4
I 16 1
• technical knowledge on science of
Magnetic
particles II 40 4
materials, process technology and types of
Radiograph I 40 3 discontinuities;
y II 120 12 • knowledge of the system of qualification
I 40 3 and certification; and
Infrared
II 120 12
I 40 3 • knowledge of at least 4 methods as
Ultrasound II 120 12 required for level 2, including UT or RT, the
I 16 1 minimum.
II 40 4
Visual This exam is common to all methods.
I 40 3
Notes:
(1) Reference: ISO 9712:2005 Main method exam:
(2) Hours required for level II training courses
include training hours for level I • knowledge on the test method;
(3) The experience is based on 40 nominal working • application of the method in the sector,
hours a week, and are totals cumulative.
including codes, norms, specifications and
procedures; and
10.6. Exams
• preparation, by the candidate, of one or
In general, the exams required to prove the more NDT procedures in the method in
knowledge of the candidate on the testing which the certification was requested. The
method is subdivided according to: procedure will include the test of some
elements or components according to a
For levels I and II: specific code or norm.

General exam: about the general theory of Practical exam: if the candidate has not
the main method. Multiple choice questions. passed level II exams before, he/she should
also approve the level II practical exam in the
Specific exam: about the specific theory of the method.
implementation of the method to certain
products or processes of the sector. May

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10.7. Certificates • to carry out a penetration in the lower part

of the tank within 12 inches of the shell;
According to the results of the qualification
• to remove, replace or add a shell plate
exams and of the documents provided, the
under the design liquid level when the
official certification organization of the
largest dimension of the replacement plate
country decides whether to grant the
is greater than 12 inches;
certification or not and issues, in this case, the
corresponding certificates. • to remove or replace the annular plate ring
material when the largest dimension of the
Certificates issued will include, at least, the replacement plate is greater than 12
following information: inches;

a) Complete name of the person who is • to remove or replace completely or

certified. partially more than half the thickness of
the vertical weld joining shell plates, or the
b) Reference to the regulation, standard or radial weld joining the annular plate ring;
certification process.
• to install a new complete bottom. If a
c) Certification date. partial installation of a bottom is carried
d) Certification expiry date. out, without changing/modifying the
critical area of the tank, it will not be
e) Certification level.
considered as a major
f) NDT method. change/modification. See API 653 12.3.3.3;
g) Identification number. • to remove or replace part of the shell
welding of the inferior part, or the annular
11. Repairs, alterations and plate ring, additionally to what is specified
quality control in API 653 12.3.2.5.1 a; or

Storage tanks that are in use may need to be • to lift a tank shell.
repaired or altered. In order to keep the It is recommended that as part of the
original performance and safety instructions to establish a repair and
characteristics, it is recommended that these alteration plan, a plan to guarantee the quality
changes are carried out according to criteria of materials, repair procedures, personnel,
and procedures established based on equipment, electrodes, source rods and
recognized laws and regulations and accepted analysis and testing methods to be used is also
by the industry. established. This quality plan should be
internally approved within the organization
It is also important that repairs are carried out and reviewed in detail by the key actors. In
with such a level of quality so as to preserve the case that other companies or contractors
its useful life and structural integrity. are selected to carry out repairs, it should be
Major repairs and changes include: required that they provide a quality assurance
plan.
• to carry out a penetration of the shell
larger than NPS 12 beneath the design A quality plan should include, at least, the
liquid level; following elements:

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 ARPEL Guideline: storage tanks inspection plans

1. Identification of a Quality Manager, and The components to be replaced that will be

Quality Chiefs to work on-site. subject to efforts, and that are
premanufactured by welded insulations,
2. Quality objectives.
should be welded according to the original
3. Deliverables and key processes of the code of construction. The supplier or
project to be reviewed to meet a manufacturer should certify that the material
satisfactory quality level. and the manufacturing are according to the
4. Quality standards. original code of construction.

5. Quality control and assurance activities. When tank components are replaced, the
6. Quality roles and responsibilities. replacement should respect the dimensional
tolerances established in the original code of
7. Identification of quality tools and testing construction of the tank. This precaution
methods. should be taken in the critical areas as the
8. Plan to report quality control and tank floor, lower part of the shell, roof, and
assurance problems. roof support columns.

11.1. Materials In the event of a conflict between codes, the

stricter code should be taken as reference.
The materials used in repairs or alterations
should meet the requirements of the original 11.3. Welding
code.
Every welding should be carried out according
The materials used should be traceable up to to the requirements of the original code of the
the original manufacture source through the project, to the corresponding repair code, or
required chemical composition and to the code adopted in the case that there is
production certificates. It is recommended to lack of information.
carry out a verification of materials and on-
site baseline NDT through the “Positive 11.3.1. Welding procedure specification
Detection of Materials” to ensure the
materials appropriateness. Welding should be carried out according to
the welding procedure specification that is
11.2. Replacement of components qualified according to the original code of
construction or, if this is not possible, the code
The components to be replaced that will be that is recognized and accepted by the
subject to efforts, made up of new materials community.
manufactured by casting, forging, extruding
and other processes that do not use welding, 11.3.2. Welder qualification and
should include identification of the identification
manufacturer, so as to be able to track the
original characteristics. Tubes with or without Welders or welding operators should be
seams, holes and connections, and metal identified and qualified for the welding
sheets are some examples. procedure used. The owner should have its
own database of welders to guarantee that

ARPEL Guideline Nº MP02-2012

36
ARPEL Guideline: storage tanks inspection plans

only qualified welders carry out the jobs and 11.6. Records updating
have a record of the welder´s performance.
Before putting the tank into operation again,
Welders should identify with their registration inspection and maintenance records of the
letter or number all the welding carried out, equipment should be updated with the
with indelible marks and they should be recommendations for the next out of service
identified in the welding registration report. inspection scheduled, and the remaining life
The use of embossed letters or numbers as of the tank should be re-calculated. The tank
marks in stainless steels should be avoided, as inventory records should be updated to make
they could lead to stress corrosion in the sure that the materials used by the project are
material as it is subjected to a corrosive replenished. The detailed quality report
environment. should be signed by the professionals in
charge.
11.4. Quality control of repairs
12. Asset management document
Repairs and alterations should be inspected
and tested, using recommended methods The last stage of the integrity
according to the requirements and inspection/evaluation is the adequate detailed
specifications of the project. Tests which recording and documentation of all that was
results are used for the evaluation of the seen, executed, tested and recommended
equipment integrity should be carried out by during the inspection. Inspection records are
qualified and certified inspectors. key components for the subsequent
evaluations of the equipment degradation,
The quality control of repairs should be and also as future references. They work as
consistent with the tank repair plan. The tools documents that are part of the operational
and the testing methods established in the record, and therefore, they should be
quality plan for the quality control should be organized and kept during the whole useful
adopted and carried out by trained and life of the equipment.
experienced personnel. Results should be
documented and presented to the project The entire inspection activity should be clearly
executor. and completely recorded, usually as an
Inspection Report, detailing the scope of the
11.5. Hydrostatic testing inspection, its extent, the techniques and
equipment used, besides including a clear
The hydrostatic testing should be carried out identification of the person responsible for
for major repairs. It is recommended that the the activities carried out, and other
execution of this testing is evaluated by a complementary information.
qualified professional, given the
characteristics of damages and repairs in As a result of this exercise, for each tank, the
question. results will be summarized in an asset
management document, as the example on
the next page.

ARPEL Guideline N° MP02-2012 37

 ARPEL Guideline: storage tanks inspection plans

Tank no. xx
Service Catalytic naphtha
Year of construction Xxx
Diameter Xxx
Height Xx
Construction material Xx
Cubage, table certified by authority (date) Xx/xx
Exterior paint (type, thickness, date)
Interior paint (partial or total, type, thickness, date)

Components Design thickness Material

Roof
Floor
Shell
Etc.

Inspection and repairs record

Inspection/repair date Findings
xx-xx-xxxx Today an inspection of the floating roof of Tank xx was carried
Inspected by xx out, which was floating in the superior part with a product
level of 12,509mm and going down. Ultrasound thickness
measurements were carried out in some metal sheets to
guarantee the safe movement of the cleaning staff, obtaining
values between 4.0mm and 5.0mm. No anomalies or product
movements were detected by the visual inspection.

Failure modes
Scenario Degradation Initial risk Mitigation Mitigated risk
mechanism measure
Small leak through Localized SHE consequence: III Scanning Probability D
tank floor. There is corrosion by inspection of the
ECON consequence: IV
perforation in the external floor every 8
floor, hydrocarbon attack. Probability C years.
leak on the soil.
Estimated repair Risk C-III
cost < 100,000USD.
D-III

Inspection plan
Component Inspection frequency Techniques to be used
Tank floor 8 years Floor scanning
Tank shell Every x years Ultrasound thickness
measurement, in operation

ARPEL Guideline Nº MP02-2012

38
ARPEL Guideline: storage tanks inspection plans

The fulfillment of a tank integrity integrity management plan, as for every

management plan, complemented by an management. They should provide the
efficient fulfillment of the corresponding resources suitable to their operative areas for
planning, allows to guarantee the operative this fulfillment to be possible, as the more
availability and reliability of storage tanks, efforts made for an efficient and optimal
avoiding unexpected failures that could cause inspection, the lower the probability of a
undesired operative pauses, accidents or degradation event in storage tanks to happen.
environmental impacts.
13. Implications and legal matters
Fulfillment best practices are part of the
national framework binding laws, and
on the inspection of
international reference standards as API 353, equipment
580, 650 and 653.
Engineering activities have a complex nature
Therefore, once the plan is established, the and inherent risks that may affect people and
historical background is known, the the entire society with different levels of
specialized inspection is carried out, the type complexity.
of failure is identified and the corresponding
analysis is carried out, it will provide the Design should abide by national regulations.
necessary tools to take the corresponding In those matters in which there are no
actions and avoid failures. technical provisions or national regulations,
foreign rules, codes, specifications should be
Top executives at organizations should be applied, as well as engineering recommended
responsible for the fulfillment of a tank practices, internationally recognized and
accepted by the local authority.

ARPEL Guideline N° MP02-2012 39

 Regional Association of Oil, Gas and Biofuels Sector Companies in Latin America and the Caribbean

ARPEL is a non-profit association gathering oil, gas and biofuels sector companies and institutions in Latin America and the Caribbean.
Founded in 1965 as a vehicle of cooperation and reciprocal assistance among sector companies, its main purpose is to actively contribute
to industry integration and competitive growth, and to sustainable energy development in the region.
Its membership currently represents over 90% of the upstream and downstream activities in the region and includes national, international
and independent operating companies, providers of technology, goods and services for the value chain, and national and international
sector institutions.
Since 1976, ARPEL holds Special Consultative Status with the United Nations Economic and Social Council (ECOSOC). In 2006, the
association declared its adherence to the UN Global Compact principles.
Mission
To foster and facilitate sector integration and development, continuous operational improvement and effective management of
environmental and social issues, by:
• sharing, enhancing and disseminating best practices;
• carrying out studies that translate in information of value;
• broadening knowledge and helping build required competences;
• promoting networking, interaction and cooperation among members and stakeholders.
Vision
A growing, competitive and integrated oil and gas industry that achieves excellence in its operations and products, and effectively
contributes to a sustainable energy development in Latin America and the Caribbean.
Value proposition
ARPEL offers a unique mean for networking, sharing knowledge, joining efforts and building synergies in favor of the sector’s integration,
growth and sustainability. Without any distinction, Members have the opportunity to alternatively lead activities and projects, contribute
with their know-how to their development, or learn from the experiences of other members.
ARPEL´s value is also reflected in its condition of strategic information center about sector activities in the region and cost-effective vehicle
for the development of publications on best practices and benchmarking, as well as on sectoral studies and executive reports aimed at
diverse stakeholders. The Association additionally stands out for its regional conferences, forums and seminars of high impact in the
industry.
ARPEL is a recognized regional body of representation for the sector that seeks to advocate in favor of the common interests of its
Membership and to enhance the industry’s public image and reputation.
Socio-environmental sustainability
Operational excellence
Sectoral development
www.arpel.org