Ageing and Life extension for structures
David Galbraith Ocean Structures / Sigma Offshore
PSA Seminar 19 November 2008
What is different for structures
Additional ageing mechanisms and other changing factors Structural failure is unacceptable
Design failure probabilities < 1x10-4 Tendency for personnel to have blind faith in the structure E.g. Wave heights based on statistics and extrapolation Can have step changes in loadings (wave impact on deck) Extremely expensive underwater inaccessible use of divers Few opportunities for inspection Fewer for remedial work (cannot be replaced) Including extended life
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Some design loadings are unknown Inspection and maintenance of substructures
Many reasons for assessment
Ageing and life extension for structures
Types of structure
Fixed steel platforms and foundations Fixed concrete platforms and foundations Floating platforms and moorings Topsides primary structure Topsides secondary structure
Different levels of criticality Different methods of maintenance Different costs of inspection and maintenance
Structural ageing and degradation mechanisms
Fatigue Corrosion and concrete degradation mechanisms Geological and Geotechnical Hazards Accidental Damage Extreme Weather Modifications and Change of Use Marine Growth Gross Errors due to Human and Organizational Factors
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Fatigue
Many cracks exist in offshore structures Early designs of fixed steel structure had weak fatigue detailing
Short fatigue lives Poor weld execution Critical and uninspectable components Early designs used a factor of 2 throughout Floating structures loss of buoyancy and/or stability Fixed steel structures loss of strength & loss of redundancy
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Current design and assessment has fatigue factors up to 10 Cracking can lead to: Fatigue also affects concrete, but not an issue for operating platforms
Fatigue life factors
PSA Seminar 19 November 2008
Corrosion
Protection provided by:
painting and coatings topsides
Corrosion under passive fire proofing can be a particular problem
Anodes and painting substructure
Many steel structures are unpainted below splash zone
Wrap plates some special areas e.g. Some splash zones Corrosion allowance particularly splash zone Repainting topsides steelwork Inspection of cathodic protection potentials Replacement / addition of anodes
Inspection, maintenance & repair
Piles are a difficult area critical area, unispectable, not directly protected Topsides condition
Often poor maintenance history EI publication on assessment of corrosion
Geological and Geotechnical Hazards
Installation foundation hazards
Pile pull-out in tension Pile punch-through in compression Degradation of pile capacity due to cyclic loading Differential settlement Seabed scour Subsidence and slope instability
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Geological hazards
Scour and subsidence can lead to wave impact on deck
Accidental Damage
Ship Collision
Supply boat (frequent) vessel masses now ~ 5000T Other service vessel (occasional) Passing vessel (rare) Range of objects and associated damage
Radios & scaffold poles to Mobile cranes, tubulars, containers
Dropped objects Particular concerns
Major impact damage Impact on risers Enhanced corrosion rates Accumulated damage
Extreme Weather
Most NS platforms design for 100 year return storm
Elastic response effectively undamaged The 100-year storm as understood at the time Deck height at 100 year storm + 1.5m (from 5 in GoM) Structure still has to stand no safety factors
Current criteria includes withstanding 10,000-year return storm But some platforms have suffered subsidence Various R&D projects have tried to compare predicted versus measured forces due to storms Effects of climate change? Life extension
Main issue is wave in deck Early designs with no subsidence often not an issue
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Modifications and Change of Use
Many changes made during life of platform Weight management tools should be used Weight audit can be necessary depending on historical weight control For life extension future uses of the platform should be considered Norwegian regulations:
Life extension and change in use are separate applications
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Marine Growth
Marine growth can add weight and environmental loading Can be very significant in warmer climates Inspection and cleaning techniques Hard (e.g. Mussels) and soft (e.g. Seaweeds) marine growths have differing significance Generally not a significant issue in Central and Northern North Sea Not particularly significant for life extension
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Gross Errors due to Human and Organizational Factors
Underdesigned structural elements Damaged elements at the installation stage Poor quality inspections and missed damage Gross errors can accumulate during the life of an installation
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Structural Integrity Management
Ongoing process throughout platform life Cyclic process Techniques and methods well understood and documented
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Structural assessment
Guidance given in ISO 19902 particularly for fixed steel structures World wide applicability Also NORSOK N-006 All platform types mainly NS applicability Also API-RP2 SIM All platform types (but mainly fixed steel) mainly GOM applicability Various assessment initiators Based on current design methodology Allows refinement of techniques
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Assessment indicators
A. Changes from design or assessment basis, including
1. 2. 3. 4. 5. 6.
Change of platform exposure level Weight or C of G changes, more onerous environmental conditions and/or criteria, more onerous component or foundation resistance data and/or criteria, excessive scour or subsidence, etc. inadequate deck height,
B. Damage or deterioration of a primary structural component: C. Exceedance of design service life, if either
the fatigue life is less than required extended service life, or degradation of the structure due to corrosion is present, or is likely to occur BUT IS A REQUIREMENT UNDER NORWEGIAN REGULATIONS
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Potential show stoppers
High fatigue utilization factor Excessive storm utilization Insufficient knowledge High fatigue utilization and limited inspectability Cumulative effect of damage excessive (including accidental damage) Widespread fatigue damage Damage tolerance requirements Not meeting acceptance criteria
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Learning from decommissioned structures
Grouted piles Repairs to joints and members Materials and welding Ring stiffened joints Flooded members Closure welds Cast Joints Verification of underwater inspection capability Protocol for recovery of components available from Ptil
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Fixed steel platforms and foundations
Fatigue damage Corrosion damage Accumulation of damage from all sources Uninspectable components (e.g. Piles, internally stiffened joints) Wave in deck
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Fixed concrete platforms and foundations
Anode usage (e.g. Unintentional electrical continuity) Wave in deck Prestressing tendonds condition Undetected corrosion
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Floating platforms and moorings
Increasing wave heights Fatigue damage Effects on buoyancy and stability Ballast control systems
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Topsides structure
Poor historical maintenance
Painting backlog
Degradation of PFP Wave in deck Walkways / grating & handrails
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