Ageing and Life extension for structures

David GalbraithOcean Structures / Sigma Offshore

1 PSA Seminar 19 November 2008

What is different for structuresó Additional ageing mechanisms and other changing factorsó Structural failure is “unacceptable”

ó Design failure probabilities < 1x10-4

ó Tendency for personnel to have “blind faith” in the structureó Some design loadings are “unknown”

ó E.g. Wave heights based on statistics and extrapolationó Can have step changes in loadings (wave impact on deck)

ó Inspection and maintenance of substructuresó Extremely expensive – underwater inaccessible – use of diversó Few opportunities for inspectionó Fewer for remedial work (cannot be replaced)

ó Many reasons for assessmentó Including extended life 2

Ageing and life extension for structuresó Types of structureó Fixed steel platforms and foundationsó Fixed concrete platforms and foundationsó Floating platforms and mooringsó Topsides primary structureó Topsides secondary structure

ó Different levels of criticalityó Different methods of maintenanceó Different costs of inspection and maintenance

Structural ageing and degradation mechanismsó Fatigueó Corrosion and concrete degradation mechanismsó Geological and Geotechnical Hazardsó Accidental Damage ó Extreme Weatheró Modifications and Change of Use ó Marine Growth ó Gross Errors due to Human and Organizational Factors

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Fatigue

ó Many cracks exist in offshore structuresó Early designs of fixed steel structure had weak fatigue

detailingó Short fatigue livesó Poor weld execution

ó Current design and assessment has fatigue factors up to 10ó Critical and uninspectable componentsó Early designs used a factor of 2 throughout

ó Cracking can lead to:ó Floating structures – loss of buoyancy and/or stabilityó Fixed steel structures – loss of strength & loss of redundancy

ó Fatigue also affects concrete, but not an issue for operating platforms

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Fatigue life factors

6PSA Seminar 19 November 2008

Corrosion

ó Protection provided by:ó painting and coatings – topsides

– Corrosion under passive fire proofing can be a particular problemó Anodes and painting – substructure

– Many steel structures are unpainted below splash zoneó Wrap plates – some special areas – e.g. Some splash zonesó Corrosion allowance – particularly splash zone

ó Inspection, maintenance & repairó Repainting topsides steelworkó Inspection of cathodic protection potentialsó Replacement / addition of anodes

ó Piles are a difficult area – critical area, unispectable, not directly protected

ó Topsides conditionó Often poor maintenance historyó EI publication on assessment of corrosion

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Geological and Geotechnical Hazardsó Installation foundation hazards

ó Pile pull-out in tensionó Pile punch-through in compressionó Degradation of pile capacity due to cyclic loading

ó Geological hazardsó Differential settlementó Seabed scouró Subsidence and slope instability

ó Scour and subsidence can lead to wave impact on deck

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Accidental Damage

ó Ship Collisionó Supply boat (frequent) – vessel masses now ~ 5000Tó Other service vessel (occasional)ó Passing vessel (rare)

ó Dropped objectsó Range of objects and associated damage

– Radios & scaffold poles to Mobile cranes, tubulars, containersó Particular concerns

ó Major impact damageó Impact on risersó Enhanced corrosion ratesó Accumulated damage 9

Extreme Weather

ó Most NS platforms design for 100 year return stormó Elastic response – effectively undamagedó The 100-year storm as understood at the timeó Deck height at 100 year storm + 1.5m (from 5’ in GoM)

ó Current criteria includes withstanding 10,000-year return stormó Structure still has to stand – no safety factors

ó But some platforms have suffered subsidenceó Various R&D projects have tried to compare predicted versus

measured forces due to stormsó Effects of climate change?ó Life extension

ó Main issue is “wave in deck”ó Early designs with no subsidence – often not an issue

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Modifications and Change of Use

ó Many changes made during life of platformó Weight management tools should be usedó Weight audit can be necessary – depending on historical weight

controló For life extension – future uses of the platform should be

consideredó Norwegian regulations:

ó Life extension and change in use are separate applications

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Marine Growth

ó Marine growth can add weight and environmental loadingó Can be very significant in warmer climatesó Inspection and cleaning techniquesó Hard (e.g. Mussels) and soft (e.g. Seaweeds) marine growths

have differing significanceó Generally not a significant issue in Central and Northern North

Seaó Not particularly significant for life extension

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Gross Errors due to Human and Organizational Factors ó Underdesigned structural elementsó Damaged elements at the installation stageó Poor quality inspections and missed damage ó Gross errors can accumulate during the life of

an installation

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Structural Integrity Management

ó Ongoing process throughout platform lifeó Cyclic processó Techniques and methods well understood and documented

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Structural assessment

ó Guidance given in ISO 19902 – particularly for fixed steel structures – World wide applicability

ó Also NORSOK N-006 – All platform types – mainly NS applicability

ó Also API-RP2 SIM – All platform types (but mainly fixed steel) –mainly GOM applicability

ó Various assessment initiatorsó Based on “current design methodology”ó Allows refinement of techniques

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Assessment indicators

A. Changes from design or assessment basis, including1. Change of platform exposure level2. Weight or C of G changes,3. more onerous environmental conditions and/or criteria,4. more onerous component or foundation resistance data and/or

criteria,5. excessive scour or subsidence, etc.6. inadequate deck height,

B. Damage or deterioration of a primary structural component:C. Exceedance of design service life, if eitheró the fatigue life is less than required extended service life, oró degradation of the structure due to corrosion is present, or is likely to

occuró BUT IS A REQUIREMENT UNDER NORWEGIAN REGULATIONS

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Potential show stoppers

ó High fatigue utilization factor ó Excessive storm utilizationó Insufficient knowledge ó High fatigue utilization and limited inspectability ó Cumulative effect of damage excessive (including accidental

damage) ó Widespread fatigue damage ó Damage tolerance requirements ó Not meeting acceptance criteria

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Learning from decommissioned structuresó Grouted pilesó Repairs to joints and membersó Materials and weldingó Ring stiffened jointsó Flooded membersó Closure weldsó Cast Jointsó Verification of underwater inspection capabilityProtocol for recovery of components available from Ptil 18

Fixed steel platforms and foundations

ó Fatigue damageó Corrosion damageó Accumulation of damage from all sourcesó Uninspectable components (e.g. Piles,

internally stiffened joints)ó Wave in deck

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Fixed concrete platforms and foundationsó Anode usage (e.g. Unintentional electrical

continuity)ó Wave in deckó Prestressing tendonds – conditionó Undetected corrosion

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Floating platforms and moorings

ó Increasing wave heightsó Fatigue damageóEffects on buoyancy and stabilityóBallast control systems

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Topsides structure

ó Poor historical maintenanceó Painting backlog

ó Degradation of PFPó Wave in deckó Walkways / grating & handrails

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