ocean stuctures
TRANSCRIPT
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Ageing and Life extension for structures
David Galbraith
Ocean Structures / Sigma Offshore
1 PSA Seminar 19 November 2008
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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
Some design loadings are unknown E.g. Wave heights based on statistics and extrapolation
Can have step changes in loadings (wave impact on deck) Inspection and maintenance of substructures
Extremely expensive underwater inaccessible use of divers
Few opportunities for inspection
Fewer for remedial work (cannot be replaced)
Many reasons for assessment Including extended life 2
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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 cr iticality
Different methods of maintenance
Different costs of inspection and maintenance
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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
Current design and assessment has fatigue factors up to 10 Critical and uninspectable components
Early designs used a factor of 2 throughout
Cracking can lead to: Floating structures loss of buoyancy and/or stability
Fixed steel structures loss of strength & loss of redundancy
Fatigue also affects concrete, but not an issue for operating
platforms
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Fatigue life factors
6PSA Seminar 19 November 2008
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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 Inspection, maintenance & repair
Repainting topsides steelwork
Inspection of cathodic protection potentials
Replacement / addition of anodes
Piles are a difficul t area critical area, unispectable, not d irectly
protected Topsides condition
Often poor maintenance history
EI publication on assessment of corrosion
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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
Geological hazards
Differential settlement
Seabed scour
Subsidence and slope instability
Scour and subsidence can lead to wave impact on
deck8
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Accidental Damage
Ship Collision Supply boat (frequent) vessel masses now ~ 5000T
Other service vessel (occasional)
Passing vessel (rare)
Dropped objects
Range of objects and associated damageRadios & scaffold poles to Mobile cranes, tubulars, containers
Particular concerns Major impact damage
Impact on risers
Enhanced corrosion rates Accumulated damage 9
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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)
Current cri teria includes withstanding 10,000-year return storm Structure still has to stand no safety factors
But some platforms have suffered subsidence Various R&D projects have tried to compare predicted versus
measured forces due to storms
Effects of cl imate 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 li fe 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 beconsidered
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 signi ficant 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 steelstructures 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, including1. Change of platform exposure level
2. 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 either the fatigue life is less than required extended service life, or
degradation of the structure due to corrosion is present, or is likely tooccur
BUT IS A REQUIREMENT UNDER NORWEGIAN REGULATIONS16
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Potential show stoppers
High fatigue utilization factor Excessive storm ut ilization
Insufficient knowledge
High fatigue utilization and limited inspectability
Cumulative effect of damage excessive (including accidentaldamage)
Widespread fatigue damage
Damage tolerance requirements
Not meeting acceptance cri teria17
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Learning from decommissioned
structures
Grouted piles Repairs to jo ints 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 18
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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 19
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Fixed concrete platforms and
foundations
Anode usage (e.g. Unintentional electricalcontinuity)
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 stabili tyBallast 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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