ocean stuctures

7/27/2019 Ocean Stuctures

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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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