Norsk Offshoredag 22. mai, Grand Hotell, Oslo

“TECHNICAL CHALLENGES RELATING TO CONTINUED

SAFE OPERATION OFEKOFISK PLATFORM STRUCTURES”

arranged by NSF (Norwegian Structural Steel Association) , 23rd May, 2007

By Michael Erik HallStructure integrity engineer,Greater Ekofisk Area.

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THE EKOFISK FIELD

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Ekofisk centre (production expected beyond 2050)

Greater Ekofisk Area: ~ 50 steel piled platforms of which ~ 20 are now disused and will be removed

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Ekofisk Complex

Monitored by: - GPS satellites,- Bathymetric surveys,- Level surveys.

(Future subsidence ispredicted by reservoircompaction models).

SUBSIDENCE

Subsidence bowl contours

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Subsidence reduces air-gap wave crest and deckSubsidence reduces air-gap (wave crest to deck)

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REASSESSMENT BASIS:

• Original design basis: API RP2A (100-yr wave with 1.5m air gap)

• NORSOK ‘N’ standards if:- API air-gap becomes < 1.5m- PSA (Ptil) requires consent for ‘continued’ operations

> PSA consent required for operations beyond nominal design life (~30 yrs)ref. Chp.2, Sect.5f NPD information duty regulations (opplysningsforskriften)

> PSA use ‘Facility regulations’ (innrettningsforskriften) as baseline for consent: e.g. Chp.3, Sect 10: ‘Accidental and environmental loads with an annual probability greater

than 10-4 shall not cause loss of main safety functions’

> A new NORSOK standard for life extension will provide further guidance

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7.7 m

1.577 m

Foundation loads / fatigue

1) Jacket assessment

Main topics for this presentation (items 1,2,3):

Fatigue loading

Boat impact (height)subs

2) Topside assessment

3) Change control System (SIMS)

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1) Jacket assessments

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Wave-in-deck loads (COMFLOW / USFOS)

‘JETTING’

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-50-40-30

-20-10

010

20304050

60708090

100110

2 3 4 5 6 7 8 9

Time (sec)

Wav

e lo

ad (M

N)

Fx-Deck Fz-DeckJacket Horisontal loadingJacket vertical loading

Direction: 225, PL NW 1000yr DNV ( H = 29.31m)

max Fzdeck

max Fxdeck

min Fzdeck

max FxJacket

Deck vs jacket loads – 1000yr - NWPCombined Deck and Jacket wave loads

Jacket

Deck

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ASSESSMENT PRINCIPLES: JACKETS

High consequence(any of)

Low consequence(all of)

Manned in storm Unmanned in storm (ref . EXWW)

Production from wells in storm Production wells secured in storm(ref . EXWW)

Financial consequence for society No financial consequence for society

2) Corresponding NORSOK limit state design criteria:High consequence

(both of)Low consequence

ULS (100-yr wave) withload/material factors 1.3/1.15

ULS (100-yr wave) withload/material factors 1.15/1.15

ALS (10000-yr wave) withload/material factors 1.0/1.0 (ref. SRA assessments)

1) Defined NORSOK consequence categories: (Ref. NORSOK N-001, §6.2.1, 6.2.2, 7.2.6)

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reduced 100-yr wave air gap

a) Slender structures (eg tripods) can satisfy NORSOK low consequence condition, but still have Pf close to 10-2 due to reduced air-gap (operating target ~10-3).

STRUCTURE RELIABILITY ASSESSMENT (SRA)

a) To quantify operating risk of NORSOK low consequence platforms b) To demonstrate low pollution risk (blow-out probability < 10-4 ) at well-head structures

b) For well-head structures, a Pf <10-3

normally required to achieve Pf <10-4

pollution risk from shut-in wells.

Pf (jacket) * -4Pf (wells) < 10

(Pf = annual probability of failure)

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‘Global’ wave ‘Local’ wave = global * 1.5 (based on local damage observations)

Cellar deck wthdrawal

Platform withdrawal

Ekofisk Extreme Wave Warning preparedness (EXWW)

Purpose: 1) Reduce consequence: Shut-in wells and unman platform prior to storm2) Protect personnel on cellar deck from ‘local waves’

LAT

Global wavethresholds

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CELLARDECK withdrawal

PLATFROM withdrawal

Inspection before re-entry

Online met.no (Meteorologisk Institut) forecast

history (measured)

extended prediction

Time

Cre

st h

eigh

t

3-hr prediction

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RISK REDUCTION MEASURES: JACKETS

a) Strengthening plan (over time)

Install new Members and/orgrout-fill existing

b) Hydrodynamic load reduction (open cellar deck, remove obstructions, wave load instrumentation programs, marine growth reduction)

Reinforce decks(tanks, girders)

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2) Topside assessments

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NORSOK consequence(life, pollution, societal)

ITEM High *Low

(+ significant cost risk)

Low(+ insignificant

cost risk)

Equipment10000-yr wave (ALS)+ Local wave (ULS)

100-yr wave (ULS)+ Local wave (ALS) repair damage

Jacket damage(falling object)

10000-yr wave (ALS)+ Local wave (ULS)

100-yr wave (ULS)+ Local wave (ALS) -

Decks10000-yr wave (ALS)+ Local wave (ULS)

100-yr wave (ULS)+ Local wave (ALS) -

ASSESSMENT PRINCIPLES: DECKS AND EQUIPMENT

( Note: 10-4 global wave height is ~equal to ‘100-yr’ local wave height)

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3) A change controlsystem (SIMS)

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Structure Integrity Status

Change evaluation

Inspection Strategy

12-yr inspection programme

Inspection Execution

Annual inspection campaigns

Database

Design and inspection data

Weight ChangeConfiguration Change

Condition ChangeOperating ChangeMetOcean Change

Reassessment

Assessment of remedial actions

Mitigation

load reduction, capacity increase

ReassessmentCycle

RBI + InspectionCycle

An integrated ‘Structure Integrity Management System’ (SIMS)

NORSOK N-004 Ch. 10: NORSOK N-005: (Reassessment of Structures) (Condition Monitoring of Structures)

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Inspection planning, execution, registration, mitigation….

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Key data, analyses, integrity status, documents, work process….

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‘SIMS’ visualises structure integrity statusand helps fulfill regulatory obligations by ensuring:

a) Necessary structure analysis data, models and documentation (storm, fatigue, earthquake, impact etc.)

are complete, compliant and accessible.

b) As-is reality is correctly simulated in structure reassessmentmodels (change control).

c) Identified non-compliances/anomalies are tracked untiladequately mitigated.