Crack Management – Part IIFeedback From In-Service Inspections

Based upon OTC 15064 “FPSO Fatigue Assessment: Feedback From In-Service Inspections”

Clemens G.J.M. van der Nat (Bluewater)

Martijn G. Hoogeland (Bluewater)

Mirek L. Kaminski (MARIN)

Content

Why a different strategy for offshore unit? Fatigue life strategy Feedback into hull fatigue strategy Conclusions

Tanker vs. FPSO

Empirical design vs. first principles design Total asset vs. (small) part of field development Weather routing vs. continuous environmental

exposure Inspection, Maintenance & Repair (IMR)

at shipyard vs. on site & in operation

Fatigue life strategy

Objective:Control fatigue accumulation over lifetime of FPSO– Hull selection– Repair & Lifetime Extension (R & LE) program– Hull monitoring & in service inspection– Fatigue defect repairs

Hull selection

Objective:– Determine hull status– Estimate R & LE scope

Current practices– Review of trading history & class records – Inspection of fatigue sensitive details– Preliminary evaluation of deck and bottom

longitudinals

Typical tanker structures

Flat bar stiffener

Bracket

Web frame

Side shell longitudinal

Life extension modification

Objective – To ensure sufficient lifetime of hull as FPSO– Definition of yard scope

Current practice– Fatigue analysis– Improving details– Provide access to structure for inspection

Fatigue sensitive details

– Fatigue damage occurs mainly between primary and secondary members

– Fatigue life prediction for longitudinals

Original detail Upgraded detail

Enlarged brackets and backing brackets

Prediction vs. Inspection

Expected damage after 20 years

Number of defects after 6 yearsBacking brackets only

Prediction vs. Inspection

as reported

Fracture as

predicted

Hull inspection at field Objective

– Check of actual integrity of hull Methods

– By periodic inspections– By continuous hull monitoring system

Current practices– 5 yearly cycle of inspections– ½ yearly cycle of defects found

Crack detection

Crack detection depends on:– Location (accessibility)– Loading condition of vessel– Cleanness/ corrosion/ colour of coating of

surfaces– Inspection method

Crack growth rate is not linear

Defect repair

Objective– Control progressive failure of structure:

Yielding / buckling Impairment of water & gas tight boundaries Unstable fracture

Method– Understanding of cause and consequences

Load path Criticality of damage

Repair schedule

– Priority by consequences– Analysis of:

Location Length Direction

1

3

2

Crack

4

Fatigue repair

Current practices– Unloading crack-tip

Drilling hole at crack-tip

– Relocation of hotspot Addition of (backing) brackets Addition of lug plates

– Renewal of material Welding New steel

Feedback from inspections

Feedback to hull selection– More locations and details must be examined

Web

1

3

2

Crack

4

Feedback from inspections

Hull modification– Analysis improvements

Use operation experience Use JIP knowledge

Cost effectiveness CAPEX <-> OPEX– Combine with coating scope– Offshore repair expensive due to operational

implications

Conclusions

Tankers are suitable as offshore unit. However:– Awareness of vulnerability for fatigue is starting

point– Control of fatigue damage in FPSO structures

requires a strategy Monitoring will give valuable information to reduce

uncertainties and improves control

Bluewater fatigue strategy

BW fatigue strategy includes at least:– Adequate selection process for hull– Selection of critical details and locations– Reliable prediction of remaining fatigue life– Principal: “Prevention is better than cure”– Consequence based repair program