Crack Management – Part II Feedback From In-Service Inspections Based upon OTC “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 n Why a different strategy for offshore unit? n Fatigue life strategy n Feedback into hull fatigue strategy n Conclusions

Tanker vs. FPSO n Empirical design vs. first principles design n Total asset vs. (small) part of field development n Weather routing vs. continuous environmental exposure n Inspection, Maintenance & Repair (IMR) at shipyard vs. on site & in operation

Fatigue life strategy n 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 n Objective: –Determine hull status –Estimate R & LE scope n 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 n Objective –To ensure sufficient lifetime of hull as FPSO –Definition of yard scope n 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 detailUpgraded detail

Enlarged brackets and backing brackets Prediction vs. Inspection Expected damage after 20 years Number of defects after 6 years Backing brackets only

Prediction vs. Inspection as reported Fracture as predicted

Hull inspection at field n Objective –Check of actual integrity of hull n Methods –By periodic inspections –By continuous hull monitoring system n Current practices –5 yearly cycle of inspections –½ yearly cycle of defects found

Crack detection n Crack detection depends on: –Location (accessibility) –Loading condition of vessel –Cleanness/ corrosion/ colour of coating of surfaces –Inspection method n Crack growth rate is not linear

Defect repair n Objective –Control progressive failure of structure: n Yielding / buckling n Impairment of water & gas tight boundaries n Unstable fracture n Method –Understanding of cause and consequences n Load path n Criticality of damage

Repair schedule –Priority by consequences –Analysis of: n Location n Length n Direction Crack 4

Fatigue repair n Current practices –Unloading crack-tip n Drilling hole at crack-tip –Relocation of hotspot n Addition of (backing) brackets n Addition of lug plates –Renewal of material n Welding n New steel

Feedback from inspections n Feedback to hull selection –More locations and details must be examined Web Crack 4

Feedback from inspections n Hull modification –Analysis improvements n Use operation experience n Use JIP knowledge n Cost effectiveness CAPEX OPEX –Combine with coating scope –Offshore repair expensive due to operational implications

Conclusions n 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 n Monitoring will give valuable information to reduce uncertainties and improves control

Bluewater fatigue strategy n 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