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Crack Management – Part II Feedback From In-Service Inspections Based upon OTC 15064 “FPSO Fatigue Assessment: Feedback From In-Service Inspections” Clemens.

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Presentation on theme: "Crack Management – Part II Feedback From In-Service Inspections Based upon OTC 15064 “FPSO Fatigue Assessment: Feedback From In-Service Inspections” Clemens."— Presentation transcript:

1 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)

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

3 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

4 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

5 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

6 Typical tanker structures Flat bar stiffener Bracket Web frame Side shell longitudinal

7 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

8 Fatigue sensitive details –Fatigue damage occurs mainly between primary and secondary members –Fatigue life prediction for longitudinals Original detailUpgraded detail

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

10 Prediction vs. Inspection as reported Fracture as predicted

11 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

12 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

13 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

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

15 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

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

17 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

18 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

19 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


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