Crack management Clemens van der Nat Bluewater Energy Services BV.
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Crack management Clemens van der Nat Bluewater Energy Services BV
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Transcript of Crack management Clemens van der Nat Bluewater Energy Services BV.
Crack management
Clemens van der Nat
Bluewater Energy Services BV
16-10-02 FRF - crack management 2
Content
Crack management Existing practice in shipping Bluewater crack management
16-10-02 FRF - crack management 3
Crack management
Management in engineering phase– Fatigue damage prediction– Classification rules vs. direct calculations
Management in operations phase– Regular survey– When cracks are found:
Risk assessment Corrosion
– Move to condition based monitoring
16-10-02 FRF - crack management 4
Existing practice in shipping
Current practice for oil tankers Location of cracks determined by loading condition Involvement of classification societies
– Survey period– Methods– Required action
Repair at shipyard
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Bluewater crack management
Current practice for FPSOs life time extension Tanker hull design (web spacing, details) Old tanker requires survey of actual status and
estimate of actual fatigue life Site specific predictions for new “design life” Critical details to be identified and improved Surveys and repairs at site
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Enlarged brackets and backing brackets
Prediction???
Expected damage after 20 years
Actual damage after 6 yearsBacking brackets only
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Critical detailsAdding a backing bracket reduces the load on the crack tip
Misalignment induces cracking, a backing bracket reduces the load on the hot spot
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Bluewater crack management
Repair of critical cracks during operational life– Repair is required, delay only by exemption– When is a crack critical?
Damage effects global strength of hull Crack length exceeds critical value More than one crack at one location Cracks in oil separating bulkheads Crack growth exceeds inspection interval
– Reduce risk on progressive cracking
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Critical crack repair
Cracks in bulkhead are not acceptable
Repair by removing crack and adding backing bracket to unload the hotspot
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Critical crack repair
Risk: crack might propagate into the longitudinal bulkhead
Repair: adding lug plate extending to the longitudinal bulkhead plate
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Comparison of cost
At the yard OffshoreProduction max. 4 $/kg min. 32 $/kgCoating Covered in
coating scopeTouch up required
Accessibility Good PoorDowntime None Depending on location
of repairTransport None By supply vessel
Note: For offshore work extra personnel for safety is required
16-10-02 FRF - crack management 12
Discussion
Discussion “Design life”, what is basis? “Critical” crack, criteria on case by case basis? Accuracy of predictions, sufficient or not? Recommendation to vessel owners: do invest
during Life Time extension or not?
