Crack management Clemens van der Nat Bluewater Energy Services BV

FRF - crack management2 Content n Crack management n Existing practice in shipping n Bluewater crack management

FRF - crack management3 Crack management n Management in engineering phase –Fatigue damage prediction –Classification rules vs. direct calculations n Management in operations phase –Regular survey –When cracks are found: n Risk assessment n Corrosion –Move to condition based monitoring

FRF - crack management4 Existing practice in shipping Current practice for oil tankers n Location of cracks determined by loading condition n Involvement of classification societies –Survey period –Methods –Required action n Repair at shipyard

FRF - crack management5 Bluewater crack management Current practice for FPSOs life time extension n Tanker hull design (web spacing, details) n Old tanker requires survey of actual status and estimate of actual fatigue life n Site specific predictions for new “design life” n Critical details to be identified and improved n Surveys and repairs at site

FRF - crack management6 Enlarged brackets and backing brackets Prediction??? Expected damage after 20 years Actual damage after 6 years Backing brackets only

FRF - crack management7 Critical details Adding a backing bracket reduces the load on the crack tip Misalignment induces cracking, a backing bracket reduces the load on the hot spot

FRF - crack management8 Bluewater crack management n Repair of critical cracks during operational life –Repair is required, delay only by exemption –When is a crack critical? n Damage effects global strength of hull n Crack length exceeds critical value n More than one crack at one location n Cracks in oil separating bulkheads n Crack growth exceeds inspection interval –Reduce risk on progressive cracking

FRF - crack management9 Critical crack repair Cracks in bulkhead are not acceptable Repair by removing crack and adding backing bracket to unload the hotspot

FRF - crack management10 Critical crack repair Risk: crack might propagate into the longitudinal bulkhead Repair: adding lug plate extending to the longitudinal bulkhead plate

FRF - crack management11 Comparison of cost Note: For offshore work extra personnel for safety is required

FRF - crack management12 Discussion n “Design life”, what is basis? n “Critical” crack, criteria on case by case basis? n Accuracy of predictions, sufficient or not? n Recommendation to vessel owners: do invest during Life Time extension or not?