Risk-Based Inspection of an FPSO
Authors: Landet E. , Lotsberg I. , Sigurdsson G. 
Published: October 2008

Risk based inspection methods have been used to develop a basis for an in-service inspection programme for the Jotun FPSO hull and topside structures with respect to fatigue cracks. Jotun is a floating production unit installed on the Jotun Field in the Norwegian Sector of the North Sea in 1999. The most critical hot spot areas' showing the shortest calculated fatigue lives have been selected for analyses. A fracture mechanics model is calibrated to the fatigue damage obtained from an S-N approach. Then the degree of accumulated fatigue damage at a hot spot can be assessed after an inspection where the crack size is a governing parameter for efficiency of the inspection.

Based on information available at the design stage an optimal target failure probability is established to achieve an acceptance criterion for probability of a fatigue crack at time for planned in-service inspections. Costs associated with inservice inspection, possible repair and shutdown have been included to determine an optimal acceptance criterion. Details in the hull structure in the production area and the turret/moonpool of the main deck, ballast and cargo tanks, bottom and side shell structure and the moonpool have been investigated. The investigated details also include welded connections in the topside structures such as the flare tower, crane pedestals, process modules and helideck. Butt welds and fillet welds are investigated for the as welded and ground conditions as found in the fabricated structure.

The time to first inspection and time interval between inspections for critical details is determined based on consequence of a fatigue crack, methods used for fabrication, reliability of in-service inspection and calculated fatigue life.

Background

Maintenance and assessment of sailing ships are based on periodical dry-docking and reclassification every 5 years. Repairs and modification due to local damages caused by corrosion, fatigue or overload are not unusual for such vessels and this is normally considered as part of the owners' maintenance procedures with acceptable safety consequences and life cycle costs. The design conditions for an FPSO are much stricter due torequirement to long service life on location with a minimum of production stops for inspection, maintenance and repairs,limited access for inspection when the unit is operating on the field,high environmental loads,special operating conditions related to loading and offloading of tanks, sloshing effects in slack tanks, etc.,increased economical risk,increased safety standards,special areas with limited operational experience like e.g. turret, turret area, fairleads, thrusters and topside supports.

In general, fatigue problems of FPSO's can be related to different factors such as:high overall dynamic stress level,local dynamic effects like side shell loads and tank sloshing,global and local stress concentrations,vibrations,structural details (design and workmanship),riser and mooring loads.

In general a low safety factor for fatigue failure, 1.0 is used in FPSO designs. However, on Jotun a design fatigue factor of 2 and a design fatigue life of 40 years are generally used for components accessible for inspection and repair.