Reliability-based service life assessment of FPSO structures. Discussion

In this paper a reliability-based formulation for assessment of FPSO hull structures for service life extension is established, as a tool in the decision taking process. The formulation accounts for possible relocation, and hence change in climate loading conditions, of either purpose-built or converted FPSO hulls. Deterioration due to combined fatigue cracking and corrosion wastage of structural components is addressed, utilizing a fracture mechanics model. The interaction between fatigue and corrosion is treated in terms of an accelerated crack growth rate in two ways: 1) increased stress range produced by the plate thickness reduction (wastage) effect and 2) corrosion fatigue itself. The reliability framework explicitly accounts for uncertainties as well as the effect of inspection. A series of inspection events are defined to update the reliability level based on detection of fatigue cracks and thickness measurement, both before and after the vessel has changed its location and sea environment. The procedure is demonstrated with sensitivity studies that show the effect of inspections on reliability updating on a typical detail of an FPSO subjected to two different climate conditions, namely harsh and benign environment.