This paper presents a reliability assessment of a FPSO hull girder subjected to degradations due to corrosion and fatigue. The corrosion defect is considered a random process with a constant corrosion-rate. The crack propagation is predicted based on a Paris-Erdogen equation. A simplified method is introduced to predict the ultimate strength of a hull girder using an effective width of plating and beam-column formula. "Failure" occurs when the combined value of still water and wave-induced bending moments exceeds the ultimate strength of the hull girder. The Ferry-Borges method is applied to combine the stochastic processes of still water and wave induced bending moments. An efficient response surface approach is used to evaluate the failure function at sampling points. A modified Monte Carlo simulation technique is applied to evaluate the failure probability. The time-variation reliability and the uncertainties in design variables, are quantified including still water & waved-induced bending moments, yielding strengths, elastic modulus, residual deflections, corrosion rate and crack size. The modeling errors, e.g. for loading and strength, are also quantified.
The most catastrophic event of a FPSO is structural failure of hull girders due to extreme bending moments. In order to estimate the probability of failure, it is necessary to address not only the random nature of the maximum bending moment loading the ship undergoes, but also the random nature of the ultimate strength of ship hull girder (Mansour, 1997). The ultimate strength of a FPSO hull girder usually degrades due to corrosion and fatigue. Both the bending moment loading and the hull ultimate strength need to be described using stochastic processes. The time-variation reliability assessment should be applied (Guedes Soares and Garbatov, 1996,1998,1999; Wirsching et al, 1997; Casella and Rizzuto, 1998, Sun et al, 1999).
