Combination of global still-water and wave load effects for reliability-based design of floating production, storage and offloading (FPSO) vessels

The purpose of this paper is to establish probabilistic models for still-water loads, based on design data, and the combined still-water and wave load effects for semi-probabilistic and probabilistic design of floating production, storage and offloading vessels (FPSO). A new still-water load model for FPSOs is proposed, based on a Poisson square-wave model, with a modified Weibull distribution for load intensity, which accounts for load control during operation. The long-term variation of wave-induced load effects is modelled by a Poisson square-wave process. A new solution for the combined effect is derived. A procedure for determining characteristic extreme values for individual and combined load effects, and load combination factors, is established. The methodology is used to illustrate load combination factors suitable for typical FPSOs. This approach is also shown to be useful in obtaining realistic load models, in terms of random variables, for use in reliability formulations.