Fatigue reliability analysis of floating offshore wind turbines considering the uncertainty due to finite sampling of load conditions

The fatigue reliability-based design is critical for the structural safety of floating offshore wind turbines (FOWTs). In principle, infinite potential load conditions that the FOWT may experience should be considered in fatigue assessment, while in practice only limited number of load conditions are used, which inevitably induces un-certainty to fatigue estimation. However, to this uncertainty, little attention has been paid. In this study, the fatigue reliability of a FOWT is investigated with a focus on this uncertainty. The C-vine copula method is adopted to model concurrent wind and wave conditions, and the probability-based sampling method is employed to determine the load conditions used for fatigue analysis. This uncertainty is assessed quantitatively via boot-strap method based on numerous simulations. The Gaussian random variable can be used to describe the un-certainty, and the standard variation is proportional to the-0.5 power of the number of load conditions. The fatigue reliability evaluation model of FOWT is established considering this uncertainty, and sensitivity analysis is performed to assess the influence of different random variables. The results indicate that the effect of this uncertainty is comparable to some other uncertainties in certain cases, and should be paid attention to in practice.