System reliability analysis of mooring system for floating offshore wind turbine based on environmental contour approach

The system reliability of the mooring system for floating offshore wind turbines (FOWT) is analyzed in this paper. Various failure modes such as mooring line breaking, fatigue and floater offset are included and solved as a series system. This work employed a new mooring breaking assessment algorithm, wherein the effect of fatigue damage on breaking strength is simulated by introducing functional dependences into the coupled dynamic analysis. In this research, the extreme design loads are estimated using the environmental contour (EC) approach and a logarithmic function is proposed to describe the relationship between the extreme response and the target return period to simplify the full long-term analysis. The time-domain coupling analysis of the FOWT is performed considering all one-year sea states, the long-term fatigue damage was approximated based on linear criterion. The fatigue damage can contribute significantly to the breaking strength of mooring lines. The results indicate that ignoring the dependence between the residual strength and cumulative fatigue damage would cause underestimated results. The calculated failure probability of the floating system is larger than that of any single failure mode, while it is mainly dominated by the main single failure mode with the largest failure probability.