Fracture mechanics-based mooring system fatigue analysis for a spar-based floating offshore wind turbine

Abstract

This paper summarized a study that aims to investigate the applicability of a fracture mechanics (FM) based approach for calculating the fatigue lives of the mooring lines of a spar-based floating offshore wind turbine (FOWT) in time-domain. Tension ranges of mooring lines are calculated from a time-domain hydrodynamic analysis, taking into account the effects of wind, wave and current. Mooring tension cycles with corresponding ranges are counted by a rain-flow counting method. A comparison between fatigue lives of mooring system predicted by S–N curves, T-N curves, and the FM based approaches is made and the results show that the FM based approach generally provides reasonable estimate of fatigue lives of the mooring lines of the FOWT. Parametric studies were further performed to investigate the effects of loading sequence, stress concentration factor, initial crack shape, initial crack size and critical crack depth on fatigue life prediction of the FM based approach. It was observed that the predicted fatigue life is sensitive to stress concentration factor and initial crack size. A comparative study on three kinds of mooring design with different chain diameters is also made and the applicability of each design is discussed.