Investigation on Dynamic Responses’ Characteristics and Fatigue Damage Assessment for Floating Offshore Wind Turbine Structures

Dynamic response analysis and fatigue damage assessment of floating offshore wind turbine (FOWT) structures are carried out in this paper with a focus on the statistical characteristics of the dynamic responses under wind–wave excitation. A proper spectral fatigue damage model is proposed to obtain better fatigue damage estimation. The OC3 floating spar (Hywind) with a 5 MW baseline wind turbine developed by the American National Renewable Energy Laboratory (NREL) is applied as the target model, and the fully coupled dynamic responses of the model are calculated in the time domain. The non-Gaussian nature is revealed based on the study of the statistical properties of the dynamic response of key components (the tower base and fairlead of the mooring line). Dirlik distribution is proved to be the best probability distribution function (PDF) that can fit the statistical distribution feature of the dynamic responses among several PDFs. The corresponding spectral fatigue damage models are applied to estimate fatigue damage of the tower base and mooring line following the proposed fatigue assessment procedure in the frequency domain, of which the results are compared with results from the time-domain analysis method. The results indicate that for non-Gaussian feature dynamic responses as FOWTs, the spectral fatigue damage model based on the Dirlik distribution can provide more accurate assessments of fatigue damage.