Calibration of wave-induced high-frequency dynamic response and its effects on the fatigue damage of floating offshore wind turbine

There is a demand to calibrate the high -frequency dynamic responses of semi -submersible floating offshore wind turbine (FOWT) induced by the second -order sum -frequency hydrodynamic loads in the numerical model and evaluate its effects on the fatigue damage of the FOWT. A modification approach for the calibration of the high -frequency dynamic responses of FOWT is proposed. In this approach, the effects of the viscous damping and additional restoring force on the second -order sum -frequency quadratic transfer functions (QTFs) are discussed in the frequency domain. The calibrated numerical model (CNM) is obtained with the tuned viscous damping, the second -order sum -frequency QTFs, and the structural damping, based on the measured data from the FOWT model test. The CNM is proved to accurately predict the high -frequency components of the strongly nonlinear responses, namely the pitch motion and tower -top shear force, with the proper prediction of the wave -frequency responses compared with the experimental model under regular wave conditions. The CNM is further applied to predict the high -frequency dynamic responses of the OUCwind under a typical sea state for an operational condition. It is found that the high -frequency component occupies a non -negligible part of the pitch motion, and dominates the tower -top shear force. The linear model (LM) cannot predict and the uncalibrated numerical model (UCNM) underpredicts the high -frequency components of responses. Furthermore, the fatigue damage of the CNM is 14 times greater than that of the LM after a 3 h sea state with a significant wave height of 3 m and a peak period of 6 s. It follows that the large frequency and intensity make the high -frequency tower -top shear force to be a huge risk to the safe operation of the wind turbine, which should attract the industry's concern about the wave -induced high -frequency dynamic responses for the design of FOWTs.