Abstract
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.
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