Comparisons of Wave Force Model Effects on the Structural Responses and Fatigue Loads of a Semi-Submersible Floating Wind Turbine

The selection of wave force models will significantly impact the structural responses of floating wind turbines. In this study, comparisons of wave force model effects on the structural responses and fatigue loads of a semi-submersible floating wind turbine (SFWT) were conducted. Simulations were performed by employing the Morison equation (ME) with linear or second-order wave kinematics and potential flow theory (PFT) with first- or second-order wave forces. A comparison of regular waves, irregular waves, and coupled wind/waves analyses with the experimental data showed that many of the simulation results and experimental data are relatively consistent. However, notable discrepancies are found in the response amplitude operators for platform heave, tower base bending moment, and tension in mooring lines. PFT models give more satisfactory results of heave but more significant discrepancies in tower base bending moment than the ME models. In irregular wave analyses, low-frequency resonances were captured by PFT models with second-order difference-frequency terms, and high-frequency resonances were captured by the ME models or PFT models with second-order sum-frequency terms. These force models capture the response frequencies but do not reasonably predict the response amplitudes. The coupled wind/waves analyses showed more satisfactory results than the wave-only analyses. However, an important detail to note is that this satisfactory result is based on the overprediction of wind-induced responses.