Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines

This paper examines the consistency between response amplitude operators (RAOs) computed using a linear frequency-domain tool to RAOs derived from time-domain computations based on white-noise wave excitation using FAST, a nonlinear aero-hydro-servo-elastic tool. The RAO comparison is first made for a rigid floating wind turbine without wind excitation. The investigation is further extended to examine how these RAOs change for a flexible and operational wind turbine. The RAOs are computed for below-rated, rated, and aboverated wind conditions. The method is applied to a floating wind system composed of the OC3-Hywind spar buoy and NREL 5-MW wind turbine. The responses from the two approaches are compared to verify the FAST model, and to understand the influence of structural flexibility, aerodynamic damping, control actions, and waves on the system responses. The results show that based on the RAO computation procedure implemented, the linear frequency-domain and time-domain approaches produce similar RAOs (as expected) for a rigid turbine in waves. However, linear frequency-domain tools are unable to model the excitation from a flexible turbine. Further, the presence of aerodynamic damping decreased the platform surge and pitch responses when wind was included. Additionally, the influence of gyroscopic excitation increased the yaw response, which was captured by both methods.