Effects of heave plate on dynamic response of floating wind turbine Spar platform under the coupling effect of wind and wave

Stability of the platform is the most fundamental guarantee for floating offshore wind turbines. In order to explore hydrodynamics of floating wind turbine Spar platform under the coupling effect of wind and wave, whole model of floating wind turbine is established based on the OC3-Hywind Spar Buoy platform and the NREL 5 MW wind turbine. Taking the research experiences of traditional marine engineering, this work proposed the structural design method of installing heave plate on main body of Spar. Based on the Bladed Element Momentum theory, this paper adopts FORTRAN language to accomplish the secondary development of hydrodynamic software AQWA to establish the coupled aerodynamic - hydrodynamic - mooring line system model. The amplitude-frequency curves of the added mass, radiation damping, wave exciting force and response amplitude operators (RAO) were obtained based on radiation/diffraction theory to investigate the hydrodynamic performance of floating wind turbine Spar platform in frequency domain. In the time-domain analysis, the three kinds of working conditions include wave-only, non-coupling effect of wind-wave and coupling effect of wind-wave were conducted comparatively to verify the importance of wind load and wind - wave interaction. In addition, the actual motions of original Spar and Spar with heave plates were analyzed to explore the effects of the heave plate and its installation position on the nonlinear dynamic response characteristics of Spar under the coupling effect of wind and wave. The research results indicated that the installation position of the heave plate has little influence on the surge response. In view of the actual heave motion, the heave plate can play a positive effect no matter where the heave plate is installed. When the heave plate is located on the upper part of the platform, the pitch response and the intensity of the fluctuation of the platform will be greatly reduced. Finally, time domain mooring analysis for Spar platform with and without heave plate is performed to illustrate the feasibility of the present approach for mooring modeling.