Coupled method for predicting motions of Spar-type offshore floating wind turbine systems

In this study, we established non-linear time-domain coupled dynamic equations and aero/hydro-dynamical models to study the interaction between wind waves and floating offshore turbine systems (FOWTs). We used Fortran code, time-frequency domain transformation, and the Runge-Kutta iteration method to solve nonlinear equations. We analyzed the 5DOF motion response characteristics of Spar FOWTs, except yawing, under steady wind wave, regular wave, and random wind-wave conditions. The results show that constant wind affects the average and peak-to-peak values of surge, pitch, heave, sway, and roll at the rated wind speeds, while it does not affect the peak-to-peak values of sway at other wind speeds. In addition, regular waves have only marginal effect on the average values of surge, pitch, heave, sway, and roll, but enlarge their peak-to-peak values. Under random wind and wave conditions, we characterized the system's sway and pitch motions for low and high wave frequencies, depending on the wind speed. The results of this study provide references for the design and hydrodynamic analysis of offshore floating wind turbine systems.