Effects of second-order wave forces and aerodynamic forces on dynamic responses of a TLP-type floating offshore wind turbine considering the set-down motion

Abstract

A coupled 6 degree of freedom (DOF) aero-hydro-restoring model is developed to study the dynamics of a tension leg platform (TLP)-type offshore wind turbine. This model includes second-order wave loads, pitch control strategies, and effects of platform motion on aerodynamic performance. The second-order wave force is calculated using the full-field quadratic transfer function. The coupled effects of the horizontal motions (such as surge, sway, and yaw motions) and the set-down motion are taken into consideration through the nonlinear restoring matrix. Different load scenarios are chosen to simulate the platform's dynamic response and the rotor's aerodynamic performance in the time domain. The analysis shows that second-order wave forces will induce slow-drift and springing resonances. Due to the large motion in surge, a significant set-down motion which is an important part of heave motion is induced by the second-order difference-frequency force. When different wind types are chosen, the aerodynamic damping effect is found in the slow-drift motion in surge and the springing motion in pitch. On the other hand, low-frequency set-down motion is excited and the heave response is amplified by the wind. Compared to the steady wind, the effect of aerodynamic damping is reduced under the turbulent wind. Slow-drift frequency, wave frequency, and springing frequency are all observed in the aerodynamic results.