Numerical Study On A SPAR Type Floating Offshore Wind Turbine Using COUPLE-FAST Code

Floating offshore wind turbine (FOWT) attracts more and more attention for harnessing wind power over the surface of relatively deep ocean water, where steady and strong wind occurs. Although it has been shown that the knowledge gained from the development of floating platforms for oil and gas production is helpful for the development of FOWTs, it alone is insufficient for understanding dynamic interactions between the supporting platform and the wind turbine. Therefore, it is desirable to conduct numerical simulations of a FOWT under the impact of different combinations of winds, waves and currents. In this study, a numerical code named as COUPLE-FAST has been developed to investigate the motions of a selected FOWT and the tensions in its mooring lines. The selected FOWT mainly consists of a 5MW NREL wind turbine and OC3-Hywind Spar support platform. COUPLE-FAST is made based two existing codes COUPLE and FAST. The former is an in-house code developed and being continuously expanded for the simulation of an offshore floating platform positioned by a mooring-line/tendon system. FAST is an open-source code capable of predicting both the extreme and fatigue loads of two- and three- bladed horizontal-axis wind turbines [1]. In COUPLE-FAST, COUPLE module is used to calculate the external loads on the support floating platform, mooring line forces and its motions, and FAST module to calculate the aerodynamic loads and flexible responses of the wind turbine. The displacements, velocities and accelerations predicted in COUPLE are transferred to FAST. The forces at the tower bases calculated by FAST are transferred to COUPLE. Total 25 cases with different combination of winds, waves and currents are simulated for calculating the motions of the FOWT and tensions in its mooring lines. Among many interesting observations made based on these simulations, it is confirmed that when the mean wind speed is above the rated wind speed the blade pitch control system may induce resonant interaction (also known as ‘negative damping’) between the surge of the FOWT and dynamic wind loads induced by the adjustment of blade pitch angle. However, the resonant effects on surge of the FOWT in the case of turbulent winds are not as significant as in the case of steady winds of the corresponding wind speed