Effect of coupled platform pitch-surge motions on the aerodynamic characters of a horizontal floating offshore wind turbine

Floating offshore wind turbines (FOWTs) work in a complex natural environment. Under the coupling effect of wind and waves, the platform experiences a six-degree-of-freedom motion, which affects the performance of the wind turbine. In this paper, a computational fluid dynamic method is used to investigate the effect of platform pitch and surge motion coupled at the same frequency as well as at different frequencies with an initial phase difference on the aerodynamic characteristics of FOWTs. The results demonstrate that the platform pitch and surge motion coupling makes the wind turbine operation more unstable. The power and thrust fluctuations are the largest when the two motions are coupled in the same phase, which leads to a dramatic change in the aerodynamic performance of the wind turbine during operation, and can easily cause hazards such as blade fatigue damage. When the initial phase difference does not affect the coupling motion frequency, the effect on the instantaneous power is more significant than that on the instantaneous thrust. However, the effect on the average power and thrust values is weaker. When the initial phase difference leads to reverse coupling of the platform pitch and surge motions, the fluctuation of power and thrust is reduced, and the wind turbine operation is more stable and safer.