## 1 Introduction
[13] investigated the combined pitch and trailing edge flap control including an individual pitch control loop and a trailing edge flap control loop for load mitigation of wind turbines. [PERSON] et al. [14] proposed a new control method by using two supplementary controllers an internal model controller with an adaptive algorithm and a standard individual pitch controller to suppress the tower vibrations and to reduce the fatigue loads of the wind turbine, with the internal model controller reducing the tower vibrations and the standard individual pitch controller reducing the fatigue loads of the wind turbine. The last group of literature look into pitch control methods to restrain the fluctuation of aerodynamic torque and output power. [PERSON] et al. [15] used the individual pitch control method to reduce the 3P fluctuating component of wind turbine output power, which is validated in different wind speed scenarios. [PERSON] et al. [16] proposed a variable pitch control strategy, which periodically adjusts each blade's pitch angle by measuring the azimuth signal and distributing the azimuth weight coefficient to reduce the 3P pulsation component of wind turbine torque. [PERSON] et al. [17] designed multivariable disturbance observer based linear controllers which can be used with individual pitch control with the objective of reducing output power fluctuation, tower oscillation and drive-train torsion by using optimal control theory. [PERSON] et al. [18] proposed a new individual pitch control strategy for three-bladed wind turbines to reduce the 1P fluctuation component in the aerodynamic loads and the multiples of the 3P fluctuation component in the aerodynamic torque caused by wind shear and tower shadow. However, at present, the effect of waves on wind turbines is mainly focused on the dynamic response, fatigue load, ultimate load, etc., of the foundation of offshore wind turbines under wave force [19; 20; 21; 22]. The research object of the above research is only the offshore wind turbine tower. Nowadays, few studies have analyzed the effect of waves on blade load and wind turbine aerodynamic torque in the mechanism. [PERSON] et al. [23] studied vibration control of offshore wind turbines induced by multi-hazard excitations and established a model consisting of the entire offshore wind turbine foundation and tower controlled by tuned liquid column dampers considering nonlinear soil pile interaction, which is subjected to wave, wind, and seismic loading. Numerical analysis results demonstrate the effectiveness of the optimal tuned liquid column dampers in reducing the fore-aft vibration of the tower. [PERSON] and [PERSON] [24] applied individual blade pitch state space control and disturbance accommodating control that reject wind speed perturbations on a 5 MW wind turbine mounted on the barge and tension leg floating platforms for performance comparison in the above rated wind speed region, and simulation results show that individual blade pitch state space control can significantly reduce tower load. [PERSON] and [PERSON] [25] also presented the development of a periodic state space controller that utilizes individual blade pitching to improve power output and reduce platform motions of floating wind turbines in the above rated wind speed region. [PERSON] et al. [26] proposed a wavelet multi-resolution based individual pitch control strategy for spar-type floating offshore wind turbines and investigated the performance of the proposed strategy under joint wind-wave-current loads considering the interaction between wave and current. Simulation results show that the aerodynamic loads can be decreased with this control strategy, which in turn reduces vibration in tower fore-aft and platform pitching motion of the wind turbine. However, in these studies, the mechanism of how waves affect the load of wind turbines and aerodynamic torque is not systematically analyzed.