In order to explore the coupling mechanism between closed-loop blade-pitch control of floating offshore wind turbine(FOWT) and platform-pitch mode of the floating body

to reveal the causes of "negative damping"

and to develop a multi-objective blade-pitch control strategy of power regulation

motion suppression and load reduction

firstly

we established a single degree-of-freedom motion model of platform-pitch mode for semi-submersible FOWT under the multi-factor influence of aeroelastic-hydrodynamic-servo dynamics. Meanwhile

we quantitatively analyzed the platform-pitch mode damping characteristics of National Renewable Energy Laboratory(NREL) 5 MW and International Energy Agency(IEA) 15 MW semi-submersible FOWT. The possible reasons for negative damping of FOWT under blade-pitch control are revealed. The impact ranges of negative damping are analyzed and the optimization directions of blade-pitch control strategies are provided. Then

based on the above analyses

an improved gain scheduling collective pitch control strategy is proposed

and an individual pitch controller is designed. Finally

under the OpenFAST/Simulink joint simulation

by using NREL 5 MW and IEA 15 MW semi-submersible FOWT as references

the multi-objective blade-pitch control performance of different controller parameters and control strategies under a wide range of wind-wave loads is analyzed. The effectiveness of the proposed control strategy is verified. The conclusion can provide reference for FOWT blade-pitch control in future.