This paper investigates the effects of nonlinearities on the design of a constrained optimal controller for spar-type floating wind turbines. The considered nonlinearities are due to wind speed variation and actuator saturation. The collective blade pitch actuator, usually employed for aerodynamic rotor speed regulation, is adopted for the mitigation of platform pitch vibrations as well. The wind speed effects and the control law are obtained from a linearized rigid-body dynamic model, taking into account persistent disturbances related to wind turbulence and wave induced loads, which are estimated using the Blade Element Momentum aerodynamic theory and Morison equation, respectively. A performance analysis is carried out for several operating points in the above rated wind speed region, considering the reduction of the platform pitch motion as the main control objective. The simulation results show that the designed fixed-gain multivariable controller can yield significant vibration reduction in comparison with a baseline gain-scheduled proportional-integral controller in the presence of the considered nonlinearities.
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