A novel resonant controller for sea-induced rotor blade vibratory loads reduction on floating offshore wind turbines

A novel multi-layer control approach for offshore floating wind turbines is presented, suitable for alleviating the vibratory loads while keeping the generator power output stable. It consists of the simultaneous application of different control strategies, each optimized for a specific objective. The proposed multi-layer scheme's main scope is to reduce the Levelized Cost of Energy. Two resonant controllers based on collective blade pitch actuation are applied for the rejection of the vibratory loads induced by sea waves. In contrast, a proportional–integral controller, fed by measured cyclic blade root flapping moments, provides the blade cyclic pitch to be actuated to reduce blade root loads at the rotor revolution frequency. The proposed control strategy is validated by simulations on: (i) the NREL (U.S. National Renewable Energy Laboratory) 5 MW wind turbine, supported by a spar buoy or a semi-submersible platform; (ii) the 15 MW IEA (International Energy Agency) wind turbine supported by a semi-submersible platform. These show that significant reductions of the vibratory blade root flapping moments and rotor nacelle assembly loads are achievable, thus demonstrating the good potential performance of the controller introduced.