Individual Blade Pitch Control of a Spar-Buoy Floating Wind Turbine

The spar-buoy floating wind turbine is one of the three main floating wind turbine concepts and one of the first to proceed to a full-scale prototype stage. Multiobjective linear state feedback controllers are implemented on the spar-buoy floating wind turbine with individual blade pitching (IBP). The spar-buoy´s deep draft results in a low platform pitch and roll natural frequencies. The low-frequency pitch and roll modes interact with other low-frequency modes of the system (i.e., surge and sway, respectively). Therefore, the linear state-space model used for control design must include the surge and sway degrees of freedom. Furthermore, a low platform pitch natural frequency limits the effectiveness of IBP at regulating the platform pitch around the first tower fore-aft (FA) resonant frequency. Simulations using a high-fidelity model are carried out according to design load case 1.2 of the IEC-61400-3 standard for fatigue load testing under normal operating conditions. Simulation results relative to a gain-scheduled proportional-integral controller show that a multiobjective state feedback controller is able to reduce tower FA and side-side bending fatigue loads by an average of 9%. This improvement is mainly due to IBP despite its limited effectiveness around the first tower FA resonant frequency.