Design Optimization of Spar Floating Wind Turbines Considering Different Control Strategies

One of the challenges related to the design of floating wind turbines (FWTs) is the strong interactions between the controller and the support structure, which may result in an unstable system. Several control strategies have been proposed to improve the dynamic behaviour, all of which result in trade-offs between structural loads, rotor speed variation, and blade pitch actuator use, which makes controller design a challenging task. Due to the interactions, simultaneous design of the controller and support structure should be performed to properly identify and compare different solutions. In the present work, integrated design optimization of the blade-pitch controller and support structure is performed for a 10 MW spar FWT, considering four different control strategies, to evaluate the effect of the controller on the structural design and associated costs. The introduction of velocity feedback control reduces the platform pitch response and consequently the fatigue loads in the tower, which leads to a decrease in the tower costs compared to a simple PI controller. Low-pass filtering of the nacelle velocity signal to remove the wave-frequency components results in reduced rotor speed variation, but offers only small improvements in costs, likely due to the limited wave-frequency response for the considered designs. Comparisons with nonlinear time-domain simulations show that the linearized model is able to capture trends with acceptable accuracy, but that significant overpredictions may occur for the platform pitch response.