UBC Theses and Dissertations
Fatigue mitigation for position controlled floating offshore wind turbines Saunders, Brendan
Improving the efficiency of electricity generation is an important component of reducing the cost of electricity. When using wind turbines to generate power, grouping them together results in losses in efficiency due to the wake effect. In order to counter those losses, it has been proposed that wind turbines in a floating offshore wind farm be dynamically repositioned to reduce wake interactions between turbines. However, this relocation strategy has raised concerns that fatigue may be increased. In response to these concerns, this thesis investigates the changes in lifetime fatigue at the tower base caused by adding position control to a Floating Offshore Wind Turbine (FOWT). The primary goal of this thesis is to answer the questions of how much the fatigue damage changes and how much the fatigue can be reduced. Building on previous works, a position controller is designed as a Proportional-Derivative (PD) controller which adjusts nacelle yaw in order to position the turbine in the crosswind direction. Then, the controller is tested with simulations in order to assess how the fatigue changes with its inclusion. Based on these results, a fatigue reducing controller is designed. This controller uses the Linear Quadratic Regulator (LQR) structure with the states in the model chosen based on the fatigue results of the position controller. Finally, the system with both position control and fatigue reduction is tested using the same environmental conditions as the position controller. When analyzing the effectiveness of the controllers with respect to fatigue, the results are compared to a baseline controller that has been designed for power capture. The results indicate that further tuning of the position controller is needed to improve station keeping, however, the controller is still reasonably successful at maintaining steady-state position. The position control goal also does not have a substantial impact on fatigue, but increases were noted in the side-to-side direction at the tower base. The fatigue controller shows success in reducing the increase in side-to-side fatigue. As well, the fatigue controller reduced lifetime fore-aft fatigue at the tower base by between 37 and 43 percent compared to the baseline.
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