ISBN 978-3-8439-3863-1

978-3-8439-3863-1, Reihe Ingenieurwissenschaften

Frank Lemmer
Low-order modeling, controller design and optimization of floating offshore wind turbines

247 Seiten, Dissertation Universität Stuttgart (2018), Hardcover, A5

Zusammenfassung / Abstract

Various existing prototypes of Floating Offshore Wind Turbines (FOWTs) proof the feasibility of placing offshore wind turbines on floating foundations, held in place by anchor lines. The motivation of this thesis is to improve the understanding of how wind and waves impact the dynamic behavior of the floating systems and how their shape can be optimized.

The thesis addresses first the dynamic modeling: A reduced-order simulation model is developed with not more than the necessary physical details for the given task. Results show that the objective of the model to represent the major system dynamics compared to a state-of-the-art model could be well satisfied while achieving a high computational efficiency. The validation of the model confirmed its validity also in comparison to experimental measurements. The experiments showed that the hydrodynamic load and damping model is important, especially in connection with the wind turbine controller. As a consequence, the simulation model was

designed with a detailed hydrodynamic model and an efficient aerodynamic model, keeping a high computational efficiency even for the nonlinear time-domain model. Based on the model, two parametric controllers are designed: Next to an advanced optimal controller a robust PI-control design procedure is developed. For both controllers the design is automated and merely a function of the linearized dynamic model and the objective functions. The comparison between both shows that especially the wind response and the difference-frequency response can be reduced by feeding back additional signals compared to an onshore controller.

Finally, the previously developed simulation model and the parametric controllers are applied in a brute-force optimization with parameterized design routines for the floating platform. Here, an improved hull shape yields a reduction of more than 30% of the lifetime-weighted fatigue damage. The analysis of the harmonic response to sinusoidal wave forces showed that this low-draft semi-submersible responds with a counter-phase pitching yielding an almost stationary hub displacement. This means that, together with advanced control, it is possible to obtain FOWTs with a very stable operational behavior, a smooth power production and only marginally increased loads compared to onshore wind turbines, while keeping the size and mass of the foundation reasonably small.