Date of Award

Spring 5-7-2021

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Andrew Goupee

Second Committee Member

Richard Kimball

Third Committee Member

Babak Hejrati

Abstract

Floating offshore wind turbines (FOWTs) have the potential to bring renewable energy to waters too deep for traditional offshore wind turbines while still being able to harness strong coastal winds in areas near population centers. However, these floating wind turbines come at a higher capital cost relative to fixed foundations and are more susceptible to vibrations induced by waves. Advances in control technologies offer the potential to reduce fatigue loads due to these vibrations, extending the life of the platform and thereby spreading the capital costs of the turbine over a longer period of time. One such advance is in blade pitch control, a standard component of most modern wind turbines. Existing solutions for adapting the blade pitch controller for use on a floating platform either detune the controller with the result of slowed response, make use of complicated tuning methods, or incorporate a nacelle velocity feedback gain. With the goal of developing a simple control tuning method for the general FOWT researcher that is easily extensible to a wide array of turbine and hull configurations, this last idea is built upon by proposing a simple tuning strategy for the feedback gain. This strategy uses a two degree-of-freedom (DoF) turbine model that considers tower-top fore-aft and rotor angular displacements. For evaluation, the nacelle velocity term is added to an existing gain scheduled proportional-integral controller as a proportional gain. The modified controller is then compared to baseline land-based and detuned controllers on semisubmersible, spar, and TLP systems for several load cases. Results show that the new tuning method balances power production and fatigue load management effectively, demonstrating that it is adaptable to many different types of hulls. This makes it useful for prototype design. Advances in hull-based structural control are also considered through the evaluation and development of a gain schedule for a novel type of adjustable tuned mass damper known as a ducted fluid absorber. This type of tuned mass damper uses compressed air to adjust its natural frequency, and so the amount of power consumed by the compressors is evaluated relative to the output of the wind turbine. Performance of a hull designed for ducted fluid absorbers is evaluated for several incoming wave directions to ensure consistent performance, and the potential for extracting electricity from the ducted fluid absorbers is considered. Finding the dampers to be feasible for use, a method of scheduling the settings of these dampers to minimize the standard deviation of a platform rigid-body mode of choice is developed. The addition of the dampers is found to produce significant reductions in the magnitude of several vibration modes, though the advantages of actively controlling the damper setting are small relative to those of simply having the dampers.

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