Robust gain scheduling baseline controller for floating offshore wind turbines

The possibility of a pitch instability for floating wind turbines, due to the blade-pitch controller, has been discussed extensively in recent years. Contrary to many advanced multi-input-multi-output controllers that have been proposed, this paper aims at a standard proportional-integral type, only feeding back the rotor speed error. The advantage of this controller is its standard layout, equal to onshore turbines, and the clearly defined model-based control design procedure, which can be fully automated. It is more robust than most advanced controllers because it does not require additional signals of the floating platform, which make controllers often sensitive to unmodeled dynamics. For the design of this controller, a tailored linearized coupled dynamic model of reduced order is used with a detailed representation of the hydrodynamic viscous drag. The stability margin is the main design criterion at each wind speed. This results in a gain scheduling function, which looks fundamentally different than the one of onshore turbines. The model-based controller design process has been automated, dependent only on a given stability margin. In spite of the simple structure, the results show that the controller performance satisfies common design requirements of wind turbines, which is confirmed by a model of higher fidelity than the controller design model. The controller performance is compared against an advanced controller and the fixed-bottom version of the same turbine, indicating clearly the different challenges of floating wind control and possible remedies.