Abstract Summary
Floating Offshore Wind Turbines (FOWTs) are have been proposed in recent years for deep water deployment where the installation of traditional fixed bottom turbines would be impossible. This opens up vast areas of the marine environment for offshore wind development. FOWTs have been realised in recent years in offshore wind farms in Scotland (Hywind & Kincardine), Portugal (Windfloat), Spain (Flocan 5 Canary), and France. However, there remain significant technical challenges to be overcome to make floating offshore wind a commercially attractive prospect. In general the structural characteristics of a FOWT are much more dynamic than an onshore turbine or a traditional fixed base offshore wind turbine. Due to this fact, novel structures and controllers must be developed specifically for FOWTs. The stability of the FOWTs (pitching and rolling of the platform) and reduction of aerodynamic loads on FOWTs is now the topic of considerable research. Much work focuses on the design of new pitch and torque controllers. Most commercial wind turbines use proportional integral (PI) collective blade pitch control to regulate rotor speed in the above rated wind speed regime. A significant drawback of this type of controller is that it assumes that the blades have identical structural properties and are subject to similar aerodynamic loads, which is seldom the case. Also, these controllers are designed to regulate the rotor speed and are not designed for structural vibration/load reduction. However, it is well known that blade pitch control can reduce structural loads on wind turbines. This opens up the possibility of designing controllers that use existing actuators and sensors like the blade pitch actuators to reduce structural loads/vibrations while maintaining the required rotor speed. Recent studies have investigated individual blade pitch control (IPC) to address these shortcomings. However, the vast majority of studies published in the literature depend on the availability of state measurement. Although sensors are commonly placed on all wind turbines, and some information is readily available, the measurement required by the typical state feedback controllers is usually not available. Displacements and velocities of the blade, the tower and the floating platform are difficult to measure. This paper develops an observer based individual blade pitch controller for load mitigation and power regulation of floating offshore wind turbines. We propose to use a Kalman filter to estimate the state from the accelerometer and strain gauge measurement for use in the state feedback controller. The state feedback controller was proposed previously by the authors and showed excellent performance. This paper extends the capability of the state feedback controller by designing an observer (Kalman filter) to estimate the state from limited measurements. The proposed observer based controller is compared against a baseline proportional integral collective blade pitch controller and full state feedback controllers to evaluate its performance. Numerical results show that the proposed output feedback controller offers performance improvements over the baseline controller, similar to the full state-feedback controller.
