Abstract
This paper presents the design of a linear quadratic (LQ) optimal controller for a spar-type floating offshore wind turbine (FOWT). The FOWT is exposed to different sea states and constant wind turbulence intensity above rated wind speed. A new LQ control objective is specified for the floater-turbine coupled control, in accordance with standard requirements, to reduce both rotor speed fluctuations and floater pitch motion in each relevant sea state compared with a baseline proportional-integral (PI) controller. The LQ weighting matrices are selected using time series of the wind/wave disturbances generated for the relevant sea states. A linearized state-space model is developed, including the floater surge/pitch motions, rotor speed, collective blade pitch actuation, and unmeasured environmental disturbances. The wind disturbance modeling is based on the Kaimal spectrum and aerodynamic thrust/torque coefficients. The wave disturbance modeling is based on the Pierson–Moskowitz spectrum and linearized Morison equation. A high-fidelity FOWT simulator is used to verify the control-oriented model. The simulation results for the OC3-Hywind FOWT subjected to turbulent wind show that a single LQ controller can yield both rotor speed fluctuation reduction of 32–72% and floater pitch motion reduction of 22–44% in moderate to very rough sea states compared with the baseline PI controller.
Highlights
A floating offshore wind turbine (FOWT) is a marine system designed to generate power from wind over deep waters
The electricity production from FOWTs is suitable for several countries, in particular Brazil where significant offshore wind resources are available over deep waters, associated with high capacity factors, several densely populated cities are close to the sea, and the long coastline is usually exposed to calm/moderate wave conditions [2]
The simulations carried out by Namik and Stol [34] using the full FAST 21-DOF model revealed that the second tower and blade bending modes are not relevant for a spar-type FOWT, only the first tower and drivetrain flexibility effects are taken into account
Summary
A floating offshore wind turbine (FOWT) is a marine system designed to generate power from wind over deep waters. Assuming limited rotor speed fluctuations and small floater motions about an operating (equilibrium) point, an FOWT can be modeled by linearized equations of motion In this case, linear control design methods based on state-space models can be applied to synthesize controllers according to specified objectives. The reduction of coupled floater-turbine motions in each sea state of interest must be considered in advanced control design to avoid poor FOWT performance in a particular environmental condition. The verification of the control-oriented model is based on comparison with a 10-DOF aero-hydro-servo-elastic nonlinear FAST model, including 6-DOF spar floater motions, drivetrain rotational flexibility, tower fore-aft and side-to-side bending modes, rotor speed, Kaimal wind turbulence model, P-M long-crested irregular waves, and a gain scheduled PI controller. The simulation results show that a single LQ controller can yield significant reduction of both rotor speed fluctuations and floater pitch motion in moderate to very rough sea states compared with the baseline PI controller
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