Control of power generated by a floating offshore wind turbine perturbed by sea waves

Abstract

Offshore wind energy is expected to provide a signi cant contribution to the achievement of the European Renewable Energy targets. One of the main technological issues affecting oating offshore wind turbines con- cerns generated power uctuations and structural fatigue caused by sea-wave/platform interactions. This paper presents a fully-coupled aero/hydro/servo-mechanic model for response and control of oating offshore wind turbines in waves, suitable for preliminary design. The wind-turbine is described by a multibody model con- sisting of rigid bodies (blades and tower) connected by hinges equipped with springs and dampers (for realistic low-frequency simulation). The aerodynamic loads are evaluated through a sectional aerodynamic approach coupled with a wake in ow model. A spar buoy oating structure supports the wind turbine. The hydrodynamic forces are evaluated through a linear frequency-domain potential solver, with the free surface deformation effects included through a reduced-order, state-space model. An optimal controller is identi ed and applied for rejection of annoying uctuations of extracted power and structural loads. The developed comprehensive model has been successfully applied to a oating version of the NREL 5 MW wind turbine for stability analysis, as well as for the analysis of uncontrolled and controlled responses to regular and irregular short-crested sea waves. The proposed controller, based on the combined use of blade pitch and generator torque as control variables and the appli- cation of an observer for non-measurable aerodynamic and hydrodynamic states estimation, has been demon- strated to be effective in a wide frequency range for alleviation of both generated power uctuations and vibratory loads.