Individual Blade Pitch and Disturbance Accommodating Control of Floating Offshore Wind Turbines

Abstract

Floating wind turbines offer a feasible solution for going further offshore into deep waters. However, using a floating platform introduces additional motions that must be taken into account actively or passively. Therefore, the control system becomes an important component in controlling these motions. In this work, the development, implementation, and simulation of multi-objective state feedback and disturbance accommodating controllers applied on the three main floating concepts are described. The three concepts are the barge, tension leg, and spar-buoy floating platforms. These controllers utilise individual blade pitching to create asymmetric aerodynamic loads in addition to the symmetric loads created by collective blade pitching to increase the platform restoring moments. Simulation results, according to design load case 1.2 of the IEC 61400-3 standard for offshore wind turbines, show that state feedback controllers improve the performance relative to a collective blade pitch gain scheduled proportional-integral controller in terms of power and rotor speed regulation as well as reducing tower fore-aft and side-side bending loads. However, the magnitude of the improvements depends on the dynamics of each platform, its responsiveness to individual blade pitching and sensitivities to external wind and wave disturbances. Furthermore, interesting physical phenomena, such as the platform roll to pitch coupling caused by the controller, are identified. Disturbance accommodating controllers for rejecting wind speed perturbations further improve rotor speed regulation and reduce tower fore-aft bending loads except on the barge platform; the barge platform motion is dominated by incident waves and therefore rejecting wind speed perturbations has no noticeable impact. Wave moment disturbance rejection is also investigated but in a limited case study. While the approach taken can theoretically cancel the effects of incident wave moments, practically, the required actuation force cannot be generated by the wind turbine blades. Furthermore, using the blades for rejecting wave moments increases the tower bending due to the load path of the restoring moment through the tower. Out of the three investigated platforms, the tension leg platform with a disturbance accommodating controller has the best overall performance with fatigue loads close or less than that of an onshore wind turbine. The other two platforms, in their current design form, experience large tower fore-aft bending loads that would prevent them from being deployed in rough sea conditions.