Influence of Control on the Pitch Damping of a Floating Wind Turbine

This paper presents the influence of conventional wind turbine blade-pitch control actions on the pitch damping of a wind turbine supported by an offshore floating barge with catenary moorings. There was a concern that the drop in steady-state wind turbine rotor thrust with wind speed above rated would lead to negative damping of the barge-pitch mode and contribute to the large system-pitch motions. It is demonstrated that neither the addition of a control loop through feedback of tower-top acceleration nor the modification to pitch-to-stall rotor-speed regulation satisfactorily improved the barge-pitch response. The latter modification helped conclude, however, that the actual barge-pitch damping was considerably greater than that implied by the steady-state rotor thrust response, but that it was still beneficial to increase the damping as much as possible. Detuning the gains in the baseline blade-pitch-to-feather controller helped, but still did not entirely resolve the barge-pitch-motion problem.