Damping analysis of Floating Offshore Wind Turbine (FOWT): a new control strategy reducing the platform vibrations

TL;DR

This paper analyzes the coupled dynamics of floating offshore wind turbines to identify instability conditions and introduces a novel control strategy that reduces platform vibrations and fatigue without impacting power output.

Contribution

It provides the first explicit analysis of the platform pitch NMPZ and proposes a new damping control strategy for FOWTs that improves structural longevity.

Findings

Identified explicit conditions for platform pitch NMPZ.

Developed a control strategy that enhances damping in platform motion.

Simulations show reduced fatigue without loss of power.

Abstract

In this paper, the coupled dynamics of the floating platform and the WTG rotor is analysed. In particular, the damping is explicitly derived from the coupled equations of rotor and floating platform. The analysis of the damping leads to the study of the instability phenomena and it derives the explicit conditions that lead to the Non Minimum Phase Zero (NMPZ). Two NMPZs, one related to the rotor dynamics and the other one to the platform pitch dynamics, are analysed. The latter is a novelty and it is analysed in this work, providing the community of an explicit condition for its verification. The domain of the instability of the platform is explicitly derived from the coupled system of equations. In the second part of the paper, from the analysis of the damping of the floating platform, a new strategy for the control of FOWTs is proposed. This strategy allows one to impose to the controller an explicit level of damping in the platform pitch motion without changing the period of platform pitching. Finally the new strategy is compared to the one without compensation by performing aero-hydro-servo-elastic numerical simulations of the UMaine IEA15MW FOWT. Generated power, movements, blade pitch and tower base fatigue are compared showing that the new control strategy can reduce fatigue in the structure without affecting the power production.