Platform yaw drift in upwind floating wind turbines with single-point-mooring system and its mitigation by individual pitch control

Abstract

This work demonstrates the feasibility of an individual pitch control strategy based on nacelle yaw misalignment measurements to mitigate the platform yaw drift in upwind floating offshore wind turbines, which is caused by the vertical moment produced by the rotor. This moment acts on the platform yaw degree of freedom, being of great importance in systems that have low yaw stiffness. Among them, single-point-mooring platforms are one of the most important ones. During recent years, several floating wind turbine concepts with single-point-mooring systems have been proposed, which can theoretically dispense with the yaw mechanism due to their ability to rotate and align with environmental conditions (weather-vaning). However, in this paper it is proven that the vertical moment overcomes the orienting ability, causing the yaw drift. With the intention of reducing the induced yaw response of a single-point-mooring floating wind turbine, an individual pitch control strategy based on nacelle yaw misalignment is applied, which introduces a counteracting moment. The control strategy is validated by numerical simulations using the 5 MW National Renewable Energy Laboratory (NREL) wind turbine mounted on a single-point-mooring version of the DeepCwind OC4 floating platform to demonstrate that it can mitigate the yaw drift and therefore maintain the alignment of the wind turbine rotor with the wind.