Abstract

Much effort has recently been put into the development of collective blade pitch controllers for floating offshore wind turbines, with the aim of overcoming negative damping issues that arise with traditional control methods. One proposed approach to this challenge involves using a two-degree-of-freedom model to inform the gain schedule of a nacelle velocity feedback term in an otherwise conventional proportional-integral controller. The model uses tower-top fore-aft and rotor angular displacements, and is used to calculate a nacelle velocity feedback gain that results in a specified increase in platform pitch damping. Earlier performance evaluations of this tuning method were favorable, suggesting its potential as an easy way for researchers to obtain an adequate controller. This paper expands on those previous results by examining the performance of the tuning method relative to baseline controllers for several hull configurations, and for several prescribed increases in platform pitch damping. Simulations were run in OpenFAST for several load cases above rated wind speed and show results consistent with trends in the earlier study. The tuning method is thus shown to be adaptable to many different types of hulls, making it useful for the evaluation of prototype designs.

Introduction

Wind energy is becoming ever more affordable, thanks to numerous engineering innovations. One of the more recent focuses of research has been offshore wind turbines, and in particular floating turbine systems. This technology allows large wind turbines to be placed in waters too deep for conventional monopile foundations (such as a great portion of the Atlantic Seaboard), while still maintaining the advantages granted by offshore monopiles like strong, consistent ocean winds and being able to put turbines out of sight beyond the horizon but still close to population centers. One issue with this arrangement is that the blade pitch controller of the turbine, traditionally tasked with regulating rotor speed, must now also ensure the stability of the floating platform. This can sometimes result in an effect known as the negative damping problem (Jonkman, 2008; Larsen & Hanson, 2007), wherein the blade pitch controller excites the natural pitch or surge mode of the system. Several solutions have been posed to this problem, including detuning of gains (Larsen & Hanson, 2007), feedforward control (Schlipf et al., 2015; Navalkar et al., 2015), state-space controllers (Lemmer et al, 2016), and feedback of the nacelle velocity (Fischer, 2013; Fleming et al, 2016). Recent research in this last strategy has involved a simple tuning method for gain schedules using a two-degree-of-freedom (two-DoF) model (Lenfest et al., 2020). The model of the turbine and platform system considers the rotor speed angular motion (φ) and platform pitch angular motion (θ) (or platform surge x in the case of a tension leg platform (TLP)), because these modes are affected most by the blade pitch controller. All the terms that couple the DoF are retained to enhance the predictions of the model. The model also considers the controller inputs, which consist of traditional kp and ki gains along with a proportional gain on the nacelle velocity, kpx.

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