Ocean Engineering xxx (xxxx) xxx Please cite this article as: Arka Mitra, Ocean Engineering, https://doi.org/10.1016/j.oceaneng.2020.108232 0029-8018/© 2020 Elsevier Ltd. All rights reserved. Sway vibration control of foating horizontal axis wind turbine by modifed spar-torus combination Arka Mitra a , Saptarshi Sarkar b , Arunasis Chakraborty a, * , Sourav Das c a Department of Civil Engineering, Indian Institute of Technology Guwahati, Assam, 781039, India b Department of Mechanics and Maritime Science, Chalmers University of Technology, Gothenburg, Sweden c School of Engineering, The University of British Columbia, Okanagan Campus, Kelowna, Canada A R T I C L E INFO Keywords: Floating wind turbine BEM Theory Spar-torus combinations Pierson-moskovitch spectrum Morison’s equation Kane’s equation ABSTRACT Spar type foating wind turbines exposed to marine environment experience signifcant vibration, which is more prominent under wind-wave misalignment. This unavoidable vibration, particularly in the sway direction, affects overall performance and can induce signifcant damage to sensitive electro-mechanical components leading to downtime for maintenance. With this in view, the present study aims to propose a modifed spar-torus combi- nation by introducing spring and dashpot in between, so that it can work as an isolator. First, a comprehensive mathematical model for this multi-body system is developed using Kane’s approach with proper aero-elastic and hydrodynamic simulation. For this purpose, 3D wind felds are generated in TurbSim and the waves are simu- lated from Pierson-Moskovitch spectrum considering wind-wave correlation and misalignment. Aerodynamic loads are computed using modifed Blade Element Momentum theory while and hydrodynamic loads are generated using Morison’s equation. Using these inputs, the response of the modifed spar-torus combination is solved, which demonstrates the effciency and advantage of the proposed vibration isolation. Different sea states and wind conditions are simulated replicating the actual scenario to investigate the performance envelope of the proposed controller for spar-type foating wind turbines. 1. Introduction Spar-type foating offshore wind turbines consist of mooring sup- ports, foating spar, a long and fexible tower supporting nacelle and rotor at the top. Unlike onshore turbines, which are exposed to turbulent wind, the primary source of vibration for offshore foating turbines is the hydrodynamic load. In this structural set-up, small vibration of spar (i.e. tower base) due to sea wave can cause an amplifed response at the nacelle (i.e. tower top). It is more pronounced under wind-wave misalignment, which is often refected in the side-to-side response of tower or nacelle. This unwanted side-wise vibration can cause signif- cant damage to electro-mechanical systems (notably, the gearbox). In this context, gearbox maintenance costs maximum revenue loss and downtime for any operational wind turbine. Therefore, nacelle vibra- tion, especially in the side-to-side direction, should be kept as minimum as possible. Due to this reason, the research activities in the recent past have been focused on different aspects of effcient modelling, analysis and design of wind turbines. Among them, vibration control has remained an open problem for the wind turbine industry. Li et al. (2020) have explored the effects of the yaw error and wind-wave misalignment on the dynamics and power generation of foating turbines using FAST. Their study in- dicates that yaw error can result in reduced power generation without adverse effects, while wind-wave misalignment signifcantly affects the dynamics of the structure. Kyle et al. (2020) have investigated the propeller and vortex ring states of NREL 5 MW foating turbine when subjected to strong waves and low wind using CFD analysis. Their study has shown that negative thrust can occur in regions of high twist angles. Li et al. (2019) have also studied the dynamics of a semi-submerged offshore wind turbine subjected to turbulent wind fow to investigate the effects of wind shear, turbulence intensity and coherence on the responses of the turbine structure. They have noticed that increased turbulence can create violent shaking of the platform, which, in turn, increases structural loads on the turbine signifcantly. They have sug- gested the use of a partial coherence structure to ensure the safety of the wind turbine. Liu et al. (2019) have analysed the reliability of foating wind turbine considering wind and wave loads with no misalignment between them. They have observed that edgewise blade response can be * Corresponding author. E-mail address: arunasis@iitg.ac.in (A. Chakraborty). Contents lists available at ScienceDirect Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng https://doi.org/10.1016/j.oceaneng.2020.108232 Received 18 May 2020; Received in revised form 9 September 2020; Accepted 11 October 2020