Passive damping systems for floating vertical axis wind turbines analysis Michael Borg Esteban Utrera Ortigado Cranfield University, UK Cranfield University,UK m.borg@cranfield.ac.uk e.utrera@cranfield.ac.uk Maurizio Collu Feargal P. Brennan Cranfield University, UK Cranfield University, UK maurizio.collu@cranfield.ac.uk feargal.p.brennan@cranfield.ac.uk Keywords: floating VAWT; dynamics; modelling; passive damping; offshore Abstract The motion of floating platforms reduces the aerodynamic performance of floating wind turbines and increases fatigue loading. One approach to combat this is to use passive damping systems to reduce the motion of the floating wind turbine. In this paper an evaluation and selection of passive damping devices for a semi-submersible support structure for floating wind turbines is presented. A preliminary design for each of the three most optimal damping devices (heave bottom plates, anti-roll tanks and open-bottom tanks) mounted on the specified support structure are presented. A dynamic response analysis in the frequency domain was carried out for each of the devices. The resulting response amplitude operators (RAOs) are compared against the original support structure RAOs in heave and roll to highlight the effects of the damping systems. It was found that the heave bottom plates produced the largest motion reduction in heave whilst the anti-roll tanks performed best in roll. These three damping devices were applied to a floating vertical axis wind turbine in a time- domain analysis through the use of a de- coupled aero-hydro dynamics model. 1 Introduction As the desire for more cost-effective wind farms has pushed for offshore projects to move further and into deeper waters, alternative support structures need to be investigated. A transition from fixed to floating foundations is essential for far offshore wind farms to remain economically viable. Floating foundations introduce additional dynamic characteristics to the wind turbine system, which may be derogatory to the operational lifetime and costs of floating offshore wind turbines (FOWTs). Suzuki and Sato [1] found that the motions of the floating platform adversely affect the fatigue life of components of a FOWT. There is a need to reduce the overall motions of the floating system to combat these adverse side effects. One approach is to make use of motion suppression systems that introduce additional damping. The reduction of motion due to these systems will lead to better aerodynamic rotor performance. This is because the large amplitude motions induced by an unmodified system leads to the rotor interacting with its own wake and subject to dynamic effects that are not yet fully understood [2; 3]. These systems also would reduce the loads experienced by the various floating wind turbine components, resulting in less fatigue. Whilst both active and passive damping control systems are available, passive damping systems are investigated in this paper as no external energy input is needed, and require less maintenance, making them more attractive for the far offshore environment. This paper presents the evaluation and selection of the most promising passive damping systems for FOWTs, and their application to a large floating vertical axis wind turbine (VAWT). The motion responses of the