The Energy Technologies Institute (ETI, UK) funded project called NOVA (for Novel Vertical Axis wind turbine) examined the feasibility of a large offshore vertical axis wind turbine in the 5 to 10 MW power range, with a view toward 20 MW. The NOVA feasibility study required a methodology to be developed to select the best configuration, based on the system dynamics. Two options have been analysed: fixed and floating support structure, with the floating option is here analysed.

The design space has been investigated, ranking the possible options using a multi-criteria decision making (MCDM) method called TOPSIS. Based on this, two configurations have been promoted to the next analysis stage: a barge and a semisubmersible.

The evolution of the configuration designs, necessary to optimise and compare these two options, is here presented, taking their dynamics and costs into account. The barge concept evolved to the ‘triple doughnut-Miyagawa’ concept, consisting of an annular cylindrical shape with an inner (to control the damping) and outer (to control added mass) bottom flat plates. The semisubmersible was optimised to obtain the best trade-off between dynamic behaviour and amount of material needed. The main conclusion is that, in general, the requirement driving the design is a good dynamic response to waves. If only basic requirements were taken into account (ability to float and ability to counteract the wind turbine inclining moment), a much smaller, lighter, and cheaper structure would fulfil them.

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