Preliminary design of a floating support structure for a 5 MW offshore wind turbine

The offshore wind industry is already exploiting near shore sites, using bottom fixed support structures, and is moving toward further and deeper sites: around 100 km from the coast and in 50–150 m water depth. As already happened for the oil and gas offshore industry in the 1960s, the floating support structure option for 5 MW (and future 7–10 MW) offshore wind turbines is becoming not only a technically feasible but also an economically viable solution with respect to the fixed solution. In the present article, taking as input the NREL 5 MW turbine and the Dogger Bank site, in the North Sea, seven preliminary floating support structure concepts have been investigated, and compared through a preliminary techno-economic analysis. Then the optimum concept among the seven, the tri-floater configuration, has been further developed and refined through hydrostatic, hydrodynamic, and structural analyses. Due to the novelty of this research field, there are no established guidelines, recommended practices, or standards to design floating support structures: this work presents a relatively simple and quick methodology to use in the conceptual and preliminary design phase, using and adapting the standards developed for oil and gas offshore floating structures.

► Wind industry is moving from shallow, near shore sites to deep, offshore sites.
► As for oil and gas in the 1950s, wind industry is moving from fixed to floating structures.
► Much can be learned from O&G industry, but there is a lack of specific standards.
► A conceptual and preliminary design method for floating supports is proposed.
► Design of a 5 MW floating support structure is presented to exemplify the method.