Preliminary Design of a Floating Wind Turbine Support Structure and Relevant System Cost Assessment

Abstract

This paper describes the preliminary design of a floating support structure for offshore wind turbines that has been developed by ERSE (former CESI RICERCA). It considered the prospects that this kind of structure could hold for exploiting wind resources in the windiest sea areas along the coasts of Italy (Sardinia, Sicily, Apulia). A review of the various types of floating structure that have been devised worldwide for wind turbines has made it possible to single out the floating structure that looks most interesting from the technical and economic standpoint. Specifically, the selected structure is of the Tension Leg Platform (TLP) type. It consists of a nearly fully submerged hexagonal platform, which carries a 6 MW wind turbine and is linked to the sea bottom by 6 ties, which assure stability and limited tilting, even under the worst loads induced by wind and sea. The construction and installation phases for this structure have been analysed, and the most effective and economically viable techniques chosen. With a view for avoiding the use of expensive sea craft, the floating structure has been designed in such a way that it can be towed to its site with the wind turbine already standing on it. A preliminary dimensioning of the floating structure has been carried out, along with technical verification of the main structural members. This work has been performed in cooperation with the engineering company TECON, which used, among others, the MOSES computation code by Ultramarine. Attention has also been devoted to the most suitable options for mooring the floating structure to the sea bottom. To assess the behaviour of the floating system under the action of wind and waves, the computational code (BM) has been developed by ERSE. This code takes into account the whole “wind turbine + floating platform” system, represented by a scheme featuring 8 degrees of freedom. Using this code no anomalous or unexpected behaviours were observed. The costs of building and installing this floating system have been calculated, drawing on experience gained with similar offshore structures in the oil and gas fields. The unique conditions of Italy as regards shipyards, harbours, electrical grid layout etc. have been taken into account. Along with estimates of operation and maintenance costs, and annual energy yields, these results have allowed the calculation of the probable unit energy production costs of floating wind farms in deep-water coastal areas of Italy.