Abstract

There is a dominant trend in the offshore wind industry towards larger turbines. Another trend is the diminishing availability of shallow coastal areas suitable for bottom-fixed wind turbine installation. As a consequence, there is an increased focus on the development of floating wind turbines to harness the great wind resources often found at deeper waters. Installation and maintenance of large floating wind turbines by crane ships is currently a main cost driver. The availability of such large crane ships as well as suitable weather windows for their operation at windy offshore sites are limited. The Bluewind concept is a design that allows the turbine tower to be raised and lowered through the center of a floater providing the needed buoyancy and stability. Installation as well as replacement or maintenance of blades and other parts of the turbine can thus be performed from a variety of vessels, as the need for heavy and high lifting cranes is eliminated. In this paper we give an outline and a reality assessment of the concept.

Introduction

In 2009, the first floating offshore wind turbine (FOWT), the Hywind Demo (Skaare et al., 2015), was put in operation at the Marine Energy Test Centre outside Karmøy, Norway, by the energy company Equinor (then Statoil). This prototype FOWT consists of a Siemens 2.3 MW turbine mounted on a spar-buoy floater. Today, a commercial floating offshore wind farm, Hywind Scotland, consisting of five 6 MW FOWTs of the Hywind type, is in operation since 2017. The latest project based on the spar-buoy concept is Hywind Tampen, where eleven 8 MW FOWTs will provide electrification of the Snorre and Gullfaks offshore field operations in the Norwegian North Sea. In the decade since the first FOWT was launched, many alternative FOWT designs have been developed, but few have yet reached maturity in terms of demonstration. A notable FOWT concept is WindFloat (Roddier et al., 2010), where the turbine is mounted at one of the corners of a triangular semi-submersible, instead of a spar-buoy. The largest FOWTs installed so far are in the WindFloat Atlantic project, where 8.4 MW Vestas turbines are mounted on three semi-submersibles off the coast of Portugal. In addition to spar-buoys and semi-submersibles, FOWT designs based on barges (Kosasih et al., 2020) and tension-leg platforms (Uzunoglu and Soares, 2020) have been developed.

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