Abstract

The concept of floating wind turbines is as old as the idea of offshore wind turbines itself. In order to use the vast offshore wind resources, Heronemus proposed floating wind turbines already in 1972. However, it took about 20 years to even produce electricity with fixed offshore wind turbines: in 1991, the offshore wind park "Vindeby" was commissioned, featuring eleven turbines with monopile substructures. Only two years after that, the FLOAT project (cf. Tong, 1998) was accomplished. FLOAT proposed to mount a three-bladed horizontal axis wind turbine on a spar-type floater with catenary mooring lines and also realized a model test with this concept. From then on, a lot of different concepts for floating wind turbines have been proposed and investigated. This lead to the world's first full-scale floating wind turbine, the Hywind prototype, which was installed in 2009. Since then, more concepts have been proposed, more prototypes have been realized, and a lot more research has been done with respect to floating wind turbines. The reasons for pursuing floating wind energy in contrast to continuing working with fixed-bottom offshore wind turbines are manifold. Floating wind turbines can be installed with less noise emission compared to fixed-bottom wind turbines, as piling is typically not necessary. Furthermore, depending on the floating concept and localocation, the system can be assembled from the quay and/or in sheltered waters, and is then towed to the location with standard offshore vessels (in contrast to specialized and expensive installation vessels that are used for fixed-bottom offshore wind turbines). The time needed for costly marine operations, such as installation of the turbines, is thereby significantly reduced.

However, the most important argument for floating wind turbines is their independence on water depth compared to fixed-bottom substructures. The feasibility of the latter has an economic limit that strongly depends on the water depth. This limit is softer for floating wind turbines. Hence, floating wind energy is especially interesting when sites for fixed-bottom wind turbines become scarce (e. g. in the UK) or when mainly deep water sites are available (e. g. in Japan, Norway, or the USA).

Offshore wind turbines are getting bigger and are being installed in deeper waters (Arapogianni et al., 2013). This results in a huge interest of both the industry and research community to bring floating offshore wind turbines into the market at a reasonable cost of energy. Henderson and Witcher (2010) and Wang et al. (2010) reviewed the state of the art of floating wind energy previously. However, since 2010 the number of publications dealing with floating wind energy has grown exceptionally (James and Costa Ros, 2015). The recent review by Tande et al. (2015) focuses more on a general overview of existing prototypes than on details and research aspects. This motivates the need for a more recent, in-depth review of the research on floating wind energy. We present and discuss the current state of the art in terms of design and dynamics of floating wind turbines, taking into account the latest literature. In addition, current challenges for floating wind turbines are summarized, special issues are discussed, and recommendations for future work and research are given.

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