Floating wind turbines are complex dynamic systems. The dynamics and design of such systems pose interesting engineering and scientific challenges. Although there already exist some informative and valuable reviews of this topic, there have been many additional and important studies in more recent years. This topical paper reviews the main developments and research avenues currently under way, focusing on recent research literature. It addresses specific challenges of floating wind turbines and introduces key issues focused on the floating platform. In addition, recommendations for further research are introduced.
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 (Heronemus, 1972). However, it took approximately 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 was accomplished (cf. Tong, 1998). FLOAT proposed to mount a threebladed horizontal-axis wind turbine on a spar-type floater with catenary mooring lines and also realized a model test with this concept. Since then, many different concepts for floating wind turbines have been proposed and investigated. This led to the world's first full-scale floating wind turbine, the Hywind prototype, which was installed in 2009 (Skaare et al., 2015). Since then, more concepts have been proposed, more prototypes have been realized, and much more research has been done with respect to floating wind turbines.
The reasons for pursuing floating wind energy as opposed to continuing to work 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 location, the system can be assembled from the quay and/or in sheltered waters and 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 from water depth compared to fixedbottom 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 deepwater sites are available (e.g., in Japan, Norway, or the USA).
