Technicians working on offshore wind turbine structures are exposed to harsh conditions. Large, slender structures with hub-heights of more than 100 m above sea level are common for next-generation offshore wind turbines, leading to a significant motion response to external factors such as wind and waves. The motion response increases when moving from fixed-bottom to floating substructure concepts - a finding that has initiated a large research study to investigate if the well-being of humans located on floating platforms during maintenance work could potentially be affected. This paper relates a motion assessment of an 8 MW monopile-based fixed-bottom offshore wind turbine to the previously achieved results for four different floating structures. It is found that three out of four floating structures show a less favorable motion behavior than the fixed-bottom structure applied for comparison. Motion response is design-specific and its implications towards operations and maintenance considerations must be assessed depending on site conditions. However, results show that the topic of motion exposure has yet not been sufficiently considered for both fixed-bottom and floating offshore wind deployment.
Dimensions of state-of-the-art offshore wind turbines have been increasing and continue to increase significantly over time. The cost of energy is positively influenced by this trend, leading to the fact that electricity generation by offshore wind assets is becoming a competitive subsidy-free alternative to other forms of sustainable energy conversion but also to conventional power plants (Web-source I and II, 2017).
The availability of suitable offshore sites is, however, limited to certain geographical areas; the predominating limiting factor being the water depth on site. From the studies of Fischer (2012), Cruz and Atcheson (2016) as well as Borisade et al. (2016), an upper feasibility level of 50–70 m of water depth is considered to be achievable for the installation of the conventional fixed-bottom structures used today. Those structures are, in their basic technical characteristics, similar to usual civil structures; i.e. load transfer and stability is provided by a solid arrangement of steel or concrete items which are rigidly linked to a foundation in the seabed.