Floating Windfarms for Shallow Offshore Sites
Author R.H.M. Huijsmans, R.H.M. van Hees, M., Henderson, A.R., Zaaijer, M.B., Bulder, B., Pierik, J., Snijders, E., Wijnants, G.H. and
Title Floating Windfarms for Shallow Offshore Sites
Conference/Journal ISOPE Conference, Toulon
Month May
Year 2004

Offshore wind energy appears to be on the verge of a phase of enormous expansion to becoming a significant source of electricity for a number of countries in northern Europe. Two major offshore windfarms at Horns Rev and Nysted are now in operation and dozens further projects are in various stages of preparation with a significant number now having planning permission and firm construction dates.
The projects built to date have been in shallow seas, of up to around 20m in depth, in the North and Baltic seas, with the planned projects extending the range to new seas and greater depths. However, numerous challenges remain in the greater depths, in particular relating to the necessary size of the support structures, the resulting wave loads, handling equipment and natural frequencies.
At one point, the inherent advantages associated with a floating support structure (of compliance due to the flexible attachment to the ground) will match the additional costs due to complexity and novelty. Questions of course remain under what conditions this will be (water depth, sea climate, distance to shore) and whether offshore wind energy can be proven to be economic at all under such greater challenges.
Significant technical and cost challenges will remain, in particular regarding:
• minimising wave induced motion
• the additional complexity for the windturbine design process
• understanding the coupling between the support structure and the windturbine
• the construction, installation and O & M procedures.
On the positive side, these challenges are accompanied by new opportunities for systems and procedures that the use of a floating support structure allows. The research project being reported on within this paper has focused on the shallow seas of around 50 m water depth in the Dutch sector of the North Sea. For floating support structures, at these depths, additional challenges beyond those listed above include:
• achieving stability,
• designing appropriate moorings (paradoxically it being more difficult to moor a vessel in the shallowest waters).
This paper provides an overview of a feasibility study, DrijfWind or FloatWind performed in the Netherlands, describing the concept generation, evaluation and selection process, reporting on ancillary issues such as grid connection and O & M and providing a report of the in-depth analysis of the concept selected to be most suitable for the conditions considered, this being a triple floater. The conclusions are that although, in this case, this technology may not yet be ready for commercial application, the margin to economic viability is closing.

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