Offshore Wind Turbines for the Norwegian Shelf

Abstract

The report discusses the appropriate design of wind turbines for placement in the North Sea on the Norwegian Continental Shelf. For wind turbines in the southern part of the Norwegian Sector, at water depths in the range of 50 m, jacket foundations supporting the wind turbines will be necessary as the technology for use of “monotower” foundations limits these to shallower depths. The development of new foundation solutions, as for example concrete mono-towers, could, however, not be disregarded. Fabrication and installation are within state of art, although equipment handling the work will be heavier than in shallow water due to heavier foundation platforms and deeper waters excluding small jackups. Personnel transfer technology using walk-to-work technology is established for the area, however, care must always be taken to ensure the safety of all personnel involved. For floating wind turbines in deeper waters, say 150m, the technology state of art is represented by Spar type wind turbines, for example the Hywind design, and of semisubmersible wind turbine support structures of the “Wind Float” design. The physical conditions of the North Sea must be considered, and the Norwegian Coastal Current could represent large forces on the anchor and cable systems. For safe design, a safety evaluation will be necessary with respect to the redundancy of the anchor system. Fabrication of the Spar type units are possible in dry docks with transfer to deeper fjords for the finalization of the construction prior to tow to the offshore site. The semisubmersible units can be fabricated in dry docks and towed fully ready for operation to the offshore site. Stability calculations must be done to ensure intact stability during the tow across most of the shallow parts of the tow-out route. The floating wind turbines will be challenging to access by walk-to-work technology and waiting for acceptable weather will limit the weather windows for safe transfer of personnel.