This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 21705, ’Use of a Vertical Wind Turbine in an Offshore Floating Wind Farm,’ by Marc Cahay and Eric Luquiau, Technip, and Charles Smadja and Frederic Silvert, Nenuphar, originally prepared for the 2011 Offshore Technology Conference, Houston, 2-5 May. The paper has not been peer reviewed.
With 2.0 GW already installed and more than 50.0 GW planned in Europe by 2020, as reported by the European Wind-Energy Association, the installation of bottom-mounted offshore wind turbines in very shallow water (less than 40 m) is now well established. The foundation and the offshore installation costs represent approximately 50% of the total development cost of an offshore wind farm, and this proportion increases with the water depth. Hence, floating wind turbines are the solution for deep offshore.
Coming from a strong and complementary partnership of utilities, industrials, and academics, the Vertiwind project plans to build, install, and operate, in real offshore conditions, a full-scale floating vertical-axis wind turbine (VAWT) of 2 MW. This is a real step change in the technology compared with most offshore wind turbines with horizontal axes.
The floater concept retained is of a semisubmersible design and is called “multifloater.” It skillfully combines an out-of-the-wave excitation response, a shallow draft to facilitate fabrication, and a very simple installation procedure requiring only one or two tugs. The VAWT is located in the center of the floater, ensuring that the center of gravity, the buoyancy, and the convergence point of the mooring lines are all on the same axis. This geometrical property highly reduces the sway and yaw response of the floater subject to noncolinearity or heading variation of wave and wind. Because of the architecture and underlying principles of this Darrieus turbine, the power production is not affected by the inclination of the turbine axis relative to the wind direction as a result of the floater motions.
The turbine design is carried out in parallel with the floater. It integrates, at an early stage, all requirements of the offshore environment in terms of loads, accessibility, and ease of maintenance. The complete unit will be anchored to the seabed and linked to the network by a compliant subsea cable.