
Original Article by Francis Rousseau. Edited and translated by Christopher Longmore.
Floating wind turbines can generate electricity where the depth is between 50 and 300 m, something that can occur sometimes very close to shore where the continental shelf drops steeply as is the case for some European countries on the Atlantic. At those depths, a fixed wind turbine is impossible and only one floating offers a viable solution. The concept of a floating wind turbne was first officially put forward by researchers at the University of Massachusetts (UMASS) in 1972. According to an american report produced by the National Renewable Energy Laboratory (NREL) : "We had to wait until the the mid 1990`s, that is to say some time after the onshore wind farm industry was well established, before there was any further scientific research". Until 2003, the installation of offshore wind turbines was limited by the use of fixed foundations, the use of which is limited to waters no more than 30m deep. Since 2003, numerous developers all over the world have become interested in floating wind turbines. To-day there are four different types of deep water wind turbine paltforms using technologies very different from those mentioned in an earlier article of 1 June 2011 (French only)
4 PRINCIPAL TYPES OF FLOATING FOUNDATIONS.




The French floating platform WINFLO developed by DCNS, Nass&Wind, Saipem and In Vivo Environnement with the support of IFREMER Brest and ENSTA, is also part of the semi-submersible category with catenary anchoring but with sufficient differences for it to be part of a different category the Free Floating Platform (FFP). Thus it is a "semi-sub" structure lighter and more independent of the anchorage system and the float. This allows it to be disconnected and towed easily for maintenance. The WINFLO floating offshore wind farm project brings together major industrial groups in the shipbuilding, oil construction and wind turbine industries. The recent French government call for tenders of the 11 July 2011 and that yet to come in January 2012 should lead the protagonists to put in place rapidly a near full scale model of 2.5Mw off the coast of Brittany and actually linked into the grid. The project provides for the use of a nacelle specifically designed for offshore use, light and resistant to the needs and aggressions of the marine environment. The machine will be installed on a semi-submersible platform secured to the sea bed with catenary anchors that are easier to use than conventional ones.

To these four ideas for floating wind turbines, there are variations like the very interesting French system by Ideol based in La Ciotat that has the effect of limiting the wake effect of a wind turbine thanks to a mechanical solution that moves the entire turbine (see photo at left), backed by software that calculates in real time the optimal array in the light of a number of parameters this maximising the energy output. Founded just a year ago (See article of march 2011), the company hopes to install a first experimental prototype in 2013.
PROSPECTIVE FLOATING WIND TURBINES
1. Vertical axis floating turbines.


In this race to floating gigantism and strange objects, mention must be made of the Danish DeepWind presented in the magazine Wind Tech ien 2010 (Click here). It is a 0Mw vertical axis floating turbine studied by the Danish Technical University (DTU). Despite its highly futuristic appearance, the Deep Wind project, launched in 2010, has received a €3m. subsidy over 4 years in the context of the european future technologies program. The concept combines a vertical axis turbine of the Darrieus type (well known technology), with a blade system and an innovative transmission and control system, linked to a floating and rotating structure. The concept includes a direct drive generator with electronic controls located at the base of the unit and links via electric cables. A full-scale 5Mw demonstrator prototype is due tp be placed in the waters of Roskilde sound near DUT. Once they have completed this phase, the developers are planning a larger unit with a turbine that could be as powerful as 20 Mw.

SWAY
Still in the field of prospective floating wind turbines, some UFOs are competing for the limelight! Leader of the pack is Sway® the first studies for which go back to 2003. This absolutely unique system works on the same principal as a half full bottle of water in the sea. The one-piece, heavily ballasted, combined turbine, mast, and foundation behaves just like the floating bottle. With the centre of gravity far below, this gives the mast enough stability to handle the strains produced by the turbine above. At sea the unit might swing through 5 to 8º. Despite its apparent fragility, the system has nonetheless been approved by the very well-known practice Garrad Hassan & Partners, stating that it is "technology that best confronts the worst sea conditions." In 2009, (see. article of 1 august 2009) AREVA Wind judged the project sufficiently convincing to start a collaboration with Sway® to build a joint turbine of 5 Mw. In 2010 (see artcile of 17 February 2010) Sway® and the public Norwegain company ENOVA.no announced a project to build a 10Mw turbine on a Sway® base, a project that was to propose the first concrete elements in 2011. A small scale prototype was launched on 10 Juin 2011 (Click here). The trials are to last for 1 or 2 years. We are still some way from the largest floating turbine in the world.

Finally at the end of the chain of prospects, so to speak is the MUFOWS (Multiple Unit Floating Offshore WindfarmS) project of which details HERE. It started to see the light of day in the 1990s when interest in offshore wind farns really got going. Developed jontly by University College of London (UCL), W.S. Atkins and the Energy research Centre of the Netherlands (ECN), the concept is based on the apparently simple idea of mounting a battery of various turbines on a single semi-sub foundation anchored to the sea bed with a cable. Two types of structure were envisaged - a backbone (photo 1) and an octagon each able to carry up to 8 turbines. Sadly, in 2000, an in-depth feasability study concluded that although the structure were perfectly doable and reliable, they had no chance of being viable in the circumstances then ruling in northern Europe. The study also concluded that the matter should be studied again if and when deep water offshore wind farms were under review. That is now the case.
Sources : Sites lnked and mentioned. Photo 1 : Eolienne Windflfoat ©Principle Power Inc. 2: Hywind floating turbine ©Technip/Statoil. 3: Descriptive Schema Windfloat©Principle Power. 4 et 5 : Winflo ©DNCS. 6 : Ideol©Ideol. 7.Vertiwind vertical axis turbine © Technip. 8 : Darrieus floaating turbine Deepwind ©DTU. 9. Eolienne Sway © Sway® . 10Mufows on backbone and 11 Mufows octagon ©UCL
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