Original Article byr 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 tachnologies very different from those mentionned in an earlier article of 1 juin 2011 (French only)
4 PRINCIPAL TYPES OF FLOATING FOUNDATIONS.
1. "Spar Platform " : a submerged foundation with stabilised ballast equipped with catenary anchors allowing it to be simply hooked to the bottom. This is the system used by Hywind for StatoilHydro, Technip and Siemens off the coast of Norway (see article of 24 July 2009). (French onlyl). A first experimental unit was installed in 2009 off Stavanger at a depth of 22m and supporting a Siemens 2.eMw turbine. The end of the two year study is due for completion shortly. The 100m long steel tube used in this first "spar" foundation was towed while horizontal to a Norwegian fiord, and then ballasted with water and rock while two floating cranes lifted and installed the mast and the turbine itself. The total cost according to Statoil was $72m. This foundation does not rest on the bottom of the sea; it floats and is attached to the bottom only by cables having a certain freedom of movement and fixed to anchors.
2. Tension Leg Platform are submerged platforms secured to the bottom with stretched cables themselves fixed to pillars partly buried in the sae bed but not to catenary anchors. The cables tense and relaxmore or less in line with the swell. This is the technique used by Blue H Technologies for their prototype described here in the article of 17 October 2008. )French only). The first test phase started in 2008, after various fruitless discussions, with the installation of a first structure supporing a twon blade turbine off the coast of Italy in depths of 113m within the context of the Tricase Project.This was the world's first test of a floatng platform. It was dismantled 6 months later. At the same time Blue H started phase 2 of the development that entailed building a platform intended to carry a 2Mw wind turbine, for delivery in 2012, and still within the Tricase Project. Phase 3 is forecast for 2014 with the deployment in the high seas of the final design (see image) supporting a turbine from an as-yet unnamed maker.
3. "Semi-submersible" or stabilised floating. As its name implies, this is a semi-submerged platform, part visible above the surface. It uses a barge type structure secured to the bottom by catenary anchors. This is the principle used in the spectacular triangular WindFloat platform used by the American developer Principle Power Inc. They are working round the clock with Energias de Portugal (EDP) and ASM GROUP to permit a first launch off the coast of Portugal at a depth of 43m. in all liklihood during September 2011. This tri-angular floating platform is an innovatory version of the semi-submersible platforms used in the oil and gas industries. It can be used in any depth of water and comprises three columns of light steel which makes it particularly stable and resistant to even the most extreme circumstances. Like the majority of of floating platforms, the semi-sub WindFloat will be attached to the bottom of the sea with four cables. This structure estimated to cost US$23m has been designed to carry a wind turbine with a 67 metre high mast., and, initialy, a Vestas Wind V80 2,0 MW unit. Initially since Dominique Roddier, technical director of the project at Principle Power, made it cleaer that " This platform in time is going to revolutionise the world of offshore wind power". If the experiments that are starting in September and lasting for a year, confirm their hopes, the developers have decided to plan a 150Mw offshore wind farm based on this technology offshore in Portugal. Ut may be based on Vestas5Mw turbines and, in the future, even the new 7Mw Vestas V164 7.0 de 7 MW.
4. Free Floating Platform (FFP)
The French floating platform WINFLO developed by DCNS, Nass&Wind, Saipem and In Vivo Environnement with the support of FREMER 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.
VERTIWIND. There is some research into vertical axis offshore wind turbines, including VERTIWIND (See article of 20 January 2011). A brief summary: vertical blades turning on a vertical mast like a domestic beater; rotation speed that varies in line with the wind strength thus eliminating all risk of blade vibration; 90m high, so muchc less than that of traditional wind turbines; the floating platform needs only shallow (10m) water that makes towing it from land cheaper and less complex than with a conventional wind turbine. Thanks to its abiility to float above depths of 200m and more, the VERTIWIND concept brings together the advantages of other floating wind turbines with that of great flexibility in use thanks to its vertical blades. Technip, whom we can say are at the forefront of companies engaged in the battle for floating wind turbines, is involved in the Hywind project in Norway, was right in launching a call for tenders in regard to Vertiwind and in setting itself the task "of designing, making, installing, and testing a pre-production wind turbine for use offshore" where the twin peculiarities are being both floating and of vertical axis. For Technip, VERTIWIND offers new perspectives for a second generation of offshore wind turbines for various countries, particularly in the Mediterranean basin, and even in the USA. The Vertiwind project has been cerrtified by Pôle Mer PACA (one of our partners) and benefitrs from one of the first grants from the Programme d’Investissements d’Avenir, (French only) launched in teh context of the Grand Emprunt (French only) under the auspces of the French Prime Minister via ADEME.
In this race to floating gigantism and strange objects, mention must be made of the Danish DeepWind presented in the magazine Wind Tech en 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.
2. Autres projets
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 jintly 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 (photo 2) each able to carry up to 8 turbines. liennes bipales, ou une structure octogonale (cf photo 2) pouvant porter aussi 8 éoliennes. 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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