Deep-sea oil rigs inspire floating wind turbine design1 June 2007
How can developers locate large numbers of offshore wind turbines in a cost-effective way without creating an eyesore for people living on the coast nearby? Perhaps the answer can be derived from the structures that have been developed for the offshore oil and gas industry. The Massachusetts Institute of Technology has been investigating the possibilities of deep-water offshore wind turbines.
Imagine a wind farm consisting of 400 huge offshore wind turbines providing onshore customers with enough electricity to power several hundred thousand homes – yet nobody standing onshore can see them. It’s surely impossible, so what is the trick? Simply this – the wind turbines are floating on platforms located a hundred miles or more out to sea, where the winds are steady and strong.
Current designs of offshore wind turbines usually stand on piles that are driven deep into the ocean floor. However, that arrangement works only in water depths of 30m or less. As a consequence, proposed installations are typically close enough to the shore to arouse strong public opposition. The construction of such a design is also an expensive process and a slow process, reducing the competitiveness of offshore wind applications.
There may now be an alternative. Professor Sclavounos, a professor of mechanical engineering and naval architecture at Massachusetts Institute of Technology (MIT) in the USA, has spent many years designing and analysing large floating structures for deep-sea oil and gas exploration. It became apparent to him that such designs could be applied to offshore wind farms.
In 2004, MIT teamed up with wind turbine experts from the National Renewable Energy Laboratory (NREL) to integrate a wind turbine with a floating platform, called a ‘floater’. The design calls for a tension leg platform (TLP), a system in which long steel cables, also known as ‘tethers’, connect the corners of the platform to a concrete block, part of a mooring system located on the ocean floor. The platform and the turbine are therefore supported by buoyancy rather than through the use of an expensive tower. As Sclavounos says “You don’t need to pay anything to be buoyant.”
In the deeps
Offshore wind development has generally been limited to waters shallower than 30m in the North and Baltic seas in Europe. At depths less than 30m, the established monopile foundation technologies can be deployed without significant R&D effort. For many European countries, such as Denmark, the Netherlands, Germany and the UK, these shallow water sites appear to be abundant, and should allow offshore wind installations to proliferate rapidly in the near term. In the USA, approximately 500 MWe of shallow water development is underway, but to date, no deep water installations have been permitted. The NREL estimates that the USA has an offshore wind resource of 907GWe located 5 miles or more from the coast. Of this, 98 GWe is in shallow water, defined as depths of less than 30m. The remaining 810GWe of offshore wind resource is located in water of greater depth. New technologies will therefore need to be developed to take advantage of this vast resource.
Analyses by MIT and NREL have indicated that floater-mounted turbines could operate in water depths of 30-200m; typically, this might represent distances of 50-150km from the shore, depending on the extent of the sea floor shelf. At such distances out to sea, the turbines do not have to conform to limitations imposed by visual aspects, and the turbine on the top of each platform can be as large as required. Larger turbines have an economic advantage in the wind turbine industry. The MIT-NREL design assumes the use of a 5.0 MWe turbine similar to the largest turbines currently under development. At present, the largest generally available units are 1.5 MWe for onshore units, and 3.6 MWe for offshore units.
Ocean assembly of the floating turbines would be prohibitively expensive because of their size; the tower is typically 90m high, and the rotors are about 140m in diameter. The design is intended to enable the turbines to be assembled onshore – probably at a shipyard – and then towed out to sea by a tugboat. Cylinders placed inside each platform would be ballasted with concrete and water in order to keep each platform stable during the towing process. Once it is at the site, the platform is hooked to previously installed tethers; water is pumped out of the cylinders until the entire assembly lifts up in the water, pulling the tethers taut.
The tethers allow the floating platforms to move from side to side but not up and down, which is a very stable arrangement. According to computer simulations carried out by MIT, under hurricane conditions, the floating platforms – each measuring about 30m in diameter – would shift by 1-2 m, and the ends of the turbine blades would remain well above the peak of even the highest waves. The researchers hope that they will be able to reduce the potential sideways motion still further through the installation of specially designed dampers similar to those that are used to steady the sway of skyscrapers during high winds and earthquakes.
The design is similar to the offshore floating platforms used by the oil and gas industry. However, there are some differences, most of which allow wind energy platforms to be easier and cheaper to construct. These include:
• Oil platforms need higher safety margins because of the need for permanent accommodation for personnel.
• Oil platforms have to make allowance for personnel evacuation. Wind plat-forms are mostly unmanned.
• Oil platforms must provide additional safety margins and stability to prevent spills. This is not an issue with wind platforms.
• Wind platforms need only be deployed in water of depths up to 600 ft. Floating tension leg oil platforms are found in depths from 1500 – 8000 ft.
• Wind turbine platforms can be submerged to minimise the structure exposed to wave loading. Oil platforms must maximise above-water deck area and payload.
Sclavounos estimates that building and installing this floating support system should be one third the cost of constructing the standard type of truss tower that is currently planned for and used in deep-water installations. Installing the tethers, the electrical system, and the cable to shore is standard procedure. Assuming sufficient technology improvement and volume production, the NREL studies suggest that costs for the deployment of deepwater offshore wind turbines could fall to $0.051/kWh by 2015.
It is also well-known that wind profiles tend to be improved in deep-water locations compared with sites near to land. As a result of this phenomenon, the floating turbines should be able to generate twice as much electricity per year per installed megawatt as wind turbines currently in operation. Because the turbines are not permanently attached to the ocean floor, they are a moveable asset. If a company had, for example, 400 such units serving the Boston area, and then found that it needed more power to supply New York city, it would be feasible to unhook some of the floating turbines and tow them to a new location by tugboat.
There have been some encouraging responses to this scheme from wind, electric power and oil companies . As a result, there are plans underway to install a half-scale prototype south of Cape Cod off the east coast of the USA. Sclavounos says “We’d have a little unit sitting out there that ... could show that this thing can float and behave the way that we are saying that it will. That’s clearly the way to get going.”
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