Machine Design

Floating windmills borrow ideas from oil rigs

Environmentalists and power companies would like to tap into wind power to supply some of the nation's electricity.

So far, windmills have been uneconomical, and even some eco-minded groups are offended by their looks and impact on birds and people. One solution, according to Paul Sclavounos, a Massachusetts Institute of Technology professor of mechanical engineering, is to place windmills hundreds of miles out at sea where winds are strong and steady. And rather than anchor them on the ocean bottom, which only works if the water is less than 50-ft deep, Sclavounos suggests putting the wind turbines on floating platforms, much like oil-drilling rigs.

In Sclavounos' design, called a tension-leg platform, steel cables run from the corners of a floating platform down to a concrete block or other mooring system on the ocean floor. The platform and turbine are supported by buoyancy. This design, according to his analysis, would work in waters from 60 to 600-ft deep. And in the Northeast, for example, they could float 30 to 90 miles offshore. These offshore windmills could be larger than current on and offshore models. Sclavounos predicts his floating windmills, with their 295-ft towers and blades 459-ft in diameter, could generate 5 MW of power, much more than the 1.5 MW of average onshore units and the 3.6 MW of offshore sites.

The downside is the prohibitive cost to construct these large floating turbines at sea. So the researcher and his team designed the turbines to be assembled onshore, probably at a shipyard, and towed to sea by a tugboat. To keep the rig stable while being towed, it would be outfitted with cylinders filled with concrete and water for ballast. Once on site, divers would hook the platform to already-installed cables. Then they would pump water out of the ballast cylinders until the rig is out of the water and the cables tighten.

The cables would let the platform move from side to side, but not up and down. This, according to simulations, means that even in hurricanes, a 295-ftdiameter platform would only shift by 3 to 6 ft and remain well above the highest waves. The team is now trying to reduce sideways motion with dampers.

Sclavounos estimates such windmills would cost a third that of truss towers now planned for deepwater windmills. And the floating windmills would still be mobile. So if a company with turbines serving the Boston area needed more power for New York City, it could unhook some and tow them south.

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