Deepwater Wind, a company based in Providence, Rhode Island, has drawn up plans for what could be the largest wind farm in U.S. waters, the company announced last week. The proposed farm would generate a huge 1,000 megawatts of power and would be located 18 to 27 miles off the coast of Rhode Island and Massachusetts at a depth of 52 meters—considerably deeper than any other large scale wind project to date. By moving into deeper waters, turbines can harness stronger, more sustained winds. And the massive turbines the company plans to use—each capable of generating more than 5 megawatts of power, with blades rising 150 meters above the water’s surface—will be nearly invisible from shore, thereby avoiding potential legal battles with coastal communities that perceive the turbines as eyesores.
Four-legged steel platforms rising from the seafloor will allow Deepwater Wind to operate in depths more than twice those of conventional steel “monopole” wind turbine platforms. As water depth increases, the diameter of monopoles must increase exponentially, making them uneconomical in water deeper than about 20 meters. By using a four-legged design, company officials say they will be able to work in depths that were previously prohibitively expensive.
The four-legged platform design is already commonly used for offshore oil and gas rigs. It was first adapted for offshore wind turbines in a pilot project in the North Sea, known as the Beatrice Wind Farm Demonstrator Project, in 2007. Two 5-megawatt turbines were each mounted on a four-legged tower in 45 meters of water. Although the towers were never connected to the electricity grid, they remain the world’s deepest offshore wind platforms.
Paul Sclavounos, a professor of mechanical engineering at MIT, says four- or even three-legged towers offer a ready way for offshore wind to expand into deeper waters. However, truly deepwater deployments—platforms in hundreds of meters of water—will require floating platforms unattached to the seafloor.
“For depths much larger than [45 meters], the only solution is floaters, but we still have to do more development on these designs,” Sclavounos says. Only two small pilot floating turbines have so far been deployed anywhere in the world.
Deepwater Wind’s proposed project would cost $4 billion to $5 billion, and state and federal approval would be required. The company is also proposing a $500 million to $1 billion offshore high-voltage transmission line to connect the wind farm to the electricity grid in Massachusetts, Rhode Island, and New York.