Stabilisers will let deep-sea wind turbines stand tall

FAR out to sea is nirvana for wind energy generation. Here the winds blow stronger and the giant turbines are less of an eyesore. But how to keep them from toppling over in a gale? That is the challenge being tackled by a consortium of universities and private companies called DeepCwind.

The consortium will install a scaled-down prototype of a new design of wind turbine near Monhegan Island, Maine, in 2012. If all goes well, two years later a full-sized version with a capacity of 3 to 5 megawatts will be installed, probably much further out. This could pave the way for wind turbines moored in deep water off coastal cities.

Wind turbines fixed in shallow water are already in operation in Europe, but local opposition has stalled construction of near-shore wind farms in the US. Putting the turbines much further out will mean they can't be seen from shore, but sidestepping objections is not the only benefit of doing so. "Most of the offshore wind resource is in deep water," says Walt Musial of the National Renewable Energy Laboratory in Golden, Colorado, who isn't involved in the project. Winds blow stronger over open water than land, where they lose energy to turbulence around buildings, trees and hills.

The biggest obstacle to building a deep-water wind turbine is that the blades make the assembly top-heavy and prone to tipping over, especially in high seas, Musial says. Shallow-water wind turbines are installed on massive piles drilled deep into the seabed, an impractical arrangement in deep water.

With no proven techniques for keeping floating turbines upright and properly aligned, DeepCwind is testing three new platform designs using one-fiftieth scale models in a wave tank at the University of Maine in Orono. One design is a large floating tube with a massive keel beneath and anchors for additional stability. The second design is secured to the seabed using taut cables, as with many modern oil rigs. The third design, likewise held in place by cables, features a pair of semi-submersible platforms balanced like a catamaran.

The tests will help the consortium pick a design for the second stage of the trials, when engineers will build a 30-metre-high turbine. In the spring of 2012 they will tow the prototype to a site 4 kilometres off Monhegan Island, where it will float in 120 metres of water.

The prototype should reveal how well the design can withstand exceptionally severe storms. The acid test will come in November, when the wind and waves are particularly rough. "The beauty of a one-third scale model is we can get a 50-year event in weeks," says Habib Dagher of the University of Maine, who is also head of DeepCwind. That's because the waves will be three times higher relative to the prototype's height as compared with the 90-metre full-sized turbine, planned for 2014.