Safe at Sea
Designing floating windmills
However strong the environmental case for windmills, many coast-dwellers don’t like the idea of a wind farm as a part of their seascape.
Paul Sclavounos is designing floating platforms that would be stable enough to support even very tall windmills and far enough offshore to mollify the view-protectors. But if that solves one problem, it also raises the question of what would happen if a hurricane sent gigantic waves crashing into the structures.
Sclavounos, an MIT professor of mechanical engineering and naval architecture, notes that in many oceanic settings, waves can indeed be massive. “The highest waves — which incidentally can also be hundreds of feet long — are about 30 to 40 meters peak-to-trough, or roughly 110 to 130 feet,” he says. But while that’s plenty high, Sclavounos says floating windmills can be designed to handle nearly anything Mother Nature throws at them.
His confidence reflects the 30 years worth of experience that seabed explorers have had with floating oil and gas rigs. “These rigs are built to withstand waves two to three times as high as anything they’re ever likely to encounter,” notes Sclavounos.“Very few have been lost in severe storms.”
A water-borne windmill’s platform would look a lot different from an oil rig’s. The platform on a common type of rig sits on four floating cylinders. The much smaller windmill platform would top a single slender cylinder, called a floater. But both achieve stability the same way.
The buoyancy of each of a rig’s hollow cylinders, like that of a bathtub sailboat pulled downward in the water, causes the cylinders to push up against rigid tethers anchored in the seafloor. That same dynamic would stabilize a windmill-bearing floater.
Yes, it would move slightly side to side in heavy seas. But even in the trough of a 30-meter wave, says Sclavounos, “its buoyancy is such that it won’t move up and down, and it won’t tilt.”
Offshore wind’s potential is huge. The winds in U.S. coastal areas alone could theoretically generate about one trillion watts of electricity, or seven percent of the world’s total energy production capacity.
What has allowed Sclavounos to become a key player in this hot area of research is his expertise in how floating objects, from oil rigs to hydrofoils, interact with water. It’s an interest that reflects the fact he grew up in Greece. “A lot of Greek kids decide to go into maritime careers because of shipping’s role in that country,” he notes. And Sclavounos not only had the interest, he also had a flair for mathematics and analysis, which helped earn him a place as a grad student in MIT’s ocean-engineering program. He’s been with the program ever since.
His main focus is optimizing the performance of ships and rigs in all types of sea conditions. And while a lot of his work has been with oil rig designers, he’s also collaborated with others, including some in the America’s Cup yacht design community.
As former chief scientist for a group that helped design the country’s cup competitors, he contributed to the shaping of several U.S. boats. Prior to the 1995 America’s Cup finals, he even got to race on one of them. (“They’re designed to move up and down very little, even in a strong breeze,” notes Sclavounos. “It’s great when you’re gliding through the water so fast, and all you hear is the sound of the wind and the sea.”)
READY FOR ANYTHING
How will the new floating windmills work? One key set of components is the tethers. These hollow steel tubes, roughly 10 inches across, have almost no give. This rigidity is one reason the floater could stay relatively still even in ferocious storms. The other is the way the tethers are anchored.
One anchoring technique is to pound steel pilings into the seafloor. “For an oil rig, they can be up to 150 meters long,” notes Sclavounos. The second involves a kind of massive suction cup that’s driven into the mud, where it’s held in place by the very high water pressures at depth. Both are proven approaches.
Windmill technologies, for their part, seem readily adaptable to sea conditions. In fact, at least one offshore Danish wind farm has been running for 15 years plus.
What remains to be done? “Our technology has been looked at by several outside groups and several companies, and they seem to feel that what we are proposing is realistic,” says Sclavounos. Indeed, he notes, at least one firm that his group will be working with hopes to build a prototype within two years.
Of course, there are pending issues related to when and how the wind energy would be used. Sea winds can blow hard at night as well as during the day, so for offshore wind farms to make economic sense, you may need associated systems for storing electrical power when electricity prices are low. “I’ve got a graduate student working on that,” he says.
In the meantime, though, the concept is stirring interest unlike anything Sclavounos has experienced before. “It’s an exciting time,” he notes. “We’ve had countries calling us to check on the status of this technology, because they need the electricity.”