Floating Wind Turbines Headed for Offshore Farms

Floating turbines could reduce the cost of offshore wind power

3 min read

Floating Wind Turbines Headed for Offshore Farms
Prototype wind turbine.
Photo: Robert F. Bukaty/AP
Floating wind power is no longer science fiction. Promising results from five test platforms operating worldwide—including three in Japan—are turning into project plans for a first generation of floating wind farms. Industry analyst Annette Bossler, who runs Bremen, Maine-based Main(e) International Consulting, predicts that the number of test platforms will nearly double over the next two years and that commercialization is within site. "By 2018-2019 you will start to see the first really large-scale commercial use of floating platforms," predicts Bossler.
Putting wind turbines on offshore platforms akin to those developed for the petroleum industry provides a means of exploiting high-quality offshore winds—which are stronger and more consistent than onshore winds—in waters too deep for today's bottom-fixed foundations. The Department of Energy calls floating wind the future of offshore wind because over 60 percent of U.S. offshore wind resources—and nearly all of those off the West Coast—blow over deep water.
Last month, Seattle-based Principle Power secured $47 million in federal funding to test that potential 29 kilometers out from Coos Bay, Oregon. It has partnered with Rhode Island-based offshore wind developer Deepwater Wind to tether platforms for five 6-megawatt wind turbines in over 300 meters of water—way beyond the 50-meter maximum depth for fixed foundations such as those that Deepwater Wind plans to use at its East Coast sites.
Floating turbines also offer potential cost savings. Floating platforms and their turbines are fully manufactured on shore, then towed out and tethered to the seabed. By contrast, fixing foundations to the seabed and then bolting on massive turbines requires specialized vessels, which cost upwards of US $200 000 per day to rent—whether or not the weather permits their use.
Bossler says floating platforms can also achieve cost savings through serial manufacturing. Whereas fixed foundations must be tailored to each turbine site's depth and seabed conditions, every platform in a floating array can be identical.

Prototype testing is assuaging doubts about floating platforms' ability to stabilize massive offshore wind turbines against wave action as well as their ability endure punishing offshore storms. Principle Power's array of 6-MW turbines will sit atop larger versions of a prototype that has carried a 2-MW turbine in Portuguese water since 2011 (photo at right / Principle Power). The semi-submersible platform is, like a glacier, mostly below water; its stability derives from water moving around the platform, as well as ballast water moving within it.
This month also marks one year of operation of a floating test turbine in Penobscot Bay (photo at top). It remains the only offshore wind turbine in U.S. waters. The 20-kilowatt prototype installed by a University of Maine-led consortium is just one-eighth the size of a 6-MW turbine. But its smaller scale actually provides an accelerated means of "de-risking" the design, according to Habib Dagher, the University of Maine structural engineer and composites expert who directs its DeepCwind Consortium.
Dagher says the platform relies only on water flowing around it for stability, yet is proving extremely stable through waves that—given its 1:8 scale—equate to 23-meter hurricane-scale assaults. "It saw 100-to-500-year storms relative to its size, and its maximum inclination angle was just 5.9 degrees off of vertical," says Dagher.
While Dagher's consortium lost out to Principle Power in the current round of DOE project funding, its plans to install two 6-MW turbines off Maine's coast may yet hold water. Maine has only deep water, and state regulators eager to jumpstart offshore wind development guaranteed DeepCwind a generous 23 cents per kilowatt-hour for its power. That power purchase deal is worth over $240 million, says Dagher, and is something that other U.S. offshore wind developers are struggling to secure.
Then there is DeepCwind's unique materials technologies, which it asserts could slim the cost of offshore wind power by more than half by the mid-2020s. DeepCwind replaces steel with corrosion-resistant concrete in its platform and with comparatively lightweight composites in its turbine tower.
Bossler says cost reduction will be critical to commercializing floating wind power. This is true even in Japan, where idled nuclear plants and soaring power costs are accelerating floating wind development. But she declines comment on whether DeepCwind's solution is the way forward. "I do work for a competitor," she says.
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