So, why floating turbines? “When you go over about 150 feet of water depth it becomes very costly to make a foundation,” Dagher explains. “You have to go to a floating system. About 60 percent of the U.S. offshore wind resources within 50 miles of our shores requires floating technology because of the water depth, both on the East and West Coasts.”
This water depth challenge is one of many reasons that Europe has such a strong lead in wind energy, with over 90 percent of the world’s offshore wind farms. Average distance from shore for those farms is around 25 miles.
“In Maine, where we have deep water, we can’t use fixed bottom foundations unless we put them within a mile of shore,” Dagher points out. Recreational and commercial boaters, fishermen and waterfront homeowners have had concerns with that option. So, floating foundations become a prime alternative for harnessing the more powerful offshore winds. “By getting 20 or 30 miles away from shore you reduce the interference with those other forms of ocean use,” Dagher says.
The Advanced Structures and Composites Center has been working toward floating turbine technology since 2011, when researchers first traveled to the Netherlands to test several 1:5 scale designs. In 2013, the team launched VolturnUS, a 1:8 scale prototype floating hull and wind turbine.
The 65-foot-tall turbine was anchored off the coast of Castine, Maine, in 90 feet of water to test design feasibility and become the first grid-connected offshore wind turbine in the U.S. After an 18-month deployment, the team brought it back in and set to work analyzing the data the turbine had collected.
“We had 50 sensors onboard and through that we were able to prove that our floating turbine stayed within 7 degrees of vertical in a 500-year storm with 70-foot waves,” Dagher says. (Conditions in the winter of 2013-2014 were so harsh they had a 1 in 500 chance of occurring.)
The success of this feat led the DOE to invest $40 million into building a full-size demonstration turbine, a project currently underway. “If all goes well, we’ll have the first commercial-scale floating turbine [in the Americas],” Dagher says.
Developing Full-Scale Technology
The UMaine team has had more than 10 patents approved for its work on floating platforms. Now, the data gathered from earlier deployments is informing fresh innovation set to go live by 2022.
At the base of the design is the semi-submersible concrete hull, fabricated by Cianbro, a Maine-based construction and fabrication company. Next comes the tower. Wind towers have typically been tubular steel sections, but Dagher’s team is exploring the potential use of composite towers for floating turbines. This could reduce the overall weight by 250 to 300 tons, or about 40% of the weight compared to steel.