Hy-Line Cruises, a ferry service operating out of Hyannis, Massachusetts, plans to offer “eco-tours” of the Cape Wind windmill installation, shown in the simulation above, once it is complete. Courtesy of Cape Wind
Geotechnical work has begun on the first U.S. offshore wind farm to receive a federal permit.
August 7, 2012—Geotechnical and geophysical surveys began recently on Horseshoe Shoal, in Massachusetts’s Nantucket Sound, for Cape Wind, the first offshore wind farm in the United States to receive a federal permit. Developed by Energy Management, Inc., of Boston, Cape Wind is a 130-turbine installation projected to have an average generating capacity of 174 MW—enough to meet about 75 percent of the average electricity demand for Cape Cod, Martha’s Vineyard, and Nantucket Island.
The geotechnical work is divided into four phases, according to Mark Rodgers, the communications director for Cape Wind. Fugro, headquartered in Leidschendam, the Netherlands, is the principal contractor for the work, which will be completed later this year and involve more than 50 scientists, engineers, archaeologists, and geologists. Fugro has performed similar work for offshore wind farms in Europe.
“Phase one is a relatively modest vessel that is towing instrumentation behind it that is using acoustic mapping,” Rodgers says. “Stage two is vibracores, which we have done before, but this just adds to the work that was done. That’s getting sediment samples of relatively shallow depths, but [that] would cover us through the last ice age.”
Stages three and four will involve deeper analysis and use cone penetration tests and deep borings. Developers are looking for paleosols and evidence of human artifacts. The surveys build upon work done during the arduous development process for the project, which extends back to 2001.
“We are hoping to come away with a very sophisticated and detailed understanding of the sediment composition and type at different substrates going into the seabed to inform our foundation design for the turbines,” Rodgers says. (Read Civil Engineering’s August 2012 cover feature on wind turbine foundation design, “Supporting the Winds of Change.”)
The turbines will be arrayed in a grid pattern several miles offshore. The turbine towers will be 258 ft tall, and the massive blades will sweep 75 ft above the ocean’s surface and reach a maximum height of 440 ft above the surface. Steel monopole foundations 16 ft in diameter with walls more than 2.5 in. thick will be driven 80 ft into the sandy soil, the sediment filling in around the poles as they are placed.
The turbines will be spaced 0.34 nautical mi apart in rows, and the rows will be separated by a distance of 0.54 nautical mi. The rows will contain various numbers of turbines, dictated by the irregular boundaries of the 25 sq mi site. Water depths at the site vary from 12 to 45 ft. The sea provides the greatest challenge for this stage of the project.
“The biggest challenge is pretty normal for offshore work—weather interruptions, sea state conditions,” Rodgers says. “We have a very favorable site in that sea stage conditions tend to be a lot more calm than they are in open ocean sites in this area. But even here, there are times that the sea status is too high. Particularly, stage one requires pretty calm seas.”
The Cape Wind turbines, simulated here as if seen from a boat
1 mi away, will be spaced 0.34 nautical mi apart in rows, and the
rows will be separated by a distance of 0.54 nautical mi.
Courtesy of Cape Wind
Because the sea isn’t always calm, developers modeled a category 3 hurricane when designing the turbines, monopoles, and foundations. Like other wind farms, the turbines are designed to shut down and lock into place when sustained winds exceed 50 mph.
“The facility is designed for the extreme storm event and what the wind and sea conditions will be like during that event,” Rodgers says. “It’s actually not often that we get storms that strong. It’s less than one event a year that our turbines would [shut down]. But we are designed for a pretty significant hurricane to survive that kind of storm.” Fortunately, Nantucket Sound is an optimal location, he adds. “When we modeled a very strong category 3 hurricane, we saw that maximum wave heights south of the islands or off of the outer cape could be 50 ft high, but within the sound only about 16 ft high.”
The project will operate under a 25-year commercial lease from the U.S. Department of the Interior, which will receive royalty payments. At the end of 25 years, the project will be decommissioned and removed with funds provided by Cape Wind
. Construction is expected to begin in 2013.
In a July 5 press release announcing the work, Cape Wind’s president, Jim Gordon, said, “Our geotechnical program this summer on Horseshoe Shoal begins our detailed design engineering and construction phase and will allow us to optimize our project to ensure that Cape Wind
will deliver its important clean energy benefits over its design life.”