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Connecting Offshore Wind Energy to the Mainland Grid

Jim Lanard,
President, Offshore Wind Development Coalition
Bill Wall,
Director, Marine Operations,
Atlantic Wind Connection

Electricity generated by offshore wind farms is coming soon to a coast near you. While this technology has been successfully deployed in European waters for more than two decades, the United States has yet to install its first offshore wind turbine. But that is about to change.

Utility-scale and demonstration offshore wind projects are moving forward off of the Atlantic and Pacific Coasts, as well as in the Gulf of Mexico and the Great Lakes. Along the East Coast of the U.S., there are four projects that will utilize fixed-bottom foundation structures. These are Cape Wind's (Boston, Massachusetts) 468-megawatt wind farm in Nantucket Sound, Deepwater Wind's (Providence, Rhode Island) 30-megawatt pilot in state waters off of Block Island, Rhode Island, Fishermen's Energy's (Cape May, New Jersey) 25-megawatt wind farm off of Atlantic City and Dominion Energy's (Richmond, Virginia) Virginia Beach two-turbine, 12-megawatt pilot. Two other East Coast projects will utilize floating-platform technologies: Statoil's (Stavanger, Norway) four 3-megawatt turbines to be sited in water depths greater than 460 feet in the Gulf of Maine and the University of Maine's two 6-megawatt turbines off of Monhegan Island.

In the Great Lakes, the Lake Erie Energy Development Corp. (Cleveland, Ohio) plans to install nine 3-megawatt turbines off of Cleveland. In the Gulf of Mexico, Baryonyx Corp. (Austin, Texas) is developing an 18-megawatt pilot project in state waters off of Port Isabel, Texas. And let us not forget the West Coast, where, in the deep Pacific Coast waters off of Coos Bay, Oregon, Principle Power Inc. (Seattle, Washington) expects to deploy five 6-megawatt turbines on floating foundations.

All of these projects will be located 2 to 15 miles offshore. And that will require significant submarine power cable design, manufacture and installation. In utility-scale wind farms, medium-voltage cables (inner array or interturbine cables) will link each of the turbines to an offshore transformer station that will boost the voltage and send it to the mainland via higher-voltage export cables. Note that these export cables are likely to be 10-to-15 miles long—in one continuous length—and weigh up to 4,000 tons. Hence, special-purpose-built vessels will be needed for installation.

As an example of connecting offshore wind to the grid, the Atlantic Wind Connection (Chevy Chase, Maryland), or AWC, is developing a high-voltage, direct-current (HVDC) submarine cable backbone system to interconnect offshore wind generation to strategic grid locations along the mid-Atlantic region of the East Coast. Developers utilizing the AWC submarine cable grid will deliver their power to the AWC's offshore converter platform, from which the power will be sent to shore to feed into the grid. All of these cables will be armored to withstand laying forces and will be buried to a target depth of approximately 4 to 6 feet below the seafloor. When approaching the coast line, the cables will utilize horizontal directional drilling (HDD) technology to avoid disruption of sensitive marine and wildlife areas and to protect our nation's valued beaches and shoreline.

The cables from the turbines themselves will run along the turbine towers and foundations to the seafloor, exiting from a J-tube before being plowed under the seabed. In the case of cables from floating foundations, the cables will be suspended midwater and could be surface laid on the seabed, depending on the water depth.

Correct initial protection of the submarine cables during the installation phase is critical to the success of an offshore wind farm. Approximately 50 percent of all insurance claims and 80 percent of the payouts related to offshore wind farms in Europe are cable-related. Hence, developers of offshore wind farms in the U.S. and their insurers will need to be extremely vigilant in the implementation of their submarine cable systems.

Offshore wind has a promising future in the U.S., given the interest, government support (including tax credits) and developing infrastructure for the industry. The technology is there; it is only a matter of time before offshore wind electricity becomes a reality to Americans.

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