Offshore Wind Energy

The first offshore wind project was installed off the coast of Denmark in 1991. Since that time, commercial-scale offshore wind facilities have been operating in shallow waters around the world, mostly in Europe. With the U.S. Department of the Interior’s “Smart from the Start” initiative, wind power projects will soon be built offshore the United States. Newer turbine and foundation technologies are being developed so that wind power projects can be built in deeper waters further offshore.   

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Wind energy has been utilized by humans for more than two thousand years. For example, windmills were often used by farmers and ranchers for pumping water or grinding grain. In modern times, wind energy is mainly used to generate electricity, primarily through the use of wind turbines. All wind turbines operate in the same basic manner. As the wind blows, it flows over the airfoil-shaped blades of wind turbines, causing the turbine blades to spin. The blades are connected to a drive shaft that turns an electric generator to produce electricity. The newest wind turbines are highly technologically advanced, and include a number of engineering and mechanical innovations to help maximize efficiency and increase the production of electricity. 

  

Offshore Wind Energy Resources

Offshore wind turbines are being used by a number of countries to harness the energy of strong, consistent winds that are found over the oceans. In the United States, 53% of the nation’s population lives in coastal areas, where energy costs and demands are high and land-based renewable energy resources are often limited. Abundant offshore wind resources have the potential to supply immense quantities of renewable energy to major U.S. coastal cities, such as New York City and Boston.  

Offshore winds tend to blow harder and more uniformly than on land. The potential energy produced from wind is directly proportional to the cube of the wind speed. As a result, increased wind speeds of only a few miles per hour can produce a significantly larger amount of electricity. For instance, a turbine at a site with an average wind speed of 16 mph would produce 50% more electricity than at a site with the same turbine and average wind speeds of 14 mph. This is one reason that developers are interested in pursuing offshore wind energy resources. The U.S. Department of Energy (DOE) provides a number of maps showing average wind speed data through its Resource Assessment & Characterization page and through National Renewable Energy Laboratory’s (NREL) MapSearch. 

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 United States wind speeds at 80m hub height
(Credit: NREL)

Wind resource potential is typically given in gigawatts (GW), and1 GW of wind power will supply between 225,000 to 300,000 average U.S. homes with power annually.  In a July 2012 Technical Report, NREL estimates a gross wind power resource of 4,223 GW off the coast of the United States. That is roughly four times the generating capacity of the current U.S. electric grid. Even if only a fraction of that potential is developed, clearly there is enough offshore wind resource to power a substantial portion of our nation’s energy needs.

Wind speeds off the Atlantic Coast and in the Gulf of Mexico are lower than wind speeds off the Pacific Coast. However, the presence of shallower waters in the Atlantic makes development more attractive and economical for now. Hawaii has the highest estimated potential, accounting for roughly 17% of the entire estimated U.S. offshore wind resource. For additional information on NREL’s assessment of offshore wind power resource, see the publication Assessment of Offshore Wind Energy Resources for the United States. Maps of renewable energy potential for multiple technologies, or state-by-state analyses, can be downloaded here

  

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United States offshore wind resource by region and depth
(Credit: NREL)

 

Commercial Offshore Wind Energy Generation

Many countries, including the United States, have coastal areas with high wind resource potential. Worldwide there are 4.45 GW of offshore wind energy installed, with another 4.72 GW under construction and an additional 30.44 GW approved. Over 50 projects are operational in coastal waters of countries such as Denmark, the United Kingdom, Germany, Norway, the Netherlands, Japan, China, South Korea, Belgium, Sweden, Italy, Portugal, and others. A list of offshore wind power projects can be downloaded at The Wind Power website, a worldwide database about wind turbines and wind power facilities. While the United States does not have any operational projects yet, there are thousands of megawatts (MW) in the planning stages, mostly in the Northeast and Mid-Atlantic regions. Projects are also being considered along the Great Lakes, the Gulf of Mexico, and the Pacific Coast.     

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Nysted Wind Facility, 8-12 miles offshore Denmark, the North Sea. Wind turbines are arranged to take advantage of the prevailing wind conditions at the project site, and turbine spacing is carefully designed to maximize cost efficiency and power production. (Credit:NREL)

 

Commercial-scale offshore wind facilities are similar to onshore wind facilities. The wind turbine generators used in offshore environments include modifications to prevent corrosion, and their foundations must be designed to withstand the harsh environment of the ocean, including storm waves, hurricane-force winds, and even ice flows. Roughly 90% of the U.S. OCS wind energy resource occurs in waters that are too deep for current turbine technology. Engineers are working on new technologies, such as innovative foundations and floating wind turbines, that will transition wind power development into the harsher conditions associated with deeper waters.  

  

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Progression of expected wind turbine evolution to deeper water (Credit: NREL)   Floating wind turbine platform configurations, one designed by NREL and a Dutch Tri-Floater concept. (Credit: NREL)

 

Offshore Wind Energy Technology

The engineering and design of offshore wind facilities depends on site-specific conditions, particularly water depth, geology of the seabed, and wave loading. In shallow areas, monopiles are the preferable foundation type. A steel pile is driven into the seabed, supporting the tower and nacelle.  The nacelle is a shell that encloses the gearbox, generator, and blade hub (generally a three-bladed rotor connected through the drive train to the generator) and the remaining electronic components. Once the turbine is operational, wind sensors connected to a yaw drive system turn the nacelle to face into the wind, thereby maximizing the amount of electricity produced.

For more information about wind turbine technology, see NREL’s “Wind Energy Basics: How Wind Turbines Work.

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Wind Turbine Nacelle Schematic (Credit: NREL)

 

Today’s offshore turbines have technical modifications and substantial system upgrades for adaptation to the marine environment. These modifications include strengthening the tower to cope with loading forces from waves or ice flows, pressurizing nacelles to keep corrosive sea spray from critical electrical components, and adding brightly colored access platforms for navigation safety and maintenance access. Offshore turbines are typically equipped with extensive corrosion protection, internal climate control systems, high-grade exterior paint, and built-in service cranes. To minimize the expense of everyday servicing, offshore turbines may have automatic greasing systems to lubricate bearings and blades as well as heating and cooling systems to maintain gear oil temperature within a specified range. Lightning protection systems help minimize the risk of damage from lightning strikes that occur frequently in some offshore locations. There are also navigation and aviation warning lights, regulated by the U.S. Coast Guard and the FAA. Turbines and towers are typically painted light grey or off-white to help them blend into the sky, reducing visual impacts from the shore. The lower section of the support towers may be painted bright colors to increase navigational safety for passing vessels. 

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The Repower 5M turbine, offshore Scotland, one of the world’s largest wind turbines.
(5-MW, 126m tall, 45m depth)

 

To take advantage of the steadier winds, offshore turbines are also bigger than onshore turbines and have an increased generation capacity. Offshore turbines generally have nameplate capacities between 2 MW and 5 MW, with tower heights greater than 200 feet and rotor diameters of 250 to 430 feet. The maximum height of the structure, at the very tips of the blades, can easily approach 500 feet, and turbines even larger than 5 MW are being designed and tested for future use. 

While the tower, turbine, and blades of offshore turbines are generally similar to onshore turbines, the substructure and foundation systems differ considerably. The most common substructure type is the monopile—a large steel tube with a diameter of up to 20 feet. Monopiles are typically used in water depths ranging from 15 to 100 feet. The piles are driven into the seabed at depths of 80 to 100 feet below the mud line, ensuring the structure is stable. A transition piece protrudes above the waterline, which provides a level flange to fasten the tower. In even shallower environments with firm seabed substrates, gravity-based systems can be used, which avoids the need to use a large pile-driving hammer. Tripods and jackets foundations have been deployed in areas where the water depth starts to exceed the practical limit for monopiles.

  

Transport of Wind-Generated Energy

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An Electric Service Platform (ESP) for an offshore wind facility.

 

All of the power generated by the wind turbines needs to be transmitted to shore and connected to the power grid. Each turbine is connected to an electric service platform (ESP) by a power cable. The ESP is typically located somewhere within the turbine array, and it serves as a common electrical collection point for all the wind turbines and as a substation. In addition, ESP’s can be outfitted to function as a central service facility, and may include a helicopter landing pad, communications station, crew quarters, and emergency backup equipment. After collecting the power from the wind turbines, high voltage cables running from the ESP transmit the power to an onshore substation, where the power is integrated into the grid.  The cables used for these projects are typically buried beneath the seabed, where they are safe from damage caused by anchors or fishing gear and to reduce their exposure to the marine environment. These types of cables are expensive, and are a major capital cost to the developer. The amount of cable      used depends on many factors, including how far offshore the project is located, the spacing between turbines, the presence of obstacles that require cables to be routed in certain directions, and other considerations.

  

Environmental Considerations

In 2007, the Bureau published the Final Programmatic Environmental Impact Statement for Alternative Energy Development and Production and Alternate Use of Facilities on the Outer Continental Shelf. This document examines the potential environmental impacts related to renewable energy development on the OCS for each phase of development (technology testing, site characterization, construction, operation, and decommissioning). Actual proposals will be evaluated in project-specific analyses under the National Environmental Policy Act.

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