Offshore Wind Energy

Commercial-scale offshore wind facilities are currently in operation in shallow waters off the coasts of Europe, but further technology development is needed for use in the deeper waters of the Outer Continental Shelf (OCS).


	Wind Facility, Sweden. © GE Energy

Wind is air in motion. Since the earth’s surface is made of various land and water formations, it absorbs the sun’s radiation unevenly. Wind is produced by the uneven heating of the earth’s surface by the sun.

Onshore, wind energy has been utilized for power generation for more than two thousand years. In modern times, wind energy is mainly used to generate electricity, primarily through the use of wind turbines. Wind flows over the airfoil-shaped blades of wind turbines, causing lift (similar to the lifting force on airplane wings), causing the turbine blades to turn. The blades are connected to a drive shaft that turns an electric generator to produce electricity.

Offshore Wind Energy Resources

Offshore wind turbines are being used in a number of countries to harness the energy of the moving air over the oceans and convert it to electricity. Offshore winds tend to flow at higher speeds than onshore winds, thus allowing turbines to produce more electricity. Much of this potential energy is near major population (and energy load) centers where energy costs are high and land-based wind development opportunities are limited.

Because the potential energy produced from the wind is directly proportional to the cube of the wind speed, 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.

Offshore Commercial Wind Energy Generation

Many offshore areas have ideal wind conditions for wind facilities. Denmark and the United Kingdom have installed large offshore wind facilities to take advantage of consistent winds. Today, just more than 600 MW of offshore wind energy is installed worldwide, all in shallow waters (<30 meters) off the coasts of Europe. Proposed offshore wind projects through 2010 amount to more than 11,000 MW, with about 500 MW each in the United States and Canada, and the remainder in Europe and Asia.


	Offshore Wind Facility © GE Energy


	Offshore Wind Facility


	Aerial Photo of Offshore Wind Facility

Commercial-scale offshore wind facilities currently are similar to the onshore wind facilities, but with modifications to prevent corrosion and protect against wave and wind interactions. Because roughly 90% of the U.S. OCS resources are over waters that are much deeper than European waters where commercial wind facilities are currently sited, new technologies are being developed (e.g., for strengthened tower foundations) to harness the wind in the harsher conditions associated with deeper waters.


	Deepwater Turbine Evolution


	Deepwater Turbine Concept Designs

Offshore Wind Energy Technology

Offshore wind facilities today are generally developed and operated as follows. Once a suitable place for the wind facility is located, piles are driven into the seabed. For each turbine, a support structure and a tower to support the turbine assembly, to house the remaining plant components, and to provide sheltered access for personnel are attached to the piles. After the turbine (generally a three-bladed rotor connected through the drive train to the generator) is assembled, wind direction sensors turn the nacelle (a shell that encloses the gearbox, generator, and blade hub) to face into the wind and maximize the amount of energy collected. Wind moving over the blades makes them rotate around a horizontal hub connected to a shaft inside the nacelle. This shaft, via a gearbox, powers a generator to convert the energy into electricity.


	Wind Turbine Schematic Diagram

Click to View Animation of Wind Turbine Operation on NREL Web Site

For more information about how wind turbines work, including an animation of a wind turbine, see How does a Wind Turbine Work? on the National Wind Technology Center Web site.

Offshore turbines have technical needs not required of onshore turbines due to the more demanding climatic environmental exposure offshore. Offshore turbines look similar to those onshore with several design modifications. These include strengthening the tower to cope with wind-wave interactions, protecting the nacelle components from sea air, and adding brightly colored access platforms for navigation and maintenance. Offshore turbines are typically equipped with corrosion protection, internal climate control, high-grade exterior paint, and built-in service cranes. To minimize expensive servicing, offshore turbines may have automatic greasing systems to lubricate bearings and blades and pre-heating and cooling systems to maintain gear oil temperature within a narrow temperature range. Lightning protection systems minimize the risk of damage from lightning strikes that occur frequently in some locations offshore. There are also navigation and aerial warning lights. Turbines and towers are typically painted light blue or grey to help them blend into the sky. The lower section of the support towers may be painted bright colors (e.g. yellow) to aid in navigation and highlight the structures for passing vessels.

Offshore wind turbines are also bigger than onshore turbines (to take advantage of the steadier offshore winds and economies of scale). A typical onshore turbine installed today has a tower height of about 60 to 80 meters, and blades about 30 to 40 meters long; most offshore wind turbines are at the top end of this range. Offshore turbines installed today are generally between 2 and 4 MW, with tower heights greater than 200 feet and rotor diameters of 250 to 350 feet. Turbines of up to 5 MW are being tested.

Transport of Wind-Generated Energy

Undersea collection cables connect multiple turbines in the wind facility and transport the electricity from them to a transformer where the combined electricity is converted to a high voltage for transmission via undersea cables to a substation. There the electricity is connected to the onshore electricity grid. Alternative approaches, such as using the wind to produce hydrogen (through the hydrolysis of desalinated seawater), which would be shipped to shore for later use, are also being investigated.

Environmental Considerations

Potential impacts on the environment that may occur during construction, operations, and decommissioning of offshore wind facilities are highlighted below.

Marine life. Foundations can act as artificial reefs with a resultant increase in fish populations from the new food supply. These increases in fish population may also have stimulating effects on bird populations in the area, which could cause collisions between birds and towers or rotors.

Migrating birds. Besides potential collisions (bird strikes), it is possible that the birds would need to consume more energy to avoid collisions and maintain their orientation when navigating around the turbines. Tower illumination may also cause navigational disorientation for birds.

Interference with navigation for endangered and threatened species. Electromagnetic fields created by the electric cables running from the turbines and underwater noises and vibrations could affect orientation and navigational ability.

Potential alteration of natural environments and diminution of habitats. Underwater support pilings, anchoring devices, scour-protection materials, and electromagnetic fields could cause a decrease in benthic communities, alter natural environments, and possibly affect migration patterns.

Emissions. Each unit of electricity generated from the wind that saves a unit generated from fossil fuels, which will help reduce greenhouses gases, pollutants, and waste products that result from fossil fuel use.

Marine traffic, recreation, and other sea space uses. It is possible that wind turbine energy plants may disrupt air traffic control and maritime radar systems, and that facilities siting could affect recreation and other sea space uses.

Visual impacts from towers, rotating turbine blades and navigation and aerial warning lights.

Noise impacts from rotating turbine blades.

For More Information

Download the wind technology white paper:

Technology White Paper on Wind Energy Potential on the U.S. Outer Continental Shelf. (255 KB)

The following presentation from the National Renewable Energy Laboratory also provides information on ocean-based renewable energy technologies, including wind energy technology. This presentation was shown at scoping meetings for the OCS Alternative Energy Programmatic EIS.

NREL Scoping Meeting Presentation: Renewable Energy Technologies for Use on the Outer Continental Shelf (3 MB)

Links to additional information on this topic are also available on the Links page.