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OWOE - Wind Power - What are offshore wind farms?
  Figure 1 - Progression of expected wind turbine evolution to deeper water (NREL)
Figure 1 - Progression of expected wind turbine evolution to deeper water (NREL)
Figure 2 - Worldwide Offshore Wind Farms (EIA)
Figure 3 - Offshore Wind Speed at 90m Height (NREL)
Figure 4 - Map of US Offshore Wind Resource Potential at 90m Height (NREL)
Video - Principle Power WindFloat Prototype - Offshore Portugal
Figure 5 - Hywind Scotland floating offshore wind platforms during constructioni
What are offshore wind farms?

Offshore wind farms are groups of wind turbines clustered together, but situated in coastal waters, generally considered within 50 nautical miles of land. Locating wind turbines offshore has a number of benefits:
  • Offshore winds tend to be stronger and more consistent that onshore
  • Over 50% of the population of the US lives in coastal regions
  • Wind turbines don't compete with other land uses
  • The concern over wind farms as an eyesore is greatly reduced
The biggest challenge with offshore wind is cost, including the additional costs associated with the turbine foundations, advanced materials to resist corrosion, installation that requires special marine vessels, and buried subsea cables to bring the power to shore. These additional costs can be mitigated by the use of larger turbines than might be desirable onshore, which will reduce the cost per kWh.

In shallow water a wind turbine can be supported directly on the sea floor. In deeper water the turbine can be mounted on a fixed or floating platform and take advantage of technology developed for the offshore oil industry. Figure 1 illustrates the progression of turbines into deeper water.

The majority of offshore wind farms are concentrated in Europe in the North Sea, as shown in Figure 2. The US has been slow to capitalize on it's offshore wind resources, but it is estimated by the NREL that offshore wind in the US has a theoretical potential capacity of 4,200 GW, which is approximately four times the total generating capacity of the US grid. Figure 3 shows average offshore wind speeds, and Figure 4 shows the NREL's estimate of each region's wind generating capacity.

The first offshore wind farm to start operation in the US is Deepwater Wind's Block Island Wind Farm. This is a 30-megawatt offshore wind farm located approximately three miles southeast of Block Island, Rhode Island, consisting of 5 turbines on fixed foundations. The wind farm is designed to generate over 125,000 megawatt hours annually, enough to power over 17,000 homes. Power is exported to the mainland electric grid via a 21-mile submarine cable that makes landfall in Narragansett, Rhode Island. Offshore installation began in the summer of 2015, and first electricity was generated in December 2016. Upon successful connection to the grid, Block Island was able to shut down its diesel generators which had been the only source of electricity on the island.

Offshore wind has primarily been focused on shallow water developments using fixed foundations. However, many future sites for development are in regions of the world with sharply sloped seabeds, such as off the West coast of the US and Hawaii. Offshore development in these areas will require floating wind platforms. Principle Power built and tested a prototype floating wind platform, called WindFloat for a potential site offshore Coos Bay, Oregon. The video shows the prototype project installed offshore Portugal. However, the first commercial floating wind farm, Hywind Scotland, has been developed by Statoil. It consists of 5 spar-type platforms supporting 6 MW offshore turbines and anchored in water depths between 90-120 meters. The turbines were installed on the platforms in a Norwegian fjord (see Figure 5), and the completed units were towed across the North Sea for installation offshore Peterhead, Scotland.

There are an additional 18 potential offshore wind developments in various stages of planning/permitting. Visit OWOE Amazing Energy: UMaine VolturnUS for a look at a 1:8 scale prototype offshore Maine and the first floating wind platform in the US to be connected to the grid.

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