Tens of kilometers from shore, the wind blows faster and more steadily than it does on land. But offshore wind farms are expensive and complicated to build and maintain. The Danish company Dong Energy is constructing what will be one of the world’s largest such farms, a 367.2-megawatt, $1.5 billion project off the coast of England. It will use Siemens wind turbines as tall as a 30-story building.

Under threatening skies, ships lay power cables between the 150-meter-high turbines and an offshore substation (left), where the voltage of the current they generate is increased before it’s transmitted the 45 kilometers to shore. The turbines can produce 3.6 megawatts each, twice as much as those commonly used on land.

Sustainable Energy

British Sea Power

See the construction of one of the world’s largest offshore wind farms off the coast of England.

by Kevin Bullis

Feb 22, 2011

Both loading and installing the 440-ton wind turbines require each component to be placed precisely. This makes it necessary to use a ship, like the one shown here, that can be jacked up from the ocean floor for stabilization. The tall cylinders at the left will support the generator housing that’s being hoisted onto a waiting ship. It takes 24 hours to load the components of a pair of wind turbines onto the ship that will take them on the eight-hour trip to the wind farm. This trip will be repeated 51 times to complete the facility.

Wind turbine blades await loading on a ship for a trip to the construction site.

A worker inspects the bolts at the end of a wind turbine’s blade. They will be used to attach the blade to the turbine hub once the components reach their destination, in the Irish Sea.

A worker inspects the turbine hub, which is where the turbine blades attach.

A ship approaches a yellow tower mounted on a 56-meter structural pile that has been hammered into the sea floor, and the process of jacking it up begins.

Once the ship is jacked up (above), the two tower pieces and the generator housing, called the nacelle, are installed with the help of a crane.

Blades are hoisted into place. The weather conditions that make the Irish Sea a promising location for a wind farm also make construction a challenge. The turbines are designed to operate at average wind speeds of over four meters per second, reaching full power generation at 14 meters per second. Installation, however, can be undertaken only in calm weather: winds swifter than 10 meters per second can shut down operations.

Installing the transmission lines that deliver power from the wind turbines to land requires a massive plow to bury electrical cables two meters under the sea floor.

Bad weather can stall construction and prevent repair work on completed turbines, forcing them to sit idle until the sea calms enough for workers to climb to the top of the tower. Here’s the view from such a precarious perch.

To prevent construction delays, Dong Energy is testing technology that will allow ships to continue working in relatively high waves. A gangway between a ship and turbine tower is connected to a system that enables it to move to compensate for the waves, giving workers stable access.

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