The winds of change

Story and photo by Walter Garschagen

A windmill icon has begun appearing on British Admiralty charts of waters off the coasts of the United Kingdom, Denmark and other European countries. In the last five years, hundreds of wind turbines have been built offshore, where they are able to capture up to 25 percent more wind than shore-based turbines.

These wind farms in coastal waters are often located near shipping lanes, causing concerns for professional mariners, recreational boaters and fishermen alike. More than 15 offshore wind farms have been built; 24 more are proposed in Europe and four have been proposed in the United States. Given the rapid increase in the number of these projects, the offshore sighting of wind turbines has become a hot-button issue.

Sixty miles east of London on the south side of the Thames Estuary stands a newly constructed $185 million offshore wind farm. The Kentish Flats offshore wind farm was built on a sandbar in 15 feet of water six miles northeast off the small fishing village of Whitstable. Thirty 210-foot-tall turbines were placed in a diamond-shaped grid four miles south of the deepwater Knock John Channel leading into the Port of London. The turbines, which came online in August 2005, have a combined capacity of 90 MW, enough energy to power up to 100,000 homes.

From the docks of Whitstable the wind turbines look tiny, like toy pinwheels. With the blades turning slowly, out of sync with one another, they appear to be almost touching each other. When a haze hangs over the water, the wind farm actually disappears from view. They become most noticeable when a glint of the sun catches a blade and is reflected toward shore.

“Out on the Thames Estuary there is always plenty of wind,” said Assistant Harbormaster Mike Gambrill. The turbines start generating electricity at wind speeds of 7.5 knots and reach peak output when the winds exceed 25 knots. At 48 knots, the turbines disengage.

“People said they would be noisy and unsightly,” Gambrill said, “but they have already become part of the landscape.”

Part of this new landscape started showing up on radar shortly after construction for the turbines began. Pilots for the Port of London reported seeing multiple images of the wind farm on the radar screen and side-lobe interference.

“This was not foreseen,” Capt. Roy Stanbrook said. As deputy harbormaster for the Port of London, Stanbrook was consulted during the planning phases of the wind farm. “It has the potential for a problem,” he said.

The port, which employs 90 pilots and handles more than 12,000 commercial vessels a year, published a report on the radar problem in 2005. In that report, Stanbrook used a radar image to illustrate the problem, giving this description:

“The Kentish Flats offshore wind farm can be seen in a diamond shape in the lower half of the screen. On this particular vessel, secondary echoes are produced on the starboard side of the ship and also astern.”
Because the Thames Estuary has several commercial ship channels intersecting one another, the potential for the secondary echo obscuring an approaching vessel is real. “The presence of a wind farm may produce irregular secondary echoes, which could mask the presence of outbound vessels, introducing an additional navigational hazard,” Stanbrook said in the report.

“This hazard becomes more acute when visibility is reduced below the comfort threshold for making collision avoidance decisions.”

For future projects Stanbrook suggested the turbines be set back far enough from the channel so that any secondary echoes appear outside the mariner’s decision-making area.

By Professional Mariner Staff