Rock laying marks start of offshore wind farm work

11 March 2010

A massive rock installation ship has begun positioning rocks on the sea bed off the Norfolk coast, marking the first phase of the construction work on the 88-turbine Sheringham Shoal offshore wind farm.

The 164 metre long Nordnes vessel arrived on Tuesday to position two to nine-inch 'filter' rocks at selected foundation locations in preparation for the start of the installation of the giant monopile structures.

With a loading capacity of 24,000 tons the vessel is the biggest rock installation vessel on the market.

Using precision techniques, the Nordnes will place the small rocks in a doughnut shape around the site of 77 of the foundation locations to reduce the likelihood of scour and protect the cables when they are installed.

Scour protection is needed for wind turbine foundations, as well as other types of marine structures, due to changes in water flow patterns that may cause a lowering of the seabed immediately surrounding them.

Dutch company Van Oord has been contracted to carry out the filter rock work.

In a second phase, larger rocks will be placed in the same pattern as 'armour' after all the foundations and cables have been installed. The contractor for this phase has not yet been decided.

Project director Rune Rønvik said the arrival of the Nordnes meant construction work had officially started on site.

“This work will prepare the site for the arrival of the first foundation for installation at the end of next month.

“Each foundation is made to individual specifications and will be between 50 and 55 metres long, weighing from 400 to 600 tonnes.”

The ship will be on site, between 10 and 14 miles off the coast, until March 18 when it will return to Norway to collect its second load of rocks.

It will arrive back on March 22, working until March 30. The final filter rock placement work will be completed around early June.

The wind farm is owned equally by Statoil and Statkraft through the joint venture company Scira Offshore Energy Limited. It is due to be operational by the end of 2011.