Sweden: Jan De Nul Lands Kårehamn Offshore Wind Farm Contract

In the Swedish part of the Baltic Sea, in close vicinity of the city of Kårehamn, E.ON has ordered the construction of a wind farm consisting of 16 wind turbines. The contract for the design, construction and installation of the 16 foundations was awarded to Jan De Nul in 2011.

These foundations are constructed on 2 pontoons in the back harbour of Zeebrugge in 4 phases: foundation base plate, the walls of the ballasting tanks, middle shaft and ice cone. These parts have heights ranging from 15 to 25 m. The heaviest element weighs 1950 tons. Once completed, the pontoons with the foundations will be tugged to the target locations in Sweden and installed on the prepared seabed. Design, construction and installation are executed by Jan De Nul Group, who has great experience in combining these type of works. The fact that the heavy foundation elements are constructed on the pontoons makes this project so unique.

 DESIGN BY JAN DE NUL

 1. Different dimensions

As the water depths range from 8 to 21m, the height of the cylindrical part is constantly changed, resulting in a specific design for each foundation. The largest foundation has a length of 24.5 m and a base plate diameter of 18 m.

 2. Gravity based foundations

Because of soil conditions using piles was not possible, therefore it was decided to keep the concrete foundation in place by extra ballasting. This is referred to as a ‘gravity based foundation’.

 3. Ice cone

To make sure that the foundations would be able to withstand the force of colliding and cracking ice, an ice cone was implemented in the design, i.e. a slanted side which forces the ice to be submerged causing it to crack. The ice cone is fitted with ladders, rails and other accessories.

The same type of wind turbine will be installed in front of the Belgian coast for the Northwind wind farm.

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Offshore WIND staff, May 14, 2012; Image: jandenul