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.
1. Seabed preparation
Before arrival of the foundations, the seabed is prepared so as to provide the perfect position for the wind turbines.
Firstly 0.5 m of the seabed is removed using a backhoe dredger to provide a solid surface without large boulders. These dredgers have enough power to excavate the hard seabed, creating a perfectly horizontal surface.
To remove any irregularities resulting from the excavations, a layer of gravel will be installed using a cable crane on a pontoon, equipped with a claw.
The gravel bed is levelled further by another excavator on the same pontoon, equipped with a levelling tool specifically designed by Jan De Nul.
Once the gravel bed is installed and levelled, ‘Rambiz’, the heavy lift vessel owned by Scaldis in which Jan De Nul has shares, will lift the foundation off the pontoon. Next the vessel, using its anchoring system, will install the foundation within 30 cm of the theoretical position.
In addition to the positioning system of the ‘Rambiz’, the foundations will be equipped with a similar system, guaranteeing perfect positioning.
After the foundations have been installed, they will be ballasted to be able to withstand the powers of the sea and the wind turbine itself. In a preliminary phase the shaft of the foundation is filled with crude iron ore, which is cast via a funnel using a cable crane. In the next stage the other compartments are filled with crude iron ore and a layer of heavy quarrystone is placed on top.
To prevent the gravel layer from being washed away a quarrystone anti-scour layer out of rubble is installed around the foundations.
4. Installation wind turbine generator
The wind turbine generator which is installed on the foundation is a VESTAS V112 with a 112 m rotor diameter which produces 3 MW. Once the wind farm is fully operational it will be able to provide green power for 50,000 families.
The backhoe dredger ‘Jerommeke’ is equipped with a Liebherr 994 excavator. ‘Jerommeke’ can be fitted out with a bucket of 4,5 to 11 m³, depending on the water depth and the type of material to be dredged.
The enormous tear-out forces of ‘Jerommeke’ guarantee that also harder layers can be cut. ‘Jerommeke’ is used for the accurate installation of rock and iron ore, as well as for the precise trimming of slopes and/or trenches under water.
Dredging Today Staff, May 14, 2012; Image: Jan De Nul