The Kårehamn windfarm, which is due to become operational in 2013, will have a capacity of 48 megawatt (MW) and is being built at a cost of around €120 million. The project will make use of 3MW V112 turbines and of concrete gravity base foundations which will be built not in Sweden, or even in Scandinavia, but in Zeebrugge in Belgium, the contract for their design, construction and installation having been awarded to Jan De Nul in 2011.

However, what really makes the project unique is that rather than building the foundations in the conventional manner, they are being prefabricated on pontoons, which will then be towed to the Baltic.

The foundations are being built on pontoons in the harbour at Zeebrugge in four phases: the foundation base plate; the walls for the ballasting tanks; middle shaft; and ice cone. The components range in size from 15-25m, with the heaviest elements weighing some 1,950 tonnes. Once completed, the pontoons and foundations will be towed to the installation locations off the Swedish coast and installed on the seabed, which is being specially prepared by Jan de Nul.

The water depths at the site range from 8m to 21m, so the height of the cylindrical part of the foundations is not the same for each of the 16 units. As Jan de Nul explained, what this has meant in practice is that each foundation has its own specific design, the largest having a length of 24.5m and a base-plate diameter of 18m. Gravity base foundations were selected for the project because soil conditions would not allow the use of piles.

Apart from the varying water depth and the fact that piling was out of the question, E.ON also had another issue to contend with, given the climate in the Baltic in winter – namely, ice. To make sure the foundations would be able to withstand the force of ice impact, the foundations also needed to incorporate an ice cone in the form of slanting sides, which forces ice moving towards the foundations to crack and submerge. The ice cone is also fitted with ladders and rails for access.

The first phase of construction of the foundations sees the base plate produced, after first ensuring that the ‘super pontoons’ selected by Jan de Nul for the job are in a completely level position. This is important, because if they are not produced totally upright, lifting the foundations from the pontoons during installation offshore could become extremely problematic.

The 0.5m-thick base plate is reinforced and cast in concrete, after which the rebars are connected. The location for the construction of the foundations and for the pontoons on which they are being built was selected because it is situated behind a series of locks, and is not subject to tidal action.

Ensuring that there was sufficient water depth was also important because the draught of the pontoons increases during the construction process. To keep the deck of the pontoons perfectly horizontal, water is regularly pumped in and out of their tanks.

Once the base plate is finished, the inner and outer walls, the tower and the ice cone are cast using a quartet of tower cranes which pour the concrete into the structure. In total around 10,000m³ of concrete and 3.5 tonnes of steel are required for each unit. The largest of the tower cranes can lift 15 tonnes of material to a height of 40m. The crane is also on rails, allowing it to reach the outermost parts of the pontoons. The concrete is transported from Bruges to Zeebrugge in trucks, which pour the concrete into pump trucks so the mixture can be pumped into the foundation formwork.

Once the foundations have been completed, powerful tugs are used to tow them to Sweden, where they will be installed. The 1,500km trip, through the Kattegat, takes approximately five days, depending on the weather conditions.

Prior to the arrival of the foundations offshore Sweden, the seabed needs to be prepared in order to provide a suitable position for the wind turbines to be installed. Half a metre of seabed material is removed using a backhoe dredger, to provide a solid, horizontal surface without large boulders. To remove any remaining irregularities left after excavation, a layer of gravel is installed using a pontoon equipped with a bucket. This gravel bed is levelled by another excavator on the same pontoon, using a levelling tool specifically designed by Jan De Nul.

The process of preparing the seabed is undertaken using Jan De Nul’s backhoe dredger Jerommeke, which is equipped with a Liebherr 994 excavator. Jerommeke can also be fitted with 4.5-11m3 bucket, depending on the water depth and the type of material to be dredged. The enormous ‘tear out’ force produced by the backhoe ensures that even the hardest layers of material can be excavated.

Once the gravel bed has been levelled, Rambiz – a heavy lift vessel owned by Scaldis, a well-known heavy lift contractor in which Jan De Nul has a share – lifts the foundation from the pontoon. Rambiz was originally designed for the construction of the Vasco Da Gama bridge over the River Tagus in Lisbon, Portugal. The unit has already been used on a number of windfarm projects, such as the installation of the gravity base foundations and the turbines on the Thornton Bank offshore windfarm in Belgium, and elsewhere in Europe. The highly specialised vessel has a catamaran-type hullform built using two pontoons – Ram and Bizon. It is fitted with two fixed cranes, and has a maximum lifting capacity of 4,000 tonnes.

Having anchored itself in position, Scaldis then lower the foundation to within 30cm of its designated position. During this process, both Rambiz and the foundations are fitted with a positioning system, in order to ensure that they are placed as accurately as possible.

Once the foundations have been installed, they are ballasted in order to be able to withstand the forces acting on them from the sea, and the wind turbine itself. The shaft of the foundation is first filled with crude iron ore, which is cast via a funnel using a cable crane. In the next stage of the process, the other compartments in the foundation are filled and a layer of heavy quarry stone placed on top. To prevent the gravel layer from being washed away by currents, an ‘anti-scour’ layer of rubble is put in place around the foundations.

The wind turbines to be installed on the foundations have a rotor diameter of 112m. Once the windfarm is fully operational it will be able to provide green power for 50,000 househgolds. Jan de Nul’s role in the project is due to be completed in the third quarter of 2012.