Iemants and Jan de Nul to construct gravity bases

Dag Pike

Belgium based Jan de Nul Group has joined forces with Iemants, another Belgian company to construct, install, ballast and protect the gravity based foundations (GFBs) and deck structures for the Krieger Flaks offshore wind farm substations.

This contract is part of this 600MW offshore wind farm in the Danish Baltic Sea, and has been awarded as part of the Combined Grid Solution interconnector project. Two companies, and 50Hertz Transmission GmbH are responsible for the connector part of the project and awarded the contract to the joint venture between Iemants and Jan de Nul.

The wind farm comprises 2 sections, each of which will have its own substation. Kriegers Flak A, the western section, will have a total capacity of 200 MW whilst the eastern section, Kriegers Flak B, will have a total capacity of 400 MW. The Interconnector will link the two substations to the Danish and German power systems on shore and this is a project that is funded by the European Energy Programme for Recovery.

The Jan De Nul Group will be in charge of the design, construction, transport and installation of the reinforced concrete GBFs, with Iemants taking care of the detailed design, fabrication and transportation of the steel shafts and decks which will be placed on top of the GBFs. These steel structures will be fabricated at the Iemants’ facilities in Belgium.

Jan de Nul plans to construct the GBFs on a pontoon that will be based in the port of Zeebrugge in Belgium and when completed the pontoon will be towed to the Baltic Sea. Meanwhile at the installation site Jan De Nul’s backhoe dredger Gian Lorenzo Bernini and the fallpipe vessel Adhamar de Saint-Venant will excavate the foundation pit and install the gravel bed as the foundation for the gravity bases.

Once the pontoon with GBFs arrives at the installation site in Danish waters, it will be submerged to offload the structures which will then be ballasted down with water onto the gravel bed foundations. Once located onsite the GBFs will be permanently ballasted with rock and protected through scour installation placed by the Adhamar de Saint-Venant.

It is planned that these gravity based foundations will be installed on site by the end of 2017 and it is hoped that by 2022, the wind farm, which will be the largest in Danish waters, will start generating CO2-free electricity for approximately 600,000 households. is an independent public enterprise owned by the Danish state owns the Danish electricity and gas transmission system. 50Hertz Transmission GmbH is responsible for the management of the overall electrical system throughout several North German states and is responsible for the secure integration of renewable energy.

Diagram of gravity base ballasting taking place

Diagram of gravity base ballasting taking place