During 2021, the 600MW Kriegers Flak offshore wind farm, in the Danish waters of the Baltic Sea, will commence the supply of electricity to 600,000 families. Grid Operators, Danish Energinet and German 50 Hertz, will connect the Kriegers Flak OWF with the German Baltic 2 OWF, thereby creating the first offshore power grid in the world to combine two wind power installations with the possibility of supplying electricity to either or both, Denmark or Germany. This part of the project is co-financed by the EU. The Kriegers Flak wind farm consists of two sites, each with its own substation. Jan De Nul was responsible for the foundations supporting both these substations.
Building on water
Project Manager Christopher Derycke explains: “The foundations are gravity-based foundations. The weight of the foundation ensures that it is kept on its place.” Jan De Nul designs these foundations entirely in-house. “In fact, such foundation consists of two huge concrete blocks, a square and a rectangular one. Both foundations have a concrete shaft and on top of that a steel conduit and the deck on which the substation will rest. In these blocks, recesses are made through which the cables connecting the substation with the mainland are led.” For the steel component, Jan De Nul set up a joint venture with steel construction company Iemants.
The two concrete structures have been built in the port of Ostend and then towed to Denmark on a submersible barge as complete foundations. The construction works started in June 2017 after a design period of only 6 months.
Structural Engineering Manager Tom Vermeersch was responsible for the design and, in the first half of 2017, headed a team of young engineers and draughtsmen. “The design is a technical masterpiece. Starting from scratch we had only six months to develop foundations that would be able to withstand the forces of water and wind and, at the same time, carry the weight of the substations.
"These are the largest foundations ever built by our company.”
Jan De Nul completed the construction of the concrete structures in November, with the steel upper section ready at the end of December. A tugboat transported the barge with the substation foundations from Ostend to Denmark. Christopher again: “For these towing works, we depended on the weather. In the end, we were able to leave on 8 January 2018 and on 17 January, we arrived in the port of Køge.”
From then on, Køge became the team’s operational base. “We prepared everything for carrying out the installation works. Still, again we were fully dependent of the weather: we could only install the structures if the height of the waves did not exceed 1 metre. When you’re in open sea, you know you have to be lucky.”
The foundations each weigh as much as the Eiffel Tower and are installed at a depth of 18 and 22 metre, with a very high level of accuracy. A tugboat brought the barge to the exact location at sea. The barge itself could be gradually lowered into the…
The weather. It's a phenomenon everyone talks about. But everyone also knows that when you’re waiting for good weather, it can take a long time and more so during the winter. Christopher knows how it feels: “For transporting the foundations,…
The construction hasn’t been entirely without challenges. Koen Van Regenmortel, Civil Works Project Manager, supervised the construction works. “We did everything in our power, and more, to speed up the construction and complete the installation…