Denmark: All Foundations for Anholt Offshore Wind Farm Placed

The last of the 111 transition pieces has now been installed on monopile no. 111 in Anholt Offshore Wind Farm. This means that all foundations for Anholt Offshore Wind Farm are in place, and DONG Energy is well on its way in the construction of Denmark’s largest offshore wind farm. The next step is the installation of 111 wind turbines.

A foundation consists of a monopile, a long cylindrical steel tube driven into the seabed, and a transition piece that connects the monopile and the wind turbine tower.

Wind is welcome – sometimes 

After having completed the approximately 12-hour long sea journey from Aalborg, where they are produced, to the offshore wind farm, it takes about 7-8 hours to drive the monopiles, up to 460 tonnes of weight, into the seabed. The 170-tonne transition piece is then lifted into place and grouted on to the monopile. This exercise has now been performed for the 111th and last time by DONG Energy and MT Højgaard, the foundation supply and installation contractor.

”The wind and weather conditions in the Kattegat where the turbines are to be erected can be rough. This is of course a good thing when the turbines are spinning as it will result in large power production. However, during construction of the wind farm, we are not too fond of strong winds, and we’ve had to suspend installation of the foundations many times,” says Flemming Thomsen, Project Director, and he continues:

”That’s why I’m very pleased that we have managed to stay on schedule, and that we’ve now installed all 111 foundations. It’s an important milestone for the construction of Anholt Offshore Wind Farm and for the large team of employees and suppliers who have contributed to the project.”

5,000 boulders had to be moved 

It has not always been easy to prepare the seabed for driving of the monopiles as the seabed holds a lot of challenges. Particularly in the northern part of the wind farm there are areas with very soft seabed, while there are many large boulders in the seabed in the southern part of the wind farm.

DONG Energy has moved a total of 5,000 boulders that have now been placed in 30 artificial reefs.

”Before we started the work, we scanned all the turbine positions. We have considered all possibilities, and we have the tools in place that are necessary to handle the challenges that the different seabed characteristics pose. Now that we have completed the foundations, we can focus on the next important tasks to come. We’re well underway with installation of cables between the foundations, and then we will start installing turbines on the foundations,” says Flemming Thomsen.

Svanen installed the monopiles 

The installation vessel Svanen, which was originally built to construct large parts of the Great Belt bridge, has driven the foundations into the seabed. Svanen installed the first foundation on 31 December 2011 at 19.00, while the installation vessel Jumbo Javelin started installation of the transition pieces on 16 April, and this weekend it finished the last of the 111 foundations.

The 111 foundations have been manufactured in Aalborg by Bladt Industries and installed by MT Højgaard.

DONG Energy expects to start installing the turbines in the beginning of September.

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Offshore WIND staff, August 1, 2012; Image: DONG Energy