The Arkona offshore windfarm in Germany is one of a growing number of projects that are reaching key milestones – and being completed – ahead of schedule, detailed pre-construction surveys having played an important role
By January 2018, all 60 transition pieces had been installed on the Arkona offshore windfarm, with the transformer station due to be delivered this spring.
Construction of the Arkona project in the German sector of the Baltic is progressing at a fast pace. After all of the foundations were installed, attention turned to the transition pieces, all of which are now in place. Monopile foundations were selected for the project and were installed in water depths ranging from 23 m to 37 m.
The transition pieces, each weighing 400 tonnes, were transported from the port of Mukran to the construction site in the Q4 2017, thus completing the second major construction phase of the project.
The Arkona project, a joint venture between E.ON and Statoil, is 35 km northeast of the island of Rügen and will generate 385 MW of power (sufficient to supply around 400,000 households).
With installation of the transition pieces completed, attention is turning to the next phases of the project, including installation of the 6 MW Siemens Gamesa Renewable Energy turbines, and preparations for the next steps are also in full swing.
STX France is building the transformer station, which will be operated jointly by E.ON and Statoil and transmission grid operator 50Hertz. It is due to be transported from France, where it is being built, to the Baltic this spring. Once it has been installed, the turbines will be connected to it. A total of 75 km of submarine cabling required for the windfarm have already been delivered by Nexans in Hanover, Germany, to the Arkona base port at Muran. Production of the turbines is well advanced at Siemens Gamesa.
One of the reasons that construction of the Arkona windfarm is progressing faster than expected is detailed preconstruction surveys that identified potential hazards and characterised the nature of the seabed for individual turbines and their foundations.
This enabled all 60 foundations to be installed in just 10 weeks, which is an exceptional achievement, particularly given that the seabed in the area is particularly challenging due to the heterogeneous nature of the subsoil and possible boulders in it.
The company said that one of the keys to such rapid progress was a 3D survey of the subsoil prior to installation. The survey provided important data on the nature of the sediment at each of the sites where a foundation was due to be installed. The company also used boreholes to characterise the site – the first time this approach has been used for an offshore windfarm in German waters. The design of the steel foundations was individually adapted to the soil conditions.
As highlighted elsewhere in this issue, the Arkona project is also the first time that a company in the offshore wind industry has used an innovative corrosion protection technique to protect the foundations. The new coating process significantly reduces the environmental impact during operations compared to conventional methods.
The company made use of what it described as “three-dimensional seismic investigation” of the subsoil prior to installation of the monopiles. This provided windfarm-wide data that characterised the nature of the seabed at every point at which a foundation was due to be pile driven into the sediment. The company also used individual boreholes to characterise potentially difficult locations for foundations, this being the first time that such as technique had been used on a German project.
Final selection of the location for each of the turbines was determined on this basis, and the design of the foundations was individually adapted to the prevailing soil conditions.