Denmark’s largest windfarm fast approaching completion

The Anholt offshore windfarm, which is being built by Dong Energy, lies between Djursland on the Danish mainland and the island of Anholt. Work first got underway on the project in 2010, when geotechnical investigations were carried out to determine the final layout of the windfarm, which although located in an area with excellent wind resources, has very difficult seabed conditions.

Construction work commenced in late 2011 and has involved a number of vessels. The Port of Grenaa was selected as the base for the project, during the construction phase and in the grid connection and operational phases of the project. Siemens, which has production facilities in Brande and Aalborg in Denmark, is supplying the wind turbines.

As a spokesperson for the project explained to OWJ, in recent months favourable weather conditions and sea state have helped speed up the rate at which turbine installation has taken place.

Of the 97 wind turbines installed by the end of April, the installation vessel Sea Power had installed 39 of them and Sea Worker had installed a total of 31. The installation vessels Sea Installer (a recent addition to A2SEA’s growing fleet of turbine installation vessels) and Sea Jack had installed 14 and 13 wind turbines, respectively, and had departed for other projects.

Jens Nybo Jensen, Dong Energy’s stakeholder relations advisor, explained that, having installed its 14th wind turbine at Anholt on 4 April, Sea Installer had departed for another of Dong Energy’s offshore windfarm projects.

“Even though Sea Installer has only just entered service it managed to install 14 wind turbines in the 40 days it was operational on the Anholt project,” he explained. The vessel completed the installation of its 14th wind turbine, A19, at 03.30 in the morning before heading for the West of Duddon Sands windfarm which is in the Irish Sea.

“Better weather conditions than had been experienced earlier in the project resulted in long consecutive working periods for the installation vessels,” Mr Jensen explained, noting that in a two week period over the end of March and early April, they succeeded in installing as many as 20 turbines, giving Dong confidence that the installation process would be completed and all turbines would be in place in May. All of the components for the remaining turbines had been supplied to Siemens’ pre-assembly site at Grenaa harbour by the end of April, by which time 59 wind turbines had also been connected to the grid, corresponding to a total capacity of 212.4 megawatts (MW), thus exceeding the capacity of the Horns Rev 2 offshore windfarm, until now the largest in Danish waters, at 209MW.

Sea Installer also demonstrated that it is technically possible to optimise the wind turbine installation by installing complete towers in a single lift, rather than the operation requiring two lifts as has been the case until now.

“Not only does complete tower installation mean a quicker installation project overall, it also means quicker commissioning of the wind turbines after installation,” Mr Jensen told OWJ. “Normally, when installing the tower parts in two lifting operations, it takes several working days to arrange lift and cable connections between the two parts; if the complete tower is installed in a single lift it means that much of the work that would otherwise be undertaken at sea can be undertaken ashore.” Interestingly, Sea Installer managed to install a complete wind turbine in less than seven hours. Ultimately, Sea Installer will be able to carry eight complete wind turbines of the size used on the Anholt project.

When the Danish Energy Agency granted Dong Energy a concession to construct Anholt, one of the conditions was that the company undergo third-party certification of the entire construction project in order to demonstrate that the windfarm was designed and constructed according to all of the applicable standards and would remain operational for its planned lifetime.

Project certification became effective at the beginning of 2013, after an intensive review completed in co-operation with Det Norske Veritas (DNV). The review included an assessment of design conditions (soil conditions, waves, wind), a review of foundation design, type approval of the turbine and approval of the manufacturing and installation processes.

Mr Jensen said that one of the most important tasks was approval of the design conditions for the foundations because the foundations on Anholt differ from the foundations used on most other projects in the North Sea as they have to be dimensioned to take into account the presence of ice and pack ice and, therefore, ice loads.

In addition, the complicated and very variable nature of the seabed in and around the Anholt project required a very high level of geophysical and geotechnical investigation. For that purpose, Dong Energy created a detailed geological model of the area.

Construction of the 88km² offshore windfarm also required careful consideration to be given to navigation by other vessels in the area. All mariners were notified of the restriction zone by regular notices to mariners, and whilst construction was ongoing the site was closed off for safety reasons, and unauthorised navigation was not allowed. This required a temporary safety zone with a radius of 500m to be established around the site and the site to be marked with buoys.

Even though turbine installation was expected to be completed in May, activity on the windfarm will continue through the summer of 2013 so the turbines can be commissioned and the remaining grid connection work completed. Windfarm service vessels will continue to transport large numbers of technicians to and from the windfarm to complete commissioning of the turbines and bring them into operation, one by one. In parallel with this activity, all of the turbines will be inspected after their first 500 hours of operation.

Cable protection on the windfarm is also due to be completed this summer, with all of the cables between the wind turbines and the substation buried to a depth of 1m in the seabed. However, at some positions, it has not been possible to bury the cables, either because the subsoil has been too hard or because of stones in the seabed, so to secure cable protection at these positions a layer of armour stone is being installed. OWJ