Coordination of the installation work for the world’s largest offshore converter platform, rock placement, transports by Dockwise and Fairmount, surveying, anchoring, ballasting: orchestrated by the Offshore Energy division various business units put an unmistakable Boskalis stamp’ on the installation of the Dolwin Beta platform off the German coast.
The DolWin Beta HVDC platform is a huge converter weighing 15,000 tons. It is the size of a soccer pitch and stands 90 meters high. With a power transmission capacity of more than 900 MW the platform can supply enough energy to power a large city. The platform converts the electricity generated by various offshore wind farms from alternating current (AC) into high-voltage direct current (HVDC), which is then transmitted to the mainland via export cables. The platform, which was constructed as a floating, semi-submersible object, was installed at a project site 45 kilometers off the coast of northern Germany, in a water depth of 30 meters. The platform stands on six columns, which are supported by two huge pontoons. During the installation these columns were ballasted using rock. Boskalis was responsible for the execution of a wide range of activities on behalf of Aibel, ABB and end client TenneT.
Flexible and versatile
“From a logistical and technical point of view this was an exceptionally complex and demanding project. For us it was an excellent opportunity to demonstrate just how flexible and versatile Boskalis is,” said Dirkjan van den Boom, Regional Manager Subsea Contracting at Boskalis. Boskalis’ involvement started in 2014, when five tugs belonging to Boskalis joint venture Smit Lamnalco towed the platform to a location 38 kilometers off the coast of Dubai. There the platform was loaded aboard the semi-submersible heavy transport vessel Dockwise Mighty Servant 1 for transportation to Haugesund in Norway, where its construction was completed. Round about the same time the fallpipe vessel Rockpiper carried out rock placement work at the project site in order to prepare the seabed for the installation of the platform. Once a team of engineers had made detailed calculations, a 15-strong project team started work on the next stage in the summer of 2015. “The first stage of the project execution took place in August and comprised the tow-out using Anchor Handling Tugs (AHTs),” explained Van den Boom. “This involved the transport of the platform from Norway to the project site by the heavy AHTs Fairmount Expedition and Fairmount Sherpa. At the same time the AHTs Union Sovereign and Fairmount Alpine were deployed to lay the anchor pattern on the seabed at the project site.”
With great speed and success
After the platform had arrived at the site four of our AHTs were deployed to keep it in position on the seabed as the supporting columns or legs were filled with water. This part of the operation was completed very quickly and successfully.” This was immediately followed by the execution of the next stage, which involved the ballasting of the platform’s legs. To enable the structure to withstand heavy storms the legs were filled with 60,000 tons of rock. “For this we deployed our fallpipe vessel Rockpiper, equipped with a ballasting module developed by Boskalis,” said Van den Boom. The rock sourced from Norway was mixed with water on the Rockpiper and this mixture was then pumped into the legs. Next, the fallpipe vessel Seahorse was deployed to deposit another 10,000 tons of rock to prevent erosion of the seabed around the legs (scour protection). “We opted for the Seahorse because this vessel is equipped with a sloping fallpipe installation, enabling precision placement of the rock around the platform,” explained Van den Boom.
The operation concluded with the installation of extra scour protection around the platform’s bell mouths, the openings on either side of the bottom of the platform that allow for connecting the infield and export cables. A total of 22 concrete mattresses measuring 6 by 3 meters were installed to protect these connections. The mattresses were designed and produced in Scotland specifically for this purpose. “To avoid damage to the bell mouths the mattresses had to be positioned with great precision,” explained Van den Boom. “Following extensive tests we decided to perform this task using one of our Remotely Operated Vehicles, operated from the Diving Support Vessel Protea. This enabled us to perform this part of the project under our own management using our own equipment. This project gave us the opportunity to prove that we can offer literally everything in-house: that we really are a one-stop shop contractor,” concluded Van den Boom.
Click here to watch the project video of the DolWin2 project.