Offshore Construction on Dutch Trailblazer to Start in June

Vattenfall plans to start the first offshore construction activities on the 1.5 GW Hollandse Kust Zuid (HKZ) wind farm in the Dutch North Sea in June.

This is when the first ships will sail out to deliver rocks for scour protection to the wind turbine locations. Rocks will be placed to protect the monopiles against waves and currents that can cause erosion of the soil around fixed structures in the seabed.

From July onwards, the first of the 140 monopile foundations without the transition pieces will be installed at the site.

”Our foundation installation vessel the ‘Seaway Strashnov’ – a 183 m long and 47 m wide heavy lift vessel – will load three complete sets of monopiles and secondary steel components from the quay side of our foundation manufacturer, situated on the Maasvlakte in Rotterdam,” Sven Mey, Construction Manager, said.

”The vessel will sail to the offshore site to install the monopiles and secondary steel parts, such as the internal airtight platform, external work platform and the boat landing.”

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The monopiles vary slightly in size and weight depending on the locations where they will be installed. The heaviest and largest monopile weights 955 tons and is 75 meters long while the lightest and shortest monopile weights 735 tons and is 62 meters long.

”Once the vessel arrives at the offshore site, it is positioned to the exact location. The crane of the vessel then lifts and upends the monopile into the water and lowers it until it reaches the seabed. Once the monopile is positioned correctly on the seabed, a hydraulic impact hammer is used to drive the pile into the soil to target depth,” Mey said.

The monopile installation will continue, with a winter break, into the summer of 2022.

Offshore Grid

Works on the offshore grid will run from March 2022 to April 2023.

”This grid consists of cables and offshore transformer platforms that are required to transport the electricity efficient from the wind turbines to the substation on land,” said May.

For Hollandse Kust Zuid, this network consists of two offshore transformer platforms, each with a base capacity of 700 MW, and two 220 kV export cables per platform which are 30-40 kilometres long. Grid operator TenneT will develop the offshore platforms and the connection to their onshore site at Maasvlakte. The cables connecting Vattenfall’s turbines to the TenneT platforms will be installed in 2022 and 2023.

”Unfortunately, we are not the first people to lay cables in the North Sea,” May said.

”For example, there are already many telephone and data cables in the area. We map these out carefully and ensure that we have appropriate distance and protective layers between the cables when they are crossing each other.”

In total, about 325 kilometers of cables will be laid in the seabed.

The Wind Turbines

The installation of the wind farm’s 140 Siemens Gamesa SG 11.0-200 DD turbines will start in April 2022. This activity will run until April 2023.

”What you may have noticed is that different activities take place side by side. This allows us to realize the project faster,” May said.

”You can also choose to do foundation installation, the installation of the cables and the turbines at different times. But then overall construction takes a lot longer. Working in parallel, it is key for the team to function smoothly. That means good communication, good collaboration with suppliers and a strong focus.”

Cadeler’s Wind Osprey will install the wind turbines. The operation starts at Sif’s Maasvlakte facility in the Port of Rotterdam, where the tower parts, the nacelle and the three blades are prepared and loaded onto the vessel and transported to the offshore site. In addition to weather conditions, which cannot be planned, the number of wind turbines also presents challenges, Vattenfall said.

”If we discover that we have forgotten something after we have installed the 100th turbine, we have to revisit all 140 locations. We have good procedures, so we hope to avoid moments like this. But we have to be prepared for it,” May said.

The First Electricity and the Commissioning

The first turbines of the Hollandse Kust Zuid wind farm are expected to start generating electricity in 2022. By mid-2023 all 140 wind turbines are planned to be operational and the wind farm fully commissioned.

Once commissioned, Hollandse Kust Zuid will become the largest operating offshore wind farm in the world. The project is also the first in the world to be built without the government subsidies.

Photo: Vattenfall/Siemens Gamesa/Illustration