On behalf of TenneT, the undercarriage (jacket) for the Hollandse Kust (west Beta) transformer platform was successfully placed on the seabed off the North Holland coast during the past Pentecost weekend. The ready-made superstructure (topside) will be installed next year. Wind farm operator RWE/OranjeWind can then connect the wind farm, which will be built at sea in a few years, to this ‘socket at sea’.
Last Thursday, the jacket was sailed on a floating pontoon from the port of Vlissingen to its final destination about 53 kilometers off the coast of Egmond aan Zee. The steel structure, 49 meters high and weighing more than 2,100 tonnes, was lifted from the floating pontoon by the lifting vessel Sleipnir (Heerema Marine Contractors) last weekend and placed on the seabed.
Anchored
The jacket is firmly anchored at the four corner points using piles driven more than 50 meters into the seabed. These piles are placed in large tubes of 10 meters that are welded to the feet of the chassis. By filling the space between the piles and the pipes with cement (grout) you create a connection that ensures that the platform can withstand even the heaviest storm on the North Sea.
Stones
Prior to installation, the soil conditions on site were carefully examined. We also carefully checked whether there were any explosives from the Second World War. A large bed of stones was then poured for a solid and level foundation. The stones also ensure that the soil under the jacket is not washed away by seawater currents. With regular maintenance, the construction has a minimum lifespan of up to 40 years.
Measure
Special equipment has been installed on the jacket to measure the ‘skewness’ after installation. Certain tolerances have been agreed in advance here, because it is important that the topside containing the transformers is level next year. This way, after piling, you can see exactly how much metal needs to be ‘cut’ from the top of the jacket to achieve this. For the safety of shipping, temporary navigation lights are placed on the jacket.
Installation of sea cables
From the beach at Velsen, contractor NBOS (Boskalis / Orient Cable) is currently installing the sea cables that will connect the socket to the electricity grid in 2025. These cables (220 kilovolts) are laid at a safe depth and connected to the land cables behind the dunes. Through this connection, the electricity ultimately ends up in the high-voltage grid at the correct voltage (380 kilovolts) via the transformer station in Wijk aan Zee.
Socket at sea
TenneT’s socket is intended for the wind farm that RWE/OranjeWind will have built at sea in the coming years. The jacket (undercarriage) and the topside (superstructure) are built by the contractor combination Equans / Smulders. The grid operator will soon bring the green energy ashore at Velsen via the 700 megawatt grid connection. This will make approximately 3.3 terawatt hours of sustainable electricity available. This approximately corresponds to the annual consumption of 1 million households.
Stay informed
Do you find it interesting to closely follow the connection of the offshore wind farms and the activities? Then download the TenneT BouwApp for free from the App Store (iOS) or Google Play (Android). This way you will continuously receive up-to-date information about the work.
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