With the help of the crane vessel Thialf, the superstructure (topside) for the offshore transformer platform Hollandse Kust (west Alpha) has been successfully placed on the undercarriage (jacket). This new milestone for the connection of offshore wind took place last night about 50 kilometres off the coast of Egmond aan Zee. From 2026, grid operator TenneT will bring sustainable wind energy onshore via this ‘socket’.

The ready-made topside left the port of Antwerp on a floating pontoon on Monday. The structure was towed via the Scheldt to Dutch waters to enter the North Sea at Vlissingen. The topside was built in Belgium by the contractor combination Equans/Smulders, and almost two years after the first steel cut, the large steel box (over 3,600 tons) was ready for the sail out this week.


Since last year, the chassis has been firmly anchored to the seabed off the coast of Egmond aan Zee. For the installation of the superstructure, the crane vessel Thialf (Heerema Marine Contractors) set sail for the jacket from the Norwegian port of Stavanger at the same time as the topside. After both vessels met at sea, the pontoon moored against the Thialf. On board the crane vessel, Matthijs Knollenburg, as construction manager on behalf of TenneT, closely followed the work.

Ice cream cones

Knollenburg: “After loosening, the topside was lifted into the air by the crane vessel. The pontoon was then towed away, after which the Thialf had to sail another 500 metres towards the jacket. So-called cones are attached to the jacket. You can compare these with upside-down ice cream cones, with which we were able to lower the topside into exactly the right place last night without too much measuring.”

Hotel ship

Now that the installation has been completed, a hotel ship will be moored next to the platform in the short term. Knollenburg: “After this jack-up barge (lift-lift vessel, ed.) has extended its legs and hangs above the waves like a platform, a walkway is hung between the ship and the platform. A team of mechanics and technicians will then stay on the ship in the coming months to weld the topside to the jacket and ensure the commissioning of the installation.”


Earlier this year, the two sea cables, with which the green energy from the wind farm will soon come ashore, were already installed by Jan de Nul Group. Knollenburg: “These cables also provided the fibre optic connection with which everything on the unmanned platform can soon be controlled from land. Inside the large steel box are the two power transformers that increase the voltage level from 66 kilovolts to 220 kilovolts for efficient transport of the power to land. In addition, there is also more than 130 kilometres of cable in the topside to connect all systems.”

Behind the dunes

The sea cables in the seabed come ashore via the beach of Heemskerk/Wijk aan Zee. Behind the dunes, these are connected to the cables that feed the power into TenneT’s high-voltage grid about ten kilometres further along the A9 in Beverwijk. The ‘socket at sea’ for the wind farm will be ready before the end of this year.


Ecowende is building the wind farm. The wind farm will be approximately 53 kilometres from the Dutch coast, near IJmuiden. With an installed capacity of approximately 760 MW, we can green approximately 3% of the current Dutch electricity demand. The plan is to put the wind farm into operation in 2026. For more information, see: https://ecowende.nl

Source: https://www.tennet.eu/
Image source: Courtesy of TenneT

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