Dutch offshore installation company Heerema Marine Contractors has installed the 2,852-tonne jacket for TenneT’s offshore transformer platform for Vattenfall’s  Hollandse Kust (zuid) offshore wind farm in the Dutch sector of the North Sea.

This is the second jacket of TenneT’s offshore transformer platform for the wind farm. Located some 22 kilometers off the coast of The Hague, the jacket was placed on the bottom of the North Sea by Heerema’s Sleipnir vessel, on behalf of main contractor Petrofac. 

The jacket, on which the transformer platform will be placed had left the fabrication yard end of May in the United Arab Emirates, where it was built. The connection for the wind farms will be ready for use in 2022.

The jacket is 50 meters long, 34 meters wide, and 44 meters high and is anchored to the seabed by six piles, each weighing 162.5 tonnes. The 57.7 meters long piles were drilled 46.5 meters into the seabed. 

The jacket was then attached to the piles. Together, this forms the foundation for one of the two transformer stations for the wind farm Hollandse Kust (zuid). The transformer platforms will have a capacity of 700MW each. The first jacket for the Hollande Kust (zuid) wind farm was installed in the North Sea in September last year. The wind farm is developed by Vattenfall, in co-ownership with BASF. 

Offshore construction of the wind farm will start in July 2021. Once fully operational, the wind farm will be the largest offshore wind farm in the world with 140 wind turbines and a total installed capacity of 1.5 Gigawatt, Vattenfall has said.



Back to the transformer platform, the superstructure will be lifted onto the first jacket at the end of this year. The topside of the second jacket is expected to follow early next year. Once the topsides have been lifted onto the jacket, the last systems will be connected, tested, and put into operation. The cables of the wind turbines will then be connected and the wind farm is expected to be fully operational in 2023.

The grid connection for Hollandse Kust (zuid) is located 22 kilometers off the coast of the Dutch province of South Holland. It consists of two transformer platforms at sea, each with two 220kV alternating current cables. The electricity is transported to land via these cables. 

TenneT has built a new onshore transformer station there and is expanding the high-voltage ‘Maasvlakte 380kV’ substation. From this high-voltage substation, offshore wind-generated electricity will be fed into TenneT’s Randstad 380kV South ring. 

“The wind energy goes to the electricity consumers in the country via the national high-voltage grid,” Tennet said.

“TenneT is realising 3.5GW of offshore grid connections for wind farms at sea until 2023. With the completion of Borssele Alpha and Beta, the first 1,4GW was realized in 2020. This will be followed by Hollandse Kust (zuid) Alpha and Beta and Hollandse Kust (Noord),” Tennet said.

For these wind farms, TenneT uses the same type of 700MW transformer platform five times, located close to a wind farm, and the same type of 220kV cable connection to the coast.
 Credit: Tennet

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