North of the Maasvlakte, TenneT is developing the Hollandse Kust (zuid) offshore grid for the connection of new wind farms. Four offshore cables will be buried in the North Sea bed. For the first ten kilometres of the offshore cable route, these cables will be sunk 5.5 metres into the seabed to cross the busy Rotterdam “Maasmond”. The first two cables were installed in September 2020 and now the remaining cables will follow. Tuesday the underwater robot ‘Deep Dig-It’ has successfully crossed the Rotterdam Maasmond with the third cable and is now on its way to the recently installed Beta jacket in the Hollandse Kust (zuid) wind area.

Preparations completed
The Van Oord – Hellenic Cables consortium is installing the two cables to the Beta platform. The interlinking cable, which connects the two offshore platforms, was already successfully installed at the Alpha and Beta jackets. The preparatory work was completed during the past few days. The cable that will be installed to the Beta jacket has been pulled ashore through the jackets to the newly built onshore transformer station on the Maasvlakte. Now, the consortium combination will lay the cable with a total length of 34 kilometres to the wind area Hollandse Kust (zuid). Van Oord developed the Deep Dig-It, a gigantic, remote-controlled underwater trencher, especially for this project.

Cable installation
Van Oord’s Deep Dig-It is a Tracked Remotely Operated Vehicle (TROV) that travels on the seabed unmanned, making a deep trench for the cables by liquefying the seabed. At the same time, this underwater trencher lays the cables in the trench and closes it. The Beta cables weigh approximately 100 kilograms per metre and consist of three cores. These three cores are made of aluminium and the cable has a diameter of 30 centimetres. Both cables were loaded onto spools on the cable vessel “Living Stone”. This cable vessel runs on liquid gas (LNG), which reduces emissions. The installation of the 34 kilometres of cable route will take about four weeks and is expected to be completed by mid-September. At the end of this year, the topside, the transformer station itself, will be lifted onto the Alpha jacket. The topside of the Beta jacket is expected to follow early next year. Once the platforms 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 to the high voltage substation on land and the wind farm is expected to be fully operational in 2023.

Hollandse Kust (zuid)
The grid connection for Hollandse Kust (zuid) is located 22 kilometres off the coast of the Dutch province of South Holland. It consists of two transformer platforms at sea, each with two 220 kV alternating current cables. The sustainably generated electricity is transported to land via these cables. TenneT has built a new high voltage substation and is expanding the high-voltage ‘Maasvlakte 380 kV’ substation. From this high-voltage substation, offshore wind energy will be fed into TenneT’s Randstad 380 kV South ring. The wind energy goes to the electricity consumers in the country via the national high-voltage grid.

Connecting offshore wind energy up to 2030
In the period up to and including 2023, TenneT will install 3.5 GW of offshore grid connections for wind farms at sea. For these wind farms, TenneT will use the same type of 700 MW transformer platform five times, located close to a wind farm, and the same type of 220kV cable connection to the coast five times. This standardisation will enable TenneT to build these projects more efficiently, faster and cheaper. According to the follow-up Wind at Sea Roadmap, a further 6.1 GW of offshore wind farms will be developed between 2024 and 2030. This concerns the wind farms in the wind energy areas Hollandse Kust (west), TenneT is again using three standardised 700 MW platforms (2.1 GW) for the first two areas. For the efficient connection of the wind energy areas further out in the North Sea, such as IJmuiden Ver (4 GW), other techniques are needed based on direct current instead of alternating current. At least two offshore connections of 2 GW each will be developed for IJmuiden Ver. In addition, a third 2 GW connection is being studied for IJmuiden Ver, in parallel with the other IJmuiden Ver projects.

Offshore grid connections Germany
In the German part of the North Sea, TenneT alone exceeded the German government’s 2020 target already a year in advance and operates today 7.1 GW. Until 2030 TenneT’s offshore grid connection capacity in Germany will be increased to approximately 17 GW.

Image source: Courtesy of TenneT

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