Export cable installation resumes at Hollandse Kust Zuid

Van Oord’s underwater robot Deep Dig-It has returned to the Hollandse Kust Zuid wind farm to connect the HKZ Beta offshore platform to the mainland.

TenneT is developing the Hollandse Kust Zuid offshore grid for the connection of the 1.5 GW wind farm.

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 for the HKZ Alpha and now the remaining cables will follow.

On Tuesday, 3 August, the Deep Dig-It crossed the Rotterdam Maasmond with the third cable and is now on its way to the recently installed HKZ Beta jacket, TenneT said.

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 HKZ Beta jacket.

Van Oord developed the Deep Dig-It, a gigantic, remote-controlled underwater trencher, especially for this project.

The 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. The installation of the 34 kilometres of cables 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.

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.