Van Oord Deploys Giant Trencher to Bury Offshore Wind Cables in North Sea

Dutch offshore installation contractor Van Oord has deployed a large remotely operated trencher called Deep Dig-It off the Dutch coast to bury offshore wind farm cables 5,5 meters deep into the seabed.

The trencher has been deployed as part of the cable installation project for the Dutch grid operator TenneT's Hollandse Kust (zuid) offshore grid construction to connect new offshore wind farms, which will see four cables buried in the North Sea.

For the first ten kilometres of the cable route at sea, the cables will have to be buried more than 5,5 meters into the seabed to safely cross the busy shipping route Rotterdam Maasmond.

"The installation of the cables started on September 22nd by pulling the cable ashore. The Deep Dig-It trencher crossed the Rotterdam Maasmond  successfully and is on its way to the recently installed offshore Alpha jacket in the Hollandse Kust (zuid) wind farm area," Tennet said.

Overall, the Van Oord/Hellenic Cable Consortium will be installing 4 subsea 220 kV AC cables that will connect 2 offshore platforms to the onshore electricity grid. The first 2 cables to the Alpha platform will be  installed this year. The other 2 cables, meant for the Beta platform, will follow in 2021.

The trencher

Van Oord's Deep Dig-It, dubbed one of the largest and most powerful of its kind, drives unmanned over the seabed and creates a deep trench  for the cables by liquefying the seabed. At the same time, the trencher inserts the cables into the trench and seals them again into the seabed. 

The trencher weighs 125,000 kilos, is over 17 meters long, more than 8 meters high, and 11 meters wide. The powerfull trencher is controlled from Van Oord's offshore installation vessel MPI Adventure, which is also equipped with a crane to launch and take out the Deep Dig-It. 

The Hollandse Kust (zuid) offshore wind farm site is located 22 kilometres off the coast of the Dutch province of Zuid-Holland. The sea cables will connect the two offshore substation platforms with the Maasvlakte high-voltage substation and the Randstad 380 kV South ring. The electricity will then be fed into the national high-voltage grid to the electricity consumers in the Netherlands.

Offshore wind developer Vattenfall sanctioned the development of the 1,500 MW offshore wind farm project in June. When fully operational in 2023, Hollandse Kust Zuid will provide a third of the total installed offshore wind capacity in the Netherlands, and will have electricity output equivalent to the annual consumption of over two million Dutch households. 

The turbines will be supplied by Siemens Gamesa. The wind farm will consist of 140 turbines of 11 MW capacity.

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