Submersible Robot Buries Power Cables For Offshore Wind Farms 5.5 Metres Below Seabed

To the north of Maasvlakte, TenneT is working on the Net op Zee Hollandse Kust (zuid) project, which will link a number of new offshore wind farms to a substation onshore. This involves burying four submarine power cables in the floor of the North Sea. Along the first 10 km of this underwater connection, the power cables need to be buried at a depth of 5.5 m so they can safely traverse the busy shipping area of Rotterdam’s Maas Entrance.

The installation of the cables started on 22 September, when the first cable was pulled ashore. After successfully crossing the Maas Entrance, the submersible robot ‘Deep Dig-It’ is now en route to the recently installed Alpha jacket in the Hollandse Kust (zuid) wind area.

The Van Oord-Hellenic Cables consortium will install the first two cable links to the Alpha platform this year, with the remaining two cables – for the Beta platform – following in 2021. Preparations for the current phase were rounded off over the past week. The cable was pulled ashore via a controlled boring procedure to the site of the newly constructed transformer substation on Maasvlakte.

tennet-launch-deepdigit - Submersible robot buries power cables for offshore wind farms 5.5 metres below seabed
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The Van Oord-Hellenic Cables consortium will be extending this cable – with a total length of 42 km – from the substation to the platform in the Hollandse Kust (zuid) wind area. Especially for this purpose, Van Oord has developed Deep Dig-It: a gigantic, remote-controlled underwater trencher.

Deep Dig-It

Van Oord’s Deep Dig-It is what is known as a ‘Tracked Remotely Operated Vehicle’ (TROV). This unmanned submersible drives across the sea floor, creating a deep trench for the cables by liquefying the soil. In the same operation, it inserts the cables in the trench, which it then refills. What sets this TROV apart is its massive size and extraordinary power.

The Deep Dig-It weighs in at 125,000 kilograms, is over 17 metres long, well over 8 metres high and is 11 metres wide. And it is tremendously powerful – equivalent to not one, but two Leopard tanks (2,500 HP). This allows the trencher to lay cables into very hard soils. The Deep Dig-It is controlled from a remote location by Van Oord’s offshore installation vessel MPI Adventure, which has been fitted with a crane used to launch and retrieve the Deep Dig-It.

The Net op Zee Hollandse Kust (zuid) project

The Hollandse Kust (zuid) offshore wind farm is situated 22 km off the coast of the Dutch province of Zuid-Holland. Its two platforms are linked by submarine cables with the high-voltage substation ‘De Maasvlakte’ and the Randstad 380 kV Zuidring. The wind power will subsequently be distributed to end users throughout the country via the Netherlands’ national high-voltage grid.

tennet-deepdigit - Submersible robot buries power cables for offshore wind farms 5.5 metres below seabed
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In 2018, TenneT awarded the Van Oord-Hellenic Cables consortium the contract for the construction of a power connection between the coast and the Net op Zee Hollandse Kust (zuid) project. The submarine cables that are installed and buried by the consortium will actually traverse the entrance of Europe’s busiest freight hub: the port of Rotterdam.

After work on the 1,400 MW high-voltage connection has been rounded off in 2022, the wind farms will ultimately supply enough electric power to cover the annual consumption of 1.6 million households. As such, the new offshore wind farm will significantly contribute to the Dutch government’s target of realising a total of 3.5 GW of installed offshore capacity by 2023.

Offshore wind power until 2030

The first 1.4 GW of this total were realised earlier this year, when the contractors rounded off work on Borssele Alpha and Beta. The Hollandse Kust (zuid) Alpha & Beta will be followed by Hollandse Kust (noord). For this project, TenneT will be using five of the same type 700 MW transformer platform, each situated close to the wind farm, as well as five of the same type of 220kV cable connection for linking the wind farm with the coast.

This standardisation offers TenneT an efficient, swift and cost-effective procedure for realising these different projects. In the period until 2030, TenneT will be laying a further 6.1 GW of high-voltage connections for offshore wind farms across the Dutch section of the North Sea. TenneT has already completed 7.1 GW in the German section, and it plans to raise its offshore network capacity in Germany to approximately 17 GW by 2030.


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