Deep Dig-It buries cables 5.5 metres in the seabed for Dutch wind farm

TenneT is building the Hollandse Kust (zuid) offshore grid to connect new offshore wind farms. The connecting cables will have to be buried more than 5.5 metres into the seabed to cross the busy shipping route Rotterdam Maasmond. Installation has now started using Van Oord’s Deep Dig-It trencher.

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 installation of the cables started on September 22nd by pulling the cable ashore. The cable was pulled ashore via a direct drill to the new transformer substation that is being built on the Maasvlakte.

From this point, the cable with a total length of 42 kilometres to the wind zone Hollandse Kust (zuid) will be laid by the cable comnsortium. For this purpose, Van Oord has specially developed the Deep Dig-It, a large remote-controlled trencher. The Deep Dig-It trencher has now crossed the Rotterdam Maasmond and is on its way to the recently installed offshore Alpha jacket in the Hollandse Kust (zuid) wind farm area.

Deep Dig-It

Van Oord’s Deep Dig-It is a so-called “Tracked Remotely Operated Vehicle” (TROV) that 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. This new trencher is one of the largest and most powerful of its kind. The trencher weighs 125,000 kilos, is over 17 metres long, more than 8 metres high, and 11 metres wide.

The Deep Dig-It is very powerful, comparable to no less than two Leopard tanks (2500 hp) and can bury cables more than 5.5 metres deep in very hard soil. The Deep Dig-It is controlled at Van Oord’s offshore installation vessel MPI Adventure, which is also equipped with a crane to launch and take out the Deep Dig-It.

Hollandse Kust (zuid)

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

In 2018, TenneT commissioned the Van Oord-Hellenic Cables consortium to build the offshore connection for the net op zee Hollandse Kust (zuid) project. The consortium will install and bury the sea cables that cross the entrance to Europe’s busiest freight port, the port of Rotterdam. The 1400-MW grid connection will be completed in 2022 and will eventually provide electricity for 1.6 million households.

The offshore wind farm contributes significantly to the Dutch government’s objective of having a total of 3.5 GW installed offshore wind energy capacity by 2023. With the commissioning of Borssele Alpha and Beta this year, the first 1.4 GW has been realized.

Author: Mariska Buitendijk

Mariska Buitendijk is one of SWZ|Maritime's journalists as well as the magazine's copy editor.