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Offshore Konverterstation

Horizontal drilling method

Nature conservation and dike security through state-of-the-art drilling technology

TenneT uses state-of-the-art technology to connect the offshore wind farms in order to keep the impact on nature to a minimum. In particularly sensitive ecosystems, such as the Wadden Sea, where nature conservation imposes strict requirements and intervention in nature is to be avoided, TenneT uses the so-called Horizontal Directional Drilling (HDD) method. The advantage of this method: trenchless and underground cable laying and at the same time an extremely environmentally friendly and gentle laying technique, as earthworks can be kept to a minimum. Moreover, the dike safety is not impaired by this procedure. In concrete terms, this works as follows: A special device is used to drill an underground channel into which an empty cable protection tube is inserted. A power cable can then later be inserted into this cable protection tube. Even flowing waters, roads or rails can be crossed under with this method. The boreholes can be over a thousand metres long and, thanks to innovative technology, can be precisely controlled down to centimetres. The horizontal directional drilling method is used in all offshore grid connections, for example for the landing of the grid connections to the mainland.

Efficient cable bundling on Norderney

The majority of offshore grid connection systems in the western North Sea run across the island of Norderney. The main reason for crossing the island of Norderney instead of passing the routes for the grid connections: the strong currents through low and high tide. These have such a strong effect on the seabed between the East Frisian islands that safe cable laying is technically impossible here. Before TenneT received the legal mandate in 2006 to connect offshore wind farms in the North Sea to the power grid, the wind farm operators planned possible routes for the grid connections at sea themselves. For the offshore wind farms planned off the East Frisian coast at that time, they had planned a total of 14 individual routes, all of which would have crossed the island of Norderney. This solution would not have been technically feasible and would have entailed massive interventions in nature and the environment. When TenneT took over the task, it was decided to bundle the planned routes across the island of Norderney. In order to avoid having to run each route individually across the island, TenneT therefore completed a one-and-a-half kilometer long intake pipe structure under the island of Norderney from summer 2007 to spring 2008. This structure provides space for several cables from various grid connection projects that can be installed in the empty conduit structure without time-consuming earthworks. The big advantage of this procedure: With the exception of the earthworks for the horizontal boreholes for underpassing the beach, the protective dunes and the dike on Norderney and on land in Hilgenriedersiel, no more digging was necessary on Norderney. The cable for connecting alpha ventus was successfully implemented for the first time at the end of May 2008. The cables from the BorWin1, BorWin2, DolWin1, and DolWin2 grid connection projects also run on their way from See Land via the island of Norderney and are bundled in the empty conduit structure. However, due to the large number of cables, the capacities of the empty conduit structure have now been exhausted.

Horizontal drilling for future offshore grid connection projects

The expansion of offshore wind energy in the North Sea is also to be driven forward in the future. This will be accompanied by the construction of offshore grid connections, some of which will run via Norderney for geographical and technical reasons. To date, horizontal drilling has been carried out on Norderney at Grohdepolder and at the oasis. In the area in between, the cables were bundled in the empty conduit structure. However, since the capacities of this empty conduit structure have now been exhausted and TenneT wants to avoid costly work for another empty conduit in the national park, TenneT was drilling horizontal wells for offshore grid connections from the centre of Norderney in a

southerly and northern direction. The entry points for the over 1000 metre long boreholes are located near the lighthouse. This work is only permitted within a narrow construction period from mid-July to the end of September. Since mid-July 2017, TenneT has been active in the respective construction window for horizontal drilling on the island of Norderney. For nature conservation and coastal protection reasons, this work is only possible within a narrow construction window from 15 July to 30 September.

Drilling began in the south and north directions for the DolWin6 and BorWin5 grid connections. These could be completed in 2021.

Drilling for the grid connection projects BorWin4 and DolWin4 is also scheduled for the coming years. These two projects are being managed by Amprion.