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Submarine cables


Marine cable laying: Cable transfer at sea. Photo: TenneT TSO GmbH
Marine cable laying: Cable transfer at sea. Photo: TenneT TSO GmbH

The construction and planning of offshore wind farms brings with it the challenge of connecting turbines to the mainland grid. This is achieved using high-voltage power cables buried in the sea floor by specially equipped ships. A few such cables are already in operation, but many more will be needed for offshore wind farms in the North Sea and the Baltic Sea in the next few years.

North Sea and Baltic Sea cables

Two high-voltage cables with a capacity of up to 900 MW are currently in operation. Additional capacity is still in the planning stage, including several cables in the North Sea. Further information is provided in the BfN overview maps on wind power deployment in the North Sea and the Baltic Sea. See Offshore wind power and the website www.erneuerbare-energien.de (only in German).

In addition, there are international interconnectors for the import and export of power between countries. One such high-voltage submarine power cable is NorNed, which has been in operation since 2008. Extending for 580 km, this is the longest electricity transmission cable in the world and leads from Norway to the Netherlands through the German North Sea EEZ. The cables are paid off specially equipped laying vessels and then embedded in the sea floor by ploughing or water jetting. The environmental effects are the same to an extent as in sediment extraction using trailing suction hopper dredgers (see below, Sand and gravel extraction. Transmission losses are minimised by using high voltage DC (HVDC) systems. In the case of offshore wind farms, this requires the generated power to be transformed into direct current at a converter station, transmitted ashore and then reconverted into alternating current that can be fed into the grid. See also Offshore wind power.


18 cm diameter, 53 kg/m: The marine cable for the alpha ventus wind farm. Source: TenneT TSO GmbH
18 cm diameter, 53 kg/m: The marine cable for the alpha ventus wind farm. Source: TenneT TSO GmbH

The turntable on the Team Oman shows the dimensions involved: About 55 km of cable is coiled up here. Source: TenneT TSO GmbH
The turntable on the Team Oman shows the dimensions involved: About 55 km of cable is coiled up here. Source: TenneT TSO GmbH
Offshore cable transfer. Source: TenneT TSO GmbH
Offshore cable transfer. Source: TenneT TSO GmbH

Impacts of power cables on marine life diversity

While this is still a new field of research, it is gaining in importance with the large-scale construction of offshore wind farms.

First of all, cable laying and embedding, and the associated sediment displacement, brings about direct loss of seabed communities and demersal fish. It also creates turbidity plumes that affect the surroundings.

In operation, high-voltage direct current (HVDC) cables emit heat and electromagnetic fields, and recent years have consequently seen initial research into the impacts on benthic organisms and adjacent sediment. The temperature increase can affect the metabolism of living organisms and lead to a change in the composition of the infauna community in sediment around a cable. A temperature rise of less than 2 °K at 20 cm below the sediment surface is considered tolerable. This can be achieved with embedding depths of 1 m or greater.

In addition, research off Great Britain has shown that the electromagnetic fields generated by marine cables affect the behaviour (notably hunting behaviour and orientation) of demersal sharks and rays. Migratory fish species that use the Earth’s magnetic field to navigate can likewise be affected by artificial electromagnetic fields. Further research is still needed here.

Communication and data cables

Besides marine power cables, several communication cables also cross the German EEZ. Thanks to its low susceptibility compared with satellite transmission, fibre-optic cable has proved ideal for transporting data between countries and continents. Today’s developments in communication make for ever-increasing volumes of data. Further projects to lay data cables in German waters can thus be expected in the years ahead.

The impact of the communication cables on the marine environment are still unresearched.


 

Infos & Maps

Further information and maps for the installation and operation of power and communication cables in the area of the German continental shelf of the North and Baltic Sea can be found on the website of the BSH.

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