Environmental effects associated with the cable connection of offshore wind farms to the interconnected power grid

Offshore wind farm south of the Danish island of Samsö Source: Prof. Dr. Horst Crome

What are the environmental effects of connecting offshore wind farms by cable to the interconnected power grid? The Federal Office for Radiation Protection dealt with this question for the second time after 2005 in a technial statement (only in german) in 2013.

The question was whether marine life is harmed by electric and magnetic fields emanating from submarine cables. Furthermore, it was a question of whether the warming of the seabed leads to changes in the sediment and the organisms in it.

In addition, the statement contains information on the technical realisation of the cable connection; this is important for understanding the possible effects.

Effects of electric and magnetic fields on organisms

Serious harmful effects (e.g. genetic damage or tissue damage in marine life) are not expected because the strength of the electric and magnetic fields is low. However, some fish species can sense electric fields. Studies describe behavioural changes in some fish species in the vicinity of submarine cables. Some cartilaginous fish (including sharks, rays, and manatees) tend to search for prey near the submarine cables as long as the fields are weak. But within only a few days, sharks learn to associate electric fields with the presence of prey; they also learn to ignore fields from power cables where no prey is found. This indicates that they can adapt well to the changed conditions and that their hunting success is not affected by the fields emitted by cables.

Some fish species (e.g. salmon and eels) orient themselves to magnetic fields during their migrations. These animals perceive the magnetic field of the submarine cables, swim more slowly in the immediate vicinity of the cables, and change their swimming direction over a short section of their journey. However, according to previous study findings, they appear to be only slightly deflected in their direction of migration.

Observational studies of coral reef fish in Florida have shown that emissions from submarine cables have no effect on the biodiversity of fish populations there.

Many crustaceans perceive magnetic fields and orient themselves accordingly. The European lobster (Homarus gammarus) prefers shelters with magnetic fields in the range of a few millitesla. The fields do have a minor physiological effect on the daily rhythm of the animals but are not harmful.

Of the general population, only divers and swimmers may come into the vicinity of the cables; according to the current state of knowledge, there is no health risk for them because of the low field strengths.

Thermal effects in the sediment

The sediment on the seabed is strongly heated in the vicinity of the laid cables. Warming in the sediment can lead to changes, the importance of which can be only partially assessed at present.

Because the living organisms that reside in the sediment or in the bottom zone of the sea are highly temperature-dependent, it is to be expected that a warming of the sediment will affect the local composition of flora and fauna. It is assumed that a stronger warming in spring could affect the reproduction of animals living in the bottom zone of the sea. It has not yet been sufficiently investigated what influence warming has on living organisms found in the different layers of the sediment.

Recommendations

For precautionary reasons, the magnetic and electric fields emitted by submarine cables should be minimised. The Federal Maritime and Hydrographic Agency recommends laying the cables at a depth of one to three metres in order to limit a temperature increase at the sediment surface and in the seawater. The distance to the sediment surface also reduces electric and magnetic fields for animals living on the seabed.

The BfS considers it advisable to bundle the cables in ducts– as has been done on Norderney. In any case, sufficient free areas where animals whose behaviour can be influenced by electric and magnetic fields remain undisturbed should be maintained.

There are still many unanswered questions about the effects of the electric and magnetic fields on animal species sensitive to them as well as about the effects of the warming of the sediment. Other aspects such as construction noise and the alteration of the seabed are not components of radiation protection but are nevertheless important for the preservation of ecosystems. The current scientific knowledge on environmental effects of marine renewable energy extraction is summarised in a review paper by Professor Andrew Gill.