Review of Cabling Techniques and Environmental Effects Applicable

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Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind Farm Industry – Technical Report a negative impact for commercial fisheries, the introduction of hard substrate creates habitat diversity, which can benefit some commercial resources e.g. crustacean fisheries. Mobilised sediment may settle on fishing gear (e.g. monofilament gill nets) and reduce their efficiency. The significance of this depends on background turbidity levels and given the dynamic hydrographic conditions of much of the UK coastline this is likely to be insignificant. Snagging The extent of the physical disturbance to the seabed along the cable route will depend on the prevailing geological conditions, which determine the cable installation techniques that can be used. For soft sediment, natural infilling or assisted infilling by the cable burial tool should ensure that no debris to potentially snag a fishing net is left on the seabed. However in rocky substrata or in clay, chunks of debris could potentially cause fishing nets to snag. Snagging can also occur where stretches of cable are exposed for example over rocky substrata, spanning sand waves or at any location where cable is surface laid prior to any post lay burial operation. However, there should not be a problem for the latter point, as the fishermen should be respecting exclusions zones until all cable burial operations are complete. Shifting sediment can also expose previously buried sections of cable. Seabed hazards resulting from the trenching of the cable route, including any exposed sections of the cable itself, can interfere with benthic fishing gear, particularly beam trawl, otter trawl and drift netting. Static gear also has the potential to become snagged on subsea cables. The small 1m long anchors used by inshore vessels to secure static gear are found to penetrate sand to a depth of around 0.2m (less in coarser sediments). These may drag in strong currents and become snagged on cable. The risk lies in attempting to recover the gear, which is often through the use of grappling hooks. The cable route assessment should however take on board all of these potential hazards and the design depth of burial for the cables should provide suitable mitigation against the measured risks. Danger arises where vessels, unknowingly snagged on a cable attempt to heave the gear free from the seabed obstruction. This can lift the cable clear of the seabed, exposing more cable that causes a significant downward pull from the weight of cable. The recovery of snagged gear is not recommended and fishing operations will be compensated by cable operators if provided with evidence of such instances. Such claims are only compensated providing that the fishermen have not operated within restricted fishing or anchorage zones. 124
5.5.2 MITIGATION MEASURES Potential impacts and mitigration measures The extent and risk ‘associated’ with interactions between cables and fishing vessels should be avoided or minimized through appropriate: ● ● ● Routing of cables and scheduling of cable laying operations; Cable laying technique to ensure sufficient cable burial and/or protection; Communication of cable laying activities and cable positions (via Notices to Mariners, Fishing News, Kingfisher Bulletins and Admiralty Charts); and ● Monitoring of the cable in situ to identify and address areas of impact and risk to fisheries. The choice of cable route and cable laying techniques would be determined following an assessment of existing commercial fishing activities in the area and the sensitivities of the resources upon which they depend. The use of best engineering practices would be employed e.g. ensuring 100% of the cable route had adequate protection with no exposed sections of cable wherever possible. Using suitable local fishing vessels as guard vessels for cable laying operations provides useful alternative income to the fishing industry. Monitoring of the cable route post installation (e.g. side scan sonar) and regular communication with fishermen should help minimise impacts and risk from construction operations. Communication with fishermen will be greatly facilitated with the use of a suitable fisheries liaison officer. 5.6 Marine Mammals Marine mammals 2 have a wide ranging distribution and, as such, there are no marine mammals considered to be exclusively British. The cetaceans most commonly encountered and described as part of offshore wind farm projects are the harbour porpoise Phocoena phocoena and the bottlenose dolphin Tursiops truncatus. Harbour porpoise are by far the most abundant 3 (Hammond et al. 2002). Two species of seal are resident in UK waters; the common seal Phoca vitulina and the grey seal Halichoerus grypus, with the grey seal being more numerous (English Nature, 2004). The range of potential impacts upon marine mammals during cable installation (and, to a much greater extent, offshore wind farm construction) are of 2 In the context of this Technical Report, ‘marine mammals’ is the collective term for seals (pinipeds) and whales, dolphins and porpoise (cetaceans). 3 For details of other cetacean species occurring in northwest European waters see http://www.jncc.gov.uk/ page-1554 125
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REVIEW OF CABLING TECHNIQUES AND EN
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Review of Cabling Techniques and En
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Abbreviations AIS Automatic Identif
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TTS Temporary Threshold Shift TWA T
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Review of Cabling Techniques and Environmental Effects Applicable

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