Review of Cabling Techniques and Environmental Effects Applicable

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