Review of Cabling Techniques and Environmental Effects Applicable

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind
Farm Industry – Technical Report
feeding efficiency. In addition many fish species such as herring rely on light
levels to aid migration and shoaling behaviour. Low light levels caused by high
levels of suspended sediment could impair the ability of species to shoal as part
of migrations to spawning or feeding grounds. Studies described in Section 4.4
indicate short term and localised increases in turbidity over a number of tidal
cycles (for those cable installation methods investigated).
Smothering
The blanketing or smothering of benthic animals and plants, may cause stress,
reduced rates of growth or reproduction and in the worse cases the effects may
be fatal (Bray, Bates & Land 1997). The impact of smothering on fish and shellfish
will be a function of the settling behaviour of sediment resulting from increased
suspended sediment concentrations relative to background levels, the sensitivity
of certain species and/or lifestages to those increases and their ability to move
to other areas. The significance of this impact is dependent on many variables
including hydrography, seasonality, sediment type, species and the technique
used to bury the cable (see Sections 4.2 and 4.3 for more details).
The main impact on fish is the irritation and clogging of gills. Juveniles are more
susceptible to this as adult fish would normally be able to detect significantly
elevated levels of suspended sediment and move away from the affected area
(ABP Research, 1997).
Smothering can result in significant mortalities on shellfish beds as they are
less mobile than fish species, with many having lifestages that are sensitive to
variations in sediment particle size within the water. Respiratory and feeding
apparatus may be clogged by the settlement of significant amounts of sediment
that is mobilised by cable-laying operations. Filter feeders such as mussel, oyster
and scallop are therefore among the most vulnerable to smothering effects.
Shellfish are particularly susceptible during spring when spatfall occurs (Posford
Duvivier & Hill, 2001). If sensitive spawning or shellfish beds cannot be avoided
entirely, seasonal avoidance may be required.
Water quality
Cable installation requires extensive use of shipboard and subsea equipment
which in many cases is hydraulically driven. There is a consequent risk of spillage
of hydraulic fluid from vessels. The spillage is, however, limited by the system
design. Perhaps the greatest risk is a major leak from a burial ROV system when
operating on the seabed. Such systems have typical reservoir capacity of 60-100
litres. Spillage risks can be minimised by good practice:
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Deck mounted hydraulic equipment should be fitted with saveall (cofferdam)
surrounds to catch leakage and prevent discharge over the vessel side;
Spare hydraulic oil should be securely stowed, preferably below decks;