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