Page 230 - Alderney Regional Environmental Assessment of Renewable Energy: Environmental Report

Alderney Regional Environmental Assessment of Renewable Energy:
Environmental Report
moving or stationary installation vessels and equipment is also likely to increase the probability
of close quarter encounters and collisions with passing vessels.
During the survey phase for tidal stream turbines and cable routeing, vessel(s) are considered
likely to be operating individually or in small numbers, using weather windows to capture good
quality marine data. The presence of survey vessels will be recognised through the use of day
and night identification lights and shapes as required by the Convention on the International
Regulations for Preventing Collisions at Sea, 1972 (COLREGs). Therefore, the risk of collision
is low as survey craft can manoeuvre to avoid close quarters situations and can plan their
surveys to avoid peak times for marine traffic. Therefore exposure to change during surveying
is considered negligible to low resulting in an insignificant/minor adverse impact.
Vessels associated with construction and decommissioning for tidal stream turbines, and
offshore substations have the potential to increase the risk of collision with other craft
navigating in the vicinity. These activities have the potential to cause small and recreational
vessels to modify their routes to use areas transited by larger vessels, which potentially
increasing the risk of encounter or collision. Exposure to change is considered to be low to
medium resulting in an insignificant to moderate adverse impact. It should also be noted
that Bailiwick waters do not have a Vessel Traffic Services (VTS) covering the area which
would assist in increased marine safety.
Similarly for cable routeing there is considered to be an increased risk of collision with vessels
along cable routes while cabling is laid. This risk is increased in the proximity to navigation
channels (for example, in port and harbour approaches) and through greater vessel activity in
these areas. An additional risk is the physical snagging of anchors on cables prior to burial,
rock dumping and/or mattressing (although less likely, but still possible). Exposure to change
during construction and decommissioning is therefore considered medium to low resulting in an
insignificant to moderate adverse impact.
The operation of stationary tidal device arrays and offshore substations are also likely to affect
the probability of collisions. This is primarily through the risk of a direct collision or snagging of
vessel lines (whilst fishing, anchoring or towing) with structures and their moving parts, whilst
vessels are underway, adrift or at anchor. Where renewable arrays are in waters of depths
greater than 75m, the structures do not present a danger to surface navigation. However, the
presence of substation platforms and submerged structures on the seabed in depths shallower
than 75m, which is the case in the REA study area, could pose a risk to navigating vessels.
This follows the rationale that tidal devices are stationed circa 20m from the bed (to avoid bed
turbulence) and have a maximum blade around 10m in diameter, providing a 30m bed-to-
blade-tip clearance. Ultra Large Crude Carriers (the deepest draughted vessels) have a
maximum draught of around 35m. An Under Keel Clearance allowance of 10m is applied as a
maximum working clearance (This approach follows the rationale laid out in the ABPmer report
‘Developing the Socio-Economic Evidence Base for Offshore Renewable Sectoral Marine
Plans in Scottish Waters Final Report’ (ABPmer, 2013) developed from a working paper
provided by the NOREL sub group on navigation (NOREL, 2012). Overall, exposure to change
is considered medium at worst resulting in an insignificant to moderate adverse impact. A
full build out of the Draft Plan, however, and the potential installation of up to 4,000 tidal
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