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 Mitigation measures to avoid, reduce or minimise environmental effects have been cited along with a discussion on their effective implementation, where known. During the planning phases of a proposed offshore wind farm many of the potential impacts are reduced through good environmental management practices, including the avoidance of known sensitive sites or areas with high proportion of fines, chalk areas, sensitive areas for fish spawning, etc. These can occur where there is enough knowledge of the baseline environmental conditions. However, in other areas it is important that a degree of flexibility is present to enable certain mitigation measures to be applied, for example, seasonal restrictions and micro-siting of cable routes. 5.2 Subtidal Ecology 5.2.2 INTRODUCTION Subtidal habitats likely to be encountered by cable routes include the following: ● ● ● ● ● ● ● 102 bedrock and boulders; reefs; chalk; gravel and shingle beds; sand and mud (silts and clays) banks; maerl beds; seagrass beds; and ● biogenic reefs such as horse mussel beds (Modiolus modiolus), and Sabellaria spinulosa (Ross worm) reefs (Johnston et al., 2002). Potentially significant effects include: ● ● Seabed disturbance; and Increase in suspended sediment concentrations and subsequent settlement. Other effects include: ● ● ● ● Potential contaminant release; Electro magnetic effects; Heating effects; and Cable coating effects.
5.2.2 POTENTIALLY SIGNIFICANT EFFECTS Seabed disturbance Potential impacts and mitigration measures Some weeks prior to the installation of the cables, a cable burial assessment survey can be carried out (see previous Section 3.10). Disturbance to the seabed, leading to an alteration of habitat and associated species will occur during the burial assessment survey and during cable burial operations. Such impacts are likely to be limited to an area 2-3m either side of the cable, depending upon the size of the installation device used. In general, the range of installation devices available would have similar impact footprints, especially those that are tracked or have skids in contact with the seabed. Recovery of the seabed is, to some degree, dependent on the substrate that is left following installation. Certain techniques will aid recovery by infilling the trench following cable placement. In respect to this potential impact, the action of a narrow blade conventional plough is expected to fulfil this criterion more than any other method of installation. With a narrow blade conventional plough, a wedge of soil is lifted, the cable placed at the base of the trench and then the displaced soil wedge is allowed to backfill naturally. Any other excavation method will disturb the sediment and not allow layers of sediment to be reinstated in the same sequence in their natural state. For example, mechanical cutter or jetting systems have aggressive cutting mechanisms which are used to cut open trenches. Certain installation devices require the use of a vessel that utilises an anchor array to stabilise the cable laying vessel which would slightly increase the area of disturbance (see Section 3.9.1). As the vessel needs to move as the cable is laid, anchors are repositioned, increasing the area of potential impact. Where cable crossing (see Section 3.8.6) is necessary there would be a requirement for the placement of cable protection measures such as concrete mattresses, which would cover a larger area, in the order of 150m 2 for each cable crossing. The exact area would depend on the specified requirements from the existing cable owner. Installation of cables close to turbine foundations using either ploughs or remotely controlled tracked vehicles will always result in a short section of cable close to the J-tube exit points not being buried as a consequence of limitations in the cable burial procedure. A common solution for these short lengths of exposed cable (typically 10m to 15m) is to use either over covering concrete, frond mattresses or rock dumping. Rock dumping can be problematic, especially in and around sensitive habitats, as careful placement would be required in order to avoid unnecessary damage to habitats and species. Burial under the dumped rock would involve a permanent loss of habitat where the placement occurs. These rocks, similarly to all surfaces in contact with the sea, would be readily colonised by a range of fouling species, and may act to cause localised increases in biodiversity. However, artificial increases in biodiversity through the 103
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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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3 Cable types and 18 installation t
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Page 82 and 83: 4 Physical change 4.1 Introduction
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Page 142 and 143: 7 Gaps in understanding Research fo
Page 144 and 145: References Introduction Sinden, G.
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Appendix A: Standards and codes of
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Printed in UK on recycled paper. De
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