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 Table 4.2: Level of sediment disturbance arising from use of plough types in different ground conditions 88 Plough Type Ground Conditions Conventional Narrow Blade Sand Silts Gravels Clay Unstructured Rock – Matrix Material Weak Rock (Chalk) 1 1 1 1 N/A N/A Advanced with Jetting 2 3 2 2 2 N/A Deep Burial 1 1 1 1 1 N/A Rock Ripping 1 1 1 1 1 4 Vibrating 1 2 1 1 2 6 Level of disturbance 1 = Low, 10 = High, N/A = Not applicable Table 4.3: Level of sediment disturbance arising from use of other burial tools in different ground conditions Tool Ground Conditions Jetting Fluidisation Erosion Sand Silts Gravels Clay Unstructured Rock Chalk Structured Rock 2 2 N/A N/A N/A N/A N/A 3 4 3 3 N/A N/A N/A Dredging 4 6 4 N/A N/A N/A N/A Rock Wheel 3 4 3 3 3 8 4 Mechanical Chain Excavators Level of disturbance 1 = Low, 10 = High, N/A = Not applicable Quantification 3 4 3 3 3 N/A N/A There is very limited research and advice on the quantification of the volume of material that is disturbed and brought into suspension by cable burial operations. In the absence of detailed information conservative assumptions will need to be made for the following parameters: ● ● ● Volume of material disturbed; Rate of sediment release into the water column; and Vertical distribution of suspended material throughout the water column. An estimate of the rate at which sediment is disturbed can be made based on the size of the slot or trench created by the tool. For cutting tools the rate of sediment disturbed can be calculated based: Depth of Deployment of Tool x Tool Width x Rate of Progress
Physical change A similar approach may be adopted for a plough but it should be appreciated that these tools displace rather than remove sediment from the seabed. As an example, a wheel cutter tool 250mm wide forming a 1000mm deep trench at a rate of 250m/hour will cut 62m 3 /hr. Reviewing evidence from land trials of these tools and also reviewing subsea video a reasonable estimate of only 10 to 15% of cut material will backfill into the trench with the remainder deposited at the sides of the cut trench or removed as suspended material. In the absence of more specific information, it is reasonable to assume all fine sediment (clays, silts and sands) disturbed during the cable burial operations would be brought into suspension. The distribution of this sediment throughout the water column will depend upon the size of the particles and level of disturbance caused by the cable burial system. Although coarser sediments are also likely to be brought into suspension this material will quickly settle back to the seabed and is unlikely to be dispersed by tidal currents. 4.4 Seabed Disturbance by Other Activities When considering the potential impact of cable installation as a whole, it is necessary to put these into context with other influencing factors including natural perturbations such as storm activity and other seabed users such as oil and gas installations and aggregate extraction, both of which cause disturbance to the seabed. The main impact associated with cable installation relates to the physical disturbance of seabed and the subsequent creation of a sediment plume. These impacts are however localised in nature and are, in general, oneoff, short-term effects, with the seabed usually returning to its original state. When compared with the area affected by other activities, the spatial extent of cable installation is very small. Table 4.4 provides an indication of the extent of physical disturbance caused by other seabed uses in the UK continental shelf. At certain sites the cumulative and in-combination effects with other activities and projects will need to be considered during the assessment process in order to determine the overall effect on existing environmental parameters. 89
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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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6 Good practice measures This secti
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7 Gaps in understanding Research fo
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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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