163. If the offshore cable corridor crosses an out of service cable it may be recovered from the sea bed before laying the offshore export cable. The removal would be dependent on depth of burial, and accessibility.
164. Crossings preparation would need surveys to locate the asset and determine actual depth of burial. Following this, placement of protection would need to be carried out.
5.4.11.4 Export and interconnection cables: installation and burial method-ploughing, trenching, jetting, depth of burial (offshore)
165. A number of specialist vessels are available for the installation of cables. For smaller inter-array cable sizes (which generally comprise multiple short lengths) it is possible to use barges to lay the cable, however for large export cables, specialist vessels are generally required. The physical characteristics of cables have a significant impact on installation in offshore environments. The use of sub-sea cable joints is costly, and time consuming in the difficult offshore environment and it is preferred where practicable to lay long lengths of cable continuously to avoid joints.
166. For long cable installation campaigns there are only a few specialised vessels available. Smaller barge type vessels are more readily available but only capable of carrying relatively small amounts of cable, resulting in numerous sub-sea cable joints. A smaller low draught vessel may be more suitable at the shallow landfall approach before handing over to the larger vessel dependent on the cable landfall site and water depth.
167. The preferred construction technique and depth of burial for the offshore electrical infrastructure would be decided after the pre-construction geotechnical ground investigation, a risk assessment and a lifetime maintenance assessment. Each of the possible installation techniques (ploughing, jetting and trenching) have constraints within which they can be effectively utilised, e.g. shallow water, depth of burial required, sediment disturbance, sediment type and minimum bend radius.
168. The offshore export cables would be buried at a depth between 0.5m and 5m for the majority of the route. The final burial depth would be decided when a pre- construction detailed study has been completed to assess the relevant factors for each part of the cable route.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 5 Description of the Development Page 39
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