not be known until sometime after the DCO has been granted. However, the installation methods and rates presently being considered are described in Table 7.9.
Table 7.9 Cable Installation Methods and Rates Technique Ploughing
Description
Cutting through the sea bed with a blade, behind which the cable is laid
Trenching or cutting Jetting
Excavating a trench whilst temporarily placing the excavated sediment adjacent to the trench and back-filling the trench once the cable has been laid
Fluidising the sea bed using a combination of high-flow low pressure and low-flow high pressure water jets, enabling the cable to sink beneath the sediment surface
Vertical injector
Using a large jetting or cutting share strapped to the side of a barge for cable laying at the foot of a trench in shallow waters
66.
Installation Rate 150-300m/hour
30-80m/hour 150-450m/hour
30-80m/hour
Of the above cable installation techniques, jetting is considered the worst case in terms of this assessment since it results in greatest suspension of sediment off the sea bed and into the water column.
67.
For purposes of the EIA, a worst case assumption has been made that some form of cable protection measures would be required at cable crossings and in areas where cable cannot be buried (e.g. areas of exposed bedrock).
68.
Alternative cable protection measures presently being considered include concrete mattresses, fronded concrete mattresses, rock dumping, bridging or positioning of gravel bags.
69.
For the inter-array cables, the worst case is that there would be up to 10 cable crossings and that up to 10% of the inter-array cables would be unburied and require protection. The total footprint area of cable protection measures would range from 0 to 720m2. The height of cable protection measures from the sea bed would range from 1 to 3m.
70.
For the export cables, there would be a number of cable crossings required. These would particularly be in the area where the East Anglia THREE offshore cable corridor crosses the export cables of the Greater Gabbard Offshore Wind Farm and Galloper Wind Farms.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 15
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