continuous trench. The equipment uses pressurised water from water pump system on board the cable vessel to fluidize sediment.
176. There are two methods of water jetting which are:
Laying the cable first and jetting at a later time - The cable is laid on the sea bed first and afterwards a jetting sledge is positioned above the cable. Jets on the sledge flush water beneath the cable fluidising the sand whereby the cable, by its own weight, sinks to the depth set by the operator. As the sediment is fluidised a minor amount of sediment spill is expected.
Laying the cable and jetting at the same time - In this method water jets are used to jet out a trench and the cable is laid into the trench behind the jetting lance.
177. Jetting tools can be pulled directly by a surface vessel or can be mounted onto self- propelled caterpillar tracked vehicles which run along the sea bed taking its power from a surface vessel.
5.4.11.4.4Burial methods: Vertical injector 178. In shallow waters a vertical injector could be used. This is a large jetting and cutting share which is strapped to the side of a barge and the cable is laid in the foot of the trench. This technique can provide deeper than traditional method burial which can be utilised through areas of high sea bed mobility or whilst crossing areas of high risk.
179. The burial depth is controlled by means of raising or lowering the tool and horizontal positioning, by means of adjusting the barge anchors.
5.4.11.5 Cable laying and burial speeds 180. The type of installation vessel would have a bearing on the overall rate of installation. A DP vessel should not restrict the surface lay rate of installation however an anchored barge would require resetting of its anchors which may reduce progress rates. An anchor reset may occur every 500m and last several hours and dependent on its location may also be hampered by tides. The installation rates as presented below, during the normal movement of a barge should not restrict the progress rates. The speed of cable laying depends on the ground conditions, sea bed profile and depth. Factors affecting vessel manoeuvrability would also affect the speed of operation. Installation rates are shown in Table 5.17.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 5 Description of the Development Page 41
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