169. The installation practices can be divided into the following classes:
Cables are surface laid by a laying vessel, and burial is carried out in a post-lay mode using a separate vessel and trenching/jetting equipment spread depending on the method
Cables are laid and buried in a simultaneous operation with burial equipment being towed by the cable laying vessel or barge, in the case of plough or burial sled, or operated from laying vessel where a self-propelled ROV is utilised
5.4.11.4.1 .Burial methods: ploughing 170. Ploughing has been specified for major HVAC systems in the past and this generally produces better burial results than post lay burial in some ground conditions. However, for HVDC if the cables are bundled, it is technically challenging due to the bundle has to pass through the plough therefore ploughing has been substituted for post lay burial on some projects.
171. A forward blade cuts through the sea bed laying the cable behind. Ploughing tools could be pulled directly by a surface vessel or could be mounted onto self-propelled caterpillar tracked vehicles which run along the sea bed taking power from a surface vessel. The plough inserts the cable as it passes through the ground.
172. Even if the primary method adopted for laying the export cables is ploughing there would still likely be local spots that would require jetting or other methods to bury and protect the cable i.e. for any jointing loops, corners areas and where ploughing would be unable to negotiate obstacles, cable crossings, etc.
5.4.11.4.2Burial methods: trenching or cutting 173. Trenching or cutting would only be used as the last option on areas where other methods for protecting the cable are not economically and / or technically feasible.
174. This method consists of three operations. First a trench is excavated or cut while placing the sediment and fill next to the trench. The cable is subsequently laid in the trench and lastly the sediment or fill is returned to the trench. Trenching can be a difficult and relatively slow method to use compared with other methods listed here; however, this technique may be required for small sections of cable runs where other methods are not practical.
5.4.11.4.3Burial methods: Jetting 175. This method involves the use of a positioned cable vessel and a hydraulically powered water jetting device that simultaneously lays and embeds the cables in one
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 5 Description of the Development Page 40
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