Table 5.17 Typical cable installation rates for different burial methods Method
Surface lay (single cable) Surface lay (bundled pair of cables) Ploughing (single cable) Jetting Trenching Vertical injector (shallow water only)
Speed (m/hour) 300 to 400
300 to 400 150 - 300 150 - 450 30 - 80 30 - 80
5.4.11.6 Cable protection 181. In some cases such as unsuitable sea bed conditions or where another cable or pipe is already in place the above methods cannot be applied and it is necessary to use alternative methods for installing the cable other than burial. Details of some of the techniques employed are given below:
Rock placement - involves the laying of rocks on top of the cable to provide protection which is effective on crossings and other areas requiring protection. Where long sections of unsuitable sea bed are encountered. It is a costly exercise and it is subject to environmental permitting restrictions.
Concrete mattresses - are prefabricated flexible concrete coverings that are laid on top of the cable, as an alternative to rock placement. The placement of mattresses is slow and as such is only be used for short sections of cable protection. Grout or sand bags, are used similarly to concrete mattressing with mattresses filled with grout and / or sand used in place of the prefabricated concrete mattresses, this method is generally applied on smaller scale applications than concrete mattressing.
Frond mattresses could be used to provide protection by stimulating the settlement of sediment over the cable. This method develops a sandbank over time protecting the cable but is only suitable in certain water conditions. This method may be used in close proximity to offshore structures though experience has shown that storms can strip deposited materials from the frond.
Uraduct is effectively a protective shell which comes in two halves and is fixed around the cable to provide mechanical protection. Uraduct is generally used for
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
Chapter 5 Description of the Development Page 42
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