Review of Cabling Techniques and Environmental Effects Applicable

Cable types and installation techniques
The installation of concrete mattresses requires diver intervention to assist in
their accurate placement. However, for safety reasons most mattresses are
deployed with automatic release systems connected to the rigging, with divers
either out of the water or well clear when each mattress is released. It is normal
practice for a number of concrete mattresses to be transported to the work site
on a host vessel with its own vessel crane which is used to individually deploy
the mattresses. Figure 3.24 shows the deployment of a concrete mattress.
Figure 3.24: Concrete Mattress being
deployed by Crane from a Host Vessel
(Photo courtesy of FoundOcean Ltd)
Concrete mattresses can each weigh up to 10 tonnes and a typical mattress
would measure 5m x 3m x 300mm thickness. However, smaller and lighter
mattresses are also available. The size, weight and density of the concrete used
will be dictated by the stability calculations which have to be undertaken in
advance of the mattress deployment. Such calculations will take into account
any potential scour effects around the periphery of the mattresses.
Where concrete mattresses are used for cable or pipeline crossings, the procedure
would involve the placement of concrete mattresses over the existing pipeline
with the new cable then laid on top of these mattresses with a further layer of
mattresses then added over the new cable to create a sandwich effect. This
approach ensures satisfactory separation between the new cable and existing
cable or pipeline whilst also ensuring a robust cable protection solution for the
crossing arrangement. This procedure is illustrated in Figure 3.25.
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