Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
Review of Cabling Techniques and Environmental Effects Applicable
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<strong>Review</strong> <strong>of</strong> <strong>Cabling</strong> <strong>Techniques</strong> <strong>and</strong> <strong>Environmental</strong> <strong>Effects</strong> <strong>Applicable</strong> to the Offshore Wind<br />
Farm Industry – Technical Report<br />
clay, with hydraulic fracture occurring very easily along existing fracture joints.<br />
However, it would be difficult to break the chalk into smaller blocks than already<br />
exist in situ, especially in medium to high density chalks. A dredging system<br />
used in combination with the jetting system would have difficulties in removing<br />
large rock fragments to create a trench.<br />
In most weak rocks including structured medium to high density chalk, other<br />
trenching systems will be more suitable, such as a plough with rock penetrating<br />
tooth or a rock wheel cutter.<br />
In structured weak rocks, the material would be mainly fractured/cut into<br />
sizeable blocks before much <strong>of</strong> the material could be eroded or fluidised. In this<br />
case, any trench would have to be formed by a dredging system, which would<br />
lift the blocks <strong>and</strong> deposit them adjacent to the trench. In structured s<strong>of</strong>t, finegrained<br />
rocks, such as chalk, there will also be some disintegration <strong>of</strong> the rock<br />
into its constituent components, <strong>and</strong> this material could become suspended in<br />
the surrounding water <strong>and</strong> transported by the current some distance from the<br />
trench. However, as mentioned earlier, jetting is highly unlikely to be used for<br />
trenching in structured rocks.<br />
In structureless rocks, the material would be broken down into a wide range<br />
<strong>of</strong> particle sizes from clay or silt-size up to boulder-size. The clay to fine s<strong>and</strong>size<br />
particles will become easily suspended in the water <strong>and</strong> transported long<br />
distances by the current. The larger sizes, that could only be removed using a<br />
dredging system, would be deposited close to the trench. As such, the impact<br />
will be a combination <strong>of</strong> the impacts for cohesionless <strong>and</strong> cohesive soils.<br />
4.3.3 DREDGING SYSTEMS<br />
Dredging systems used on tracked cable burial machines (Section 3.8.3), ROVs<br />
(Section 3.8.4) <strong>and</strong> burial sleds (Section 3.8.5) can be used for a range <strong>of</strong> soil<br />
types including s<strong>and</strong>, silt <strong>and</strong> certain clays. However, the larger the particle<br />
size, the harder it is to move material in a slurry suspension. Dredging systems<br />
remove soil to create a trench by a process <strong>of</strong> suction. The suction tubes are<br />
sometimes referred to as eductors. The educted soil is normally “blown” into<br />
the surrounding sea to the sides <strong>of</strong> the trench, where it is also transported as<br />
suspended sediment by the current until it settles out on the seabed. Dredging<br />
systems work best when the soil is in a slurry state. For this reason they are<br />
normally used in combination with a jetting system that is used to fluidise the<br />
soil prior to dredging.<br />
Dredging systems can remove the soil material <strong>and</strong> deposit it on a barge or,<br />
more conveniently, disperse the sediment into the sea away from the trench.<br />
The dispersal distance <strong>and</strong> thickness <strong>of</strong> deposition will depend on the particle<br />
size <strong>and</strong> current speed, in the same way as for eroded materials. The impact that<br />
the dispersed sediment makes on the local aquatic environment will depend on<br />
how well adapted the marine life is to this type <strong>of</strong> sediment deposition.<br />
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