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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Physical change<br />
also used, some weakened material <strong>and</strong> lumps <strong>of</strong> disturbed soil will remain on<br />
the bottom <strong>of</strong> the trench to receive the pipe or cable as it is lowered into the<br />
trench by the forward movement <strong>of</strong> the burial device. The depth <strong>of</strong> pipe or cable<br />
sinkage will depend on the size, number <strong>and</strong> remoulded shear strength <strong>of</strong> the<br />
soil lumps.<br />
Although the trench will be wider than the jet tubes, due to outward pointing<br />
jets, this will be limited by the smaller separation normally required between the<br />
jetting system <strong>and</strong> the cutting face in cohesive soils. In many instances where a<br />
high power jetting system has been deployed in a hard soil seabed the trench<br />
width will be close to the spacing <strong>of</strong> the jet tubes. The trench sides will st<strong>and</strong><br />
vertically for all but the very s<strong>of</strong>test clays, although it will be broken <strong>and</strong> irregular<br />
due to the jet slotting action <strong>and</strong> the effects <strong>of</strong> hydraulic fracture.<br />
In very s<strong>of</strong>t to s<strong>of</strong>t clays, much <strong>of</strong> the soil will be eroded <strong>and</strong> mixed with the<br />
surrounding seawater. Clay sized particles will remain in suspension much longer<br />
than s<strong>and</strong> or silt sized particles, <strong>and</strong> could be transported long distances by the<br />
prevailing current. However, this means that when the sediment is eventually<br />
re-deposited on the seabed, the thickness <strong>of</strong> deposition will be very small.<br />
In firm to hard clays, most <strong>of</strong> the soil will be broken into sizeable lumps before it<br />
can be completely eroded. The lumps would need to be removed by a dredging<br />
system in order to form a trench. Even in strong currents, the heavy weight <strong>of</strong><br />
the lumps will ensure that they are deposited close to the trench. This means<br />
that the thickness <strong>of</strong> deposition may be large, but the extent only very limited<br />
<strong>and</strong> localised adjacent to the trench.<br />
Weak rocks (including chalk)<br />
Only jetting systems which have a very high power delivery will have any impact<br />
on weak <strong>and</strong> fractured rock seabed. It is very rare for a subsea jetting system to<br />
be deployed to attempt to cut trenches into a rocky seabed.<br />
The mechanisms for forming a trench with a jetting system in weak rocks will<br />
be a combination <strong>of</strong> those that take place in cohesive <strong>and</strong> cohesionless soils. In<br />
structured rock, the mechanisms will be very similar to those in hard cohesive<br />
soil, with fluidisation being non-existent, erosion very localised, <strong>and</strong> rock breakup<br />
being dominated by hydraulic fracturing. In unstructured rock that easily<br />
breaks down into small hard fragments, both erosion <strong>and</strong> hydraulic fracture will<br />
dominate, with fluidisation contributing where the fragments are small.<br />
The mechanisms for chalk will depend on the grade <strong>and</strong> density <strong>of</strong> the material.<br />
Structureless Grade D chalk will probably behave like a cohesionless material<br />
with the silt matrix becoming easily suspended in water by the processes <strong>of</strong><br />
erosion. Since structureless chalk typically has a large range <strong>of</strong> particle sizes<br />
(from silt-size to cobble-size), it may not be possible to fluidise the material<br />
sufficiently to enable a cable to sink without losing a substantial proportion <strong>of</strong><br />
the finer material to erosion. Structured Grade A chalk would behave like a hard<br />
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