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 />
Structureless chalk, as for silty soils, is also susceptible to mixing <strong>and</strong> dispersion<br />
in the surrounding sea water during ploughing.<br />
4.3.2 JETTING SYSTEMS<br />
Jetting systems are one <strong>of</strong> the most commonly employed cable burial tools<br />
used by tracked vehicles (Section 3.8.3), ROVs (Section 3.8.4), <strong>and</strong> burial sleds<br />
(Section 3.8.5). The mechanisms employed by jetting systems for developing a<br />
trench will largely depend on the soil type.<br />
Cohesionless soils<br />
In cohesionless soils, the soil can be liquefied or fluidised with jets at relatively<br />
low pressures (i.e. low jet exit velocities). Liquefaction or fluidisation occurs<br />
when the pore water pressures in the soil become equal to the total overburden<br />
stresses, reducing the effective stresses to zero. In both cases, the soil particles<br />
<strong>and</strong> water behave very much like a dense fluid. However, there is a distinct<br />
difference in the two conditions. In liquefaction, the volume <strong>and</strong> bulk density<br />
is more or less constant (Ishihara, 1993). In fluidisation, the water content is<br />
increased <strong>and</strong> the soil structure is completely broken down to give a material <strong>of</strong><br />
lower density. An increase in the jet flow rate at low jet pressures simply causes<br />
an increase in the volume <strong>of</strong> the liquefied/fluidised soil (N.B. an increase in flow<br />
rate at constant pressure can only be achieved by increasing the total nozzle<br />
area). If the jet pressure/velocity is increased, the velocity <strong>of</strong> the fluid flowing<br />
over the surface <strong>of</strong> the soil will increase until eventually the soil particles are<br />
lifted <strong>of</strong>f <strong>and</strong> transported away from the soil mass as suspended sediment. This<br />
is the process <strong>of</strong> erosion or scour.<br />
A trench in cohesionless soil is thus created by a process <strong>of</strong> erosion/scour.<br />
Jetting systems are sometimes used in combination with a dredging system<br />
to increase the rate <strong>of</strong> removal <strong>of</strong> soil (see Section 4.3.3). The problem with<br />
liquefaction in cohesionless soil is that the trench walls collapse <strong>and</strong> flow back<br />
into the trench. This means that a lot <strong>of</strong> soil has to be removed before there is a<br />
significant increase in trench depth. The final trench shape in cohesionless soils<br />
tends to have very gentle sloping sides (Lincoln, 1985). It may require several<br />
passes before a trench is created with sufficient depth.<br />
Due to the difficulty in forming a trench in cohesionless soils, some jetting<br />
systems do not even attempt to create one. The “fluidisation train” used for<br />
burying <strong>of</strong>fshore pipelines in cohesionless soils is one such example (Boom,<br />
1976). The train works by fluidising the soil beneath a pipe so that the pipe can<br />
sink under the combined weight <strong>of</strong> itself <strong>and</strong> the train. Water is injected into the<br />
soil around the pipe at relatively low pressure, which is just enough to cause<br />
fluidisation but not necessarily erosion.<br />
The likely effects will depend on which mechanisms take place. If a trench is<br />
formed by erosion, a substantial amount <strong>of</strong> material may be transported away<br />
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