Review of Cabling Techniques and Environmental Effects Applicable

Review of Cabling Techniques and Environmental Effects Applicable to the Offshore Wind
Farm Industry – Technical Report
from the trench area as suspended sediment. The extent to which the sediment
will be spread is dependent on the current velocity and the particle size. For
example, coarse sand and gravel will settle out on the seabed very close the
trench, especially in a slack current. In contrast, silt and fine sand will remain
in suspension longer, and could be transported for a significant distance in a
strong current.
If the jetting system only fluidises the soil to allow the cable to sink through it,
the impact will be negligible, since there will be no sediment displacement.
Cohesive soils
Liquefaction/fluidisation is a principal mechanism in the breaking up of
cohesionless soils during water jetting because the pore pressures in the soil
mass are able to respond relatively quickly to the increase in water pressures
caused by the jets. In contrast, in cohesive soils (clays), the response is relatively
slow, by which time the soil has been broken up by a different mechanism.
In cohesive soils, the general erosive potential is much reduced due to the
cohesive bonds between particles (Dunn, 1959; Flaxman, 1963). However,
localised erosion and scour enables the jets to begin to form cuts in the solid
material. Each jet tends to cut its own slot based on the various directions of
the jets. The rate at which this cutting process takes place depends on the soil
strength and the jet velocity/pressure at the cutting face (Lincoln, 1985). However,
in very soft clays the jets may erode soil in a similar mechanism to that for sand.
As the jetting proceeds, the high water pressures in the cut slots rapidly cause
hydraulic fractures to develop (Hubbert & Willis, 1957; Jaworski et al., 1981;
Murdoch, 1993). These develop along planes of weakness and have a tendency
to propagate perpendicular to the direction of minimum principal stress.
Propagation may therefore be expected in both vertical and horizontal directions
according to the depth of the jet slot. It is probable that most propagation will be
in the vertical plane causing cracking and failure of the forward face.
In stiff or hard clays, certain jetting systems utilise a number of high pressure
jets which are used to cut sections of the clay face. These are combined with
lower pressure jets on the same jetting tool to then ‘break up’ the fractured clay
face.
As the jetting system advances, the inward pointing jets, particularly at the base,
will further break up the already broken lumps of soil and finalise the cutting of a
complete trench, provided the progress is sufficiently slow. If an attempt is made
to advance the system too quickly, the jetting tool will come into contact or near
contact with the soil face and the tow force will significantly increase.
Normally, a dredging system needs to be used in combination with the jetting
system in firm to hard clays, otherwise it is very difficult to remove the cut
material away to form a trench. However, even when a dredging system is
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