Abstract:
Fibre
optic telecommunication cables laid across the seaoor are buried in shallow
water
depth (<2000m) for protection against hazards arising from commercial shing
and
shipping activity. The cables are buried in a trench created by a sea plough, often
jet assisted and towed from a ship, or by a ROV with jet legs straddling the cable and
uidising the soil around it. Recent trends in the industry require more versatile burial
tools, so a sound understanding of their fundamental mechanics is required to enable
their
optimun design and performance. The aim of this research program was to study
the mechanics of force reduction on
jet assisted cable burial tools.
The
experimental program consisted of two stages, both conducted in controlled
submerged conditions. The rst studied the effects of jet parameters, tool rake angle
and
pore pressure on tool force reduction. The second stage studied the action of a
single horizontal buried jet on the surrounding soil, in which the rst series of
experiments studied a static jet nozzle in sand and clay, and the second a dynamic jet
nozzle. The flow rate or nozzle Velocity was varied in each respectively.
The rst
stage showed force reduction was caused by the reduced soil stress on the
tool face in areas
intersecting uidised sand. The larger the uidised area (FA), or the
lower its intersection, the greater the force reduction. Evenly spaced nozzles gave
greater FA coverage of the tool face. Interaction between jet and rake angle and force
was
complex, but upward angled jets and forward raked tools gave least force
reduction. Results of the second
stage showed cavity formation in sands characterised
by shear erosion whereas in clay by pressure fracturing. The cavity size in sands was
directly proportional to jet momentum ux and inversely proportional to tool Velocity.
Mathematical models were developed from each stage, the rst to simulate tool force
reduction created by the jets, given knowledge of the FA, and the second to simulate
the FA created by a single jet. The second over predicted cavity length by a average
of 7% over the
range of tool velocities tested. The combined models over predicted
tool force, and suggested reasons for the discrepancies are given. Further research is
required to rene the model and provide a useful tool for the design and operation of
jet assisted cable burial tools in saturated sands.
