Review of Cabling Techniques and Environmental Effects Applicable

Cable types and installation techniques
to then attempt re-burial of the final repair splice of cable. The Free Swimming
Burial ROVs used jetting systems to attempt the cable burial, but with low
power budgets they only had limited success in sandy seabeds. SCARAB 1 and
SCARAB 2 were the first vehicles of this type. Both of the vehicles were equipped
with subsea power packs, control and telemetry functions via a control umbilical
and had depth capabilities of 1000m. As subsea ploughs were introduced for the
burial of subsea telecoms cables, the Free Swimming Burial ROVs developed into
more sophisticated ROVs capable of a range of subsea intervention tasks with
larger power budgets for cable burial. This next generation of Free Swimming
Burial ROVs had cable burial tools with low and high pressure jetting tools and
dredge units to allow the vehicles to work in sandy, clay and gravel seabeds.
The SCARAB 4 vehicle which came into service in 1990, was fitted with a state
of art rotating cylinder jetting tool with a series of high pressure jets capable of
working into 400KPa clays.
Free Swimming Burial ROVs have generally stayed in the niche market area of
cable repair and maintenance. However, these vehicles have seen significant
technological development with jetting tools now capable of deeper burial using
“jetting swords” which sit either side of the cable to be buried, and with depth
ratings down to 2500 and 3000m.
Table 3.12 provides a listing of a number of free swimming cable burial vehicles
which are used worldwide.
Some of the current Free Swimming Burial ROVs can interface to a tracked work
package. This provides the Free Swimming Burial ROV the opportunity to have
a stable work platform for burial operations and to revert to free swimming
mode when inspection and intervention tasks are required as well as more
manoeuvrability, especially in the deep water operations. Figures 3.20 and 3.21
show the CM ROV3 cable burial vehicle. This vehicle can operate either in free
swimming mode or can have a modular tracked unit fitted (as shown in the
Figures). Figure 3.21 shows the jetting lances in the fully deployed condition.
Some Free Swimming Burial ROVs now have power budgets of over 300kW and
are equipped with manipulators for handling tasks, together with cable cutters,
cable grippers and burial tools fitted to both the forward and rear sections of the
ROV. In addition jetting lances, which are fitted to the end of a manipulator arm,
allow for localised burial.
Dredging units fitted to Free Swimming Burial ROVs have a particular use when
working far offshore in deeper waters, where seabeds can be encountered which
consist of shell beds held together by fine sands. These seabeds are particularly
difficult to trench using jetting systems, as the energy from the water jets deflects
off the hard shell surfaces. This can be countered by using a combination of
dredging and jetting as the dredging sucks in the sand and shells and breaks up
the layer composition.
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