Submarine cables are very reliable,
however, there are occasions when a repair to a cable becomes
Cable faults are caused by many
events, both man-made and natural. In water depths greater than
1,000 metres faults are almost always caused by natural events such
as underwater seismic activity, underwater landslides, current
abrasion etc. In water depths less than 200 metres, faults are
nearly always caused by man-made activities such as fishing and
Around 70% of all cable faults are
caused by fishing and anchoring activities and about 12% are caused
by natural hazards, e.g. current abrasion or earthquakes.
When it becomes necessary to repair
a cable, a cableship needs to be mobilised. Cableships are placed
at strategic locations worldwide and are on 24 hour standby to
carry out any repairs.
Cable systems are monitored
constantly so when a system becomes faulty, it is known
immediately. The staff in the cable stations at each end of the
cable system then commences a series of tests to locate the fault.
Once the fault location is known, this location is passed onto the
cableship so it can make final plans for the repair.
The cableship will load the
necessary spare cable and subsea plant required to carry out the
repair. It will then sail to the fault location.
Once the cableship reaches the
repair site, it will commence operations to recover the cable. The
ship grapples for the cable using a large cutting grapnel (a
hook), towing it along the seabed until it catches the cable.
The cable is then raised off the seabed and cut. In buried sections
of cable, an ROV might also be used to locate the cable and cut it
on the seabed.
|Repair Operations Video
Once the cable is cut, the grapnel
is recovered to the cableship and replaced with a holding grapnel.
This grapnel is then lowered to the seabed on a rope and towed
along the seabed until one end of the cut cable is engaged/caught.
The cable is then raised to the surface and recovered on board the
cableship. The cable is then tested and any damaged sections
removed. The cable is sealed to protect it from seawater ingress
and lowered back to the seabed and buoyed off.
This recovery process is then
repeated for the other cable end. Once all damaged cable has been
removed and the original fault identified and cut out, the cable is
jointed to a section of spare cable on board, sufficient in length
to replace the damaged section which has been cut out. This cable
is then paid out back to the cable which had been previously buoyed
off. The buoy and other cable end is recovered back to the
cableship and following tests to ensure there are no further
faults, the two ends are then jointed together. This is called the
final splice. The final splice is then lowered back to the seabed
and final tests made to ensure the cable is working correctly.
Where the repair has been made in a buried section of cable, the
cable is reburied by means of an ROV, water jetting the cable back
into the seabed to a target burial depth equivalent to that when
the cable was originally laid. A repair operation may take many
days to complete.
The final splice represents a
further hazard to fishing gear since the final splice is
essentially a bight of wire leading from the seabed to the
ship. This is held onboard by rope stoppers, then the
procedure is simply to slip both stoppers and lower the bight
of cable back onto the seabed. Cable burial of this section
is often not completely successful due to the sharp turns in the
cable and poor visibility conditions and therefore avoidance of
final splices is highly recommended.
Final Splice Bights are laid on the
seabed at right angles to cable line. The crown of the bight
may be up to 2 x water depth from chartered cable line.
|The process of jointing a cable