The shore ends of ocean cables are frequently damaged or broken by surface ship operations (such as trawling, clamming, anchoring etc.). To protect them from such damage the cables are now buried 12 to 24 inches beneath the seabed by the SEAPLOW at the time of installation. Although this step reduces the amount of damage to cable, it increases the difficulty of doing any required maintenance.
SCARAB is an unmanned submersible designed specifically for surveying, recovering and re-burying existing buried cables. The SCARAB weighs 6,200 lbs., is provided with 20 hp for forward/aft thrust, 10 hp for up/down thrust, 10 hp for lateral thrust and 25 hp of hydraulic/water power for operation of tools. It also is equipped with two manipulator arms each capable of carrying special cable gripping and cutting tools, and a jetting tool for burying both cable and repeater.
This paper will describe some of the special design requirements of this vehicle and describe some of the problems encountered in the assembly, testing and first use of the system.
The need for continued improvement of the service reliability of submarine communications cables calls for the use of the latest technology in providing physical protection to those cables. Measures are needed, not only to protect cables from natural catastrophes, but also to protect them from human activities on the sea bed, of which the most damaging is trawling.
The most effective protective measure used against damage inflicted by trawling is burial of the cable. The primary method has been by use of a plow. Although plowing is successful, it has the following limitations and drawbacks:
Plowing is presently limited to a depth of 500 fathoms (3000 feet) while trawling activities are approaching depth of 1000 fathoms (6000 feet).
The plow, under adverse conditions, can leave cable sections and repeaters unburied.
The plow cannot be used to bury cable which is already in situ on the seabed.
Various means have been available to do the work associated with burying cables and maintaining the cables once they are buried.
Manned submersibles have been used to bury cables by jetting, locate buried cables using magnetic probes, uncover cables by jetting, cut cables, attach lifting lines to cables and conduct surveys of buried cables. However, manned submersibles typically have a limited functional bottom time of 8 hours or less. They also require a specialized mother ship so that a cable repair becomes a two-ship operation, and they are not always available on a demand basis for cable repairs.
Digging grapnels can be used to recover buried armored cables but the recovery of buried light weight cables requires a multiple-pass technique with the digging grapnel, the success of which is not guaranteed.
Divers can be used to help recover buried cables but depth and bottom time are limited.
The ocean cable owners have therefore been faced with a variety of tasks associated with the installation and maintenance of buried cables and no readily available, efficient means of performing them.
