Subsea Cables

print this page

Cable Burial

At the planning and installation phase of a submarine cable project one of the most effective methods of protecting a submarine cable from damage caused by external aggression is to bury the cable, usually with a sea plough.

This burial is very much dependent on suitable seabed conditions and sediments along the cable route, it should be noted that in areas where seabed conditions do not allow for cable burial other methods of cable protection may be employed, such as additional cable armouring, articulated ducting or rock placement.

Due to the nature of some areas of seabed where mobile sediments are found, cables that were buried at the time of installation may become exposed over time, therefore it should not be assumed that all submarine cables are completely protected by burial as they may become exposed and on the surface.

In water depths up to approximately 2,000 metres, the cables may be buried in a narrow (<1 m wide) trench cut by water jet or plough. The usual and most efficient burial method is by use of a subsea cable plough which is towed on the seabed behind the cableship. The cable passes through the plough and is buried into the seabed. The plough lifts a wedge of sediment so that the cable can be inserted below, thus minimising seabed disturbance to a very narrow corridor. Burial speed depends on cable type and seabed conditions but for an armoured cable, the burial speed is typically 0.2 km/hr.

If it is planned to plough bury the cable, following the survey and usually a few weeks before the main laying and ploughing operations take place, a seabed clearance operation called a Pre Lay Grapnel Run (PLGR) operation is carried out. This is to remove items of debris such as abandoned fishing nets, wires, hawsers etc. Removal of any debris ensures a clear route for the plough to negotiate so that burial can be maximised.

Cable Plough

Cable_plugh_on_beach_600px

There are also old submarine cable systems remaining on the seabed and these too are cleared from the burial route for a short distance each side. The cable ends are weighted with clump anchors so that the cable cannot be snagged by other seabed users such as fishermen.

Following plough burial, a post lay burial and inspection is normally carried out in areas where the plough could not bury, such as at cable and pipeline crossings, locations where the plough may have been recovered for repairs etc. This burial is carried out by a Remotely Controlled Vehicle (ROV), which buries the cable to the same target depth as the main lay plough but by use of water jetting. This operation also minimises seabed disturbance. At pipeline crossings, due to pipelines often being situated proud of the seabed, further protection to the cable and pipeline is normally made by means of a post lay rock placement operation. The rock placement is carried out under highly controlled conditions to place a layer of rock over the cable at the pipeline crossing location, to add a final protection layer. The rock berm is designed to minimise risk to fishing gear by specific selection of rock size and berm side slopes.

Submarine cables have been laid on the seabed since the 1850's. In most cases, recently installed cables are buried beneath the seabed to a target depth of 1 metre, unfortunately there remains a percentage of cable unburied. Cables can be scoured out by tides and currents or moved by anchors and fishing gear. Therefore cables that were considered safe from subsea activities at the time of installation may become unburied.

 

Bulletin