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| Home | Submarine cable network | Laying, burying and maintenance cables | Fishing activities | Fishing vessels |
Causes of damage in submarine cables
90% of damage to cables is caused by activities related to fishing
The most common causes are:
- Trawling, hooking, rocky sea floor, deep sea depressions
- Dredging
- Attempted recovery of lost gear
Trawling has always been the most frequent cause of damage to telecommunications cables, due as much to inaccurate plotting of the real path of cables, as to imprecise positioning on the part of fishing vessels.Hooking
As a general rule, cables are most often snagged by trawling gear with otterboards, or beam trawling gear.Trawling on very hard or rocky sea bottomsIn most cases, the cable breaks, and so does not recover to the surface.
The cable's steel outer casing can be damaged and the wires exposed in this way damage the nets.
If the frayed ends get caught in the trawl net, they form a dense tangle of knot that is both bulky and inextricable, and so holds the gear on the sea bottom.
It is very rare that a cable actually recovers to the surface, as this would require considerable power on the part of the trawler. But if this ever should occur, the cable should be left uncut at all costs, due to the high risk of a fatal electrocution accident. If one of the otterboards has become entangled, the only solution is to abandon the trawling gear.
Until very recently, trawlers scrupulously avoided working over rocky bottoms as these had the reputation of being so hazardous for trawling gear as to make fishing unfeasible.Deep sea trawlingFor several years now, advances in technology and new developments in gear have given some vessels relative freedom to fish these waters. "Diabolos" or custom designed chains, equipped with rock hoppers allow nets to pass over the rocky areas that in the past were "no-go" areas.
These new fishing zones in areas where cables cannot be buried because of the hardness of the seabed are made even more dangerous by the fact that cables are likely to be suspended tens of centimetres above the sea floor.
Trawlers undertaking this kind of fishing should take care to note the plotted route of any cables in the area, and to keep their distance from them, as it is certain that any sustained fishing in these areas will carry a high risk of hooking a cable.
The current rend for some fleets, faced with the rarefaction of some common species is to the fish the deep seas in search of other marine resources.Dredging
Trawlers working in these areas are relatively powerful, having a hefty tonnage as well as strong locomotive power.Up to the present day, the ocean deep was considered as a low risk zone, and cables located in these areas were less protected.
Hooking a cable in one of these deep-sea zones is extremely dangerous for a trawler. The length of the fishing cable (1,000 metres or more) plus the huge weight of the cable itself (heavier cables are used for the deepest water) multiply the dangers.
In addition, many of these ships tow their fishing gear with a single warp. If this should break, the equipment will be irremediably lost.Taking into consideration all the different forces involved, it is certain that despite their great power to haul gear, trawlers will have very little chance of ever recovering fishing gear hooked on a cable lying in very deep water.
Given the large area of the very deep ocean zones, it should not be too difficult to keep at a reasonable distance (one mile or more) from high-risk zones. Nowadays, the use of GPS allows vessels to position themselves with the required degree of accuracy.
The dredging of shellfish on sites where cables pass presents the same risks of hooking as for trawlers. For this reason, dredgers should strive to avoid these areas.Other causes of cable deteriorationThe teeth of the dredging bucket are particularly dangerous to the cables' protective covering. Once this has been damaged, the cable becomes more fragile and the risk of mal-functioning is increased. With time, and repeated exposure to dredging gear, the cable will deteriorate.
Dredging is usually undertaken by relatively small vessels. In the event of a hook, the danger of the boat being de-stabilised is greatly increased. The stronger the current, the great the danger, and currents are often a feature of the preferred shellfish habitat.
Anchoring (damping) is forbidden in zones across which cables pass. Protected areas are marked on navigation charts with the recognised symbols.
Vessels contravening the prohibition are punishable by law.Retrieval of trawling gear from a supposed cable hooking event is actively discouraged because of the danger involved. In any case, the chances of being able to retrieve the gear are practically nil, and moreover, the risk of hooking the cable again, this time with the grappin, is very great.
This action risks causing substantial damage to a cable. The equipment used to retrieve gear may have sharp edges which can easily pierce the cable's protective covering. Also, the risk of losing equipment in the event of hooking a cable is almost 100%. For the same reasons, the technique of towing trawling gear through the area where gear has been lost in the hope of recovering it is strongly discouraged, as it is clear a cable is present in that area, and so the vessel is liable under the terms stipulated by the legal clause stating that any deliberate attempt to damage (a cable) is a punishable offence, under the terms of article 10 paragraph 1 of the legislation in force.Anchoring of deep-sea nets can also damage cables if an anchor hooks the cable.
The technique of fileyeurs provides an easy way to avoid this risk. Knowledge of the cable's route is essential so that anchors can be laid at a safe distance (1 nautical mile).
The strong incentive for fishermen to avoid cables
- In the event of hooking a cable, the fishermen may be held responsible
- It is very difficult, if not impossible, to free gear once it has been caught in a cable
- Subsequent compensation for gear depends on what manoeuvres the vessel makes and the risks these pose to the cable
- In any event, trying to release gear will result in considerable loss of time to the fishermen.
- Subsequently the fishermen will be obliged to replace his gear, which in itself requires a great deal of work for the crew and expenditure for the ship owner
- Some ships can find themselves in especially dangerous situations when a cable is hooked
- In addition to being destabilised, the danger to the ship may be increased by currents and adverse weather conditions
- These three effects can combine and cause the boat to suddenly capsize.
The practice of attempting to recover gear is actively discouraged
Some fishermen, having attempted diverse manoeuvres to free the vessel, subject the two towing warps to maximum tension and cut one of the warps in the hope that this will free an otterboard.
THIS MANOEUVRE RESULTS ALMOST IMMEDIATELY IN CAPSIZE
The tension in the towing warp added to the weight of the warp and the cable acting in conjunction with a rebound effect will cause a sudden de-stabilisation of the vessel.How to avoid hooking a cable
Maintenance of fishing gear
Regular maintenance of gear can help reduce the risk of hooking active or inactive cables.Care of otterboards
- The leading edges of the otterboards should always be kept rounded so they pass over cables instead of hooking on to them
- Always avoid having otterboards with a rectilinear front.
- Under no circumstances should the base of the otterboard be coming away from the panel
- The attaching bolts fixing the panel bases should be screwed up with sufficient play to stop the base from hitting the otterboard. Otherwise the resulting jaw-shape or V-shape could catch on the cable and damage it.
- The soles of panels must be whole. Otterboards should not have soles that have bits broken off them.
- Where the bases are welded, they should be brought up well over the front part of the boards and nailed securely avoiding any indentations.
- As far as possible, links should be interconnected with a forged ring, which allows the warp to be attached in turn by a smaller shackle.
- All bolts on the inner surface of the otterboard should be rounded.
- Nuts on the outer surface should be countersunk to the maximum, and/or rounded.
- All the shackles associated with the trawling gear should be mounted with the rounded part going in the direction of trawl.
- Any sharp edges or protruding parts should be removed.
Variation in quality of sandy sea beds
The quality of the sea floor (sandy, muddy, rock, etc...) is determined by the materials present. The consistency of coarse sediments, pebbles or stones, as well as finer sediments such as mud, means they move very little.On the other hand, sandy bottoms are very unstable and prone to significant variations in height and position.
Very weak currents (of the order of one knot) can alter Sand bottoms. The movement of the sand forms furrows and ridges. The peaks are perpendicular to the dominant current.For these reasons, a zone crossed by a cable may carry no significant risk over a period of time owing to the fact that the cable is well buried, but later, because of displacements caused by currents or tide and then made worse by violent storms, the cable may be found lying on the sea-bottom in an exposed position without any protection from the substrate.
