Burying Fibre Optic Subsea Cables In Shallow Waters: The How And Why

It is standard practice to bury subsea cables in shallow waters. This generally means the ocean or sea lying above the continental shelf. The shelf is really just that part of the continent that is submerged under water during the warm periods between the earth's recurring ice ages. During interglacial periods like now the shelves remain submerged and during each ice age the shelves become dry land as the sea levels fall due to less precipitation. Precipitation declines as the earth becomes colder and water is locked up in snow, ice, and glaciers on dry land. Indeed, during the last ice age the oceans were about 130 meters lower than today's levels and the continental shelves were dry land. In general, the continental shelves range from 100 to 200 meters below the water surface. At the continental shelf's edge the depths plunge down a steep slope to the bottom of the ocean.

The purpose of burying is to protect a fibre optic cable from its most ferocious predators and enemies - ships and undersea debris slides. Ships typically damage cables by dropping or dragging their anchors in rough weather or by use of fishing nets that literarily scrape the ocean's floor and are strong enough to break a cable. The risk is so serious that maps are published so that fishing boats can avoid these areas. Click on this link to see samples of these maps: https://marinescotland.atkinsgeospatial.com/nmpi/default.aspx?layers=519. A cable ship buries a cable by pulling a subsea plough behind it which uses high pressure water cannons to scoop out a ditch on the sea's floor into which the cable is dropped. Ocean floors rarely have exposed solid rock, but rather consist of soft sediment and silt meters deep. This reflects the cycle of life as living creatures die and their remains descend to the ocean floor. In the early days of fibre optic cable laying Nature was allowed to gradually cover up the exposed ditch via deep sea currents which would feed silt and sediment into the ditch and fill it up. More recently, the standard practice is to use high pressure hoses to blow sand and marine debris into the ditch after the cable has been inserted. In part, this reflects the fact that these subsea cables have enough slack to drift for kilometers if not properly weighed down. If sediment layers are not stable and can be brushed aside by unusually strong currents, then the cable becomes exposes and can travel. Contacts tell me that the recent SWM5 cable repair in the Malacca Straits took longer than expected because the subsea cable drifted 15 kilometers from the original burial path. Properly executed burial would have made this less likely as well as protected it better from ship-related damage. 

Burial depths have dramatically increased over time. Initially, they were one to two meters. As technology has improved and with strong financial pressure to extend the life expectancies of expensive new subsea networks, burying two to three meters has become common. For example, EUNetwork's Scylla cable lies a full three meters below the English Channel sea floor. Reducing outages not only improves network performance, but also sharply increases life expectancy. Fewer repairs mean the submarine cable accumulates less total optical loss due to fibre splicing and hence extends its life. A cable that requires lots of repairs may reach its end after 20 years of service versus a cable lasting 30 years if repairs are few and far between. Obviously the rate of return on the infrastructure is much higher in the latter case. Once optical loss reaches a critical threshold, available power is no longer sufficient, and the cable will be shut down and decommissioned.