Causes of Subsea Cable Outages

Academic studies generally suggest that ships cause anywhere from a plurality to a majority of subsea cable outages. The percentage varies over time due to natural variability. The number one villain among fishing boats are the bottom trawlers. Their nets scoop up fish and shellfish on the sea floor generally in shallow waters where sea life is more abundant due to higher oxygen and nutrient levels (plankton need light). It is in the range from 100 to 200 meters below sea level that trawlers cause the most damage.  Frequently the otter boards that support the nets dig deep into the sediment cutting or damaging subsea cables. See the diagram below. 

The general consensus is that over the last 40 years fishing's relative contribution to subsea outages has been falling. This reflects depleted open sea  fisheries (fish farming has largely replaced them) as well as deeper cable burial and more emphasis on prevention. Another reason is the rise in global shipping as exports grow relative to global GDP. As international shipping becomes more important, anchor damage has risen. Anchors generally weight five to ten tons. This means when a ship drops anchor it will sink through the seafloor sediment destroying cables buried a meter or less. But the most likely cause is anchor dragging like what happened in the Red Sea this spring. It is standard procedure for ships to drag anchors in rough weather to steady themselves and prevent capsizing. 

As the chart below shows, other factors play a role as well. For example, icebergs close to shore in arctic regions often scrape the seafloor taking out communication cables. Dredging the coast line, ports or rivers to enable bigger ships to pass has snapped cables. Geological threats include debris slides lin subsea canyons ike the one that crushed 4 African cables earlier this year or earthquakes triggering enormous undersea mud slides like the ones that severed 11 cables off Taiwan's cost in 2006. Component failure is relatively rare and declining over time as technology improves. Abrasion is a deep sea problem where cables are tossed around by surprisingly strong deep sea currents. Consequently, the polyethylene layer is eroded away. This leads to an electrical short or eventually damaged fibre strands.