Anchor Dragging: The Dominant Cause Today of Subsea Cable Damage

The conventional wisdom has been that trawlers are the primary culprit of outages. These ships deploy fishing nets attached to sand skies (my term) that slide along the sea floor. Typically they sink 40 centimeters or even more into the muck catching the very thin fibre optic cables. Sometimes the cables break. In other cases the crew intentionally cuts the cable in its haste to resume harvesting. It is illegal, but the sea has few spectators. BT studied causes of subsea damage in the vicinity of the UK using the AIS (automatic identification system) to track ships involved in incidents. The chart below shows that prior to 2006 fishing was indeed the primary villain. But an evolution has occurred. Global trade expanded over the period while at the same time governments and voluntary organizations like Kingfisher worked hard to protect subsea cables by providing accurate maps of cable routes. Since global trade has only expanded since 2010 it is likely that the trend has continued. Ships dragging anchors caused all the outages of power and fibre optic cables in Baltic Sea in 2024. An abandoned ship, the Rubymar, drifted across the Red Sea with its anchor slicing through the AAE1, Seacom, and Eassy cables in the spring of 2024. This evolution probably also reflects depletion of fish stocks close to shore as well. Ships must travel farther from shore to harvest. 

Obviously ships drop anchor from time to time. Once upon a time Pakistan lost all its submarine international connectivity when a ship dropped anchor near shore and crushed the only two cables serving that country. People unschooled in subsea cables have used anchor dragging as evidence of Russian government involvement. This is a bad argument. A Chilean ship severed three Trans-Atlantic cables in 2002 when it dragged its anchor for 350 kilometers. The anchor had not been properly secured and sank to the bottom of the ocean. The captain thought the slowdown in speed was due to a gale. These anchors are typically five to ten tons. Not really a lot of weight for a cargo ship that can weigh from 50,000 tons to 200,000 tons at the high end. 😃 An oil tanker off  Sicily's coast in 2008 dragged its anchor for 300 kilometers cutting 6 subsea cable outages at shallow depths up to 180 meters deep. Anchor dragging often occurs during bad weather suggesting to me that captains use it to stabilize the ships or perhaps they are less likely to notice they have not properly secured the anchors because they are distracted. In any case sabotage should not be the prime suspect, but rather incompetence.