Explainer: How submarines avoid collisions at sea

On November 22, the Ministry of Defence said that an Indian Navy’s Scorpene submarine collided with a civilian fishing vessel, Marthoma, in the Arabian Sea about 70 nautical miles off the coast of Goa. This afternoon, 11 crew members of the fishing vessel had been rescued, with search operations involving six the Navy ships and aircraft were continuing to find two missing persons.

Presumably, to collide with a fishing vessel, the Scoprene would have been sailing on the surface or at shallow depth. Submarines generally operate underwater at a depth of 200-300 meters, though this varies depending upon the type of submarine, mission profile and seabed topography.

Sailing at shallow or “periscope depth”, where the submarine’s hull is just below the surface and only the tip of the periscope just above the water, is the riskiest part of sailing. This depth is maintained for carrying out a visual sweep of the area, attack, recharge batteries or for enabling certain operational activities.

There are established protocols and procedures to be followed when ascending to periscope depth or surfacing, and measures are taken to plot the positions of sailing vessels in the vicinity and ensure that the requisite distance is maintained from them.

At depth, submarines rely on sonar to detect the presence of surface and submerged vessels. Sonar is an acronym for ‘sound navigation and ranging’, a system that relies on the use of sound waves bouncing back from a surface to determine the distance and direction of an object.

Modern submarines carry a suite of sonars and other sensors to help them navigate and detect the presence of other ships. Sonars are of two types – active and passive. Active sonars emit sound waves and analyse the return signal. Given a submarine’s requirement for stealth, their use is restricted as these can give away a submarine’s position. On the other hand, passive sonar does not emit any signals but listens to sounds and audio signatures emitted by other vessels. Sonars can be hull mounted or towed in a bouy linked to the submarine with a cable.

India’s Scorpene submarines are equipped with the Low Frequency Analysis and Ranging (LOFAR) sonar system that is integrated with the Submarine Tactical Integrated Combat System which enables long-range detection and classification.

Christened locally as the Kalveri Class, the Indian Navy currently has six such submarines that are of Franco-Spanish origin and form a key component of India’s underwater attack arm. The first boat was commissioned in 2017 after being constructed in India and three more are on order.

It is not uncommon for submarines to collide with ships sailing on the sea’s surface or even with other submarines lurking in the depths of the ocean. Besides technical incidents such as fires, leaks or other malfunctions, there have also been instances of submarines hitting underwater rocks or running aground. Collisions between surface ships also happen.

The US Navy, Royal Navy Russian Navy and South Korean Navy are among maritime forces whose submarines have been involved in collisions with other warships as well as civilian vessels.

In February 2009, two nuclear submarines, the Royal Navy’s HMS Vanguard and the French Navy’s Triomphant, collided in the Atlantic Ocean. There have also been instances of US Navy submarines colliding with their surface counterparts during drills.

Poorly executed sonar sweeps, ineffective periscope search, human error, poor teamwork, failure to follow established procedures, communication gaps are among reasons attributed to collisions at sea. At times, outdated sea charts or miscalculations in navigation and poor maintenance have also been cited for accidents like grounding, leaks or on board fires.

There have also been reports of a few other incidents involving Indian submarines, both at sea or in harbour. The Indian Navy’s worst submarine-related accident took place in August 2013, when there was a heavy explosion on board the INS Sindhurakshak, a Russian Kilo-class submarine, following which it sank at its berth in Mumbai dockyard. Eighteen sailors were killed in the incident. Though the submarine was later salvaged, it was beyond repair.