Argentina sub: What happens when a submarine vanishes

 In U.S.

Handout picture and released by the Argentine Navy showing submarine ARA San Juan docked in Buenos Aires, 18 November 2014Image copyright

Image caption

A search is under way for the ARA San Juan

A submarine with 44 crew on board remains missing after disappearing off the Argentine coast on 15 November.

In the last reported contact with the ARA San Juan, the sub’s captain reported a breakdown relating to a “short circuit” in the sub’s batteries.

On Thursday, eight days after going missing, the Argentine navy said an event consistent with an explosion was recorded near where the submarine disappeared.

Why can’t subs be detected?

Submarines are built to be difficult to find. Their role is often to participate in secret surveillance operations.

Dr Robert Farley, a lecturer at the University of Kentucky who has written on the subject, says that a sub is very hard to trace if resting on the seabed because under such circumstances it will not be making any “noise”.

“Noise, which would otherwise be picked up by what is known as passive sonar, is distorted and [the sub] looks – to active sonar pings – like the sea bottom,” he says.

So how can subs be found?

There are a number of ways that the captain or crew can make their location known if in distress.

These methods include sending signal calls to contacts at naval bases or allied ships, or releasing a device that floats to the surface but remains attached to the submarine.

Stewart Little, a submariner with the Royal Navy for several decades who now runs the Submarine Rescue Consultancy, says that the standard searching protocols provide two phases in each hour when searching forces go silent and listen out for signals.

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“Those submariners will be furiously making those signals when called to do so,” he told the BBC.

What do we know about what happened?

Before the submarine went missing, the crew had reported a malfunction in the vessel’s batteries caused by an intake of water through the snorkel system (by which it renews oxygen), reported Argentina’s La Nacion newspaper. However, the commander reported that the problem had been resolved.

On Wednesday, the navy said there had been a loud noise detected about 30 nautical miles (60km) north of the last-known location of the submarine. A spokesman called this a “hydro-acoustic anomaly”. On Thursday, the same spokesman – Capt Enrique Balbi – said there was an event consistent with an explosion nearby but the cause was not known. He called it an “abnormal, singular, short, violent, non-nuclear event” and said the search would continue in the same area.

What if it has sunk?

“If it had sunk in waters of less than about 180m (600ft) it’s likely that someone would have tried to escape from the submarine,” speculates Mr Little. “As that hasn’t happened, it’s probably in waters deeper than that.

“I’m hopeful that it’s less than 600m – because that’s the depth at which the assembled rescue forces are capable of operating. If they were to search at contours between 180m and 600m… that would give them the best chance of finding the submarine.”

Rescuers can’t reach below 600m, Mr Little said – and below that, each submarine has a “crush depth” at which point its structure will not be able to withstand the water pressure.

It is extremely rare for submarines to sink, Mr Little points out. It last happened in 2000, when the Russian submarine the Kursk sank in the Barents Sea, killing all 118 crew on board.

How long can a crew survive submerged?

The number of days that a crew can survive depends on how long they have already been performing duties underwater and how well prepared they are for losing power.

“If batteries were charged and air refreshed,” Dr Farley says, “then outlook is hopeful”.

In relation to the Argentine sub he adds: “Outer range appears to be 10 days if they were well prepared.”

How is the crew trained for this?

One of the most important practices is for trapped crew members to slow down their breathing rates in order to conserve oxygen.

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