SINGAPORE: As rescuers race to find the missing Indonesian submarine KRI Nanggala 402, the worry is whether this could be done before stricken crew members run out of oxygen.
But a naval expert said there is a more lethal killer: The carbon dioxide build up in the cabin could suffocate crew even before the oxygen is used up.
This comes after the Indonesian Navy chief said on Thursday (Apr 22) that during a power blackout, the submarine would have enough oxygen for 72 hours, or until 3am (4am Singapore time) on Saturday.
The navy has said that a blackout could have occurred when the submarine was carrying out a static dive, preventing crew from controlling it or executing emergency procedures to resurface.
READ: Indonesian navy searching for missing submarine with 53 on board; oil spill found near dive position
The German-made submarine went missing on Wednesday with 53 people on board when taking part in a torpedo drill in the waters north of Bali. Contact with the vessel was lost at about 4.30am, after it asked for permission to dive at 3am.
A total of 21 vessels, five planes and two submarines have been deployed in the search operation, authorities said.
Singapore’s submarine support vessel MV Swift Rescue will join the operation and most likely reach Bali on Saturday, while Malaysia’s MV Mega Bakti is set to arrive on Sunday afternoon.
READ: Missing submarine: Oxygen will run out in 72 hours during a power blackout, says Indonesian navy chief
Time is of the essence in submarine rescues, and senior fellow at the US' Hudson Institute Bryan Clark told CNA that the "most significant problem" was the build up of carbon dioxide.
"There should be absorbent material on board to pull carbon dioxide out of the (cabin) atmosphere, but that will eventually run out," said Mr Clark, an expert in naval operations and a former submariner.
"Oxygen can be generated by chemical candles on board, but carbon dioxide will suffocate the crew before the oxygen runs out."
Mr Clark said the power blackout will have an impact on how long crew can survive, as power is needed to run the fans that move air through the oxygen candles and carbon dioxide absorbent material.
WHAT COULD HAVE CAUSED POWER BLACKOUT?
But what could have caused the power blackout in the first place?
Submarines that have gone underwater depend on their battery for power, and a battery casualty, like a fire or flooding of the battery compartment, could have caused a blackout, Mr Clark explained.
"Otherwise, because of redundant circuits and switchboards, multiple (equipment) casualties would be needed to cause a complete power failure," he added.
A torpedo explosion could also have led to a power blackout, said Mr Ben Ho, a naval analyst at the S Rajaratnam School of International Studies.
"Throughout history, a range of factors, including structural defects as well as accidental flooding and weapons explosions, have been responsible for major submarine accidents," he told CNA.
"Given that the Indonesian submarine was reportedly conducting a live torpedo firing exercise, it is possible that one of these weapons exploded."
LOCATING THE SUBMARINE
If the submarine remains intact, it will be large enough to be located "relatively easily" using magnetic and acoustic sensors, Mr Clark said.
Magnetic sensors, similar to a mine detecting system, can look for the submarine's steel hull. Acoustic sensors include active sonars that bounce and detect sound off the submarine.
"But the area over which they need to search is relatively large and the sensors can only see over a small area," Mr Clark said. "The search can therefore take a long time."
Mr Ho said the active sonars could also find it difficult to sniff out the wreckage if the seabed is cluttered.
"By their nature, submarines are already difficult to detect during regular operations, hence the nickname of ‘silent service’ being bestowed on their community," he said.
"What more one that is possibly distressed and unable to communicate with the outside world."
Rescuers are searching waters 96km from Bali, including near the dive position where an oil spill was found. The oil spill could mean damage to the submarine's fuel tank or a signal from its crew, the Indonesian Navy said.
The navy had said on Wednesday that the submarine could have fallen to a depth of between 600m to 700m, but the next day authorities said they could not confirm this.
The submarine was built to sustain pressure at a maximum depth of around 250m, an official said.
If the submarine is indeed stuck at depths of 600m and below, Mr Ho said its crew have a "virtually zero" chance of survival.
"The grim reality is that once a submarine exceeds its crush depth, it will implode from the tremendous water pressure being exerted on it," he said.
If this happens, it will be picked up by a listening sensor in the vicinity, he added.
Once the submarine is found, this is where Singapore's MV Swift Rescue could come in.
According to the Ministry of Defence website, the Republic of Singapore Navy launched this submarine support vessel in 2008, making it the first in Southeast Asia to acquire submarine escape and rescue capabilities.
The vessel, which can stay at sea for four weeks before needing to refuel, is equipped with a submersible rescue vessel called Deep Search and Rescue Six (DSAR 6). This is used to evacuate submarine crew.
The DSAR 6 is launched from the MV Swift Rescue before going underwater and docking with the distressed submarine.
"Submarine hatch mechanisms are common to almost all nations’ submarines to enable rescue," Mr Clark said.
However, Mr Clark warned that the "biggest challenge" is the orientation of the distressed submarine. "If it is lying on its side, for example, the DSAR 6 may not be able to dock to it," he added.
According to the navy news website Naval Technology, the 9.6m-long DSAR 6 can reach a depth of 500m. It is operated by two crew members and can accommodate up to 17 personnel.
When DSAR 6 resurfaces and is recovered by the MV Swift Rescue, the submarine crew would be transferred to a recompression chamber for treatment. The chamber can hold up to 40 people.
The MV Swift Rescue also has an eight-bed high dependency ward and helipad for further medical evacuation.
Singapore and Indonesia signed a submarine rescue support and cooperation agreement in 2012, allowing both countries to send resources and help each other if their submarines are in distress.
"Submarine rescue is a complex and multi-faceted undertaking by its very nature and having such agreements facilitates the process should it ever come to that," Mr Ho said.
"They usually entail cooperation on technical issues such as drawing up a common standard operating procedure, as well as the sharing of information and expertise."