Strong underwater wave may have caused Indonesian submarine to sink, navy says

The KRI Nanggala-402 submarine was due to carry out a torpedo drill after it asked for permission to dive early on April 21, but contact was then lost. PHOTOS: REUTERS, EPA-EFE

JAKARTA/BANYUWANGI - A strong underwater wave might have been what caused an Indonesian submarine to descend quickly and sink to a depth of 850m in the north of Bali last week, killing all 53 crew members on board.

Two senior naval officers said the movement of a strong internal wave in the spot where the submarine was reported missing was on the image report of Japanese weather satellite Himawari-8 as well as a European one.

Internal waves, or underwater waves, are hidden entirely within the ocean. The difference between an underwater wave and the water around it is their densities because of different temperatures or salinity.

"If we are hit by an internal wave (coming from above), that would be nature we are up against. We would be dragged by the waves, sending us to a quick descent. No one can fight nature," said Rear-Admiral Iwan Isnurwanto, a former submariner, during a media briefing in Jakarta on Tuesday (April 27).

The commander of the Navy Staff and Command School explained that there is a seawater density difference between the Lombok Strait and the much deeper waters in the north of Bali, where the KRI Nanggala-402 submarine was found.

On April 21, the day of the accident, there was a massive movement due to density difference - from high to low - that spawned an enormous internal wave coming from Lombok Strait, between 200m and 400m deep, to waters in the north of Bali, where the depth is generally more than 1,000m.

Rear-Adm Iwan, citing the satellite image report, said: "We are talking about 2 million to 4 million cubic m of water hitting you. Could anyone overcome that? The Nanggala submarine descended 13m and could have been caught in the internal wave."

Rear-Admiral Muhammad Ali, the planning and budget assistant to Indonesia's navy chief, said such weather phenomenon will be taken into account in future operations and the navy will encourage further research into it.

"An investigation is still ongoing. It would take a while. We will invite submarine experts, not only from within, but perhaps also from overseas. We have international submariners' conferences. The Asia-Pacific submarine conference is held every two years and has been attended by all submariners worldwide," he said at the media briefing.

Indonesian navy spokesman Julius Widjojono had earlier said contact with the submarine had likely been lost at 600m to 700m underwater, while by design, the vessel could withstand a depth of up to 500m.

The 44-year-old submarine was due to carry out a torpedo drill after it asked for permission to dive early on April 21, but contact was then lost.

The navy also said earlier that a possible power blackout during the static dive may have caused the submarine to lose control and made it unable to perform emergency procedures. The navy has not ruled out this possibility.

Singapore's MV Swift rescue vessel on Tuesday helped in salvage efforts, Rear-Adm Ali said. Torpedo missiles the submarine was carrying, each weighing about 2 tonnes, were also located.

"Efforts now are focused on retrieving smaller items, as the remotely-operated underwater vehicle deployed is capable of lifting materials up to 150kg. Arrangements to lift heavier items are being made," he added.

Meanwhile, the families of those who died in the incident have called for the authorities to retrieve their bodies from the sea.

People throwing flowers and petals with names of the sunken KRI Nanggala-402 submarine crew members from a boat in Bali on April 26, 2021. PHOTO: REUTERS

"We understand it may take time," Mr Wahyudi, 52, the father of 22-year old torpedo shooter Pandu Yudha Kusuma, told The Straits Times in Banyuwangi.

"Families - wives, children and parents - want to be given a chance to take care of them for the last time."

Mr Wahyudi, an army officer, said while he grieved, he was also proud of his son, a Banyuwangi native.

"This is the last show of pride to his parents - passing away while performing a state duty and becoming a hero," he said, noting that Mr Pandu had understood well the risk of being a submariner.

President Joko Widodo is set to present compensation to the families of the submarine crewmen at Surabaya's Juanda naval air base on Thursday.

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