US submarine may have made detectable wakes before South China Sea mishap, study suggests

On October 2, 2021, a US submarine hit a seamount while cruising at high speed in the northern part of the South China Sea.

The USS Connecticut (SSN-22) was travelling at 24 knots when it was involved in the serious accident. A subsequent US military investigation assigned blame to the negligence of the officers and crew on board as well as their unfamiliarity with the terrain.

A Seawolf-class attack submarine, a submarine as big as the SSN-22 is considered one of the most powerful underwater combat platforms of the US Navy. It can maintain silence, even at high speeds, making it difficult for enemies to distinguish its sound signature from background ocean noise.

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But when travelling in excess of 20 knots, and in such a complex and intensely monitored waterway as the South China Sea, a large submarine such as the SSN-22 can produce detectable ripples on the surface, thus giving away its position, according to a new study by Chinese scientists.

The team of researchers, led by engineer Li Yuhang with the 713th Research Institute of China State Shipbuilding Corporation, used real-world environmental data collected by Chinese sensors in the South China Sea to look at water surface wake in a bid to “offer some suggestions for the safe navigation of Chinese submarines”. They detailed their research in a peer-reviewed paper published in the Chinese Journal of Theoretical and Applied Mechanics on May 27.

When a submarine moves, it inevitably disturbs the surrounding water. Some of this disturbance rises to the ocean’s surface, and it can be detected by aircraft or satellites equipped with high-resolution radar.

These fine ripples can give clues to important information such as the submarine’s position, speed, direction or even make. However, the formation and spread of these waves are greatly influenced by environmental factors like ocean currents, temperature and salinity, resulting in significant differences in signal strength and patterns across different sea areas.

Due to military sensitivity, publicly available research in the past has focused on theoretical models while avoiding real-world data.

“Currently, research on [submarine] water surface wake in the actual ocean is still relatively scarce,” Li and his colleagues wrote in their paper.

The novelty of Li’s work lies in the use of real data from the South China Sea for the first time. This has allowed scientists and engineers to conduct “systematic research on the characteristics of the water surface response wave field excited by submarines in the actual ocean”.

And they obtained some unexpected results.

The team’s study area focused on the Dongsha Islands, southwest of Taiwan. The Dongsha Islands, also known as the Pratas Islands, is one of the most important submarine passageways in the northern South China Sea, and is frequently visited by People’s Liberation Army (PLA) anti-submarine aircraft during military exercises targeting Taiwan.

When navigating in this area, the Chinese researchers found that submarines around 100 metres long (328 feet) and more than 10 metres in diameter – similar to the size of SSN-22 – may resonate with the surrounding water, causing unexpected disturbances.

When the submarine’s depth is 100 metres and the speed exceeds 20 knots, a large number of ripples with heights up to several centimetres can be generated on the ocean surface, according to their estimate.

The Seawolf-class submarine can reach a top cruising speed of 25 knots, according to the Submarine Industrial Base Council, which involves more than 5,000 American companies providing critical materials for US submarine programmes.

The speed reading of the SSN-22 before the collision appeared in a footnote of the US military investigation report released in 2022. The investigators said the Connecticut was travelling at high speeds in the South China Sea as it headed towards Okinawa for a “humanitarian evacuation” when the mishap occurred.

While the Seawolf-class is the most advanced attack submarine of the US military, its design was completed 40 years ago when surface ripple detection mainly relied on visual observation. High-speed cruising has been considered safe even if it is close to the enemy’s coastline.

Increasing the submarine’s depth can also reduce surface waves. But in recent years, wake detection technology has made rapid progress. According to a maritime test announced by Chinese scientists in August last year, a new type of anti-submarine radar based on 6G technology can detect submarine ripples as small as tens of nanometres on the undulating sea surface.

On May 20, researchers from China’s Naval University of Engineering published a paper claiming that artificial intelligence technology can help detect submarine wakes in complex sea conditions with 96 per cent accuracy.

But it does not mean that a submarine is always safe at lower speeds. Li’s team found that significant resonance can also occur when the submarine travels at around two knots, increasing the risk of position exposure.

This particular discovery could add another challenge to any potential submarine warfare in the South China Sea. Before launching a surprise attack on land or sea targets, submarines generally need to enter preset positions at speeds of two to eight knots and rise to a depth of less than 100 metres to release submarine-launched missiles, according to some military experts.

If a submarine’s position is exposed during this period, it could have disastrous consequences, they said.

Anti-submarine forces rely on different reconnaissance platforms to confirm a target, including traditional sonar, seabed sensor networks, air-based geomagnetic anomaly detectors, high-precision gravity detectors and lasers.

Significant progress and breakthroughs have been achieved by scientists and engineers in detection technology in these areas in recent years, posing serious challenges to the stealth combat capabilities of traditional submarines.

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