"The sea is wide and the fish can leap for it" is just a beautiful imagination of submarine underwater navigation

Produced by: Science Popularization China

Produced by: Dahui

Producer: Computer Network Information Center, Chinese Academy of Sciences

On October 2, 2021, the U.S. Seawolf-class attack nuclear submarine USS Connecticut had an underwater collision accident.

In addition to sonar systems, modern submarines also rely on multiple systems to complement and verify each other to ensure that underwater submarines can safely navigate at a predetermined course and depth.

The red part indicates the possible damage to the USS Connecticut (Photo source: CCTV News)

You may ask: Since submarines have such advanced navigation, positioning and collision avoidance technology, why do accidents of submarines colliding with surface ships, submarines with submarines, and even submarines with seamounts on the seabed still occur from time to time?

Today, let’s continue to talk about the “dangerous factors” that endanger the safety of submarine underwater navigation.

"The sea is deep and the road is narrow", the safe depth of submarine navigation is limited

Submarines, especially nuclear submarines, are large in size, have limited underwater space, and the seabed topography and the water bodies in which the submarines operate are complex and changeable. It is indeed difficult for nuclear attack submarines, which are hundreds of meters long, have an underwater displacement of tens of thousands of tons, and can reach a submerged speed of more than 30 knots, to achieve safe navigation.

Data shows that the average depth of the world's four oceans is about 4,000 meters for the Pacific Ocean, 3,600 meters for the Atlantic Ocean, 3,800 meters for the Indian Ocean, and 1,200 meters for the Arctic Ocean. With such great depths, can submarines "catch turtles in the five oceans" at will?

In fact, contrary to everyone's intuition, the ocean is very deep, but the underwater navigation area of ​​the submarine is very narrow. It can be said that the submarine's activity space in the ocean is very cramped.

The ocean range in which a submarine navigates underwater is expressed in depth, which is usually divided into periscope depth, working depth and limit depth.

The depth of the periscope is generally about 10 meters. When the submarine's periscope is raised and exposed to the water, it can conduct radar and visual observation, reconnaissance and communication on the sea surface and the air. However, the submarine's concealment and safety are poor, and it is very easy to be discovered and tracked by the enemy, or to collide with or entangle with ships on the sea surface, fishing nets in the water, etc.

Virginia-class nuclear submarine USS Indiana (Photo source: China National Defense News)

The limit depth is the maximum safe diving depth of a submarine without permanent deformation of its pressure-resistant hull. It is generally maintained at 300 to 600 meters, depending on the performance of submarines in different countries. Submarines usually stay at this depth for a short time during tests or emergencies. If they stay at the limit depth for a long time, or if the operation is slightly improper during navigation, and part of the hull exceeds the limit depth, the external seawater pressure may break the weak parts of the hull and enter the submarine, which may cause the submarine to sink in serious cases.

The working depth is the depth at which a submarine can safely sail for a long time and launch torpedoes and missiles. To prevent being discovered by satellites, anti-submarine patrol aircraft, etc., and to avoid collisions with supertankers with a draft of 20 to 30 meters, fishing boats trawling in the water, etc., conventional submarines usually choose a water depth of about 60 to 100 meters, and nuclear submarines usually choose a water depth of about 100 to 300 meters as a safe working depth.

At present, the length of a conventional submarine is usually about 80 meters, that of an attack nuclear submarine is about 100 meters, and that of a strategic nuclear submarine is about 150 meters. The safe working depth of a submarine is theoretically only 60 to 300 meters underwater, that is, a conventional submarine can only operate in an underwater space about three times its hull length, and a nuclear submarine can only operate in an underwater space about two times its hull length.

Although the distant ocean seems vast and bottomless, the navigable space for submarines is relatively limited, with only a very shallow water layer close to the ocean surface.

There are many causes of collision, and submarines face many difficulties in underwater navigation

The ocean is vast, but the underwater navigation environment for submarines is very dangerous. The description of "the sky is high and birds fly freely, the sea is wide and fish leap freely" is usually not suitable for submarines sailing underwater.

In addition to the limitation of safe depth of submarine navigation, the seabed topography, water movement and underwater moving objects all have a great impact, limitation and danger on the safety of submarine navigation. For submarines bravely entering the "underwater maze", it can be said that "danger exists everywhere and accidents are thrilling at every step."

First, the seabed topography is very complex and varied.

It can be said that all the landforms and geomorphology on land are also found under the sea, while some landforms and geomorphology on the seabed may not be found on land. Common landforms and geomorphology on the seabed include sea slopes, trenches, sea valleys, terraces, basins, seamounts, sea hills, submarine canyons, mid-ocean ridges, depressions, submarine plains, and fan-shaped accumulations, etc. The more complex the seabed landforms and geomorphology, the greater the changes in ocean currents and tides, the worse the underwater navigation environment, and the lower the safety of submarine navigation.

Schematic diagram of complex seabed topography

(Image source: https://www.maritimeherald.com/2018/secret-pictures-of-the-deep-sea-finally-revealed/)

For example, when a submarine is sailing in a canyon, if the speed of the current in the canyon is too fast, it is easy to press the submarine against the shore or reef. This does not include the impact of sudden changes in the seabed terrain caused by volcanoes, tsunamis and artificial construction on submarine navigation.

Secondly, the changes in seawater are very rapid and complex.

Seawater is constantly changing and flowing, and the flow of seawater varies greatly at different times, depths, sea areas, and seabed topography. For example, the speed of currents in narrow waterways will increase, tidal changes will be more obvious near steep shores, and there will be eddies (seabed whirlpools) with extremely strong suction near trenches and seamounts.

When a submarine sailing underwater suddenly enters waters with large changes from an area with stable ocean currents, improper operation or untimely response may cause it to tilt, pitch, suddenly change its course, or even collide with reefs, rock walls, or touch the seabed.

In addition, sudden changes in seawater density can cause submarines to suffer fatal "depth-falling" accidents.

Due to differences in salinity and temperature in different areas, the density of seawater is not always evenly distributed as depth increases. When the density of seawater changes suddenly with depth, a density jump layer will be generated in the vertical direction. When the density of the seawater above the jump layer is less than the density of the seawater below, the buoyancy of the seawater increases sharply from top to bottom, and a barrier will be formed between the upper and lower layers, and the submarine will feel like it is touching the seabed.

Schematic diagram of density jump layer (Image source: Reference 1)

Although people figuratively call this jump layer a "soft liquid seabed", if the density of the seawater above the jump layer is greater than that of the seawater below, a negative gradient density jump layer will be formed. The buoyancy of the seawater will decrease sharply from top to bottom, and the submarine will fall sharply to the seabed (i.e., fall in depth). If the depth cannot be controlled quickly in time, the submarine will be destroyed and people on board will die when it descends to the maximum depth.

There are many causes of collisions, and underwater collisions will still be difficult to avoid in the future

In addition to the factors that may cause submarine collisions analyzed above, delayed updating of seabed charts, sudden terrain changes without warning, improper navigation positioning, tit-for-tat hydroacoustic confrontation, tailor-made anti-submarine obstacles and moving objects that meet on a narrow road may all have a "close contact" with submarines sailing underwater, damaging the hull and instruments, injuring personnel, and even causing irreparable losses such as submarine destruction, death, and nuclear leakage.

It is worth noting that within the submarine's operating depth range, there are also underwater spaces with intensive activities such as floating underwater icebergs, schools of swimming whales, and an increasing number of unmanned submersibles.

Existing statistical data analysis shows that the probability of underwater collisions is relatively high in seas with a large number of nuclear submarines and a large number of sailing batches. For example, during the Cold War, the Soviet Union's Barents Sea was the sea area with the highest probability of nuclear submarine collisions.

In the future, with the increase in the number of nuclear submarines and underwater activities of many countries, more and more nuclear and conventional submarines will be operating in underwater space at an altitude of only about 2 to 3 times their own length. The possibility of collision between submarines and between submarines and other moving objects and underwater fixed objects still exists.

In particular, the probability of nuclear submarine collisions may increase under certain circumstances. In this case, in addition to the increased risk of nuclear submarine accidents, the possibility of underwater nuclear leaks may also increase.

Therefore, for the sake of global peace and development and the safety of people in neighboring countries, we should be more vigilant and take precautions in advance against various accidents that may occur during the use of submarines.

References:

(1) Zhang Jianhua, Huang Haifeng, Hu Kun, Liu Guangxu. Research on the influence of density jump layer on submarine maneuvering and countermeasures[J]. Journal of Ordnance Equipment Engineering, 2021, 42(04): 118-122.