Producer: Popular Science China
Production: Dahui
Producer: Computer Network Information Center, Chinese Academy of Sciences
On October 2, 2021, the US Seawolf-class attack nuclear submarine "Connecticut" had an underwater collision accident.
In addition to sonar systems, modern submarines also rely on a variety of systems to complement and confirm each other to ensure that underwater submarines sail safely in a given course and depth.
The red part is the part of the "Connecticut" that may be damaged (Source: CCTV News)
You may ask: Since submarines have such advanced navigation and collision avoidance technology, why do submarines collide with surface ships, submarines and submarines, and even submarines collide with seabed seamounts from time to time?
Today, we go on to talk about the "risk factors" that threaten the safety of submarines' underwater navigation.
"The sea depth is narrow", and the safe depth of submarine navigation is limited
Submarines, especially nuclear submarines, have a large body, limited space for underwater activities, the topography of the seabed and the complex and changeable water bodies in which the submarines operate, with a length of hundreds of meters, an underwater displacement of tens of thousands of tons, and a submarine speed of more than 30 knots.
Data show that the average depth of the world's four oceans is about 4000 meters in the Great Flat Ocean, about 3600 meters in the Atlantic Ocean, about 3800 meters in the Indian Ocean, and about 1200 meters in the Arctic Ocean. With such a deep water depth, can a submarine ride freely in it to "catch turtles in the five oceans"?
In fact, contrary to everyone's intuition, the ocean water is very deep, and the submarine's underwater navigation area is very narrow. It can be said that the space for submarines to move in the ocean is very stubborn.
The ocean range of submarines underwater is expressed in depth, usually divided into periscope depth, operating depth and limit depth.
The depth of the periscope is generally about 10 meters. The submarine's periscope rises and emerges from the water for radar, eye observation, reconnaissance and communication on the surface of the sea and in the air. However, the concealment and safety of submarines are poor, and they are easily discovered and tracked by opponents, or collide with surface vessels, fishing nets in the water, etc.
Virginia-class nuclear submarine "Indiana" (Source: China National Defense Daily)
The limit depth is that the submarine's pressure-resistant hull does not undergo permanent deformation, and the maximum safe diving depth of the submarine. According to the performance of submarines in different countries, it is basically maintained at 300 to 600 meters. Submarines usually stay at this depth for a short time in test or critical situations, if they are at the limit depth for a long time, the operation is slightly improper during the voyage, and some of the hulls exceed the limit water depth, the external sea pressure may crush the weak parts of the hull into the boat, and when it is serious, it can cause the submarine to sink.
The working depth is the depth at which the submarine can safely sail for a long time and carry out torpedo missile launches, etc. In order 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 deep, fishing vessels and trawls, conventional submarines usually choose about 60 to 100 meters, and nuclear submarines usually choose 100 to 300 meters left water depth as the safe working water depth.
At present, in terms of submarine hull length, conventional submarines are usually about 80 meters, attack nuclear submarines are about 100 meters, and strategic nuclear submarines are about 150 meters. The safe working water depth of submarines is theoretically only an underwater space with an altitude of 60 to 300 meters, that is, conventional submarines can only operate in underwater space with a hull length of about 3 times and nuclear submarines with a length of about 2 times its hull.
Seemingly vast and bottomless oceans, submarines have a relatively limited range of airspace, only a very shallow section of water close to the surface of the ocean.
There are many collision triggers, and submarines are difficult to navigate underwater
The ocean is vast, but the underwater navigation environment of submarines is very dangerous. The description of "the sky is high for birds to fly, the sea is wide for fish to leap" is usually not suitable for submarines that sail underwater.
In addition to the restrictions on the safety depth of submarine navigation, the seabed terrain, water body movement and underwater moving objects all have a greater impact, restriction and danger on the safety of submarine navigation. The submarine that bravely breaks through the "underwater labyrinth" can be said to be "dangerous everywhere, and the unexpected steps are frightening".
First of all, the topography of the seabed is very complex and changeable.
It can be said that there are topographic landforms on land and under the seabed, and there are not necessarily topographic landforms on the seabed. The usual topography of the seabed includes slopes, trenches, sea valleys, terraces, basins, seamounts, mounds, submarine canyons, mid-ocean ridges, depressions, submarine plains and fan-shaped accumulations. The more complex the seabed topography, the greater the change of currents and tides, the harsher the underwater navigation environment, and the lower the safety of submarine navigation.
△Schematic map of complex terrain and landform on the seabed (Source: https://www.maritimeherald.com/2018/secret-pictures-of-the-deep-sea-finally-revealed/)
If the submarine is sailing in the submarine canyon, if the flow rate of the current in the canyon is too large, it is easy to press the submarine to the shore wall or reef. This does not take into account the impact of sudden changes in the topography of the seabed caused by volcanoes, tsunamis and artificial construction on submarine navigation.
Secondly, the changes in seawater bodies are very rapid and complex.
Seawater is constantly changing at all times, and the flow and change of seawater bodies at different times, at different depths, in different sea areas and under different seabed topography. For example, in narrow waterways, the speed of the current will become larger, the tidal changes at the steep shore wall will be more obvious, and there will be extremely strong suction vortexes (submarine whirlpools) near the trenches and seamounts.
When an underwater submarine suddenly enters a largely changing water from a stable place of the current, improper maneuvering or response may occur in tilt, bumps, sudden changes in heading, and even collisions with reefs and rock walls or touching the seabed.
In addition, a sudden change in the density of seawater can cause a fatal "drop-off" accident in the submarine.
Due to the different salinity and temperature of seawater in different regions, the density of seawater does not always show an even distribution state with the increase of depth. When the density of seawater suddenly changes with the increase of depth, a density jump is created vertically. When the density of the upper seawater of the jump is less than the density of the lower seawater, 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 have the feeling of touching the seabed.
Schematic diagram of density jumps (Image source: Reference 1)
Although people figuratively refer to this kind of jump as the "soft liquid seabed", if the density of the upper seawater of the jump is greater than the density of the lower seawater, a negative gradient density jump layer will be formed, the buoyancy of the seawater will be drastically reduced from top to bottom, and the submarine will fall sharply to the seabed (that is, fall to the depth), if the depth of the fall cannot be controlled in time, the submarine will fall to the limit depth and the submarine will be destroyed.
There are many causes of collision, and it is still difficult to avoid underwater collisions in the future
In addition to the factors that may trigger submarine collisions analyzed above, the delayed updating of submarine nautical charts, abrupt changes in terrain without warning, improper navigation and positioning, tit-for-tat underwater acoustic confrontation, tailor-made anti-submarine obstacles and moving objects that meet narrow roads, etc., may have a "close contact" with submarines sailing underwater, crashing into hulls, instruments, injuring personnel, and even causing irreparable losses such as ship destruction and nuclear leakage.
It is worth noting that within the working depth range of the submarine, it is also a densely active underwater space such as floating underwater icebergs, swarms of whales swimming, and more and more unmanned underwater vehicles.
The existing statistical data analysis shows that the probability of underwater collisions is relatively high in sea areas with a large absolute number of nuclear submarines and a large number of navigation batches. For example, during the Cold War, the Barents Sea in the Soviet Union was the sea with the highest chance of nuclear submarine collisions.
In the future, with the increase in the number of multinational nuclear submarines and underwater activities, more and more nuclear submarines will be active in underwater space at an altitude of only about 2 to 3 times their own length, and the possibility of collisions between submarines, between submarines and other moving objects and underwater fixed objects still exists.
In particular, the probability of a collision between nuclear submarines will increase in some cases. In this case, in addition to the risk of nuclear submarine crashes, the possibility of underwater nuclear leakage may also increase.
Therefore, for the sake of global peaceful development and the safety of the people of neighboring countries, we should be more vigilant and prevent various accidents that may occur in the use of submarines.
bibliography:
(1) Zhang Jianhua,Huang Haifeng,Hu Kun,Liu Guangxu. Research on the Influence of Density Jump on Submarine Maneuvering Motion and Countermeasures[J].Chinese Journal of Ordnance and Equipment Engineering,2021,42(04):118-122.