Underwater explosion analysis (UNDEX)
The underwater explosion is a chemical phenomenon in which an explosive explodes under the surface of the water due to chemical reactions. The product of this explosion is a very hot gas (about 3000 degrees Celsius) with high pressure (14 to 28 MPa). When an explosion occurs, a wave called a pressure wave is formed, which if it is faster than the speed of sound is called a shock wave.
Accidents that occur in the marine environment can be prevented from recurring in the future by investigating the incident and carefully investigating it. In the past, for example, a single-walled tanker structure was built; But in light of the high number of oil tanker crashes over the past 100 years and the spill of large amounts of oil and other derivatives into the seas that destroyed many animals, in 1992 the IMO passed a law requiring all tankers to January 2014 Two bodies will be built. This law emphasizes the importance of taking a closer look at the proper design of high-reliability offshore structures.
Underwater explosions are different from explosions that occur on the surface of the earth due to the characteristics of water. Water has a much higher density (about 1000 times) than air, which makes the water more difficult to move than air (higher inertia). On the other hand, water is more incompressible than air. That is, it must be at a pressure of about 100 atmospheres (about 10 MPa) to condense and increase its density. Hence it absorbs less of the explosion energy. These two things make an underwater explosion more intense than an explosion in the air.
Because an underwater explosion causes a difference in pressure and velocity, cavitation usually forms at the boundary between a solid and a fluid body, as well as at the water surface. Investigating the effect of cavitation during an underwater explosion is an issue that has been studied by many researchers. The effects of an underwater explosion depend on various factors such as the depth of the explosion, the depth of the water, the material of the explosive, and so on.
This phenomenon is more common in naval wars. Because an explosion that occurs underwater and near the bottom of a float is far more destructive than an explosion of the same size on the surface of the water. The initial destruction of the structure is formed by the shock wave caused by the explosion and then by the impact of a large volume of water and bubbles produced. If the explosive is enough and explodes just below the main bow of the ship, it will lead to complete failure of the ship and eventually sinking.
To prevent damage from this explosion, the float must be designed to withstand it. Experimental experiments related to underwater explosions are very expensive. Therefore, the best way to investigate this phenomenon is to use finite element software. Abacus and LS Dyna software have made it possible for users to simulate this phenomenon. There are several methods for simulating an underwater explosion, each with advantages and disadvantages. When simulating an underwater explosion, the effect of free water surface, cavitation, and bubble formation is important.
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Underwater explosion modeling methods
- ALE in software (Abaqus and LS-Dyna)
- Pressure-time equivalence method (Abaqus and LS-Dyna)
- Load_SSA in LS-Dyna software
- Surface and submarine vessels
- Explosive shaping