Numerical study on effects of the cofferdam area in liquefied natural gas storage tank on the leakage and diffusion characteristics of natural gas

The leakage and diffusion characteristics of natural gas were investigated in the condition of the leakage of liquefied natural gas (LNG) in the storage tank. Fluent was adopted to simulate the process in a series of three-dimension unsteady state calculations. The effects of different heights of the cofferdam (1.0 m, 2.0 m and 3.0 m), wind directions, ambient temperature, leakage location, leakage volume on the diffusion process of natural gas were investigated. The diffusion characteristics of the natural gas clouds over cofferdam were found. Under windless condition, when the gas clouds met, the gas clouds rose due to the collision, which made them easier to cross the cofferdam and spread out. The higher the ambient temperature was, the higher the gas concentration around the cofferdam was, and the smaller the gas concentration difference was. When the leakage occurred, the higher cofferdam was more beneficial to delay the outward diffusion of gas clouds. However, when the leakage stopped, the higher cofferdam went against the dissipation of gas clouds. Under windy condition, the time to form stable leakage flow field was faster than that of windless, and the lower cofferdam further reduced this time. Therefore, considering the effect of barrier and dissipation, it was suggested that the rational height of cofferdam should be designed in the range of 1.0 m to 2.0 m. In case of emergency, the leakage of gas should be deduced reasonably by combining the measurement of gas concentration with the rolling of gas clouds. When windless, the leakage area should be entered between the overflows of gas clouds.