Rollover phenomenon in liquefied natural gas storage tank

Abstract

Liquefied natural gas (LNG) rollover refers to the sudden mixing of stratified LNG layers, which can cause the generation of significant amounts of boil-off gas which create safety issues significantly in LNG storage tanks. Therefore, understanding of the phenomenon is very important for prevention purpose. Heat leak is a major challenge as it causes slow LNG boil-off leading to LNG stratification and LNG rollover phenomenon which has negative impact on people, environment, company asset and reputation. In this work, rollover phenomenon evolution is modeled and simulated using ANSYS FLUENT 16.0 software to study and analyze the effect of different ambient temperatures on the rollover phenomenon in LNG storage tanks and to study the stress distribution due to rollover and its effects on inner wall of the tank and determine the rollover time. This study examined the effect of different ambient temperatures ranged from 283 to 323 K on the LNG rollover inside a 5000 m3 LNG storage tank. Results show that higher temperature will cause higher heat transfer leaks to the LNG tank. The heat transfer through the tank walls is higher than the heat leaks to the tank through tank base. The pressure inside the LNG storage tank is strongly affected by the surrounding temperature. Higher temperature will cause higher pressure to build up in the tank especially along the area between the roof of the tank and the side walls. The simulations results show that significant initial density differences (∆ρ) between the stratified layers lead to late rollover events but last for longer time. At ∆ρ = 30, 15 and 7 kg/m3, the rollover will last for 3.1, 2.5 and 2.2 h, respectively.