Simulating on rollover phenomenon in LNG storage tanks and determination of the rollover threshold

• The rollover evolution is modeled and simulated, the results show that the evolution consists of three phases.
• A rollover coefficient is defined to describe rollover intensity and confirm the rollover threshold.
• Critical density difference 3, 5and 7 kg/m3are proposed to be rollover thresholds of the 160,000, 30,000and 5000 m3 tank.
• Critical density difference is not influenced by small diameter changes but increases with the increasing layer depth.

Rollover is a potential risk to the safety of LNG storage tanks during the LNG storage process, so study of its prevention method is very important. In this paper, rollover phenomenon in a liquefied natural gas (LNG) storage tank is modeled physically and mathematically. Its evolution is simulated using FLUENT™ software from the breakdown of stratification to the occurrence of rollover. Results show that the evolution consists of three phases: the initial phase where rollover occurs near the side wall of the storage tank; the turbulent phase where rollover transfers to the center of the tank; and the final phase where new layers evolve. Based on these phases, rollovers in 160,000, 30,000, and 5000 m3 LNG storage tanks are simulated at varying initial density differences, and a rollover coefficient is defined to describe rollover intensity. The simulations show that the rollover coefficient initially increases within a small scope and then increases rapidly with the increment of initial density difference. This turning point is chosen to be the rollover threshold, which is regarded as the critical density difference in this study. The critical density differences obtained from the simulation results of the 160,000, 30,000, and 5000 m3 LNG storage tanks are 3, 5, and 7 kg/m3, respectively, which can be used as their rollover criteria to ensure the safety of LNG storage tanks.