Explain what is 'rollover' in an LNG tank and how is it prevented?

Rollover in an LNG tank is a potentially dangerous phenomenon where a rapid and spontaneous mixing of LNG layers with different densities and compositions occurs, resulting in a sudden release of a large volume of vapor. This can overwhelm the tank's pressure relief system and potentially lead to tank damage or failure. Rollover typically occurs when LNG with a higher density is added to a tank containing LNG with a lower density. The denser LNG settles to the bottom of the tank, forming a distinct layer. If there is insufficient mixing, the two layers can remain stratified, with minimal heat transfer between them. The bottom layer gradually heats up due to heat leak from the tank walls and bottom. As it warms, its density decreases, eventually becoming less dense than the upper layer. This creates an unstable situation where the bottom layer suddenly rises to the surface, causing rapid mixing and release of vapor. To prevent rollover, several strategies are employed. One is to avoid adding LNG with significantly different densities to the tank. When adding LNG, it is important to carefully monitor the density of both the incoming LNG and the LNG already in the tank. Another method is to mix the LNG in the tank to prevent stratification. This can be done using submerged jet mixers or by circulating the LNG through external pumps and heat exchangers. Temperature and density probes are installed at various levels in the tank to monitor stratification and provide early warning of potential rollover conditions. Regular monitoring and analysis of tank conditions are essential for preventing rollover. Operating procedures are in place to guide operators in managing tank levels, densities, and temperatures to minimize the risk of rollover. The frequency and intensity of the mixing are also determined through a detailed risk analysis.