History of Rollover Events in LNG and LPG Industries

However, there are also auto-stratification mechanisms, and if flie above mixing is inadequate, then a follow-up mixing procedure is needed urgently to get rid of any residual stratification in every tank. 

In addition, the prerequisite of rollover, namely the development of density stratification, needs to be identified in each tank and detected with appropriate instrumentation. 

Historically, the LPG industry had, in 1969, recognised the rollover sequence and acted upon the perceived hazard of stratification and excessive vapour production from uncontrolled convective mixing. 

The first recorded rollover event with LNG took place at La Spezia, Italy in 1971 [1]. Briefly, a cargo of 33,700 m of weathered LNG was bottom-filled into the shore-tank under a lighter heel of 9500 m. There was minimum ntixing during filling and the density difference between the two layers in the tank was about 0.70 %. 

Eighteen hours after the filling was completed, an uncontrolled spontaneous mixing or rollover took place. The tank experienced a sudden rise in pressure, and the rollover event was accompanied by the evolution of a huge amount of LNG vapour lasting one and a quarter hours, with the tank vent valves open to the atmosphere (see Fig. 5.1). 

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