Some Comments on Handling an LNG Rollover

First of all, read Sect. 5.3 on the fast sequence of uncontrolled happenings in a rollover event, so as to anticipate actions needed, as the sequence develops. Do not panic  

If the pressure and BOR in a tank starts to rise unexpectedly, then all the valves to the dedicated vapour lines should be opened. This should be carried out as soon as possible in anticipation that the BOR will rise 5-10 fold, following the Mode 1 sequence. 

If the pressure continues to rise, and the vapour lines cannot accept the increasing BOR, then all the vents to the atmosphere should be opened early, in anticipation of Mode 1 converting rapidly (in a few seconds) to a Mode 2 rollover with a much higher BOR. 

There are no working correlations to help determine whether a conversion from Mode 1 to Mode 2 rollover will happen. Therefore, it is advisable to anticipate the conversion will take place, by having aU the available vents open to the atmosphere, when the rising BOR exceeds the vapour line limits. 

The size of the vents should be large enough to be able to exhaust a 200 fold increase in BOR without approaching the choked (sonic) flow condition. If not, then a Mode 2 rollover could lead to structural failure. 

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