Release of Thermal Overfill During Rollover

In some of the rollovers, when the amplitude of the convective penetrative oscillations increased so that the oscillations penetrate through the surface sublayers to the liquid surface, the BOR rose very rapidly by 10-50 fold. This behaviour is called Mode 2 Rollover in which the convective oscillations are believed to break up the morphology of the surface sub-layer, particularly the high impedance thermal conduction region. This results in the unimpeded and much higher molecular rate of surface evaporation to be realised (see Sect. 4.6.2 on molecular evaporation rates). 

Optical observations of the transition through the critical point in a critical point cell (when the density of vapour and liquid phases equalise) show similar oscillatory, penetrative convection across the phase boundary. 

This model, whereby penetrative convective oscillations reach the surface and break up the surface sub-layer, enables the magnitude of (he very high peak boil-ofif rates to be predicted, for Mode 2 Rollover. 

the estimated evaporation coefficient for LIN is 10 , the value for LNG is not known it could be larger by a factor of 10, thereby allowing the peak boil-off rate to rise to 190 times the normal BOR, close to that observed in the La Spezia incident. 

In terms of thermal overfill, we have seen how the heat flow entering the lower liquid layer by thermal conduction from ambient through the insulation is trapped in the lower layer, causing its temperature and energy content (superheat or thermal overfill) to rise with time. The build up of thermal overfill within the lower layer is clearly triggered if, or when, the density difference between the two layers exceeds some critical value, which appears to be the order of 0.5 % (0.70 % for the La Spezia event). 

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