Stratification and rollover are two least understood operationally critical phenomena in liquefied natural gas (LNG) storage. Stratification is a prerequisite for rollover, but very little is known about the conditions instigating stratification inside an industrial LNG storage tank. The conditions leading to stratification are identified, and a detailed computational fluid dynamics analysis of rollover is presented. The critical density difference between heel and refilled LNG above which stratification occurs is 0.8 kg/m(3) for bottom filling and 2.2 kg/m(3) for top filling. Density equilibration and interface disruption due to saturation in the higher density layer are the two underlying rollover mechanisms. Rollover case studies are performed covering a wide range of heel and refilled LNG combinations. The time history of boil-off gas generation rate during rollover is a measure of the intensity of mixing, and the peak rate can be over 150 times higher than the normal rate.