Floating Storage and Regasification Units (FSRU) are an expanding fleet (22 in service) handling liquefied natural gas (LNG) in a more-complex and dynamic way than LNG carriers and land-based terminals. Rollover is a physical mixing process in a single tank with two or more different parcels of LNG of different compositions, temperatures and densities that can manifest in large boil-off rates, beyond handling-equipment capacities, and large tank pressure increases culminating rapidly. Such tank-pressure rises pose safety, environmental and commercial risks. The consequences of some rollovers, if prevention/mitigation actions are not implemented, are uncontrolled venting of boil off gas in vapor form to the atmosphere, flammability hazards, tank structure over pressurization with potential damage. Existing industry guidance for handling FSRU rollovers remains limited because: 1) there are a range of vessel designs with different tank pressure ratings; 2) tanks are typically not equipped with specific "anti-rollover "equipment; 3) most FSRUs are not equipped with "densitometers" throughout their tanks; and, 4) FSRU industry procedures are often ill-suited to the operational patterns imposed on the vessels. This study provides in-depth analysis of FSRU rollovers based on observations of more than twenty rollovers on many different FSRU. The analysis leads to a new rollover-magnitude-assessment methodology integrating saturated vapor pressure (SVP) of the LNG versus actual tank pressure and tank strength. This provides operators with a rollover rating that enables them to better prepare and implement efficient and effective prevention and mitigation actions integrating cargo-stock management, internal transfers and standard techniques available on most FSRU (e.g., top spraying). Novel rules of thumb for predicting time to the onset of a rollover, LNG temperature and tank pressure based on many observed FSRU rollovers are developed to provide operators with a better "on the spot" insight to rollovers. This better understanding improves preventive and mitigating actions taken in specific circumstances.