LNG rollover challenges and their mitigation on Floating Storage and Regasification Units: New perspectives in assessing rollover consequences

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.