Floating storage and regasification units face specific LNG rollover challenges: Consideration of saturated vapor pressure provides insight and mitigation options

Abstract

Floating Storage and Regasification Units (FSRU) form a rapidly expanding sector of LNG business. In many cases, FSRU now provide a more cost-effective and very flexible way to deliver natural gas to end users in comparison with shore-based terminals. Due to enhanced operations FSRU are more complex compared to LNG carriers (LNGC). FSRU are essentially merge of the attributes of shore-based terminals and LNGC. The existing FSRU fleet is formed of new-build vessels and converted LNGC. Together with their advantages FSRU come with the inherent problems of handling and storing LNG. Here we focus on the rollover issues that occur on FSRU and suggest ways to improve handling to minimize the impacts of those events. 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. If prevention/mitigation actions are not implemented, uncontrolled venting of boil off gas in vapor form to the atmosphere is a likely consequence involving flammability hazards and tank structure over-pressurization with potential damage. This study provides in-depth analysis of FSRU rollovers based on observations of more than twenty rollovers on many different FSRU. The analysis focuses on LNG saturated vapor pressure (SVP) rather than the traditional approach of focusing on boil-off rate (BOG). This approach allows efficient rollover management without any in-built rollover prevention means. Strategies are developed for managing a combination of FSRU tanks utilizing rollover prevention and mitigation actions, as well as efficient pre-planning for LNG stock management. Novel rules of thumb for predicting time to rollover onset, based on many observed FSRU rollovers provide operators with real-time insight to what rollover preventive and mitigating actions are effective in specific circumstances.