A review: Optimizing performance of Floating Storage and Regasification Units (FSRU) by applying advanced LNG tank pressure management strategies

Abstract

Summary The demand for Floating Storage and Regasification Units (FSRU) has grown rapidly worldwide since the first unit was commissioned in 2005 and opened many new markets to LNG trade. Technology has evolved leading to a fleet of FSRU ranging from relatively cheap conversions from LNG carriers (LNGC) to larger-capacity, new-build vessels with higher-tank-pressure ratings. The different types of FSRU ideally require operating procedures tailored to their specific designs, capacities, and tank-pressure ratings. Currently, many of the operating practices developed for the lower-tank pressure ratings of LNGC are applied as a matter of routine across the FSRU fleet, leading to operating inefficiencies. There are therefore significant opportunities to improve operating efficiencies on many FSRU and thereby reduce onboard gas consumption and atmospheric emissions, particularly regarding ship-to-ship (STS)-transfer procedures and LNG cargo rollover-protection strategies. Additional non-standard use of recondensing equipment, recirculating condensed boil-off gas as warm LNG back to the cargo tanks, can also reduce unnecessary shipboard consumption of gas. These savings can be achieved without any capital investments or modifications. STS transfers for LNG were first performed in 2007 and are a relatively new, but now routine necessity for FSRU operations. The operating efficiency associated with STS transfers can be significantly improved by adopting 12 straightforward measures. Some of the STS transfer improvement measures recommended involve an increase (beyond current practices) of cooperation, planning, and coordination of tank pressures between discharging and receiving vessels. The adoption of proactive prevention and mitigation measures to limit the potential tank pressure increases associated with LNG cargo rollover events can also appreciably reduce shipboard gas consumption and improve FSRU operating efficiency at no additional cost. Other operating efficiency gains focused on tank-vapor-handling equipment are also attainable for the higher-pressure rated FSRU.