Optimal Design of BOG Reliquefaction Systems for LNG Carriers: A Focus on GMS Performance During Loaded Voyages

Abstract

This study proposes a strategy for evaluating efficient design of the Gas Management System (GMS) on LNG carriers by decomposing its performance to subsystems: the reliquefaction system (RS) and the fuel gas supply system (FS). With increasingly stringent maritime regulations on greenhouse gas emissions, the need for efficient LNG carrier operations has become critical. A major factor in reducing fuel consumptions and carbon emissions is optimizing the design of the RS, given its significant power demand for processing NBOG. However, effective GMS design must account for variations in RS operation performance, as well as the contributions of the FS in treating NBOG with changes in ship speed. This study compares GSM configurations with reliquefaction systems based on two representative refrigeration cycles: the nitrogen reverse Brayton cycle (NRBC) and the single mixed refrigerant cycle (SMRC), both analyzing effects of cold BOG utilization. Results indicate that the RS of GMS4A-aSMRC [the aSMRC is the refrigeration cycle which utilizes cold BOG within the Single Mixed Refrigerant Cycle (SMRC)] demonstrates superior RS design performance. However, the most efficient GMS configuration varies with the Boil-off Rate (BOR): GMS2-aNRBC [the aNRBC is the refrigeration cycle which utilizes cold BOG within the Nitrogen Reverse Brayton Cycle (NRBC)] is optimal aligning with its RS performance for a 0.11%/day BOR, while GMS3-SMRC without cold BOG in RS is the most efficient for a 0.075%/day BOR, owing to increased contributions from the FS. In this study, a performance index with a consistently comparable baseline is derived to accommodate compositional deviations from flash gas recirculation at NBOG disposal streams, enabling the GMS performance to be correlated with compatible values of its decomposed subsystem.