Transient Thermodynamic Analysis of Cargo-Tank Pressure and Vapor-Handling Requirements During Loading and Unloading of a Liquid Hydrogen Carrier

Liquid hydrogen (LH2) cargo transfer can cause transient pressure excursions because the vapor-space volume changes rapidly while heat ingress, phase change, and pump losses alter the tank energy balance. This study quantifies the return boil-off gas (RBOG) and make-up boil-off gas (MBOG) required during loading and unloading, respectively, for a 2015.7 m3 LH2 carrier with three Type-B spherical cargo tanks. A transient lumped-parameter model based on mass and energy conservation was developed. Loading and unloading were simulated over 12 h between liquid levels of 5% and 95%, with pressure-control setpoints of 2.4 barg (RBOG discharge) and 0 barg (MBOG supply) and initial tank pressures of 0–2.4 barg. Without pressure control or relief, loading from 1.2 barg increased the tank pressure to 6.79 barg, whereas unloading reduced it to 0.66 barg. As the initial pressure rose from 0 to 2.4 barg, RBOG increased from 9351 to 18,148 kg (7.6–14.3% of the loaded cargo). MBOG was required only below 0.1 barg in the base case (286 kg at 0 barg; 0.23% of the unloaded cargo) and below 0.25 barg in all cases examined. Initial tank pressure has opposite effects on loading and unloading, which are linked through tank-pressure management during the voyages.