The accurate assessment of boil-off gas rate is essential for the design of FLNGs and LNG carriers. Unfortunately, existing engineering methods used to estimate these rates are fairly primitive. Current methods can account for conductive heat transfer but the details of convective mixing are not included. Since convective mixing can be the larger of the two, better methods are needed to account for the impact of LNG movement due to filling, vessel motions, and off-loading. Modern CFD methods are able to account for all these effects, and this paper describes the development of one such technique using the commercial software package StarCCM+. The approach builds a 3D CFD model for one entire tank, including insulation, filling and discharge pipes, gas vents, and (if necessary) cofferdam areas and internal tank structure. Hydrodynamics, thermodynamics, heat transfer through the insulation and phase change are all computed simultaneously. Results are included for two distinct cases: one depicting a filling operation and one simulating the effect of vessel motions. In each case, 3D, time-dependent distributions of temperature, fluid velocity, pressure, and LNG volume fraction are presented. In cases where gas pressure increases beyond a preset limit, time histories of vented gas property and flow rate are given at the vent. Videos are available that demonstrate the complexities of spatial and temporal variations. Accuracy is demonstrated by comparing to known model problems for which validation data exists, and best practices are provided to assist other practitioners start their own applications.
