Modeling of LNG Spills Into Trenches and Troughs

Trenches and troughs are commonly used in LNG storage and transfer facilities to contain potential LNG spills. The trenches are concrete-lined rectangular channels dug into the ground. They are typically slightly inclined from the horizontal to direct the flow of LNG towards the sump and they are often configured with sharp turns and dead ends.

In sizing the trench, it is important to ensure that it can contain the intended design spill. The trench design also influences the rate of wetting during a design spill, which in turn contributes to the rate of vapor generation. The latter dictates the dispersion of vapors and associated exclusion zones. Despite the regulatory importance associated with trenches, there is little formal guidance for proper trench design in LNG facilities, or guidance for the tools that are needed for modeling the liquid propagation.

This study examines the use of Computational Fluid Dynamics (CFD) to model the flow of LNG within trenches and troughs during a design spill. Detailed calculations of liquid flow are obtained using Star-CCM+, a commercially available, broadly used, generally accepted and validated CFD software package. Calculations performed with this software reveal the dynamics of design spills and show how sharp turns and dead ends in trench systems can result in overspilling of LNG. These software tools are useful in sizing trenches and troughs to contain worst-case scenario design spills.

In this work, transient 3-dimensional calculations are compared to a transient 1-dimensional model that uses the shallow-water equations to predict liquid propagation in trenches and troughs. The study also shows how these liquid propagation models can be used to determine the LNG evaporation rates during the propagation of the LNG in the trench.