Modeling the Vapor Source Associated with the Spill of LNG Into Troughs and Trenches

Modern LNG receiving terminals are often configured with LNG piping outside of the tank impoundment area with troughs and trenches to capture any releases from such piping. The trenches and troughs are inclined to guide the spill toward a bund, sump or an area that is within impoundment walls. Because the trenches and troughs are occasionally located near property boundaries, the US Department of Transportation (DOT) and the Federal Energy Regulatory Commission (FERC) has requested applicants to analyze the generation and dispersion of the vapors associated with such spills.

The analysis of spills into trenches and troughs consists of two largely independent elements. The first element is the hydraulics of the flowing LNG and the generation of vapors that then spread downwind as a dense gas. In the second element, the time varying vapor source, often called the source term, is entered into a vapor dispersion model, such as a CFD model (for example Fluent, CFX, Star-CCM+ and others). The geometrical complexity of typical trenches and troughs prevents the use of integral models such as DEGADIS. To date, the channel hydraulics of the flowing LNG has mostly been modeled using the steady state channel flow equation called the Manning equation.

This paper presents a new model for the hydraulics of flowing LNG within a trench or a through that is based on the “shallow water equations.” This model captures the unsteady complexities that occur even with a constant flow rate spill as it travels downhill in the trench or the trough. The rate of evaporation of LNG is based on the amount of heat that is available from the underlying concrete which is initially at ambient temperature. The model quantifies the amount of LNG that evaporates along the trench based on the transient one-dimensional Fourier conduction law within the concrete walls and floor. The hydraulic model is presented along with validation cases.