CFD investigation of natural gas leakage and propagation from buried pipeline for anisotropic and partially saturated multilayer soil

Abstract

The incident of natural gas leakage from buried pipelines in the soil, due to the financial and environmental consequences has become one of the most serious concerns, especially in urban areas. The main objective of the present study is the development of a three-dimensional numerical model to investigate the rate of leakage and propagation of natural gas from buried pipes with respect to the real soil conditions. The intended conditions include soil anisotropy in three directions, soil stratification and layering, time-dependent moisture content of soil matrix in each layer, and slope of soil layers. These conditions are important due to their influence on leakage velocity and dispersion process, which are not investigated up to now in literature. The used mechanical and hydraulic properties of the soils in the model are based on the database of soil samples which collected from different provinces of Iran. Despite many studies that have assumed natural gas as a pure substance with ideal behavior, the presented model assumes natural gas to be the mixture of methane (90%), ethane (7%), and propane (3%) with deviation from ideality. The equation of state applied to the model is the Soave modification of Redlich-Kwong. The results indicate that hole diameter alteration on the pipeline from 10 to 50 mm causes 86.51% incremental impact in discharge rate. Also, for the dry soil case, there is no gradient of gas concentration in the soil. For multilayer soils with different slope, gas propagates along the slope gradients. Evaluation of relative concentration of thiol in comparison with the concentration of other gas components in different soil contaminated areas indicates that up to 6.7% of the thiol is adsorbed onto soil particles’ sites.