A new methodology to estimate the gas ascending time from underwater gas releases

Abstract

Accidental subsea gas releases can pose a threat to people, equipment, and facilities since gas can be toxic or flammable at the concentrations in which the leak occurs. The accurate prediction of the behavior of the gas plume formed in the leaks can be fundamental to the development of techniques of accident prevention or, in some cases, remediation measures, avoiding the emergence of more serious consequences. Among the different ways to analyze the behavior of gas plumes formed underwater, the Computational Fluid Dynamics (CFD) tool stands out for allowing the study of plume behavior to be done in a safer, simpler, and less expensive way, if compared to experimental studies. Inspired by the accidental release of the subsea gas scenario, this work validated a CFD setup of a 2D two-phase air–water flow using the VOF method in Ansys Fluent. The use of the VOF method differs this work from other works that use a hybrid Eulerian–Lagrangian methodology to model such types of flow. In this validation, simulations with a 9 m base tank, and 7 m water depth, and 0.050, 0.100, and 0.450 m3/s gas flow were performed. The simulated data were compared to experimental results available in literature. After the validation of the setup, a study was carried out varying the size of the leak to 0.24 and 0.17 m, and the gas flow from 0.006 to 0.150 m3/s aiming to verify how some plume characteristics are affected by the changes. Finally, following the directions from literature for analyzing the ascending gas behavior, and combining it with a dimensional analysis of the data, we proposed a mathematical model for calculating the gas ascending time using only properties of the gas leak. With future modifications of the proposed methodology, we hope that soon it will be possible to simulate gas releases under more realistic conditions. Even so, the findings of this work are already a significant step forward in the study of underwater gas releases.