Abstract
The numerical simulation of gas dispersion and estimation of consequence impact is of importance in Oil and Gas industry's process safety management. For natural gas fields with toxic components like Hydrogen sulfide, the toxicity impact zone drives business decisions related to equipment design, facility siting, layout, land use planning, and emergency response measures. Proprietary tools or empirical models which are calibrated using experiment database are often used for carrying out consequence modeling.
The selection of a software tool and a suitable dispersion model, based on the cloud behavior, at the source of dispersion is critical for the impact zone estimation. It is observed that, the fluid phase and the cloud density are key for determining the appropriate dispersion model. Incorrect parameter selection could lead to an inaccurate consequence impact zone estimation. This in turn could result in disproportionate process risk management efforts especially for toxic impacts from exposure to a very low concentration.
This paper discusses the results from consequence modeling studies done for a selected set of toxic natural gas release events related to onshore pipeline transfer using approved software. The study analyses the modeling inputs, parameters and determines the key release source terms and atmospheric parameters that impacts the estimation of impact zones. The study determined that the natural gas dispersion behaviour is dependent on the natural gas molar mass and the composition of Hydrogen sulfide. The study provides the guidance on overcoming uncertainty in dispersion modeling through sensitivity assessments and lists key parameters to be subjected for toxic natural gas dispersion modeling sensitivity analysis.
•Phase equilibrium properties of the release should be considered in determining the release phase as low temperature and high-pressure releases can have longer impact zone distances. Detailed review (prior to implementing risk mitigation) should be carried out for high pressure releases of compositions with >18 mol% H2S & molar mass >29 lb/lbmol and for low temperature releases of compositions with molar mass >30 lb/lbmol.•Downwind dispersion of toxic cloud is dependent on hole size, release rate and composition. The failure mechanism and related hole size for larger releases need to appropriately be determined. Dispersion from small hole releases are not sensitive to the composition of natural gas.•Release rates and downwind dispersion are sensitive to low temperature for those compositions with >30 lb/lbmol. For such cases with significantly higher impact zone, further analysis should be carried out before implementing risk reduction measures.•Downwind dispersion for high pressure releases are sensitive for compositions with greater than 18% H2S content. For such cases with significantly higher impact zone, further analysis should be carried out before implementing risk reduction measures.