Gas Explosion hazard in offshore facilities such as enclosed modules with ventilation systems is a crucial factor to be assessed while conducting a Quantitative Risk Assessment (QRA) and has always been a challenge. Computational Fluid Dynamics (CFD) methods help to charecterize the consequences of hazardous events such as cloud dispersion over complex process facilities in a detailed manner. This paper emphasize on using CFD tool, OpenFOAM for detailed assessment of transient gas dispersion and ignition probabilites inside an enclosure. This custom made tool accounts for the transient gas leak behaviour based on real time gas detection and emergency shutdown (ESD) systems as well as initialtion of Blow Down (BD). The consistent results from CFD tools and the ability to prenest them clearly help in taking concrete safety actions against consequences. These measures in clude positioning and number of gas detectors, ignition source isolation, voting philosophy in terms of initiation of emergency shutdown systems, structural integrity of explosion barriers, layout of evacuation routes, venting of the affected area and evacuation of personnel.