Abstract
Fire and Gas detectors are used on Oil and Gas facilities to trigger an alarm and potentially initiate automatic safety measures in response to hazardous leaks. The effectiveness of the Fire and Gas system strongly depends on the layout and coverage of the detectors implemented. Prescriptive approaches using industry guidelines (e.g. SHELL DEP 32.30.20.11-Gen or BP GP 30-85) outline a target gas cloud size for detection for enclosed, semi-enclosed, and open areas. The objective of this study was to assess the conservatism inherent in prescriptive approaches in comparison to a Computational Fluid Dynamics (CFD) approach. The employment of a CFD approach to model the dispersion of flammable gas from the potential loss of containment was used to determine whether the prescriptive approaches are overly or under conservative for flammable gas detection. The analysis found for the same assessment parameters such as zone volume and a 90% single detector coverage criterion, geographic coverage (prescriptive approach) requires a significant increase in gas detectors to meet the criterion, an extra 13 detectors. Further to this analysis, the method of using the volume of a sphere to calculate the equivalent diameter was assessed. It was found by modelling the characteristics of a flammable gas release and using the volume of a cone to calculate the equivalent diameter, a more accurate and realistic method was obtained. The study also introduces a 3D frequency plot which is a novel tool for positioning the detectors in the most effective areas irrespective of equivalent diameter. Incorporating the proposed CFD method with the prescriptive approaches raises a number of questions in the area of Fire and Gas Mapping. Whilst the future for this industry may be the use of scenario based coverage, more research is required in order to develop the most effective method.
