QRA Limitations & Criticality in Sour Hydrocarbon Development
Authors: Abri A. A , Jewitt I.. 
Published: January 2011

Abstract
QRA (Quantitative Risk Assessment) is a powerful decision aiding tool which can assist in the ranking of acceptable solutions for complex safety challenges using specific risk acceptance criteria. In addition, QRA allows for the comparison of risk reduction options for a particular hazard on an equivalent basis. The process of QRA includes hazard identification of major hazards (fire, explosion and toxic release) and definition of credible accidental scenarios. Then, consequence modelling of accidental scenarios is conducted along with estimation of their likelihood/frequency. The consequence impact and the frequency are then evaluated to assess the risk levels of the identified hazards against the risk acceptance criteria.
There is a vast growth in the oil and gas industry through the exploration and production of more inherently hazardous reserves containing high sour gas concentrations at very high pressures. Therefore, QRA studies and the analysis of the results are becoming more critical and in particular their accuracy.
The risk levels evaluation for any particular scenario require extensive preparation for gathering several data, such as leak frequency, metrological data, hole sizes releases to be modelled, ignition probabilities..etc. Within each set of the data there is a limitation of the certainty which could be achieved. Therefore, the tools/software packages and techniques used (for example to predict the consequences, failure databases, QRA) are continuously updated. For example, several dispersion modeling and risk integrator software have been recently updated to account for an added feature to integrate toxic releases more realistically. However, the techniques and the understanding of the criticality of the updates and modification still require more insight as the experience around the world in the QRA techniques is limited. This paper discusses the implication of some of the QRA limitations by presenting examples of recent applications and learnings from sour hydrocarbon developments/projects and how to account and manage their criticality and limitations.
It can be concluded that with high pressure high H2S concentration sour hydrocarbon developments, the current QRA tools require further improvement to include stringent controls to account for these new and challenging hazards and risks. In addition, considering the limitations associated with QRA application could imply avoiding utilising the tool for seeking absolute number of risk to demonstrate acceptable risk. The magnitude of the uncertainties with the data required to generate the risk values might limit the use of QRA to be used as comparison tool only.
The representation of toxic risk from sour hydrocarbons with H2S contamination is not as well established as fire and explosion hazards. Misinterpretation of the toxic risk might result in a very conservative QRA results and hence result into cost and operation implications. This is very evident in predicting separation distances to allow for online maintenance while adjacent sour units are live. Another challenge is the definition of toxic criteria for different group of workers and public.
It is recommended to improve the reliability of the data used in the sour hydrocarbon QRA study such as leak frequency, ignition probability and different release hole sizes, to include the increase detail in equipment design and material selection.