Impact of downward releases on the risk profile in hydrocarbon process plants

Abstract

Quantitative risk assessment (QRA) calculations for major hazard installations often involve consequence analysis calculations for thousands of events, and therefore, some simplifying assumptions are generally required. The simplifications are usually designed to make the analysis reasonably practicable and reduce the cost of the QRA. Nevertheless, the overall methodology and the applied parameters should be chosen conservatively to cover possible uncertainties. One of the key assumptions in many QRAs is the release direction, which is usually assumed to be horizontal. This is generally assumed to provide a conservative representation of all other possible release directions, which may occur vertically (upward or downward) or at an angle. A sensitivity analysis has been performed and presented in this paper to investigate how different release direction assumptions affect the results of consequence analysis, and eventually, QRA outcomes, i.e. individual and societal risk results. A high-pressure hydrocarbon system is considered as a case study and SNC-Lavalin’s (formerly Atkins) in-house QRA software, ‘RiskTool’, has been used to carry out the QRA modelling. The overall conclusion is that the assumption that all releases are horizontal may lead to a significant underprediction of risks for some types of high-pressure release events. This is because an unimpeded horizontal jet may entrain air, and hence, dilute much more rapidly than a jet that impinges on the ground or nearby obstacles.