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Event tree analysis

RR1034 Review of the event tree structure and ignition probabilities used in HSE’s pipeline risk assessment code MISHAP

The Health and Safety Executive (HSE) uses the MISHAP01 (Model for the estimation of Individual and Societal risk from HAzards of Pipelines) model to calculate the risks associated with Major Accident Hazard (MAH) pipelines in Great Britain. The risks calculated are used to determine the distances to land-use planning (LUP) zones around the MAH pipeline. The UK Onshore Pipeline Operators Association (UKOPA) has queried some of the assumptions and methods currently used within the tool and has compiled a briefing note outlying areas of concern. In response to the briefing note, HSE asked the Health and Safety Laboratory to review the MISHAP tool. The review examined the natural gas and non-natural gas event tree structures used by the tool as well as the probability values used to populate them. The review proposes replacing the generic natural gas and non-natural event trees with three event trees that take into account the minimum ignition energy of the substance and the substance reactivity. The derived event trees will feature in future versions of the MISHAP tool.

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