Weighted expectation: a new risk-based method for assessing land use development proposals in the vicinity of major hazards

BG Technology and the Major Hazard Assessment Unit (MHAU) of the Chemicals and Hazardous Installations Division (CHID) of the HSE have developed a novel approach for the management of the risk of harm to people in the vicinity of major hazards. Particular attention is given to such risk in the vicinity of high-pressure gas pipelines.The objective of the approach is to provide a means of allowing reasonable development of land whilst ensuring that a tolerable level of risk is maintained.One means of managing this type of risk is known as the F/N approach. The F/N approach expresses and manages the risk in terms of the frequency, F, of N or more casualties occurring. However, the F/N approach may lead to a perceived inconsistency as it may allow some initial development of a particular type of dwelling in a particular location, then preclude further development in adjacent locations.The approach derived herein provides some alleviation to this perceived inconsistency by discouraging localised regions of high population density, thereby promoting a more uniform distribution of development of land.The essence of the approach is to apply a weighting (≥1) to casualty density prior to the evaluation of casualty expectation. The resulting parameter, which has been aptly named weighted expectation (WE), is assessed according to a criterion based on true casualty expectation (E). The weighting on casualty density is derived directly from an appropriately constructed F/N criterion line (or envelope), thereby ensuring a close relationship between the outcomes of WE and F/N assessments.The Scaled Risk Integral (SRI), which has been developed previously, is shown to be a special case of WE.The practical application of WE and its relationship with F/N are demonstrated through simple high-pressure pipeline case studies.