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 nov el approach for the management of the risk of harm to people in the vicinit y of major hazards. Particular attention is given to such risk in the vicin ity 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 maint ained. 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 pe rceived inconsistency as it may allow some initial development of a particu lar type of dwelling in a particular location, then preclude further develo pment in adjacent locations. The approach derived herein provides some alleviation to this perceived inc onsistency by discouraging localised regions of high population density, th ereby promoting a more uniform distribution of development of land. The essence of the approach is to apply a weighting ( greater than or equal to 1) to casualty density prior to the evaluation of casualty expectation. The resulting parameter, which has been aptly named weighted expectation (W E), is assessed according to a criterion based on true casualty expectation (E). The weighting on casualty density is derived directly from an appropr iately constructed F/N criterion Line (or envelope), thereby ensuring a clo se relationship between the outcomes of WE and F/N assessments. The Scaled Risk Integral (SRI), which has been developed previously, is sho wn to be a special case of WE. The practical application of WE and its relationship with F/N are demonstra ted through simple high-pressure pipeline case studies. (C) 1999 Elsevier S cience Ltd. All rights reserved.