System Wide Risk Assessment (SWRA) is an integral part of an Integrity Management Program (IMP), and it is the first step in most IMPs. Risk is the expected value of loss (often expressed as damage per year, i.e. expected number of annual injuries or fatalities). Risk is calculated as the product of the Probability/Likelihood of Failure (LoF) and the consequence of failure, where failure is defined as a loss of containment event. Hence, it is necessary to calculate both the Likelihood and the consequences of failure in order to accurately model risk.

For natural gas pipelines, consequence is primarily human safety-based. The primary threat to the population is the effect of the thermal radiation due to ignited pipeline ruptures. Currently, most pipeline industry system wide risk assessment models are qualitative risk models, where consequence is generally characterized by class, relative population measures, or some other relative measure. While this may be adequate for some relative risk ranking purposes, it is generally not accurate in representing the true consequences and the arbitrary nature leads to poor representation of actual consequences. Qualitative risk models are also highly subjective, and can have a high degree of bias. Thus, in this study, quantitative LoF assessment and a rigorous quantitative consequence model were used to make the risk assessment process more accurate, more objective, and transparent. The likelihood algorithm developed in this study is described in a companion paper. It should be noted that a quantitative estimate is never completely objective as subjective assumptions and idealizations are still involved, however it provides a framework to make it as objective as possible.

The consequence model implemented in this study is highly quantitative, and it depends on the pipeline properties (i.e. diameter, MAOP etc.) in addition to the structure properties (i.e. precise location and type of structures). The lethality zone utilized in the consequence model is a curve which has 100% lethality at the point of rupture but recedes in lethality as the point of concern moves away from the rupture location. The lethality curve is calculated using the PIPESAFE software [6] that is developed by rigorous analytical, experimental, and verification work. This ensures that the lethality curves are pipeline specific. Furthermore, the position of the structures inside the lethality zones is taken into consideration, which means the structures located closer to the pipeline see a higher degree of lethality than the structures further away from the pipeline.

Risk is represented by two specific, well defined measures: Individual Risk (IR), and Societal Risk (SR). These two measures are well accepted concepts of risk that go beyond the pipeline industry, and are particularly used in the pipeline industry in countries where quantitative risk is required by regulation (e.g. UK and Nederlands). IR takes into account the inherent risk of the pipeline to the single individual who may happen to be in the vicinity of the pipeline. SR, on the other hand, takes into account known population centers, settlements, and structures to define the risk to communities. When risk is calculated quantitatively, it is possible to use well defined and widely accepted criteria to determine the acceptability of risk in terms of IR and SR criteria for all pipelines. The advantages of using IR and SR are discussed and shown through implemented examples.

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