While the discussion of societal risk criteria for the process industry began in the 1970s – led primarily by the Dutch and British governments – the literature on the societal risk of hazardous goods transportation, particularly by pipeline, has been much more limited.
For fixed facilities, F-N curves and individual risk calculations are broadly used to assess the risk to the general public.
Various governments have established “tolerable risk” limits based on these analysis methods. Many corporations have also adopted these methods for internal evaluation of the relative risk of projects, plants and businesses, presumably setting their own criteria.
F-N and individual risk analyses have also been applied to pipelines, generally with F calculated on a per-length-of-pipeline basis. Such an analysis is useful for comparing the risk of hazardous goods transport via pipeline to that of another mode of transport. Given a “tolerable risk” limit on a per length basis, a determination of the appropriate level of risk mitigation can also be made. Such an analysis does not, however, provide any indication of the overall level of risk of the pipeline or allow the risk of a pipeline network to be compared to that of other business ventures. It is useful to compare pipeline to fixed facility risk so that risk reduction resources can be appropriately allocated. 
The Dutch government began risk quantification after 1953 floods that claimed the lives of 2,000 people. Following the floods, the Netherlands established a criterion of 10-6 probability of fatality/year for the sea dike systems. This criterion was the precursor to the Dutch individual risk criteria .
The 10-6 fatality/year individual risk criterion is based on 1% of the natural death risk for 10 to 14 year-olds in the Netherlands. The purpose of the individual risk criteria is to ensure that the risk to an individual from a facility handling dangerous goods is only a small component of the overall average risk of death. Safety zoning distances based on individual risk have been established in the Netherlands to ensure adequate separation between hazardous materials and populations. The same individual risk criterion is applied for stationary activities and the transport of hazardous materials.
The criteria for “tolerable risk” adopted by the Dutch government are shown in Figure 1. The solid line represents a limit of fatalities/year for fixed facilities. Risk above this line is considered to be intolerable. Below this line, the ALARP (As Low As Reasonably Practicable) principle is applied to reduce risk. In the 1990s, the Dutch Parliament made the societal risk criteria non-mandatory. Local permitting agencies were given the responsibility to maintain risks below the maximum tolerable level, but the criteria could be waived when justified .
In 1996, the Dutch established criteria for the transport of hazardous materials . The societal risk line is one order of magnitude higher in frequency and is applied to a kilometer of the transport route. The transportation criteria line, shown in Figure 1, is applied to road, rail, water, and pipeline transport.
For pipelines, societal risk is calculated for the worst-case kilometer of the pipeline route per municipality. When the length of the pipeline is less than 1 kilometer, the societal risk for the whole pipeline is determined. The worst-case kilometer is assessed by an evaluation of the consequence area and the surrounding population density.
In the United Kingdom, the Institution of Gas Engineers and Managers (IGEM, formally IGE) developed a methodology for the assessment of natural gas pipelines . This methodology was largely adopted by the UK Health and Safety Executive (HSE) in the BSI Code of Practice for pipelines carrying any flammable . The IGEM Communication and the BSI Code of Practice both recommend the use of individual risk and societal risk in the assessment of pipelines.
The BSI quotes the following HSE established individual risk criteria for setting land use planning zones for hazards sites, including pipelines carrying flammables:
<10-6 – broadly acceptable. 10-6 to 10-4 tolerable if alarp is applied.>10-4 – unacceptable for the public.
The HSE established the societal risk criteria shown in Figure 2 for fixed facilities as part of the Control of Major Accident Hazards (COMAH) regulation. The BSI applies the same criteria to pipelines with F in fatalities/year/km. The 1 km basis was chosen because 1 km was judged to expose the public to the same level of risk as a typical medium-size COMAH site.
Figure 2:- Risk Criteria Established by British Government.
IGEM chose to plot F-N data for the existing UK natural gas pipeline network on a per mile basis. They then drew a curve above all existing data and established this curve as a “broadly acceptable” limit for natural gas lines on the basis that the existing network was accepted as tolerable by society . The resulting curve lies very close to the lower line in Figure 2 when scaled to a 1 km basis.
When a pipeline is of a single construction and the nearby population can be assumed to be of a single density, a generic evaluation can be made. In this case, the risk at any point along the line is the same. The f,n pairs for such a line are summed over a km or mile and the resulting curve compared to the appropriate length scaled F-N criteria.
The BSI and IGEM documents also describe how a site specific assessment can be made for a given community or development. The first step is to calculate an interaction length defined as the length of the pipeline through the community plus two times the impact radius for the most severe pipeline event. An event occurring anywhere along the interaction length could affect the population within the community, so, f,n pairs along the interaction length are summed. The resulting F-N curve is normalized to per km (mile) by dividing the calculated F by the interaction length.
Purpose Of Risk Criteria
As discussed in the CCPS Guide for Developing Quantitative Safety Risk Criteria, the selection of appropriate criteria depends upon the objective of the analysis . Governments set limits to protect their citizens. Corporations need to compare alternatives and assess investment opportunities. When evaluating a pipeline, a company may wish to: compare hazardous goods transport by pipeline to other transportation modes; compare alternative pipeline routes; establish appropriate pipeline specifications and safeguards, including making a comparison of the effectiveness of various mitigations; or compare the overall risk of a pipeline to other business ventures.
Basis For F-N Calculations
The Dutch and British governments have chosen to establish F-N criteria for a given length of pipe. While the TNO and BSI methods both use one km as the reference pipe length, they differ in how the criteria are used. TNO recommends evaluating the worst-case km within a metropolitan area while BSI recommends summing the f,n pairs over the entire length through a community and then normalizing to 1 km.
The BSI method has two drawbacks. First, it is often difficult to identify distinct population clusters along a pipeline in order to define a community. Secondly, normalizing F averages the risk over the interaction length. In this way, information is lost and high risk segments of the pipeline could be missed. To illustrate this point, consider a hypothetical 6-km interaction length of pipeline with a constant frequency of each failure scenario. The F-N curves for the individual 1 km segments would be represented by the dashed curves in Figure 3. Clearly, some of these hypothetical curves fall above the criterion line, indicating a higher societal risk within those 1 km segments.
Figure 3: Hypothetical Risk Curves for 1 km Segments.
If the f,n pairs are summed over the entire interaction length and then normalized to 1 km as per the BSI methodology, the risk of the entire length of pipeline is judged to be tolerable as shown in Figure 4. This example is based on 6 km for simplicity. The averaging effect could be more pronounced with longer interaction lengths. The CCPS Guideline on Chemical Transportation Risk Analysis suggests 10 -20 miles (16-32 km) as an appropriate length for evaluating risks to a community. 
Figure 4: Hypothetical Risk Curve Normalized over the Interaction Length.
Selection Of F-N Criteria
If one chooses to establish criteria on a length basis, the question then arises as to the appropriate segment length. If one wishes to establish a benchmark against which to compare future pipelines as IGEM did, criteria can be established on any convenient length basis. To compare a pipeline project to other investments, a segment length “equivalent” to a fixed facility must be selected. Fixed facility criteria can then be applied to the chosen length of pipeline. The basis for this equivalence could be an impact area as suggested by the BSI, a definition of a typical community, or some measure of project size such as capital invested or income generated.
Individual and/or societal risk can be used to set pipeline specifications, including safeguards. The most conservative application of individual risk calls for specification of the pipeline such that the chosen individual risk criterion is met even at a minimal distance from the pipeline such as the edge of the easement. The advantage of this approach is that it ensures a low event frequency and increases the likelihood that the risk from potential population encroachment is tolerable.
The selection of analysis method and risk criteria depends on the objective of the risk evaluation. Options for pipeline reviews are summarized in Table 1. If a length-scaled F-N analysis is chosen, care must be taken not to miss identifying high risk sections by summing the F over too long a length of pipeline. To avoid this, the F-N curves of individual segments can be plotted and compared to F-N criteria scaled to the segment length.
Table 1: Risk Evaluation Options.
This article is based on a presentation at the AIChE 8th Global Congress on Process Safety, Houston, April 1-4, 2012.
Joan M. Schork, Ph.D., is the Global Functional Lead for Process Safety at Air Products. In her 24 years with the company, she has held a variety of technical and management positions in research, engineering and operations. She can be reached at 610-481-4014.
Elizabeth M. Lutostansky, Ph.D,, is a lead process safety engineer. She leads the risk and consequence modeling at Air Products.
Steven R. Auvil, Ph.D., is an Air Products Fellow. He leads the Chief Engineer’s Office and is accountable for the identification and management of the technical risks associated with the introduction of new technologies.
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