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Quantitative Risk Assessment

AFAA undertake QRAs, using industry leading software, for housing developers, pipeline operators and local authorities.

Gas escaping from a pipe wall rupture of a high pressure pipeline can lead to very serious harm, or even fatality, to anyone in the vicinity of the release, if the gas ignites. For this reason pipeline design codes do not generally allow the presence of occupied buildings within a minimum distance of the pipeline known as the Building Proximity Distance (BPD).

Although rare, it is inevitable that some occupied buildings will occur within the allowable distance at various locations along the length of the pipeline (proximity infringements). Also along the route of most ‘rural’ pipelines, isolated occurrences of suburban type locations can occur as the pipeline passes through small villages and housing estates.

At the design stage it is possible to mitigate the risks in these locations through the introduction of proximity pipe. The purpose of this is to reduce the stress levels. However, since stress is only one of a number of factors that can affect the risk, the introduction of a ‘one size fits all’ proximity pipe can still lead to varying risk levels. Thus, whilst proximity pipe will always reduce the risk, it is still possible, in principle, that some locations may require further mitigation, e.g. protective slabs or thicker proximity pipe.

Contrarily, in other locations it may be possible to reduce the level of mitigation, e.g. install a shorter length of proximity pipe. However, such requirements can only be identified by a case-by-case assessment of each location. Quantitative Risk Assessment (QRA), that explicitly takes account of both the likelihood of failure ( derived using techniques such as Structural Reliability Analysis (SRA)) and the consequences (and hence risk), is used for this purpose.

Likelihood of Failure

Structural Reliability Analysis (SRA) is a structured process for determining the failure frequency of a loaded structure. The process comprises four main stages:

Identification of potential failure causes
Identification or construction of limit state function for each failure mode
Construction of probability density functions describing uncertainty in key variables
Evaluation of failure frequencies

The results of the SRA are combined with an assessment
of the consequences to determine the risk levels. Other approaches for determining the likelihood of failure can also be used, such as the approach detailed in IGEM/TD/2.

Consequence Analysis

If an unplanned release of gas from a high pressure
pipeline ignites, the effect of the resulting heat source, on the surrounding environment, can be very significant in terms of the
potential damage to building structures and harm to individuals.
The potential harm that could be sustained by any individual depends on a number of factors including the quantity of gas that is released, the time taken for the gas to ignite, the distance between the individual and the heat source at the time of ignition, the distance to shelter and the speed at which the individual can reach shelter.

Following ignition, the basic series of events thus consists of an individual present in an unsheltered location acknowledging the
presence of the heat source and attempting to escape the harmful effects by running away to seek shelter. The likelihood that the individual will become a casualty depends on the thermal dose that is received by the individual during this activity.

In our software each of the relevant issues are considered in detail and mathematical models are used that determine the nature of the consequences in terms of the key parameters. The consequence modelling is subsequently combined with the results of the likelihood of failure analysis to determine the risk levels.

Risk Assessment

The results of the likelihood of failure and consequence analysis are combined to determine the risk levels. Some of the key outputs from the analysis and how they are used are detailed below:

Information on the Hazard Range and the Wood Burning Distance
The contribution from each pipeline segment to the Societal Risk calculation. This allows the User to pinpoint pipeline segments where further analysis and perhaps even additional mitigation measures are required.
The distance from the pipeline for each of the infrastructure inputted to the model, along with the individual risk levels at that point. The individual risk level is determines by considering all segments of the pipeline that could affect the infrastructure.
Individual risk contours surrounding the pipeline are also a graphical output of the software and this provides a good visual output of the risk levels surrounding the pipeline.
The Societal Risk, expressed in the form of an F-N curve and the expected number of casualties per year is also an output of the software. Along with the Individual Risk levels at infrastructure and surrounding the pipeline, the Societal Risk is used to determine what further mitigation (if any) is required along the pipeline.