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.
Although rare, it is inevitable that some occupied buildings will occur within the allowable distances 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 (structural reliability analysis (SRA)) and the consequences (and hence risk), is used for this purpose.
AFAA have developed very sophisticated in-house software for undertaking QRA studies, it has been subjected to extensive validation and verification processes. The software is able to assess the potential consequences of an ignited release of gas, and to combine the assessment of the consequences with the results of the SRA in order to determine risk levels. Both Individual and Societal risk parameters are determined.
The individual risk is used to determine the general acceptability of the proposed design along the entire length of the pipeline, societal risk parameters are used to assess the risk levels at infringements such as those described above. The results of the risk assessments are used to assess the effectiveness of the proposed mitigation, and subsequently to allow and support recommendations for any changes (increases or decreases) in the level of mitigation.