An Operator was commissioned to construct an onshore high pressure gas pipeline. During the route selection phase it was discovered that it would be very difficult to avoid passing the pipeline through a locally highly populated area. In view of this it was naturally decided that the pipeline should be constructed from heavy wall sectioned pipe to mitigate the threat of failure due to causes including mechanical damage and corrosion. However, there was still a concern that the residual risk, even when the above mitigating measure had been taken, would still be unacceptably high.

In view of this the Operator commissioned AFAA to assess the remaining risk levels using a quantified risk assessment technique in accordance with the UK pipeline design code, IGE/TD/1 Edition 4, which provides for the use of such techniques. The technique used by AFAA involved detailed Structural Reliability Analysis (SRA) combined with an assessment of the consequences of failure.

AFAA used SRA to determine the likelihood of failure due to all the credible failure causes taking account of key areas of uncertainty including variations in defect dimensions and material properties.

A consequence model was used to determine the possible effects on the local population if a rupture of the pipeline was to occur. The consequence model was used to determine the amount of thermal dose that personnel in the vicinity of the release might receive, taking account of the transient nature of the gas flow.

The mitigating effects of nearby buildings, that would afford shelter from the effects of the thermal radiation levels, were naturally taken into account.

The results were expressed in terms of an F/N curve, and assessed against the risk criteria contained in the appropriate design code, IGE/TD/1.

It was concluded from the analysis that the proposed design did not pose an unacceptable level of risk, and moreover that part of the proposed heavy wall section was unnecessary. However, in the interests of conservatism the Operator proceeded with the original design.