Risk Based Design Solution for Routing a High Pressure Gas Transmission Pipeline Through a Region of Geological Instability

Abstract

If present, regions of geological instability are normally identified during the route selection study and a route through such locations will normally be rejected if a suitably practicable alternative route can be found. Therefore the consequential effects of a landslip on pipeline integrity rarely need to be considered. However, when an alternate route is not practicable, then a means of negotiating the landslip zone in a safe manner needs to be determined and adopted. One means of negotiating a landslip zone is to route the pipeline well beneath the slope using techniques such as horizontal directional drilling (HDD). However, the success of an HDD cannot be guaranteed in such situations and hence alternative solutions need to be considered even if only as a back-up. This paper describes a technique that was developed by a major UK high pressure gas pipeline operator with support from Andrew Francis & Associates Ltd (AFAA) to achieve a viable engineering alternative to HDD. The technique is based on a combination of structural reliability analysis (SRA) and quantified risk assessment (QRA) which was developed by modifying and customising an approach that the operator had developed and used previously for demonstrating the safe operation of gas transmission pipelines at design factors in excess of 0.72. The overall objective is to demonstrate that all associated risks have been reduced to levels that can be regarded ‘As Low As Reasonably Practicable’ (ALARP). SRA begins with an identification of all credible failure causes and these are then analysed using a combination of structural mechanics based techniques and probability theory to determine failure frequencies. For the present application, significant attention was given to the interaction between the moving land mass and the pipeline using 3-dimensional finite element analysis. The analyses were performed for a range of credible scenarios assuming a range of soil properties to establish the likelihood that failure would occur in the event of a land slide. These were then combined with an assessment of the event frequency to determine estimates of the failure frequency. Having established ‘raw’ failure frequencies, the model was developed further to investigate the effects of introducing mitigating methods to reduce the failure frequencies, and hence risks, to levels that could be regarded as ALARP. The paper describes the philosophy and the salient features of the approach and illustrates the application using a case study.