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 means of negotiating the landslip zone in a safe manner need to be determined and adopted.
AFAA were contacted to investigate the feasibility of a pipeline design routed down an escarpment where it had been noted that there was a potential for landslides to occur at various locations, with some future landsliding being virtually inevitable owing to the natural processes of slope degradation. The design was required to meet the criteria given in IGE/TD/1.
The solutions provided by AFAA were based on a combination of numerical stress analysis and structural reliability analysis (SRA). The stress analysis was conducted using the commercial finite element software ABAQUS. The SRA was undertaken using relevant and documented limit state functions, which took account of all potential failure causes to determine the relevant frequencies.
A Quantified Risk Assessment (QRA) was then performed by combining the results of the SRA with the results of a failure consequence analysis in order to determine whether mitigation methods would be required to reduce risks to levels that can be regarded As Low As Reasonably Practicable (ALARP).
Where mitigation methods were required, a combination of pipeline route straightening, the use of soft-fill to take up soil displacements, and the specification of improved material properties and welding inspection procedures allowed the pipeline to be demonstrated to be acceptable by QRA.