PR676-233801-R09 Pipe Vulnerability Analysis Methodology

Continuous buried pipelines are generally required to traverse long distances and as such, inevitably pass through unstable landforms which may impart critical levels of stress beyond the intrinsic capacity of the pipelines. The deformational response of buried pipelines subject to relative ground movement is strongly influenced by the geotechnical properties of the immediate backfill and surrounding soil, the size and direction of the soil movement in which the pipeline is embedded, as well as mechanical soil-pipe interaction behavior. With respect to the mode of deformation, relative ground movement may be classified as transient ground deformation (TGD) or permanent ground deformation (PGD). Studies show that pipelines are more susceptible to brittle fracture due to cyclic loading when subjected to TGD whereas, under PGD, they tend to exhibit a more ductile response. The orientation of the moving soil mass relative to the pipe axis is a major factor that determines the deformational response of a buried pipeline under PGD: transverse and oblique PGD tend to induce predominantly bending strains while longitudinal PGD is more likely to induce axial tension and compression close to the margins of the PGD zone. Ductile behavior in metallic pipelines is characterized by a definitive yield point in the stress-strain curve beyond which the pipe material experiences a significant loss of stiffness (i.e., significantly less loading capacity relative to material deformation); hence, strain-based design and assessment (SBDA) methodology is well suited to account for geohazard-related failure mechanisms in pipelines subjected to landslides. Pipeline vulnerability evaluation relies on the SBDA methodology as it represents the limit state function which describes the relationship between the strain demand on the pipeline and the strain capacity of the pipeline. The applicability of the vulnerability analysis method presented in this report is limited to strain demand in the tensile region of the pipe; where non-bending strains are more likely to initiate cracking. Finite element models of seven real-world cases were simulated using a commercial finite element analysis (FEA) software package, ABAQUS CAE (version 2019), and the resultant axial strains in the tensile zones of the models were compared to the strain demand predicted using the analytical methods outlined in this report. The results showed reasonable correlation between the FEA results and the analytically predicted strain demand. Appropriate implementation of the vulnerability analysis methodology presented herein requires subject matter and state of the art familiarity to determine the appropriate margin of safety to account for differences between real word soil-pipe interaction and the simplified assumptions inherent in the strain demand models.

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