EFFECTS OF POST-PEAK SOFTENING BEHAVIOR OF DENSE SAND ON LATERAL AND UPWARD DISPLACEMENT OF BURIED PIPELINES

Buried pipelines are extensively used in onshore and offshore for transportation of hydrocarbons. The response of pipeline due to lateral and upward relative displacements is one of the major concerns in pipeline design. Both physical modeling and numerical analyses have been performed in the past to understand pipeline-soil interaction mechanisms. The numerical analyses are generally performed using finite element (FE) modeling techniques. For the pipelines buried in sand, a large number of analyses available in the literature have been performed using the Mohr-Coulomb model assigning constant values of angle of internal friction (phi') and dilation (psi). However, dense sand shows post-peak softening behavior and the behavior of sand also depends on mode of shearing, such as triaxial (TX), direct shear (DS) or direct simple shear (DSS) conditions. In the present study, FE analysis of buried pipelines in dense sand is presented. The first set of analyses are performed using the built-in Mohr-Coulomb model in Abaqus FE software with constant angles of internal friction and dilation, as typically used in previous FE analysis of pipeline-soil interaction. The second set of analyses are performed using a modified Mohr Coulomb model where pre-peak hardening, post-peak softening, density and confining pressure dependent friction and dilation angles are considered. The FE analyses are performed using the Arbitrary Lagrangian-Eulerian (ALE) approach available in Abaqus/Explicit FE software. The modified Mohr-Coulomb model is implemented in Abaqus FE software using a user defined subroutine. Shear band formation due to strain localization and failure patterns for both lateral and upward pipeline-soil interactions are discussed from the simulations with MC and MMC models. FE results show that the MMC model can simulate the load-displacement behavior and failure pattern better than the simulations with the MC model.