Failure analysis of a natural gas pipeline subjected to landslide

Abstract

This paper presents the results of failure analysis of a 16-in. natural gas pipeline that ruptured due to ground movement caused by a 40 m wide landslide. The rupture occurred at a girth weld in the middle of a double bend located 120 m from the center of the landslide. The Finite Element Analysis (FEA) method was used to evaluate the response of the pipeline to the ground displacement. The pipeline and the surrounding soil were simulated by the Winkler-type beam-on-spring model and nonlinear static analyses were carried out to establish the root cause of the failure. The results of the analyses indicated that the ground movement was primarily resisted by the membrane action of the pipeline. The pipe axial force due to the membrane action extended well beyond the landslide zone and encompassed the bend region. The axial force at the bend location caused high local shear and bending moment in the bend region. Assessment of the state of stresses on three girth welds in the bend region indicated that the rupture was mainly due to the high shear demand on the girth weld in the middle of the bend.This study indicates that fabricated bends are vulnerable if they are placed within a critical length of the pipeline where the landslide-induced axial force is substantial. To reduce the risk of pipe rupture in pipelines crossing a potential landslide, fabricated bends should be avoided along this critical length. The critical length of a pipeline crossing a potential landslide includes the landslide zone and the anchor length on either side of the landslide. This study also indicates that the anchor length calculated by the relevant equation in the PRCI guidelines may underestimate the actual anchor length.