Journal of Pipeline Systems Engineering and Practice

This paper presents the study of the sequential failures of a nominal pipe size 8 (API 5L X42) carbon steel and SDR 17 medium density polyethylene (MDPE) pipes near a high-pressure water pipe. The study was conducted by reviewing the background information and visual inspection on failed specimens. The investigation revealed that a jet issuing from a water pipe joined with surrounding soil to form a slurry with highly erosive properties. The impact of this slurry upon the pipe caused the loss of pipe coating material and thinning of the pipe surface, which eventually led to its failure. Subsequently weaker ground support caused the MDPE pipe to sink into the high-speed gas jet from the failed API 5L X42 pipe. The high impact of the gas jet physically hit the MDPE pipe, causing its rapid erosion.

A study is presented to investigate the fatigue and local buckling-induced fracture incident of a buried continuous pipeline during the 1985 Mexico City earthquake. Assuming pipeline wrinkling is caused by longitudinal wave propagation, peak ground strains are estimated from recorded velocity time histories near the site of the fractured pipeline and critical buckling strains are calculated from experimentally based empirical relationships. A physics-based model of the postbuckling deformation of the pipeline provides estimated deformation demands on the pipe, whereas a micromechanical fatigue-fracture model is used to determine the potential for fracture considering the estimated axial deformations from the recorded ground motion, plastic strain demands, and stress state at the critical fracture location. The ground motion at the Ciudad Nezahualcóyotl site in Mexico is assumed to have approximately seven cycles of ground deformation large enough to exceed the critical buckling strain of the member. From a postevent reconnaissance, photographs demonstrate that the pipeline did experience a circumferential fracture near a butt weld region; thus, fracture in the base metal along with the heat-affected zone (HAZ) is considered. Although there is no evidence as to the specific time of fracture during the earthquake, the results from the fatigue model are presented in a probabilistic manner to demonstrate the likelihood of failure in the base metal and HAZ. The model and general procedure developed herein is promising in the context of improvingseismic design criteria to prevent pressure boundary loss in buried steel pipelines.

The vertical uplift resistance of long pipes buried in sands and subjected to pseudostatic seismic forces has been computed by using the lower-bound theorem of the limit analysis in conjunction with finite elements and nonlinear optimization. The soil mass is assumed to follow the Mohr-Coulomb failure criterion and an associated flow rule. The failure load is expressed in the form of a nondimensional uplift factor $F_{\gamma }$. The variation of $F_{\gamma }$ is plotted as a function of the embedment ratio of the pipe, horizontal seismic acceleration coefficient ($k_h$), and soil friction angle ($\varphi$). The magnitude of $F_{\gamma }$ is found to decrease continuously with an increase in the horizontal seismic acceleration coefficient. The reduction in the uplift resistance becomes quite significant, especially for greater values of embedment ratios and lower values of friction angle. The predicted uplift resistance was found to compare well with the existing results reported from the literature.