Soil displacement along Balboa Boulevard during the 1994 Northridge earthquake ruptured natural gas transmission and distribution pipelines as well as two pressurized water trunk lines. Four other buried pipelines in the ground displacement zone were not damaged. This study probabilistically assesses the performance of the buried pipelines in the framework of performance-based earthquake engineering. The main aspects of pipeline performance follow from the geotechnical characteristics of the site. Uncertainty in each of the key soil-pipeline system parameters is estimated, including length of the seismic ground displacement zone, amount of seismic ground displacement, soil-pipeline interface shear stress, pipe steel yield strength and Young's modulus, and shapes of the pipe steel stress-strain curves. Monte Carlo simulations are performed with an analytical model to assess the pipe strain response. New fragility functions are proposed to evaluate pipeline performance in response to tensile or compressive longitudinal strain. The resulting probabilities of failure are compared with the results of a conventional analysis in which the modeled pipeline strains are evaluated with respect to the critical strains that cause either tensile or compressive failure. The failure probabilities compare well with the pipeline performance observed during the Northridge earthquake, except for one natural gas transmission line. A sensitivity analysis is performed for this line to investigate the reasons for the discrepancy. Advantages and limitations of probabilistic analyses are discussed.

Buried pipelines are subjected to various types of loads, including external pressure from soil overburden and internal pressure from pressurized fluids. These loads can induce axial and hoop stresses, which are the primary factors leading to the formation of integrity threats, such as cracks. The presence of cracks can render a pipeline susceptible to failure, posing a significant threat to its operation, safety, and the environment. This underscores the importance of promptly detecting and evaluating even seemingly minor surface defects, as they can significantly damage the structural integrity of the pipeline. It is also crucial to accurately predict the failure pressures of pipelines with cracks to ensure that the operating pressure remains below this critical limit with an adequate margin of safety. A variety of approaches exist for assessing cracks in pipes, including empirical approaches such as MAT-8, Ln-Sec and CorLASTM models, as well as numerical approaches like the extended finite element method (XFEM). XFEM is a powerful tool to estimate the failure pressures of pipelines containing cracks. It extends the capabilities of the traditional Finite Element Method (FEM) and offers a more effective means of simulating crack propagation. In ABAQUS, initial cracks can be modelled in either sharp or blunted shapes. However, it is uncertain whether the shape of the crack affects the failure pressures of cracked pipelines. For this purpose, detailed parametric studies are necessary to investigate the implications of pre-existing cracking shapes on the ductile fracture response of pipes subjected to pure mode I loading.