Objective Permanent ground displacement from faults can lead to significant bending and axial deformation of buried oil and gas pipelines, ultimately resulting in failures due to tensile rupture or compressive buckling. Traditional analytical methods lack sufficient applicability and accuracy; thus, there is an urgent need for the development of a more precise and efficient analytical approach to quickly assess the safety of pipelines in fault zones.

Methods An analytical method for assessing axial strain in oil and gas pipelines based on equivalent bending stiffness was proposed. The linear hardening constitutive model was used to capture the nonlinear behavior of the pipe material, and the nonlinear soil spring model was used to represent the soil’s nonlinear constraints on the pipeline in both axial and lateral directions. Utilizing the axial tensile force and the nonlinear soil constraint force, the governing equation of the pipeline deflection curve under the condition of large-deflection deformation of the pipeline in the fault zone was constructed. By considering the equilibrium between the stress in pipeline cross-sections and the overall axial tensile force, along with the internal and external bending forces, a novel iterative solution algorithm for equivalent bending stiffness during elasticoplastic deformation of the pipeline in large-deflection deformation segments was established, enabling accurate solution of the elasticoplastic state of pipeline cross-sections in fault zones.

Results An X80 pipeline crossing a fault zone was examined, with operating conditions involving a fault crossing angle between 15° and 90° selected for the analysis. The calculation results from the nonlinear finite element method were used as reference values, and comparisons were made between the newly developed analytical method and three mainstream analytical methods to verify the applicability of the new approach for combined tensile and bending deformations induced by varying crossing angles. Under different combined load conditions, strong consistency was observed between the new analytical method and the finite element simulation results, with an average relative error of 10.32％ and a mean square error of 6.7×10⁻³. This performance was significantly better than that of commonly used mainstream analytical methods, enabling rapid and accurate predictions of strain for buried oil and gas pipelines in fault zones.

Conclusion The new analytical method offers significant advantages in calculation efficiency and accuracy, providing essential theoretical support for the strain design and evaluation of pipelines crossing fault zones. Additionally, it can serve as a reference for revising seismic design standards for oil and gas pipelines.