The complex relationship between irregular corrosion defects and pipeline failure presents substantial challenges for pipeline safety assessments. To investigate the interaction between irregular corrosion defect parameters and pipeline failure bending moments, a theoretical calculation model for the ultimate bending moment of pipelines with irregular corrosion defects was first established under the individual or combined effects of internal pressure, axial force, and bending moment, based on the Net Section Collapse criterion. Then, the accuracy and reliability of the proposed model were verified through experimental data from the literature and finite element analyses. Finally, taking API 5L X80 pipeline steel as an example, the impacts of defect shape, depth ratio, length ratio, and relative position of shallow and deep defects on the ultimate bending moment of pipelines with irregular corrosion defects were analyzed under three different working conditions. The research results indicate that the ultimate bending moment of the pipeline is related to the shape of the corrosion defect. Considering the shape of the corrosion defect in the calculation of the ultimate bending moment can avoid the underestimation of residual strength. The increase in the depth length ratio of irregular corrosion defects reduces the ultimate bending moment of the pipeline. When the depth length ratio remains constant, the larger the depth/length of the corrosion defect, the faster the rate of decrease of the ultimate bending moment. Additionally, adding internal pressure and axial force loads also reduces the ultimate bending moment. When a deep corrosion defect is located in the center of a shallow corrosion defect, the ultimate bending moment reaches its minimum. As the deep corrosion defect moves from the center towards the edge of the shallow corrosion defect, the ultimate bending moment of the pipeline gradually increases but the change is not significant. This paper presents a failure assessment method with the merits of clear physical concept and convenient calculation for pipelines with irregular corrosion defects, which enriches and develops the relevant theories of pipeline safety assessment.