Pipelines are important facilities for the development of oil and gas, and it is of great significance to ensure their safe and efficient operation. The existing pipeline ultimate load evaluations and researches lack the consideration of pipe characteristics and failure mechanism, there are certain errors, and the types of defects involved are relatively single. In order to give full play to the potential of pipes, consider the strain hardening effect of pipeline steel during the true failure process of pipelines, and study the influence of different types of defects on the ultimate bearing capacity of the pipeline. This paper uses the twin shear stress yield criterion and based on the Ramberg-Osgood material strain hardening model to derive an equation which is used to predict the ultimate load of the defect-free X60 pipeline. Using finite element method to study different kinds of defects on the ultimate load of X60 pipeline. A model for predicting the ultimate load of the X60 pipeline with a flat-bottomed rectangular single-point external defect is fitted, and the accuracy of the model is verified by comparison with real data. The failure modes of pipelines with different defects are also studied. On the basis of the above prediction model, the ultimate load prediction models of X60 pipeline with axially adjacent double-point external defects were fitted. Besides, the critical distance of interaction between the double-point defects has been discussed. The above analysis provides a basis for the safety assessment of X60 pipeline.

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