ELASTIC-PLASTIC FINITE ELEMENT ANALYSIS UTILIZING DETAILED CRACK PROFILES

Operators are required to mitigate any known risk identified during a pipeline inspection, meaning all detected crack-like defects must be investigated. While defects with depths or lengths exceeding the limits set by the pipeline owner will be repaired, shallower or shorter crack-like defects are often evaluated using engineering analysis and judgement. Deterministic assessment based on modern fracture mechanics is commonly used to inform the mitigation process. However, these assessments often assume lower bound material properties and generalize the crack shape as semi-elliptical for simplicity and conservatism, potentially rendering overly conservative results. Through evolving in- line inspection (ILI) technology, ultra-sonic imaging technologies can now capture the true profile of a crack-like defect. Rather than assuming a semi-elliptical crack based on a single depth and length measurement, detailed crack shape analysis now offers reduced conservatism and improved accuracy. However, averaging techniques are often used to characterize a detailed crack shape given that closed-form fracture mechanics models such as API 579-1/ASME FFS-1 Fitness-for-Service (API 579) or MAT-8 require an assumed semi-elliptical geometry. An alternative deterministic assessment is provided that can be executed with the detailed crack profile using elastic-plastic finite element analysis to calculate burst pressure and remaining life. Three-dimensional (3-D) crack meshes of the detailed crack profiles are generated using the ILI data, enabling the J-integral to be accurately computed along the entire crack front. The maximum crack front J-integral value is compared to the pipe's toughness to determine the predicted burst pressure. The 3-D custom shape crack results can then be compared to the API 579 and MAT-8 fracture models. This alternative approach reduces conservatism, which can aid operators and reduce costs while maintaining asset integrity.