The J-Integral Method Compared to the API 579-1/ASME FFS-1 Standard to Calculate Stress Intensity Factor (SIF): Leak-Before-Break (LBB) Application with Uncertainty Quantification

Leak-before-break (LBB), as a part of the fitness-for-service (FFS) assessment, is a critical requirement to ensure pressure vessel structural integrity LBB generally means a leak will be detected before an in-service catastrophic failure occurs. Despite some established procedures in API 579-1/ASME FFS-1 or BS 7910 standards, performing a robust LBB assessment is not a regular and straightforward practice in the oil, gas, and petrochemical industries. A mix of different sources has been commonly used in case studies, which could lead to non-consistent results. This paper presents, firstly, a three-dimensional finite element analysis (FEA) within an LBB assessment framework for a cylindrical pressure vessel. The stress intensity factor (SIF) of a defective vessel with a through-thickness crack is numerically calculated using the J-integral method and based on linear elastic fracture mechanics (LEFM) approach. The accuracy of the numerical solutions is then compared with the analytical results proposed by the API 579-1/ASME FFS-1 standard. The maximum (limiting) through-thickness flaw size, which will not grow to an intolerable size during the vessel service life, is calculated analytically and numerically. Afterward, errors in measuring the exact length of the crack during inspections, the internal pressure fluctuations due to the vessel's operational conditions, and uncertainties in characterizing the mechanical properties of the base material, including its minimum yield strength and toughness, are quantified. A reliability analysis is finally evaluated to assess the probability of failure considering these uncertainties.