Crack Combination Criteria for Internal Semi-Elliptical Surface Cracks in Pressurized Pipes

Failure of pipes containing multiple cracks is a critical concern, commonly assessed using the Mode I stress intensity factor (KI). API 579-1/ASME FFS-1 provides a criterion that allows two closely spaced, semi-elliptical internal cracks in the same plane to be treated as a combined crack for fitness-for-service (FFS) evaluations. However, this criterion has not been thoroughly validated. This study uses finite element analysis (FEA) to calculate KI for pipes with two identical cracks and their corresponding combined-crack models. A conservatism factor (η) is introduced to evaluate the safety and applicability of the criterion across a wide range of conditions, including pipe size (t/Ri), crack depth (a1/t), crack shape (a1/c1), and distances between cracks (s/a1). Results show that η decreases with increasing t/Ri and s/a1, but increases with higher a1/t and a1/c1 ratios. Underestimation (η > 1) occurs in thin-walled pipes with deep, semi-circular cracks, indicating non-conservative predictions. To address this, a modified criterion incorporating a correction factor (α = 3.5) into the distances between cracks term is proposed, effectively eliminating underestimation and improving safety. Given the high computational cost of FEA, the analytical KI solution from API 579-1/ASME FFS-1 was also applied with α = 3.5 and found to consistently yield conservative results. The modified criterion also showed safe performance for crack distances beyond API limits. These findings support the enhanced reliability of FFS assessments for pipes with interacting cracks.