Fatigue reliability analysis of dented pipelines

Abstract

The present study carries out fatigue reliability analyses of oil and gas pipelines containing unconstrained plain-smooth dents under the variable amplitude loading caused by cyclic internal pressure. The API 1156 S-N curve and Miner’s rule are employed to establish the limit state function for fatigue, and the stress concentration at the dent is accounted for based on findings reported in the literature. The reliability analysis is carried out on two pipeline examples containing dents with depths ranging from 2% to 5% of the pipe diameter. The examples are subjected to cyclic internal pressures at the discharge and suction ends of a compressor station on an actual in-service oil transmission pipeline. In the context of the cyclic pressure records considered in the present study, the analysis results suggest that fatigue failure of dents under suction-end pressure cycles is highly unlikely, whereas the probability of fatigue failure of dents under discharge-end pressure cycles can be significant if the diameter-over-wall thickness ratio of the pipeline is large. This study provides a methodological framework to carry out reliability-based fitness-for-service assessments of dented pipelines with respect to fatigue.

Keywords

• Pipeline
• Dent
• Fatigue reliability
• S-N curve
• Miner’s rule
• Variable stress ranges