PR-328-133702-R02 F-S Fatigue Testing of Crack-in-Dent with Framework for Life Prediction
Authors: Arumugam Udayansankar , Elboujdaini Mimoun , Gao Ming , Vanoye Ramiro 
Published: August 2023

ASME B31.8 states that "Dents that contain stress corrosion cracking or other cracks are injurious to the pipeline" and therefore, requires immediate attention by the Operators. Dent containing crack fields (colonies) are often observed in liquid pipelines. The recently completed PRCI research project MD-1N "Study of the Mechanism for Cracking in Dents in a Crude Oil Pipeline" showed evidence of a mechanism for fatigue cracking. The crack growth rate as a function of stress intensity factor was estimated using the measured spacings of fatigue striations from fracture surfaces based on the assumption that the formation of fatigue striations on a cycle-by-cycle basis. However, due to the lack of full-scale fatigue crack growth data, the success was limited. This gap prompted PRCI to launch a full-scale experimental investigation of crack growth rates of cracks in dents under cyclic pressure load in the simulated groundwater NS4 environment (PRC-328-133702, MD-1Q). The objective of the study was to determine the crack growth rate as a function of stress intensity factor, the number of cycles to failure, and the failure modes of cracks in dents. The test results would be used to evaluate the validity of cycle-by-cycle based assumption for crack growth rate estimation from the measured fatigue-striation-spacing. The investigation was also aimed at establishing a framework for remaining fatigue life prediction of cracks in dents in liquid pipelines. This framework would benefit liquid pipeline Operators to manage better the integrity of dents associated with corrosion fatigue cracking in groundwater.

A total of six pipe samples containing cracks in shallow dents excavated from a retired 24-inch diameter liquid transmission pipeline were available and used for the full-scale fatigue tests. The test system developed under the project consisted of four components: (1) a computer-controlled hydraulic pressure cycling system, (2) an environment chamber containing a simulated groundwater NS4 solution mounted on the pipe in around the dent region to provide a simulated field environment condition; (3) real-time crack growth monitoring systems including direct cur-rent potential drop (DCPD), Clip gage and Strain gage; (4) data acquisition system. The cyclic pressure range used in the fatigue tests was 78 to 780 psig (72%SMYS) with R=0.1, which was based on historical operational pressure data and the Rain flow analysis. A constant frequency of 0.0526 Hz was selected for the testing to ensure the frequency requirement for corrosion fatigue is met.

The remaining fatigue life of cracks-in-dents and failure modes were evaluated using the full-scale fatigue test results. Further, fatigue crack growth rates were established. Finally, a framework was developed for the life prediction of cracks in shallow dents based on the findings from six full-scale fatigue cyclic tests. This framework will assist liquid pipeline operators to estimate the remaining fatigue life for cracks in shallow dents utilizing inputs from ILI and pipeline's historical operational pressure fluctuation data and to mitigate the threat of cracks in dents in a timely manner.

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