Stress Corrosion Cracking (SCC) is a wel l- known threat to the integrity of oil and gas pipelines. SCC is a damage mechanism that typically results in axial cracks on pipelines due to simultaneous action of tensile stress, potent environment (soil, coating, electrolyte etc.), and the pipeline material (poor microstructure). Small cracks are in itially formed around the corroded area (Initiation) which tend to coalesce under the combine d impact of applied stresses (pipeline hoop stress), cyclic stresses (pressure cycling) and residual stresses (bends, change in direction, and point of inflection). SCC remains a significant issue l argely because the industry’s understanding of this phenomenon is still evolving and practical methods of addressing SCC are not as mature as methods for addressing other failure causes.
ASME B31.8S provides a basic framework for determining the SCC suscep tible segments of buried pipelines. This framework is based on pipeline operating p arameters, environment, age and condition. The variables considered in ASME.
B31.8S were subjective in nature and do pr ovide a detailed methodology of how to prioritize pipeline segments for their susceptibility to SCC and optimize the cost of Crack Dete ction In-Line- Inspection (CD ILI). Besides other variables, cyclic pressures variations are very critical to analyze the initiat ion of cracking on pipelines. The differential stresses produced in the pipeline as a result of pressure cycles are capable to initiate cracking in vintage pipelines even at relatively low amplitude.
Saudi Aramco studied the behavior of these cyclic stresses on their liquid pipelines and found that consistent cyclic pressure fluctuations of medium to low magnitude coupled with po tent cracking environment and vintage pipe material may generate conditions that make the pipelines susceptible to SCC. This paper describes the enhanced SCC susceptibility criteria that was used by Saudi Aramco to op timize CD ILI inspection cost and effectivel y address the SCC threat on their pipelines.... (read more)
