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In this blog, we share how ILI run-to-run comparison and fatigue growth modelling revealed that factors beyond cyclic loading may have contributed to the evolution of crack colonies in a vintage pipeline.\",\"nodeType\":\"text\"}],\"nodeType\":\"paragraph\"}],\"nodeType\":\"document\"}"},"text":{"raw":"{\"data\":{},\"content\":[{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"The pipeline was constructed in 1963 using a low-frequency ERW pipe. NDT Global first inspected the line in 2018. The pipeline has historically operated with two pump stations, while a third pump station remained idle. The operator planned to return this station to service, which would increase the maximum operating pressure (MOP) and change the pipeline's operational conditions. \\n\\nTo establish a baseline understanding of the crack population before these operational changes, the operator undertook a detailed assessment of the inspection results. In addition, a hydrostatic test was conducted in 2022 as part of the integrity management program.\\n\\nIn 2024, the pipeline was inspected again using the same inspection technology deployed in 2018. Throughout the inspection interval, the operator maintained moderate cycling conditions. Based on the operating history, significant crack growth was not anticipated. However, the 2024 inspection results told a different story.\\n\\nExisting crack colonies had grown, and additional crack colonies were reported. The extent of the observed growth, along with the appearance of potential new anomalies, was unexpected for a pipeline operating under moderate cycling conditions.\",\"nodeType\":\"text\"}],\"nodeType\":\"paragraph\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"Investigating the Change\",\"nodeType\":\"text\"}],\"nodeType\":\"heading-4\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"To better understand the evolution of the crack population, the operator requested a detailed signal-to-signal run comparison between the 2018 and 2024 inspections.\\n\\nThe assessment identified an average growth of approximately 0.020 in., with crack field anomalies in the 2024 inspection data showing a clear trend toward higher signal amplitudes. This increase in amplitude is observed in both the repaired and unrepaired populations and exceeds the variation typically expected between inspections. \\n\\nA detailed signal-to-signal comparison of anomaly profiles indicates potential growth across multiple sections of the crack field. The results suggest that small individual cracks within the colonies propagated between inspections, increasing interaction and coalescence between adjacent cracks.\\n\\nAs these cracks grew and interacted, their combined response may have surpassed the tool's detection thresholds, resulting in higher reported amplitudes and greater visibility in the 2024 data set.\\n\\nThe signal-to-signal run comparison was a critical component of the assessment because it provided evidence of population-wide changes that could not be explained by inspection variability alone. The statistical and signal-level analysis indicated genuine evolution within the crack field population between inspections.\\n\\nThe increase in reported crack colonies was also influenced by improved discrimination between true crack colonies and manufacturing-related surface indications. Nevertheless, the presence of potential new anomalies could not be ruled out.\",\"nodeType\":\"text\"}],\"nodeType\":\"paragraph\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[{\"type\":\"bold\"}],\"value\":\"Understanding the Growth\",\"nodeType\":\"text\"}],\"nodeType\":\"heading-4\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"To determine why these anomalies were showing growth and what the main driver of this growth was, NDT Global modelled the growth of the 2018 anomalies following API RP 1176, using Paris Law. The simulation of fatigue cycling growth was based on the actual cycling conditions of the line before and after the hydrostatic test.\\n\\nThe fatigue calculations overestimated growth at the lower range, attributing an average growth of 1 mil per year to all anomalies. However, the ILI results indicated that some of the linear anomalies were not affected by the cycling conditions.\\n\\nNevertheless, the discrepancies between the predictions and the ILI measurements in the mid-range growth population were noticeable. Crack colonies showed a median growth four times higher than predicted, and an average growth twice as high as predicted.\\n\\nThe impact of an unaccounted growth driver cannot be discarded. The potential influence of the hydrostatic test on the evolution of the crack colonies, as well as the environmental growth component, had not been considered.\",\"nodeType\":\"text\"}],\"nodeType\":\"paragraph\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[{\"type\":\"bold\"}],\"value\":\"Considering the Environmental Factor\",\"nodeType\":\"text\"}],\"nodeType\":\"heading-4\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"To better understand the discrepancy between the fatigue calculations and the ILI observations, environmental growth mechanisms were considered. The total crack growth rate can be expressed as:\\n\\n\",\"nodeType\":\"text\"},{\"data\":{},\"marks\":[{\"type\":\"bold\"},{\"type\":\"italic\"}],\"value\":\"Total growth rate = da/dt (environmental) + da/dN (fatigue)\\n\\n\",\"nodeType\":\"text\"},{\"data\":{},\"marks\":[],\"value\":\"An environmental growth rate of 2 mpy overestimated growth in the mid-range. This raises the question of whether all evaluated SCC anomalies were equally affected by environmental conditions.\\n\\nWhen an environmental growth rate of 2 mpy was applied only to anomalies showing fatigue growth of at least 1 mpy, the predicted combined fatigue and environmental growth was closer to the growth observed in the ILI results.\",\"nodeType\":\"text\"}],\"nodeType\":\"paragraph\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"Key Findings\",\"nodeType\":\"text\"}],\"nodeType\":\"heading-4\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"The signal-to-signal run comparison was a critical component of the assessment, demonstrating measurable change across the crack field population. The analysis identified an average growth of approximately 0.020 in. and a consistent increase in signal amplitudes that exceeded normal inspection-to-inspection variation. These findings provided strong evidence of crack colony evolution between inspections.\\n\\nThe run-to-run statistical comparison demonstrated measurable growth across the crack field population, while no statistically significant growth was observed among the reported in-weld anomalies.\\n\\nFatigue modelling showed that growth driven solely by cyclic loading could not fully explain the changes observed between the 2018 and 2024 inspections. While fatigue contributed to crack extension, measured growth within many crack colonies exceeded model predictions.\\n\\nIncorporating environmental growth mechanisms improved the correlation between predicted and observed behavior. The best agreement was achieved using an environmental growth rate of 2 mpy applied selectively to anomalies that were already experiencing fatigue growth.\\n\\nThe 2022 hydrostatic test may also have influenced the evolution of the crack colonies. Small pre-existing cracks within these colonies may have grown between inspections, leading to increased interaction and coalescence. As these cracks combined and became more interconnected, their collective response became more apparent in the inspection data.\",\"nodeType\":\"text\"}],\"nodeType\":\"paragraph\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"Conclusion\",\"nodeType\":\"text\"}],\"nodeType\":\"heading-4\"},{\"data\":{},\"content\":[{\"data\":{},\"marks\":[],\"value\":\"This case study demonstrates that operational cycling alone does not always provide a complete explanation for crack colony growth.\\n\\nAlthough the pipeline operated under moderate cycling conditions, the 2024 inspection revealed significant changes in the crack population. The signal-to-signal run comparison identified measurable growth within existing crack colonies and provided evidence that the observed changes were not simply the result of normal inspection variability. Fatigue modeling showed that cyclic loading alone could not fully account for the observed behavior.\\n\\nThe addition of environmental growth considerations resulted in a significantly improved match between modelled and observed growth, suggesting that multiple mechanisms were influencing crack development. The potential contribution of the hydrostatic test further highlights the complexity of crack colony evolution in vintage pipelines.\\n\\nBy combining structured monitoring programs, robust ILI run-to-run comparison methodologies, and both fatigue and environmental growth assessments, operators can develop a more complete understanding of crack behavior over time. This broader perspective supports more informed integrity management decisions, enabling operators to anticipate changes within crack populations and proactively manage evolving threats to pipeline integrity.\",\"nodeType\":\"text\"}],\"nodeType\":\"paragraph\"}],\"nodeType\":\"document\"}"},"disableGlossaryAutolinking":false},{"__typename":"ContentfulCtaArrowsModule","image":{"description":"NDT Global Employees Standing and High-Fiving Around Inspection Tool","gatsbyImageData":{"images":{"sources":[{"srcSet":"//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png?w=781&h=437&q=80&fm=webp 781w,\n//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png?w=1563&h=875&q=80&fm=webp 1563w,\n//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png?w=3125&h=1750&q=80&fm=webp 3125w","sizes":"(min-width: 3125px) 3125px, 100vw","type":"image/webp"}],"fallback":{"src":"//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png?w=3125&h=1750&q=80&fm=png","srcSet":"//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png?w=781&h=437&q=80&fm=png 781w,\n//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png?w=1563&h=875&q=80&fm=png 1563w,\n//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png?w=3125&h=1750&q=80&fm=png 3125w","sizes":"(min-width: 3125px) 3125px, 100vw"}},"layout":"constrained","width":3125,"height":1750,"placeholder":{"fallback":"data:image/jpeg;base64,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"}},"file":{"contentType":"image/png","url":"//images.ctfassets.net/vs1cvwqhrsnd/mmwNhAzRUbWbeUrxDS6Zq/b8a133ba57df26b59c7e6b06daa93966/Untitled_design__2_.png"}},"heading":{"heading":"Discover the Power of Clarity™."},"ctaText":"Contact Us","text":{"raw":"{\"nodeType\":\"document\",\"data\":{},\"content\":[{\"nodeType\":\"paragraph\",\"data\":{},\"content\":[{\"nodeType\":\"text\",\"value\":\"Whether you're planning an inspection, evaluating asset integrity, or exploring solutions to complex challenges, our team is ready to help.\",\"marks\":[],\"data\":{}}]}]}"},"ctaTarget":{"__typename":"ContentfulContactUsPage","id":"ee079353-7784-57d0-97ac-2e89c9cb1cdb"}}]}},"pageContext":{"id":"ea5247f8-b6d9-5571-a521-b774ecff7a6f","slug":"when-moderate-cycling-does-not-tell-the-whole-story-understanding-crack-colony-growth-through-ili-run-comparison","category":"Blog","secretPage":false,"secretResourcesGroup":null,"resourceCategory":{"plural":"Blogs","singular":"Blog","slugPlural":"blogs","slugSingular":"blog"}}},"staticQueryHashes":["1058762779","1334403470","2665121642","3475427234","660649468","732355233"]}