Working Out the Wrinkles: Combining FEA Analysis and Low-Cycle Fatigue Testing to Assess Wrinkle Bends

For pipelines installed before the 1950s-60s, the practice of intentionally bending a pipe to achieve a change in the routing direction would typically result in the formation of localized circumferential bulges on the compressive side of bent pipe. These bulges are known as wrinkles, and the bent pipe sections that manifested from this bending practice are known as wrinkle bends. While this approach to bending pipes was gradually phased out after the 1960s, there are a lot of vintage pipelines in the ground with these wrinkle bend “features”. A vast majority of these wrinkles have been existing without any incidents, but there have been occasional failures associated with wrinkle bends that warrants an understanding from the point of view of their threat potential.

Unraveling the Mystery of Wrinkles

Understanding the mechanical response of these unique pipeline features under various loading conditions is not straightforward. On account of their peculiar geometry (the outward bulge looks like a “U” in a sectional view), when a pipe with a wrinkle is subjected to internal pressure or to tension or bending loads, a small region near the apex of the wrinkle experiences very high stress (and associated high strain) – much higher than the nominal stress in straight base pipe. Due to the high-localized nature of this amplified stress, it is extremely difficult to accurately measure or track this stress from the point of view of integrity management, whether in the field or in the laboratory. This makes performance and failure predictions for wrinkles complex and necessitates a carefully planned approach. At ADV Integrity we use full-scale and sub-scale testing in conjunction with finite element analysis (FEA) to unravel the mystery of wrinkle bends in vintage pipelines. For more realistic predictions, we use data from laser scanning of wrinkles to build the finite element (FE) models. These FE models are then used for two purposes: 1) to plan the loads and displacements during the full-scale or sub-scale testing, and 2) to predict the peak stresses at the wrinkle apex.

The Disappearing Wrinkle Problem

Did you know that a force-controlled sub-scale testing approach for wrinkles results in an unexpected outcome? The wrinkles gradually disappear during such a test! Which means that the wrinkle shape and therefore the problem definition keeps changing during such a test! ADV Integrity handles this issue by designing displacement-controlled, low-cycle fatigue testing for wrinkles. The results from this testing approach are then used with the results from FEA for interpretation of stresses and strains at the wrinkle apex, and then combined with a strain-based damage model to predict the wrinkle performance in terms of strain range and associated cycles to failure. The beauty and the simplicity of this approach is that it allows a direct and practical correlation of global strains on the pipe (arising from geohazard-related loading and/or thermal loading) to the predicted performance of the wrinkles.

Thinking Outside the Box

Traditional fatigue analysis – the type of analysis that most engineers first cut their teeth on in school – is associated with high-cycle fatigue (large number of cycles to failure, low magnitude of cyclic loads). We instinctively think of stress concentration (or amplification) factors and S-N curves (stress vs cycle curves) when thinking of fatigue. However, low-cycle fatigue (low number of cycles, high magnitude of cyclic loads) does not readily fit that traditional image and necessitates the engineer to think in terms of strain ranges, strain-based damage models, and ε-N curves (strain vs cycle curves). The physics is still the same, but the perspective needs a bit of out-of-the-box thinking – even more so if one is integrating analysis and testing programs.

Assurance from Integrated Testing and Analysis .. and More

The ease with which ADV Integrity can transition between analysis and testing, under the same roof, for understanding a complex issue like wrinkles makes us truly unique. Additionally, integrity considerations for wrinkle bends also overflow into interpretation of ILI data for wrinkle bends as well as material properties and metallurgical considerations. Our team handles all these aspects seamlessly for a more complete integrity management solution for wrinkle bends. As more opportunities arise to study wrinkle bends, we aspire to continuously learn and improve on our understanding and approaches, and continue generating vital information for the industry to help manage pipelines with wrinkles.

Originally published on June 28, 2022