Pipeline operators are often presented with decisions which could be supported with scientifically-vetted and situationally accurate stress thresholds for no crack growth. The threshold stress-intensity factor, ∆K~th~, is the value for ∆K where crack growth, da/dN, approaches the threshold crack growth rate. Stress-intensity factors at or below this value result in crack growth small enough for operators to practically ignore it in pipeline integrity assessment. Previously, a ∆K~th~ value of 2.0 MPa\*m^0.5^ had been suggested for general use in a PRCI study, these laboratory-determined values appear conservative when compared to industry experience, established ∆K~th~ for similar steel alloys, and conclusions reached in a USDOT report regarding post-hydrostatic test susceptibility. By establishing an on-shore pipeline specific ΔK~th~ which considers a pipeline-specific da/dN threshold and stress ratio effects, operators are afforded the opportunity to:

Exclude certain pipelines or portions of pipelines from crack growth susceptibility

Identify features with no life limit where loading is known

Adjust load and boundary conditions to preclude crack growth

Improve computational efficiency by discarding load cycles below threshold

 

A large pipeline operator has researched pipeline crack growth testing and has derived reasonable and prudent negligible crack growth values through a closer examination of ∆K~th~, including exploration of how different loading scenarios and environments affect ∆K~th~. A da/dN threshold for when diminishingly small crack growth rates can be neglected for typical pipeline assets was determined based on pressure fluctuation frequency. Applications and value derived from deployment of ∆K~th~ are illustrated for North American pipeline assets, and there are significant implications of determining growth susceptibility of low-pressure pipelines after hydrostatic test. Growth limitations are developed for circumferential cracks based on ∆K~th~ screening for buried pipelines. Establishing threshold ∆K~th~ paves the way toward adoption of next-level fracture mechanics best practice models such as the NASGRO equation; and facilitates crack growth simulations and root-cause analysis.


Negligible Crack Growth Thresholds

Division / Sponsors: 
Pipeline Systems Division

Paper Proceedings: https://asmedigitalcollection.asme.org/IPC/IPC2020/volume/84447

IPC2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.