Negligible Crack Growth Thresholds

This paper initiates use of fracture mechanics best practice growth models and tools for pipeline steels with full tri-region da/dN characterization. The utilities associated with establishing negligible crack growth thresholds are demonstrated.

Pipeline operators are often presented with decisions that could be supported with scientifically vetted and situationally accurate stress thresholds for negligible crack growth. The threshold stress-intensity factor, ΔK_th, is the value for ΔK where the crack growth rate, da/dN, approaches the minimum threshold crack growth rate. Stress-intensity factors at or below this threshold value result in crack growth small enough for operators to practically ignore it in pipeline integrity assessments. Previously, a ΔK_th value of 2.0 MPa*m^0.5 had been suggested for general use in API 579[1]. The API 579 value appears conservative when compared to industry experience and established ΔK_th for similar steel alloys across all stress ratios.

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,

• adjust load / boundary conditions to preclude growth,

• improve computational efficiency by discarding load cycles below threshold, and,

• more accurately simulate crack growth scenarios

Pipeline crack growth testing has been researched to derive reasonable and prudent negligible ΔK_th values through a closer examination of loading scenarios and environments which affect ΔK_th. A da/dN threshold for when diminishingly small crack growth rates can be neglected for typical pipeline assets was determined based on observed pressure fluctuation frequencies. Applications and value derived from deployment of ΔK_th are illustrated for North American pipeline assets. Environmental and blunting effects on ΔK_th for near-neutral pH stress corrosion cracking previously developed are shown for comparison and utility.

Fully established negligible growth thresholds pave the way toward adoption of next-level fracture mechanics best practice models and tools such as AFGROW and NASGRO, and facilitates crack growth simulations and root-cause analysis.