scope:

This recommended practice (RP) is applicable to any pipeline system used to transport hazardous liquid or natural gas. This RP includes detailed technical discussion on dent formation, strain and fatigue, and failure modes and mechanisms. These details are provided to give pipeline operators the information and knowledge necessary to make informed integrity management decisions regarding the management of dents on their systems. This RP describes preventive and mitigative measures that pipeline operators can apply to manage dents after detection. The in-service response of dents to a range of loading conditions is discussed in detail.

Mechanisms that promote denting are discussed, methods to inspect dents are described, and approaches and tools to estimate dent fitness-for-service (i.e., dent formation induced cracking, failure pressure, and pressure cycling fatigue life) are presented. Selection of the appropriate dent integrity assessment methods and integration of pipeline operating data is also discussed.

This RP is specifically designed to provide the operator with guidelines on industry-proven practices in the integrity management of dents. The guidance is largely targeted to the line pipe along the right-of-way, but some of the processes and approaches can be applied to pipeline facilities, including pipeline stations, terminals, and delivery facilities associated with pipeline systems.

This RP includes a review of currently available in-line inspection (ILI) technologies for detecting and characterizing dents, and provides guidelines for collecting data in the ditch when excavation is performed based on ILI data review and the pipeline and dent is exposed. Data integration practices are also addressed. Mitigation and repair techniques and approaches are discussed.

This RP provides general information on the dent formation process and describes approaches to evaluate fitness-for-service of dents regarding their potential to reach a structural limit state. A limit state represents an operator-defined condition where the pipe is no longer fit for purpose. This document provides tools to address three primary limit states: formation strain, failure pressure, and fatigue damage. The document provides screening tools and detailed assessment methods to address each of these limit states. This methodology is consistent with other design and fitness-for-service codes that address both static and fatigue limit states¹.

In the context of plain dents, the assessment of formation strain is considered to be an indication of the potential for cracking due to dent formation, and has been included as part of ASME B31.8 since 2006.

The assessment of failure pressure is typically not performed for most dents. Plain dents regardless of shape with depths up to 10 % of the pipe diameter, without coincident features such as metal loss, welds, or cracks have been shown in testing to have the same failure pressure as plain line pipe². Consequently, failure pressure is typically addressed for dents with coincident features or in detailed fitness-for-service assessments.

Pressure cycling fatigue determines the remaining life and susceptibility of a dent to failure as a result of operational cyclic pressure.

Dent assessment is also addressed when coincident or closely-aligned features are present and could be affecting the fitness-for-service.

¹ ASME Boiler and Pressure Vessel Code

² Bood, R., Gali, M., Marewski, U., Steiner, M., Zarea, M., "EPRG Methods for Assessing the Tolerance of and Resistance of Pipelines to External Damage (Parts 1 + 2)," European Pipeline Research Group (EPRG)