PR350-224502-R02 Managing Longitudinal Stresses from Uneven Support and Settlement

High longitudinal stresses in pipelines can be a major contributor to failures of girth welds and circumferentially oriented flaws. These high stresses are frequently associated with construction and maintenance activities, such as excavation and backfill in new construction, pipe replacement, and integrity digs. There are currently no industry-wide guidelines for the management of construction- and maintenance-related stresses. The objective of this project is to develop such guidelines by extracting and streamlining the best practices from pipeline operators and other sources.

Evidence collected in this project suggests that settlement after excavation and backfill can be a major source of high longitudinal/axial stresses. Crossings and locations near a body of water are particularly susceptible to such settlement-induced high longitudinal/axial stresses. The review of 675-mile ILI IMU reported strain features suggests, even without or with minimum geohazards, locations with longitudinal/axial strain exceeding that corresponding to 90%SMYS (ASME B31.4 and B31.8 design limit for most pipelines) are not rare, occurring at frequency of approximately 1.5 instances per mile.

Excavation and backfill practices were reviewed through a targeted survey of project team members and direct communication with other SMEs identified by the project team members. General guidelines have been developed for (1) supporting the unsupported pipeline segments during excavation, (2) excavation, (3) backfill and compaction. When implemented, these guidelines will likely lead to reduced longitudinal/axial stresses. It is also recognized, however, that more specific procedures may be needed for pipeline sections that are within a deep overly excavated section (i.e., such as crossings and locations near bodies of water).