PR-320-113706-R01 Neutron Diffraction Measurements of Residual Strain from Dents and Gouges in Pipelines

Mechanical damage is one of the most prevalent causes of pipeline failure in North America and Europe.   Gouged dents are much more likely to produce a failure than “plain” dents (i.e., a dent with no coincident metal loss or crack features), however the residual stresses around gouged dents are more difficult to model and predict.

The Pipeline Aggression Rig (PAR) located in the St. Denis facility of GdF Suez was used to introduce backhoe-type gouging into pipe samples, which were nine pressurized pipe sections of varying grades.    Five (5) samples were created using high-impact dynamic aggression (termed High Dynamic) and the other four (4) samples were created using lower impact energy and an axial-dragging mechanism (termed Low Dynamic).   This project involved using neutron diffraction to measure the residual stresses in and around these gouged dents.   Key findings are as follows:

• In the undamaged regions of all samples, residual stresses were low (typically <±200 MPa) and had “mild bending” residual stress distributions typical of axially-welded pipe.  
• Point-to-point variation of residual stresses in and around the gouge region often varied considerably, indicating that deformation behavior is highly complex, even for these samples where deformation was carefully and consistently introduced.
• Residual stresses were low at the immediate gouge base, even for High Dynamic gouging.   This was the most surprising finding in the study.
• Residual stresses in and around four (4) High Dynamic medium gouges were localized and residual stresses were largely bending-related: mild tensile at the outer wall/gouge base and compressive at the inner wall. 
• Residual stresses in and around one (1) High Dynamic severe gouge were tensile and critically high.   
• Residual stresses in and around Low Dynamic gouges had inconsistent and unpredictable residual stress magnitudes and distributions.   

PRCI members will find value in this information for understanding the severity of gouges in pipelines, for use in stress modelling verification, and, finally, for understanding characterizing MFL signals from ILI tools.