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Study to determine residual stress on a pipe spool

A one-year collaborative study to quantify the relaxation of residual stresses on a pipe spool subjected to 3% strain has produced results expected to improve existing fracture integrity assessment by challenging the current treatment of residual stresses. The outcome of the study, carried out by TWI and ISIS, has important implications for the oil and gas, and nuclear industries and for the future of fracture mechanics assessment.

TWI and ISIS have carried out a programme of study using the non-destructive neutron diffraction technique to quantify the redistribution of residual stresses in a pipe spool before and after subjecting it to 3% strain. The engineering teams performed full three-dimensional residual stress measurements on a 273 mm (10.75 in) diameter pipe spool in 450 grade steel (18.3 mm wall thickness) containing a girth weld. Pipes of this kind are used in the offshore industry to make hydrocarbon flowlines and pipelines. These are welded and inspected onshore, then installed by a process known as ‘pipe reeling’, which imposes high axial strains (up to 4% total strain) on the pipe, and thus high transverse strains on the girth weld.

The study began at the ISIS neutron diffraction facility at the Rutherford Appleton Laboratory near Harwell, Oxford. The first stage of work characterised welding residual stress distribution in the as-received weld (in the hoop, axial and radial directions relative to the pipe), measuring residual stresses at the 3 o’clock position. Engineers performed two line scans at this location; one 3 mm above the root level and one 3 mm below the cap level. They then measured approximately 25 positions along each measurement line extending -40 mm and +40 mm from the weld centre, and in three directions (axial, hoop and radial).

The next step involved taking a set of stress-free reference combs from similar locations on a pipe containing a girth weld manufactured using the same welding parameters. These matchsticks were focused on weld and heat-affected zone regions as well as the base material. TWI/ISIS measured approximately 60 positions on the stress-free combs.

Determination of residual stresses in the as-welded pipe spool at ISIS laboratory
Determination of residual stresses in the as-welded pipe spool at ISIS laboratory

The spool was returned to the laboratories at TWI, where the team welded loading flanges onto the ends, allowing application of an axial load, uniformly distributed around the circumference of the pipe. Engineers instrumented the spool with displacement transducers and strain gauges, and applied loading via a horizontal test rig to produce a remote longitudinal strain of 3%. The spool was then returned to ISIS for repeat determination of residual stresses.

Early results from the programme of study have shown a marked reduction in the peak value of the hoop stress (the stress parallel to the weld) after plastic straining, as would be expected. In addition, when considering residual stress profiles before and after the introduction of 3% strain, the TWI/ISIS teams observed the following:

  • Residual stresses in the hoop direction relaxed significantly after the introduction of 3% strain. In the vicinity of the weld (±4mm from the weld centreline), the peak value of residual stresses drops to a negligible value from yield magnitude. However, stress state is balanced in the far field, approximately 15mm away from weld centre.
  • Residual stresses in the axial direction decrease in general. However, relaxation in the vicinity of the weld is not as pronounced as the relaxation in the hoop direction.

  • Residual stresses in the radial direction relax also but are limited to the vicinity of the weld and the distribution of residual stresses before and after straining follows the same trend in the far field, approximately 15mm away from the weld centre line.

  • In a fracture assessment, the stresses perpendicular to the orientation of the flaw (in this case, axial stresses, assuming the presence of flaws parallel to the girth welds) play an important role in the performance of the structure. However, it should be borne in mind that the triaxial stress state will also have an effect on the fracture behaviour of the structure.

TWI and ISIS have interpreted the full results of the study in relation to current models of residual stress relief by mechanical loading. The findings, which have relevance for the future treatment of fracture mechanics assessment in the oil and gas and nuclear sectors, were reported to the R6 Panel in November 2012 and are under preparation for publication 

This work was carried out as part of TWI’s in-kind contribution to the R6 Panel and also under ISIS’s Industrial Collaboration R&D Scheme, funded by the Science and Technology Facilities Council (STFC).

For further information about our work please visit our Structural Integrity web pages or email

Pipe spool under test at TWI
Pipe spool under test at TWI