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).