Pipelines used to carry fluids must be robust and able to withstand production and installation. Leading engineering consultancy TWI have been using Engin-X at ISIS to assess how pipes used in the offshore oil and gas industry are affected by installation processes introducing high plastic strain. The results give an insight into how residual stresses introduced by pipeline welding change after strains imposed during installation, and what this means for the structural integrity of the pipe.​

The outcome of a study carried out by engineering consultancy TWI and ISIS has important implications for the oil, gas and nuclear industries. Pipelines used to carry fluids must be robust and able to withstand production and installation. Leading engineering consultancy TWI have been using Engin-X at ISIS to assess how pipes used in the offshore oil and gas industry are affected by installation processes introducing high plastic strain. The results give an insight into how residual stresses introduced by pipeline welding change after strains imposed during installation, and what this means for the structural integrity of the pipe.​

The study looked at a girth (circumferential) weld in the type of steel pipes used in hydrocarbon flowlines and pipelines. These are usually welded and inspected onshore (or on the vessel used for installation), then installed by a process known as ‘pipe reeling’, which imposes high axial strains (up to 4% total remote strain) on the pipe, and thus high transverse strains on the girth weld. The welding procedure tends to introduce a residual stress field and flaws. The installation loads acting externally on the pipe containing the girth weld together with the residual stresses can in principle lead to failure, with disastrous consequences.

The first stage of work characterised welding residual stress distribution in the as-received weld. Next the pipe was returned to TWI where engineers used a test rig to replicate the strains the pipe would see during installation. Then it was returned to ISIS for measurement of residual stresses after welding.

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. 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. Close to the weld (±4mm from the weld centreline), the peak value of residual stresses drops to a negligible value from yield magnitude. However, the stress state is balanced in the far field, approximately 15mm away from the 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 also relax 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 stress in all three directions has an effect on the fracture behaviour of the structure.

Dr Elvin Eren and Dr Isabel Hadley from TWI state that this study has important implications for the oil and gas industry and these findings have been submitted to the R6 panel, a group of industrialists and academics working on the development of the UK nuclear industry’s fracture integrity assessment procedure. Ultimately the work will also influence the standards on fracture mechanics used by the oil and gas industry.