Machining of simulated defects in pipeline steels: A neutron diffraction study of local stress effects

Abstract

The most commonly used in-line pipeline inspection tools utilize the magnetic flux leakage (MFL) technique, which is sensitive to the stress state of the pipe wall. Calibration of these MFL tools is often carried out using unstressed pipeline sections containing simulated defects, usually produced by mechanical drilling. There is also strong interest in creating simulated defects in stressed pipe walls, thus simulating corrosion in the field. In this study, neutron diffraction is used to map the local stresses surrounding mechanically and electrochemically drilled holes in a steel plate sample. Holes were drilled in unstressed samples, as well as samples held at a constant 80 MPa uniaxial stress. Mechanical drilling into a stressed sample was found to induce considerable residual stresses around the hole. Conversely, electrochemical drilling did not create a residual stress field in the vicinity of the hole.

Factors Affecting Stress Concentrations Near Pit Defects, as Monitored by Magnetic Flux Leakage, Magnetic Barkhausen Noise, and Neutron Diffraction

“…Figures S(a) and S(b) show the results from the hoop and axial directions, respectively. The introduction of plastic deformation was confirmed by measurements following unloading, where compressive residual stresses of > 100 MPa were observed at the hole edge [2).…”

“…Details can be found in ref. 2. ND measurements were carried out using the NS spectrometer at the NRU reactor in Chalk River, Canada.…”

“…As a result, the study of stress concentration around pits of different depths and made under different external conditions is important in estimating the size of the defect. This stress concentration around a defect can be measured directly by neutron diffi"action [2] and photo-elasticity measurements [3]. We also measure stress concentrations indirectly using the magnetic Barkhausen noise (MBN) technique [4].…”

“…This device has been termed the 'hydraulic stress rig'. Artificial circular pits ofdiameter -15 mm and depth-4.5 mm (-50% ofthe wall thickness) were created in the pipe wall either by mechanical drilling using a ball-mill drilling tool, or by electrochemical drilling [2].…”

Residual stress state behaviour under fatigue loading in pipeline welded joints

“…Oil and gas pipelines operate at up to approximately 70 per cent of their yield strength [9]. Based on this data and considering the preliminary results on specimen 1 (Figs 3 and 4), the next tests with four specimens were carried out at 70 per cent YS (403 MPa).…”