The Effect of Hoop Stress on the Burnthrough Susceptibility During In-Service Welding of Thin-Walled Pipelines

Most companies control the risk of burnthrough by prohibiting welding on pipelines with wall thicknesses below a specified thickness. This is a safe approach but the risk of burnthrough depends not only on the wall thickness, but also on the welding parameters and the operating parameters of the pipeline which include pressure. It is generally acknowledged that the hoop stress caused by pressurizing the pipeline has a relatively minor effect on the risk of burnthrough since the size of the area heated by the welding arc is small. While this has certainly been shown to be true for thicker materials, previous research has shown that the pressure can have a dramatic effect on burnthrough risk for thinner materials. The objective of this project was to further investigate the effects pressure and hoop stress has on the burnthrough risk of welding onto thin-walled pipelines in service. For circumferential welds, pressure and wall thickness determine the burnthrough risk and pipe diameter appears to have no effect. The failure mechanism for circumferential welds is consistently a burnthrough. For longitudinal welds, pipe diameter does appear to affect burnthrough risk even though the effect appears to be secondary to pressure and wall thickness. The pipe diameter is believed to be more influential for longitudinal welds because of the larger area of heated material that is exposed to the hoop stress. Also, the results indicate that the magnitude of the hoop stress has a direct effect on the failure mechanism for longitudinal welds (i.e., burnthrough or weld centerline cracks). For longitudinal welds, the failure mechanism is commonly burnthrough for welds made onto pipes with a hoop stress below 30% specified minimum yield stress (SMYS) which indicates that the internal pressure of the pipe is the main driving force for failure. Longitudinal welds made on pipes which are experiencing hoop stress above 30% SMYS commonly fail by weld cracking. It is important to note that even though pressure does have an effect on the burnthrough susceptibility of welds made on thin-walled pipelines, pressure only becomes a factor for welds made at heat input levels in excess of what is predicted safe by thermal analysis modeling.