Experimental Study of the Residual Stresses in Girth Weld of Natural Gas Transmission Pipeline

In order to achieve integrated condition in the girth welding of high pressure natural gas transmission pipelines, the weld zones and its surrounding area should have good mechanical properties. Residual stresses are an important defect especially in the girth welding of the pipeline. In this study, two API X70 steel pipes (with spiral seam weld) of 56 inches outside diameter and 0.780 inch wall thickness were girth welded first. The hole drilling tests were conducted for residual stress measurement on the surfaces of the pipes. The hoop tensile residual stress on the external surface of the pipe with the maximum value equal to 318-MPa was measured on the weld centerline. Hoop residual stress distributions in the internal and external surfaces of the pipe were approximately similar. The maximum axial residual stress was observed in the heat affected zone (a distance of approximately 30 mm from the weld centerline). The maximum axial residual stress on the external surface of the pipe was tensile, equal to 137 MPa, and on the internal surface of the pipe was compressive, equal to 135-MPa. Axial residual stress magnitudes in the weld centerline on the internal and external surfaces of the pipe were close together. Away from the weld centerline, axial residual stresses on the internal and external surfaces showed the opposite behavior. Therefore, in the girth welding of natural gas transmission pipelines, the peripheral direction on the internal surface of the pipe is the critical zone and have the highest tensile residual stresses.