Corrected Stress Concentration Factor for Hi/Lo at the Root of a Pipe Girth Weld

Abstract

Abstract. The stress concentration factor (SCF) for a hotspot at the root of a pipe girth weld due to high–low misalignment offset (hi/lo) is used in fatigue and fracture assessments, since some such offset is inevitable due to fabrication and welding alignment tolerances. Where the hi/lo is not already accounted for by the SN fatigue resistance curve used, an SCF is needed. Following established practice, the SCF is determined to account for bending stresses across the wall thickness of the pipe, but does not account for stress concentrations arising from the reentrant corners that can arise at both the root and the cap of the girth welds. This SCF arises from a linear distribution of stress across the wall thickness, which is statically equivalent (in terms of membrane force and bending moment) to the actual stress distribution. A simple approximation to determine this SCF using the line of tension from axisymmetric shell theory and considering the actual details of the geometry at the girth weld leads to the same result as in the Recommended Practice DNV-RP-F108 at the cap, but at the root, the value of SCF-1 is found to be more than double that from DNV-RP-F108. The result is confirmed by axisymmetric finite element analysis.