This paper presents methodology and design criteria for assessment of pipeline girth weld defects that may result in circumferential failure. It covers major failure modes such as plastic collapse, local buckling and fracture for the limit state design. The existing strength equations for circumferential failure are reviewed. The new strength equation based on the ultimate bending moment capacity equation (Bai and Haueh, 1998) is derived and compared with existing strength equations. The ultimate bending moment capacity equation is also extended to the fracture assessment of the corroded pipeline.
Circumferential failure may occur in defective girth weld when it is under high longitudinal stress (Rosenfeld and Kiefner, 1995). The defects in girth welds may be corrosion defects or due to lack of sufficient fusion in welding process. Corrosion defects are primarily confined to the lower part of the pipe, e.g. from 0400 to 0800 at the most. B31G criteria (ASME 1993) are not suitable for evaluating whether circumferential failure is possible, because it has been made for longitudinal failure under excessive hoop stress. Rosenfeld and Kiefner (1995) suggested the following assessment procedure:
Evaluate the longitudinal defect profile by B31G criteria to verify that the defect is acceptable at the maximum operating pressure.
Evaluate the longitudinal stress state of the pipeline, considering factors such as internal pressure, thermal expansion, spanning loads, and other loading as appropriate. Calculate the ratio (Rs) of longitudinal stress (σL) to circumferential stress (σC) Rs=σL/σC. Calculate the ratio (RL) of the circumferential dimension of the defect (L) to longitudinal dimensions (b) of the defect, RL=L/b. If RS=<0.5, or if Rs=<R L, further evaluation for circumferential failure is unnecessary. If RS>0.5, and Rs>R L, evaluate the defect for circumferential failure by the appropriate methods.
