Effects of Bending Moment and Torsion on the Burst Pressure of Locally Thinned Pipes

Finite element analysis (FEA) model for pressure limit load is validated using pressure burst test data of locally wall thinning flaws in 4″OD (107.1 mm) carbon steel pipes. It is shown that with a flow stress of 3*Sm, the FEA pressure limit load can reasonably predict the test burst pressure for non-planar flaws in carbon steel pipes. The models are then used to investigate the effects of a combined pressure, bending and torsion loading on piping non-planar flaws. For application, membrane stress structural factors (SF_m) for design pressure are calculated for the locally thinning pipe flaws given in AMSE Section XI, Code Case N-597-2, Table -3622-1 for limited transverse extend and t_pred≥ 0.3*t_min. The calculated SF_m are consistently greater than 2.7, the minimum SF_m for Service level A membrane stress given in C-2622 for axial flaws. An improved t_aloc/t_min curve based on SF_m of 2.7 is proposed. It is also shown that a SF_m of 2.7 on design pressure is maintained when the flaws are also subjected to a combined moment of bending and torsion, which is equivalent to the bending limit load given in C-5430 for planar circumferential flaws.