Pipes with local wall thinning can be simultaneously subject to multi-axial loads such as the internal pressure loads, axial loads, bending and torsion (twist) moments. Continued operation of such items makes use of fitness-for-service assessments using codes such as the ASME Boiler and Pressure vessel Code, Section XI. Currently the non-mandatory Appendix C of the Code gives guidance for performing assessment for various modes of failure. However in the current guidance there is no provision for inclusion of the torsion loads for either planar (crack) or non-planar (wall thinning) type flaws under predominant membrane and bending loading.
This paper is a continuation of the study reported at the earlier PVP conferences, whereby the intent is to provide guidance on inclusion of small amounts of torsion with the dominant pressure and bending loading for a pipe containing a wall thinning. In this paper the results are reported for a 4″ (100mm) nominal diameter Schedule 160 pipe. Two shapes of the wall thinning are examined; a cylindrical with round edges and an ellipsoidal shape. These are chosen to span a range of real shapes. The limits on the applicability of the equivalent moment, defined as the root of the sum of the squares (RSS) in construction codes is presented in terms of the wall thinning dimensions.