Plastic collapse of circumferential surface defects in pipeline materials

Abstract

Fitness for the purpose techniques for the evaluation of defects in pipeline girth welds as described in Appendix K of the CSA Z662 have proven to be overly conservative for high applied strains. Conventional limit loads, including CSA Z662, were not consistent in assessing the plastic collapse of surface defects. The effect of crack geometry was not successfully accounted for using available limit loads.

The full pipe subject to remote bending was simplified to obtain a better understanding of plastic collapse behaviour. An extensive program combining analytical, experimental and numerical study was performed for all simplified geometries. As a result of detailed finite element analyses, a plastic collapse solution was introduced and compared with available experimental data.

The ligament failure criteria, which was introduced for tensile and single edge notch tension (SENT) specimen analyses, provided good agreement with experimental data for wide plate specimens and full pipes. In general, it was observed that geometries with shallow surface cracks produced high plastic collapse loads even when the cracks were long. Deep and long cracks produced much lower plastic collapse loads, and the occurred by ligament necking. A plastic collapse solution for surface defects in pipeline girth welds was proposed and showed significant improvement over existing solutions for assessing such surface defects.

The proposed plastic collapse solutions for wide plates and full pipes provided better, more consistent agreement with experimental data than available conventional limit load solutions, and, above all, these solutions reflect the physical behaviour of pipeline defects. This method should be used to investigate defect interactions and embedded cracks.