Plastic Collapse Analysis of Pipelines Containing Surface-Breaking Circumferential Defects

摘要

Girth weld failure by plastic collapse is often viewed more likely than brittle fracture in modern pipelines, where material toughness is typically "high." Selection of an appropriate plastic collapse solution is often difficult partly due to the "over-abundance" of the available solutions, but also because there is often very limited information on the applicable range of these solutions. This paper focuses on plastic collapse solutions relevant to the girth welds of gas transmission pipelines containing surface-breaking circumferential defects. A total of nine plastic collapse solutions are reviewed and compared with available experimental data. Both pure bending and the combined internal pressure and bending are considered. It is not the intention of the authors to provide an absolutely lower-bound solution with respect to all the available experimental data. Instead, the differences between the plastic collapse solutions and the experimental data are rationalized whenever possible. It was found that experimental data relevant to large D/t pipes containing relatively long circumferential defects are few, although there is a large number of experimental data of pipes with smaller D/t ratios from the stand point of typical gas transmission pipelines. For pipes containing defects of depth less than 50 percent of wall thickness and length less than 10 percent of the circumference, the solutions of Miller, Wilkowski, Kastner, and ASME NSC predict similar plastic collapse stresses when pure bending loads are considered. These solutions also agree well with experimental data from pipes with D/t >= 28 and relatively short cracks (up to 10 percent of the circumference). The Kastner solution provides the best agreement with experimental data for pipes of smaller D/t ratios with with longer and deepen cracks and loaded in pure bending. For pipes loaded in a combined internal pressure and bending, the solutions of Miller, Wilkowski, and ASME NSC outperform the Kastner solution. For crack length up to 60 percent of the circumference, the Miller and the ASME NSC solutions provide almost identical plastic collapse stresses when pure bending is considered. The review of the experimental data shows that more tests are needed if the defect length greater than 10 percent of the circumference were to be allowed in gas transmission pipelines.