Girth Joints in Steel Pipelines Subjected to Wrinkling and Ovalling

摘要

Water utilities commonly use welded steel for larger diameter pipelines. The most common way to join these pipelines is using a single lap weld girth joint, made in the field. This type of joint is susceptible stress concentrations and wrinkling, factors commonly ignored in design. It is not surprising that welded steel large diameter water pipelines have regularly failed in past earthquakes where these pipelines are exposed to large ground deformations due to fault offset, landslide, or liquefaction. Theoretical ypical wrinkling formulae can be used to predict the onset of wrinkling in butt welded pipes. Field experience suggests that even lap welded pipe can undergo substantial wrinkling prior to rupture, and typical wrinkling formulae may be too conservative. This paper presents analytical and empirical evidence that a double lap welded joint can wrinkle significantly prior to rupture of the pressure boundary. It is proposed that an allowable strain of 5% in the wrinkle provides adequate margin against tearing of the pressure boundary. A typical guideline to limit the effects of pipe ovalization is to keep the diameter-to-wall thickness (D/t) ratio of a pipeline, at a fault crossing, under 90. This paper presents analytical results from a detailed model of a 66-inch diameter pipe with wall thickness varying from 0.375-inch to 0.75-inch, when subjected to a five foot fault offset, when buried in a pea gravel trench. The findings show that at a D/t ratio of 176, severe ovalization occurs, with buckling of the pipe walls; for a D/t ratio of 88, moderate ovalization still occurs. The pipe strains in the hoop direction due to ovalization are not excessive in either case.