Fracture Analysis for Pipeline Girth Welds in High Strain Applications

Over the last several years, Pacific Gas & Electric Company (PG&E) has designed and installed seismic upgrades at several locations where their transmission pipelines cross active fault zones. As part of the process of evaluating the seismic upgrade designs, PG&E commissioned SSD, Inc. (SSD) and B erkeley E ngineering A nd R esearch, Inc. (BEAR) to perform buried pipe deformation and fracture assessments of the pipeline fault crossings to develop capacity estimates for compressive wrinkling and girth weld tensile fracture. This paper describes the elastic-plastic fracture analysis used to determine girth weld tensile fracture capacity and the relatively simple equations derived that have wide application for high toughness pipe and weld material. The equations have the form: e (%) = α · Sf · D2c0.5 (1) where the Sf is flow stress, D is pipe diameter, 2c is flaw length and α is a function of a/t where the a is flaw depth and t is the pipe wall thickness. The tension strain capacity depends on girth weld material toughness, flow stress and the length and depth of flaws that may exist in or near a girth weld. The analysis method used is based on the interaction of ductile tearing and elastic plastic fracture. Crack tip opening displacement (CTOD) is used to characterize material toughness. The derived equations can be used to predict allowable tension strain for X-60 and X-65 pipe with diameters ranging from 10 to 36 inches (273 to 914 mm) and for weld flaw depths of up to 1/3rd of wall thickness. Adequately tough pipe girth welds containing flaws can be shown to have safe tension strain capacities above 4%.Copyright © 2004 by ASME