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There exists in the pipeline industry a potentially catastrophic phenomenon known as ductile fracture. This report presents new technology which minimizes the effect of a ductile fracture if preventive measures fail.

It is assumed that the axial bending stresses which arise from this type of weld have a linear distribution. From these stresses, axisymmetric solutions in the literature enable the stress intensity factor to be calculated for an internal circumferential crack. In this paper it is shown that the same solution may be obtained from the plane strain solution for bending of a cracked beam. This approach applies to any radius to thickness ratio for thin-walled pipe while the axisymmetric solutions apply only for the specific ratios taken in the numerical solutions. Also, the present procedure allows the radial deflection and moment on the crack plane to be determined as a function of crack depth with simple calculations.

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 Berkeley Engineering And Research, 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: ε(%)=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%.

During the summer of 1996, the TransAlaska Pipeline System (TAPS) experienced vibrations in a section of the pipeline near Thompson Pass, north of Valdez, Alaska. Alyeska Pipeline Service Company, operator of TAPS, initiated an extensive investigation, and determined that the vibrations were caused by pressure pulses originating near a slackline-packline interface. The pressure pulses are thought to have been caused by the collapse of vapor bubbles trapped in the flow. The vibrations occurred only when the interface was positioned near a terraced portion of the pipeline topography on the downstream side of the pass. This knowledge allowed Alyeska Pipeline to control the vibrations by back-pressuring the pipeline to move the slackline-packline interface well above the terrace location.