The Effect of Internal Pressure on the Tensile Strain Capacity of X52 Pipelines With Circumferential Flaws

Abstract

Wide plate testing has been traditionally applied to evaluate the tensile strain capacity (TSC) of pipelines with girth weld flaws. These wide plate tests cannot incorporate the effect of internal pressure, however, numerical analysis in recent studies showed that the TSC is affected by the level of internal pressure inside the pipeline (Wang et al. 2007, "Strain Based Design of High Strength Pipelines," 17th International Offshore and Polar Engineering Conference (ISOPE), Lisbon, Portugal, Vol. 4, pp. 3186–3193). Moreover, most of the past studies focused on the effect of circumferential flaws on the TSC for pipelines of steel grade X65 or higher. The current Oil and Gas Pipeline System Code CSA Z66211 provides equations to predict the TSC as a function of geometry and material properties of the pipelines. These equations were based on extensive studies on pipes having grades X65 or higher without considering the effect of internal pressure. This paper investigates the TSC for pipelines obtained using an experimental technique considering the effect of internal pressure and flaw size. Eight fullscale tests of X52 NPS 12 in. pipes with 6.91 mm wall thickness were conducted in order to investigate the effect of circumferential flaws close to a girth weld on the TSC for vintage pipelines subjected to eccentric tensile forces and internal pressure. The tensile strains along the pipe length and on the outer circumference of the pipe were measured using biaxial strain gauges and a digital image correlation (DIC) system. Postfailure macrofractography analysis was used to confirm the original size of the machined flaw and to identify areas of plastic deformation and brittle/ductile fracture surfaces. From the experimental and numerical results, the effect of internal pressure and flaw size on the TSC and the crack mouth opening displacement (CMOD) at failure were investigated and presented.