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 Z662-11 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 full-scale 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.

Prediction of Tensile Strain Capacity for X52 Steel Pipeline Materials Using the Extended Finite Element Method

“…The fracture behavior of vintage API 5L grade X52 steel pipes was investigated experimentally by Abdulhameed et al [21] and Lin [23]. The experimental work included full-scale and small-scale tests.…”

“…A circumferential crack-like defect was created in each pipe sample on the outer surface of the pipe close to the girth weld. The circumferential flaw length was 5% or 15% of the pipe circumference, while the flaw depth was 25% or 50% of the pipe wall thickness [21]. For small-scale tests, 25 tension coupon tests were used to obtain the tensile properties of the X52 grade of pipeline.…”

“…Failure in the model was defined as the onset of the crack tip (or element damage) reaching the inner edge of the last element of the wall thickness. Since the experimental results from Abdulhameed et al [21] showed that failure in the X52 occurred in the base metal, the material properties of the base metal were applied for the whole pipe. To compare the predicted XFEM results with experimental results, the ratios of test-to-model predictions and the concomitant coefficient of variation (COV) of the ratios were calculated in the current study, in which COV = the standard deviation/the mean of the ratios of test-to-model predictions of all eight tests.…”

“…Apart from models 5 and 6, it is seen in these two figures that the damage parameters selected in the XFEM model (Maxpe = 0.085 and G c = 900 N/mm) are appropriate for the modeled X52 grade. It must be emphasized that the tensile strain capacity is defined as an "average" value of the strain around the flaw since the strain profile is disrupted at the flaw location [21]. The simulation cannot accurately predict the tensile strain results with a distance of less than 20 mm from the crack tip due to the complex stress-strain state near the crack tip.…”

“…In contrast to Maxps, which was used previously by Lin et al [13] to predict the fracture response of this grade of vintage pipeline, Maxpe critical values are relatable to the physical critical strain values expected in the vicinity of the crack tip [12]. The crack initiation and propagation are numerically predicted through simulating eight published full-scale tests of X52 vintage pipes that are subjected to the combination of internal pressure and external eccentric tension which were reported by Abdulhameed et al [21]. A proper set of damage parameters for X52 grades of pipes is obtained by calibrating the XFEM model implemented in ABAQUS [22].…”