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A Novel Tool For Predicting The Tensile Strain Capacity Of Welded Onshore Vintage Pipelines
- Author / Creator
- Agbo, Sylvester I
Pipelines can be exposed to a wide variety of loads, depending on the environments and the area of application. These loads especially the displacement-controlled types may impose large longitudinal plastic strain on pipelines, which could constitute a significant threat to the structural integrity of the thin-walled cylindrical shell structure. The fracture and failure mechanism of the ductile shell structure is inherently influenced by a variety of parameters that may exhibit direct linear and nonlinear relationships with the resultant stresses and the consequent straining of the structure, under these complex loading conditions. Traditional stress-based design methods may become uneconomical in the design of pipelines subjected to large plastic strain. In view of the extensive use of such pipelines in remote areas where oil and gas resources are currently being extracted, reliable calibration of the tensile strain capacity (TSC) plays a critical role in strain-based design (SBD) methodology. This study focused on the development of a novel predictive tool capable of characterizing the TSC of lower grade vintage pipelines, specifically of X42 steel grade, which is currently in service, in their numbers across North America, by investigating the parameters that significantly affect the TSC response of such pipelines under load. Firstly, eight full-scale pressurized four-point bending tests were conducted on X42, NPS 22 vintage pipes with 12.7 mm wall thickness to investigate the effect of internal pressure and flaw size on the TSC. The pipes were subjected to internal pressure levels equivalent to 80% and 30% of the specified minimum yield strengths (SMYS) and different girth weld flaw sizes machined at the girth weld centerline.
Secondly, the extended finite element method (XFEM) in the ABAQUS CAE was used to simulate the four-point bending tests to demonstrate the capability of the XFEM in simulating full-scale ductile fracture response of pipelines subjected to biaxial loading, and the numerical results were validated against the full-scale test results. Moreover, a nonlinear parametric study investigating the effects of pipe and defect geometries, as well as loading conditions on the pipe TSC was conducted using the XFEM technique to fully understand the influence of these parameters on the TSC. The trends of TSCs obtained for the various combinations and interactions of the parameters considered were examined to derive an appropriate individual variable function representing each parameter combination. Nonlinear regression analysis was then employed to develop a nonlinear semi-empirical model (tool) for predicting the TSC of welded X42 vintage pipe.
The TSCs predicted using the new tool (TSCvin.) was compared with those evaluated using the full-scale-test-validated XFEM models. Excellent goodness-of-fit with the TSCs obtained from the validated XFEM simulations was obtained for the developed predictive tool.
Finally, I conducted a statistical analysis to ensure the model is unbiased and can predict conservative TSCs by running a probabilistic error analysis to quantify the error and use the result to modify the predictive model. The modified predictive model is useful in practical applications because it provides a quantifiable degree of conservatism to the predicted TSCs.
The modified TSC model was applied to some hypothetical X42 pipeline cases to demonstrate the applicability and accuracy of the new tool, which confirmed the efficiency of the model by replicating the trend obtained from both the experimental tests and the XFEM numerical simulations
- Graduation date
- Spring 2020
- Type of Item
- Doctor of Philosophy
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