Pipelines can be exposed to a wide variety of loads, depending on the environments and the area of application. These loads may impose large longitudinal plastic strain on pipelines, which could constitute a significant threat to the structural capacity of the pipeline. Reliable calibration of the strain capacity of pipelines plays an important role in the strain-based design (SBD) method. In this paper, a tensile strain capacity (TSC) predictive model (an equation) for welded X42 vintage pipes has been developed by conducting nonlinear parametric analysis followed by nonlinear regression analysis. First, our previously validated extended finite element method (XFEM) model was used to demonstrate the applicability of the XFEM in simulating full-scale ductile fracture response of pipelines subjected to biaxial loading, using pressurized American Petroleum Institute (API) 5L X42 vintage pipes subjected to four-point bending. Second, a parametric study investigating the effects of pipe and defect geometries as well as loading on the pipe TSC is presented. The nonlinear parameterization using XFEM was conducted in abaqus/standard. The TSC trends obtained for the various parameters considered were examined to derive appropriate individual variable functions for each parameter while taking any significant interactions between the parameters into consideration. Also, a nonlinear regression analysis is employed to develop a nonlinear semi-empirical model for predicting the TSC. The results obtained from the developed TSC predictive model (TSCvin.) was compared with those evaluated using the validated XFEM models. The results showed good agreement. Finally, statistical analysis was conducted to ensure the model is unbiased and predicts conservative TSCs by modifying the model using probabilistic error analysis. The modified model is capable of increasing the confidence level in the predicted TSC hence becoming a practical tool for reliable prediction of TSC of X42 vintage pipes needed for conducting pipeline integrity assessment. This modified predictive model is useful in practical applications because it provides a quantifiable degree of conservatism and reliability to the predicted TSCs.

The structural integrity of buried pipelines is often threatened by permanent ground deformation (PGD) within the pipeline route. Oil and gas transmission steel pipelines, which usually traverse several kilometers, are mostly buried. Inevitably, they can cross over areas with considerable geohazard concerns due to different soil types or extreme topographies. PGD usually occurs with various patterns, magnitudes, and directions relative to the pipeline axis with varying influences on the structural performance of buried pipelines. This paper discusses a comprehensive investigation of the effect of PGD patterns on the strain demand of pressurized buried continuous steel pipelines subjected to ground movement. The maximum strains induced in buried pipelines by longitudinal and transverse ground displacements of different patterns are presented. A three-dimensional nonlinear beam-Winkler spring finite-element (FE) analysis is used. The FE model accounts for the elastic-plastic behavior of pipe and nonlinear pipe-soil interaction. A straight NPS 42 Grade X70 steel pipeline with a wall thickness of 14.3 mm was considered and assumed to be fully embedded in two different soil types (stiff clay and dense sand). The pressurized pipeline was subjected to six different PGD patterns (i.e., sinusoidal, trapezoidal, ridge, ramp, block, and step) of three different magnitudes (1.0, 2.0, and 3.0 m). Each pattern was applied in the transverse (vertical and lateral) and longitudinal directions. The results show a significant variation in the strain demand of buried pipelines subjected to the various PGD patterns. The longitudinal strain induced in a pipeline due to PGD of the same magnitude but different patterns varies by up to 752%. Moreover, stiff clay is found to impose more strain on buried pipelines than dense sand. The results presented in this paper can be a useful reference in the safety assessment of straight sections of a similar buried pipeline under PGD.

Vintage pipelines subjected to geohazard-induced ground movements require strain-based assessment methods that account for weld defects and material properties. This study develops and validates a semiempirical tensile strain capacity (TSC) model for NPS 6, API 5L X42 pipelines, taking into consideration the existence of girth weld flaws. Three full-scale pipe specimens containing machined flaws at critical weld locations were tested under combined internal pressure (7230 kPa, 53% SMYS) and four-point bending. Specimen X1 (50% wall thickness flaw in a defect zone) exhibited brittle fracture at TSC of about 0.1162% (approximately 0.12%), while specimen X3 (25% flaw in defect-free weld zone) showed ductile tearing with TSC of approximately 0.36%. Fractography revealed that weld misalignment (0.56-1.0 mm) and undercut features significantly influenced crack propagation paths.,An extended finite element method (XFEM) model, calibrated using single-edge notched tension (SENT) test data, accurately reproduced the experimental failure modes and TSC values. A parametric study of 64 XFEM simulations varying flaw depth (0.25t - 0.75t), flaw length (4t - 10t), and internal pressure (0.4 Py - 0.7 Py ) was used to develop a semi-empirical TSC model, termed TSCCM , based on circumferential material properties. The model showed strong predictive capability (R² = 0.9985) and provided better agreement with experimental data than existing industry models (ExxonMobil, PRCI) when applied to small-diameter, thin-walled vintage pipelines. The TSCCM model offers a practical engineering tool for integrity assessment of the vintage X42 pipeline under investigation, operating under ground-movement hazards. The paper provides a methodological approach to generate similar tensile strain capacity models for any vintage pipeline using a combination of full-scale and small-scale experiments, aided by the extended finite element analysis method.