The present study utilizes the coupled Eulerian-Lagrangian finite-element modeling technique to simulate three-dimensional piles with true helices in sand under axial compression. The aim is to develop a numerical methodology that accounts for installation-induced disturbances of the helices, evaluate the influence of the true-helix geometry, and investigate the axial failure mechanisms of helical piles in sand. The models include single-helix and double-helix piles with interhelix spacing ratios of 1.5 and 2.5. The results are validated against centrifuge model tests of helical piles in sand. The disintegration of axial loads between the helices and the shaft suggests higher loads in the helix in single-helix piles and lower helix loads in double-helix piles. The structural integrity of the helix, assessed against the ultimate axial load capacity, validates the assumption of elastic pile behavior and confirms its adequacy. The axial behavior of multihelix piles in sand was governed by an individual bearing mode (IBM). IBM failure also dominates for larger helix wings, while larger-diameter shafts lead to global failure, accompanied by ground surface heave. As pile embedment increases, the failure envelope around the upper helix of a double-helix pile transitions from a near-circular to a funnel-shaped pattern. A shaft-to-helix ratio of 1-to-3 presents an ideal selection for pile design.

Shallowly buried offshore pipelines operating under high pressure and high-temperature conditions are susceptible to upheaval buckling. Such pipelines may contain pre-existing defects, including fabrication-or operation-induced cracks. If an offshore pipeline with an initial defect experiences vertical movement due to upheaval buckling, the crack can propagate in the tensile stress region, leading to fracture. This study presents a numerical modelling technique using an eXtended Finite Element Method (XFEM) to analyze the initiation and propagation of tensile fractures in a post-buckled pipeline. Conventional fracture mechanics commonly employ damage initiation criteria based on maximum principal stress (MAXPS) or maximum principal strain (MAXPE) with fixed values. However, these criteria have limitations when considering crack-tip constraints (stress triaxiality and Lode angle) during the numerical analysis. A modified Mohr-Coulomb (MMC) fracture criterion is implemented in the finite element program, Abaqus, using a user-defined subroutine to address this limitation. The MMC criterion considers shear slip and ductility, providing a more realistic representation of ductile materials than MAXPS and MAXPE models. This study also examines the influence of various fracture parameters under different damage degradation models. The findings provide practical insights for assessing crack initiation and propagation in post-buckled offshore pipelines.

Overtime, continuous research efforts have been devoted to investigating the effects of permanent ground deformation (PGD) on buried pipelines. These efforts have faced major uncertainties in the ground movement patterns that impose displacement-controlled loads on pipelines. This study presents a systematic review of the PGD patterns available in the literature for the analysis of buried pipelines, with particular emphasis on the PGD directions relative to pipeline axes. Depending on the directions, PGD patterns are divided into three groups: longitudinal, transverse and oblique. The relevant literature is organized into four main categories: experimental, numerical, analytical and probabilistic. Seven different PGD patterns (block, ramp, step, ridge, sinusoidal, asymmetric ridge, and asymmetric sinusoidal) available in the literature are discussed in this study. All these PGD patterns, except asymmetric sinusoidal, were applied in the longitudinal direction. In contrast, only three (block, sinusoidal and asymmetric sinusoidal) were applied transversely, which indicates a potential gap considering the randomness of PGD relative to pipeline axes. Besides, one additional PGD pattern (trapezoidal) is suggested in the present study based on the literature review. The findings of the present study show that 84% of the reviewed cases considered either block or sinusoidal PGD pattern, while only 16% considered the other PGD patterns (ramp, step, ridge, asymmetric ridge, and asymmetric sinusoidal). Almost all these 16% reviewed cases assumed pipelines primarily as linear elastic materials. Based on these observations, the present study recommends a comprehensive study on the effects of all the mentioned PGD patterns on pipelines in all three directions.

Inline pressure isolation tools are widely used for offshore pipelines for planned maintenance and emergency repair work, e.g. valve replacement, repair, tie-ins. Design and engineering analysis of such tools for offshore application have been well developed. Although inline pressure isolation technology is widely used in offshore pipelines, its use in onshore application is limited. However, because of its capability of providing reliable in-line isolation with significantly reduced blown-down length and methane emission for integrity and capital projects that traditionally require pipeline outages, the inline pressure isolation tools are getting increased attention from the onshore pipeline operators in recent years. Consequently, there is a growing need for better understanding of applicability and limitations of using established offshore design rules for onshore applications and developing engineering assessment workflow to ensure successful execution and prevent potential irreversible damage to existing assets. One of the challenges for the application of pressure isolation tools to onshore pipelines is to better understand the pipelines stresses that are exerted by these tools during their operation. This paper investigates the impact of these tools on the integrity of onshore pipelines from stress perspective through leveraging 3-D continuum finite element (FE) analysis using Abaqus FE software, with consideration of actual loading and boundary conditions that onshore pipelines could involve. The FE model was first calibrated with a full-scale test to ensure accuracy and validity of the simulation. Subsequently, a total of 137 FE models were developed for a wide range of pipe sizes (NPS 10 to NPS 48) and wall thicknesses (4 mm to 26.4 mm). The resulting stresses were compared with the allowable stresses for the pipelines. Furthermore, a sensitivity study was performed for various pipe material properties to ensure that an adequate factor of safety is maintained throughout the analysis. Outcomes of the investigative study was fed into a streamlined assessment workflow that includes a series of simplified graphs for determining the allowable pressure for various pipe sizes for future applications. The results demonstrate advantages of using FE based streamlined approach over conservative case-by-case analytical solutions. The outcome of this study provides insights on the various stress conditions that could result from the inline isolation tool for onshore pipelines and could potentially impact pipeline integrity.

Buried offshore pipelines transporting hydrocarbon at high temperatures and pressures might experience upheaval buckling. Although pipelines are designed to remain in place, field evidence shows that the buried pipeline might displace a significantly large distance in the upward direction. In the present study, the pre- and post-buckling behaviour of offshore pipelines buried in sand is investigated considering the degradation of uplift soil resistance with the upward displacement of the pipe. The soil resistance degradation models are implemented in a finite element program, and the analyses are performed for large upward displacements even when a buckled section moves above the seabed. The simulation results show that the capacity of the pipeline to carry the load generated from an increase in pressure and temperature reduces significantly if post-peak degradation of uplift soil resistance is considered. The effects of the internal pressure modelling approach, uplift soil resistance degradation model, burial depth and pipe dimensions (diameter and thickness) on upheaval buckling for varying initial imperfections are investigated. Based on a simplified model for developing initial stresses in a pipeline during installation, it has been shown that initial stresses primarily affect the upheaval buckling at lower ranges of upward displacement of the pipe.

A series of full-scale experiments was conducted to estimate lateral soil constraints on pipes buried in dense sandy slopes at different burial depths. The experimental data indicated that the soil force on the pipe increases with increase in the slope grade and burial depth ratio. The lateral soil force against relative pipe displacement response observed from the experiments is presented and compared to those arising from level ground conditions. The study was extended to larger burial depth ratios by simulating pipes under sloping ground conditions using a numerical (finite-element) model that was initially calibrated using the results from physical modelling. The findings from the study in terms of the variation of peak lateral soil restraint as a function of the slope grade and burial depth ratio are presented for consideration in pipeline design.

Finite element (FE) results might suffer from significant mesh dependency, especially when modeling shear bands in strain-softening soils. On the other hand, the shear bands in the field are generally thin. The computational costs might dramatically increase when modeling such thin shear bands, especially for large-scale problems, such as progressive landslides. To overcome these issues, FE analyses are generally performed with larger shear band thicknesses by adopting ‘softening scaling’ rules based on local and nonlocal (averaged) strains at the shear bands. The present study compares the performances of local and nonlocal approaches, with a specific focus on softening scaling and large deformation. Analyses are performed using a Eulerian-based FE program, which can model large strains in the shear band without numerical issues related to mesh distortion. Implementing strain-softening behavior in local and nonlocal methods, two idealized cases are simulated: (i) biaxial compression test, (ii) slope failure due to upslope surface loading. FE simulations show that the macroscopic response (i.e., load–displacement behavior) can be modeled using both local and nonlocal regularization techniques. Shear band thickness increases with the progress of shearing. In local analysis, mesh orientation has a considerable effect on shear band thickness. The existence of neighboring shear bands could affect nonlocal strain calculation and the simulation results.

The dependency of finite element (FE) results on mesh size is a major concern for the numerical analysis of strain-softening materials. The local methods of strain regularization rely on the shear strains of a solitary point. However, the nonlocal methods incorporate strain-softening, including the strain in surrounding soil elements, which show less mesh dependency. Previously, nonlocal methods were mostly implemented in Lagrangian-based FE programs and simulated the response for small to moderate strain levels. However, many geotechnical problems, such as large-scale landslides in sensitive clays, involve extremely large deformation. This study presents the implementation of the “original” and two modified nonlocal methods in a Eulerian-based large deformation FE program using a relatively simplified approach where simple soil models, such as von-Mises criteria for undrained behaviours, can be used. Two biaxial compression tests are simulated by using the nonlocal Eulerian-based FE program, and the results are compared with a nonlocal Lagrangian-based FE analysis and a nonlocal Material Point Method (MPM) of simulation, respectively. Among the three, the modified nonlocal methods, especially the over-nonlocal method, show a better performance in mesh convergence analysis. Several approaches have been proposed to minimize the computational costs, as nonlocal modelling is generally computationally expensive.

Thermal stress analysis is an integral part of the design and integrity assessment of buried pipelines. Pipe bends can be subjected to significant cross-sectional deformations due to bending moments induced by thermal cycles, compared to straight pipes, and therefore are the most crucial component of the pipeline's structural integrity. Fatigue fracture, which is the primary failure mode in pipelines under the thermal cycle, may occur at the crown region of the pipe bend in the form of a longitudinal crack. This specific failure pattern is primarily the result of excessive circumferential stress that may develop in the crown region of a pipe bend. The present paper suggests a novel approach to reduce the stress range at the crown region of pipe bends using carbon fiber-reinforced polymer (CFRP) wraps. This approach has been used in the pipeline industry to reinforce and repair corroded pipes. However, a very limited study on the use of CFRP wrap to enhance the mechanical behavior of undamaged pipe bends is available in the literature. This study employs an advanced finite element (FE) method to investigate the performance of buried pipe bends reinforced with CFRP composite wraps and subjected to thermal expansion-induced bending moment. A combined beam and shell-based FE model has been used in this study to ensure reasonable accuracy and remarkable computational efficiency for engineering practice. The FE results show that a 6 mm CFRP wrap around the pipe bend can decrease the von-Mises stress imposed by thermal expansion by up to 27.4%. In short, reinforcing pipe bends with CFRP wrap has a strong potential to decrease the stress range imposed in the pipe bend under thermal expansion-induced moments and consequently prevent fatigue failure in pipe bends. (C) 2022 American Society of Civil Engineers.

Blending hydrogen into existing natural gas pipelines is being pursued as a means of delivering hydrogen to markets. However, as stated in ASME B31.12, high-strength steel pipelines under stress can be susceptible to hydrogen embrittlement, which is a phenomenon that could induce brittle fracture in steel. This study proposes a numerical framework using phase-field fracture modelling techniques to model the hydrogen embrittlement phenomenon in high-strength steels. The proposed numerical framework is validated against a Compact Tension experimental test specimen, which is deemed suitable to capture the crack-tip constraint observed in high strength steel. The finite element results show a good agreement with experimental results, which demonstrate the capability of the phase-field fracture model in reasonably predicting hydrogen embrittlement in high-strength steel. As such, the proposed numerical modelling framework could also be applicable to typical high strength steel pipelines.

Buried transmission pipelines generally traverse long distances, which potentially increase their susceptibility to geotechnical hazards. Geohazards triggered by permanent ground deformation (PGD) in the vertical plane, e.g., subsidence, frost heave, thaw settlement, and uplift, can potentially induce large plastic strain on buried pipelines. Not only the magnitude and direction but also the pattern of this vertical ground movement can result in different pipeline strain demands, which are crucial for assessing the structural performance of pipelines. This paper presents the effects of the asymmetrical soil stiffness in the vertical plane on the response of buried pipelines subjected to two different patterns of transverse vertical PGD (Step and Block) patterns. Using the nonlinear beam-spring finite-element modelling technique in Abaqus, the analysis is conducted for an NPS 42 X70 pipe with a uniform wall thickness of 14.3 mm buried in four different types of soil and subjected to a PGD magnitude of 3.0 m. The pipeline is internally pressurized to the maximum operating pressure of 9.93 MPa and temperature variation of 50 degrees C. The results show a significant variation in the strain demand of buried pipelines subjected to the Step PGD pattern due to the asymmetric soil stiffnesses in the vertical direction. Not only the ratio of upward and downward soil resistances but also their values affect the strain demand of the pipeline against the Step pattern.

Onshore pipelines are usually designed and constructed for burial conditions. However, pipeline exposures can develop from environmental and geohazard conditions such as creek crossing, soil washout due to significant rainfall events, short-term flooding, etc. Engineering assessment of the exposed pipeline sections is a vital consideration to assess the fitness of the exposed pipeline sections for continued service. A fitness for service engineering assessment methodology that was successfully used for the assessment of an NPS 10 onshore natural gas pipeline exposure is presented herein. The assessment is based on finite element analysis to evaluate several damage mechanisms, e.g., lateral and vertical longitudinal bending stresses and strains, buoyancy, hydrodynamic forces, vortex-induced vibration, mechanical damages due to foreign objects impact, and brittle fracture assessment as per failure assessment diagram approach. The proposed framework for the fitness for service engineering assessment of exposed pipe sections presented in this study helps identify potential integrity concerns and prioritize required corrective actions. The proposed methodology offers a robust framework that can significantly improve the operation and maintenance of the existing buried pipelines.

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.

Mechanical damages such as dents are one of the most common threats to the structural integrity of buried pipelines traversing high population areas or mountainous terrains. Dents are defined as local inward depression in the pipe surface caused by external forces that produce pipe wall plastic deformation and a disturbance in the curvature of the pipe. Since not all dents affect the fitness for service (FFS) of a pipeline, dents identified by in-line inspection (ILI) tools need to be properly evaluated to determine the severity. This article provides a methodological framework to carry out a comprehensive fitness for service engineering assessment of dented natural gas pipelines using finite element analyses. The assessment is based on the evaluation of the dent formation strain, plastic collapse, local buckling, and pressure cycling fatigue failure modes. This FFS engineering assessment methodology addresses the susceptibility of dents to ductile fracture damages and stress-corrosion cracking integrity threats that can develop due to high membrane and bending loads in a dented pipe. This FFS assessment methodology has successfully been utilized to assess dents identified by ILI in gas pipeline systems. The results and the associated interpretation and discussions of the FFS assessment findings are presented herein.

Horizontal directional drilling (HDD) is one of the popular pipeline trenchless construction techniques for sites where surface excavations and conventional trenching are not desirable. An integral part of the pipeline design and construction process is to perform stress analysis on the HDD overbends, which can be subjected to significant cross-sectional deformations due to stresses/strains imposed by thermal expansion and internal pressure. This paper proposes a novel approach to reduce the stress range in the HDD overbends using carbon fibre reinforced polymer (CFRP) wraps. Although this reinforcement technique is primarily used in the pipeline industry for repairing damaged pipes, there is a handful of recent studies that showed the promising effect of using CFRP reinforcement on undamaged pipe bends. A total of 259 finite element analyses are conducted with a different combination of pipe diameter to thickness ratio, CFRP length and thickness, fibre orientation, and internal pressure. An exploratory data analysis is then performed to demonstrate the impact of each variable on the maximum equivalent stresses imposed on the HDD overbend. The finite element results show that multi-directional fibre orientation leads to the highest reduction of peak equivalent stress on the HDD overbend. Besides, an increase in CFRP thickness results in a greater reduction of stresses on the HDD overbend. However, CFRP length does not have a noticeable effect on decreasing the stresses on the HDD overbend.

This paper presents a thermo-hydro-mechanics theory and corresponding computational framework to capture the freezethaw action of frozen porous media and associated frost action under chilled gas pipelines. Based on the mixture theory, frost-susceptible soils are formulated to capture the Darcy flux and thermal actions below the pipelines. Constitutive models that combine the cryo-suction are presented to reproduce the changes in volume, strength, and thermal characteristics of solid grain, pore water, and ice crystal. A generalized hardening rule is adopted to replicate the elasto-plastic responses which strengthens the frozen porous media due to ice crystallization. Changes in permeability and thermal diffusivity are also incorporated by considering the phase transitions of pore water and ice crystal. Numerical examples for pipeline applications are designed to analyze the influence of the freezing and melting process around the pipelines.

In the current industry practice guidelines, the soil restraints to assess the behaviour of pipelines subject to permanent ground displacements are numerically characterized using independent “soil springs”. These guidelines have been primarily generated by considering the typical configurations of buried pipelines in level ground. The assumption of level ground does not always hold true when assessing pipelines located on sloping ground in mountainous areas and riverbanks. This research presents the outcomes from a set of full-scale physical model tests conducted on a pipe buried in slopes. The results highlight the significance of the slope grade effects on soil-pipe interaction. The results are useful as input to modify soil springs accounting for the ground surface inclination.

Finite-element modeling of pipelines buried in dense sand under oblique loading in vertical (upward)-lateral load space is presented. The prepeak hardening, postpeak softening, density, and confining pressure-dependent behavior of sand are considered in simulations. A monotonic displacement-controlled loading is applied where pipe is displaced at an angle ranging from 0 degrees (vertical upward) to 90 degrees (lateral). The plots of vertical and lateral components of oblique force (load paths) resulting from the displacement-controlled loading show a significant nonassociated response for shallow burial depths and lateral loading cases. The restraint to vertical displacement during lateral displacement increases the lateral resistance. A simplified approach based on maximum vertical and lateral restraints, together with an inclination factor, is proposed to estimate the maximum oblique resistance. (c) 2020 American Society of Civil Engineers.