Numerical modeling of pipe-soil and anchor-soil interactions in dense sand

Buried pipelines are one of the most efficient modes for transportation of hydrocarbons, in both onshore and offshore environments. While traversing large distances through a wide variety of soil, buried pipelines might be subjected to lateral or upward loading. Pipelines are generally installed in a trench and then backfilled with loose to medium dense sand. However, in many situations, the backfill sand might be densified even after installation due to natural phenomena, such as wave action in offshore environments. Proper estimation of force/resistance due to relative displacement between soil and pipe during lateral or upward movement is an important engineering consideration for safe and economic design of pipelines. In the development of design guidelines for pipelines, theoretical and experimental studies on anchor behaviour are also used, assuming that a geometrically similar pipe and anchor behave in a similar fashion. Pipelines and anchors buried in dense sand are the focus of the present study. Improved methods for analysis of complex pipe and anchorsoil interactions are developed in the present study through finite element (FE) analysis using Abaqus FE software. Recognizing the limitations of the classical MohrCoulomb (MC) model, which is typically used for modelling sand in FE modeling of pipe–soil interaction, a modified MohrCoulomb (MMC) model is proposed, which considers nonlinear variation of angles of internal friction and dilation with plastic shear strain, loading condition, density and confining pressure, as observed in laboratory tests on dense sand. The proposed MMC model is implemented in Abaqus using a user-defined subroutine. The response of buried pipelines subjected to lateral ground movement is investigated using FE analysis with the MC and MMC models. The FE results (e.g. force–displacement behaviour including the peak and post-peak lateral resistances) are consistent with the results of physical model tests and numerical analysis available in the literature. The uplift resistance against upheaval buckling is a key design parameter, which is investigated for a shallow buried pipeline across a range of pipe displacements. An uplift force– displacement curve can be divided into three segments: pre-peak, post-peak softening and gradual reduction of resistance at large displacement. A set of simplified equations is proposed to obtain the force–displacement curve for a shallow buried pipe. Although many pipelines are embedded at shallow burial depths, deep burial conditions are also evident in many scenarios (e.g. ice gouging prone regions). The uplift resistance and its relation to progressive formation of shear bands (i.e. zones of localized plastic shear strain) are also investigated for deep buried pipes across a range of burial depths and pipe diameters. A simplified method to calculate the peak and post-peak uplift resistances, using an equivalent angle of internal friction, is proposed for practical applications. A comparative study is conducted to explain the similarities and differences between the lateral response of buried pipes and strip anchors, which shows that the anchor gives approximately 10% higher peak resistance than does a pipe of diameter equal to the height of the anchor. The lateral resistance increases with burial depth and becomes almost constant at large burial depths. The transition from shallow to deep failure mechanisms occurs at a larger burial depth for anchors than pipes. Finally, a set of simplified equations is proposed to estimate the lateral resistances for a wide range of burial depths

Black Tea and Theaflavins Assist Healing of Indomethacin-Induced Gastric Ulceration in Mice by Antioxidative Action

The healing activities of black tea (BT) and the theaflavins (TF) against the indomethacin-induced stomach ulceration were studied in a mouse model. Indomethacin (18 mg/kg, p.o.) administration induced maximum ulceration in the glandular portion of the gastric mucosa on the 3rd day, accompanied by increased lipid peroxidation and protein oxidation, depletion of thiol-defense and mucin, as well as reduced expressions of cyclooxygenases (COX) and prostaglandin (PG) E synthesis in the gastric tissues, and plasma total antioxidant status of mice. Treatment with BT (40 mg/kg), TF (1 mg/kg), and omeprazole (3 mg/kg) produced similar (74%–76%) ulcer healing, as revealed from the histopathological studies. Treatment with all the above samples reversed the adverse oxidative effects of indomethacin significantly. BT and TF also enhanced the PGE synthesis by augmenting the expressions of COX 1 and 2, but did not modulate acid secretion

Finite element analyses of soil/pipeline interactions in sand with an advanced soil constitutive model

Inexorable demand of energy intensified the search for oil and gas in remote and harsh regions. The operations related to exploration and production of oil and gas has been increased significantly over the last 30 years. The liquid hydrocarbon and natural gas products are usually transported through pipelines, which traverse large distances through a variety of soils. Failure of such pipelines could cause a significant economic loss and environmental damage. Geohazards and the associated ground movement represent a significant threat to pipeline integrity that may result in pipeline damage and potential failure. Safe, economic and reliable operation of pipeline transportation systems is the primary goal of the pipeline operators and regulatory agencies. Pipelines are usually buried or partially embedded into the seabed. To develop a reliable pipelines design method the complex behaviour of seabed sediment (soil) and soil/pipeline interaction should be properly modeled and analyzed. -- Finite element (FE) modeling has been widely used for predicting the response of buried pipelines. One of the main challenges in FE modeling of buried pipelines is to use appropriate soil constitutive model. Most of the FE analyses used built-in soil constitutive models in available commercial FE software. However, their prediction might be better if an advanced soil constitutive model is used. -- The main objective of this research is to perform finite element modeling for analyzing the response of pipelines buried in sand. The primary focus of this research is to adopt an advanced soil constitutive model which might have a significant impact on pipeline response due to soil movement and to implement it in the commercial finite element software package ABAQUS with a user defined subroutine UMA T. All the analyses presented are in drained condition. -- In this study numerical analyses of soil/pipeline interaction are performed using the built-in Mohr-Coulomb soil constitutive model in ABAQUS FE program. This study shows the limitations and advantages of this constitutive model. Reviewing available soil constitutive models in the literature, it is identified that NorSand soil model proposed by Jefferies (1993) could better simulate the behaviour of sand particularly the dilation under monotonic loading. NorSand soil constitutive model implemented in ABAQUS FE software using user defined subroutine UMA T is used for modeling the response of pipelines under lateral, vertical (upward) and oblique loading events. Finite element analyses are also performed with built-in Mohr-Coulomb model. It is shown that the NorSand UMA T can simulate better the force displacement behaviour including the post-peak softening, which cannot be done with Mohr-Coulomb model with a constant dilation angle. The failure envelope obtained with NorSand UMAT for combined lateral and vertical (upward) oblique loading for a deep burial pipeline in dense sand is comparable with the analytical solution and previous numerical analyses

Finite Element Modeling of Lateral Pipeline-Soil Interactions in Dense Sand

Finite element (FE) analyses of pipeline-soil interaction for pipelines buried in dense sand subjected to lateral ground displacements are presented in this paper. Analysis is performed using the Arbitrary Lagrangian-Eulerian (ALE) method available in Abaqus/Explicit FE software. The pipeline-soil interaction analysis is performed in the plane strain condition using the Mohr-Coulomb (MC) and a modified Mohr-Coulomb (MMC) models. The MMC model considers a number of important features of stress-strain and volume change behaviour of dense sand including the nonlinear pre- and post-peak behaviour with a smooth transition and the variation of the angle of internal friction and dilation angle with plastic shear strain, loading conditions (triaxial or plane strain), density and mean effective stress. Comparing FE and experimental results, it is shown that the MMC model can better simulate the force-displacement response for a wide range of lateral displacements of the pipe for different burial depths, although the peak force on the pipe could be matched using the MC model. Examining the progressive development of zones of large inelastic shear deformation (shear bands), it is shown that the mobilized angle of internal friction and dilation angle vary along the length of the shear band, however constant values are used in the MC model. A comprehensive parametric study is also performed to investigate the effects of pipeline diameter, burial depth and soil properties. Many important aspects in the force-displacement curves and failure mechanisms are explained using the present FE analyses.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

A comparative study between lateral and upward anchor-soil and pipe-soil interaction in dense sand

Buried pipelines are extensively used in onshore and offshore environments for transportation of hydrocarbons. On the other hand, buried anchors have been used for many years to stabilize various structures. In the development of design guidelines for pipelines, theoretical and experimental studies on buried anchors are sometimes used assuming that pipeline-soil and anchor-soil interaction are similar. In the present study, finite element (FE) modeling is performed to simulate the response of pipeline and anchor buried in dense sand subjected to lateral and uplift forces. The similarities and differences between the responses of these two types of structures are examined to justify the application of anchor theory to pipeline behaviour. The stress-strain behaviour of dense sand is modeled using a Modified Mohr-Coulomb (MMC) model, which considers the pre-peak hardening, post-peak softening, density and confining pressure dependent friction and dilation angles. A considerable difference is found between the lateral resistance of pipeline and vertical strip anchor of similar size. Progressive development of shear bands (shear strain concentrated zone) can explain the load-displacement behaviour for both lateral and upward loading

Lateral resistance of pipes and strip anchors buried in dense sand

The response of buried pipes and vertical strip anchors in dense sand under lateral loading is compared based on finite-element (FE) modeling. Incorporating strain-softening behaviour of dense sand, the progressive development of shear bands and the mobilization of friction and dilation angles along the shear bands are examined, which can explain the variation of peak and post-peak resistances for anchors and pipes. The normalized peak resistance increases with embedment ratio and remains almost constant at large burial depths. When the height of an anchor is equal to the diameter of the pipe, the anchor gives approximately 10% higher peak resistance than that of the pipe. The transition from the shallow to deep failure mechanisms occurs at a larger embedment ratio for anchors than pipes. A simplified method is proposed to estimate the lateral resistance at the peak and also after softening at large displacements

Upward pipe-soil interaction for shallowly buried pipelines in dense sand

Uplift resistance is a key parameter against upheaval buckling in the design of a buried pipeline. The mobilization of uplift resistance in dense sand is investigated in the present study based on finite-element (FE) analysis. The prepeak hardening, postpeak softening, density-dependent, and confining pressure-dependent soil behavior are implemented in FE analysis. The uplift resistance mobilizes with progressive formation of shear bands. The vertical inclination of the shear band is approximately equal to the maximum dilation angle at the peak and then decreases with upward displacement. The force-displacement curves can be divided into three segments: prepeak, quick postpeak softening, and gradual reduction of resistance at large displacements. Simplified equations are proposed for mobilization of uplift resistance. The results of FE analysis, simplified equations, and model tests are compared. The importance of postpeak degradation of uplift resistance to upheaval buckling is discussed