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