Uplift mobilisation resistance of subsea pipelines in loose sand

Peer reviewedPublisher PD

Şerit ankraj plakalarının çekme kapasitesi davranışının farklı koşullar altında sayısal analizi

Bu çalışmada, kumlu zeminlere gömülü şerit ankraj plakalarının çekme kapasitesi davranışı sayısal olarak analiz edilmiştir. Bu amaçla, prototip bir model oluşturulmuş ve farklı koşullar için sonlu elemanlar analizleri gerçekleştirilmiştir. Çalışmada, sonlu elemanlar yöntemi ile çözüm yapan PLAXIS bilgisayar programı kullanılmıştır. Analizlerde, ankraj genişliği, gömülme derinliği ve kumun sıkılık derecesi parametrelerinin, şerit ankraj plakalarının çekme kapasitesi davranışına etkileri incelenmiştir. Tüm analizler iki farklı malzeme modeli (Mohr-Coulomb ve Hardening Soil) kullanılarak gerçekleştirilmiş ve elde edilen çekme kapasitesi değerleri, teorik sonuçlarla karşılaştırılmıştır. Analizler sonunda elde edilen sonuçlara göre, kuma gömülü şerit ankraj plakalarının çekme kapasitesi, plaka genişliği, ankraj gömülme derinliği ve kumun sıkılığının artmasına bağlı olarak artış göstermektedir. Hardening Soil malzeme modeli kullanılarak gerçekleştirilen analizlerden elde edilen çekme kapasitesi değerleri, Mohr-Coulomb malzeme modeli ile elde edilen değerlerden daha büyüktür. Ayrıca, sayısal analizlerden elde edilen çekme kapasitesi değerleri ile teorik çözümden elde edilen değerler arasında genel bir uyum söz konusudur. Ancak, teorik çözüm ile elde edilemeyen deplasman ve gerilme davranışının sonlu elemanlar yöntemiyle gözlenebilmesi konunun daha rahat anlaşılmasına olanak sağlamaktadır

Uplift resistance of horizontal strip anchors in sand: a cavity expansion approach

This letter presents an analytical cavity expansion theory-based method for predicting peak uplift resistance of shallow horizontal strip anchors buried in sand. Based on an analytical two-dimensional stress solution for loading analysis around a cylindrical cavity, the method was developed by assuming that the peak anchor uplift resistance can be approximated by the cavity breakout pressure. In the new cavity expansion model, the ultimate failure is reached once the plastic zone develops to the ground surface, and the biaxial state of in-situ ground stresses is taken into account. A database consisting of 75 model tests on shallow strip anchors in sands was compiled to valid the new method. The predicted results and measured data are in reasonable agreement, with a mean over-prediction of the peak uplift resistance by 1.6%. The reliability of the new solution was also checked by comparing with other commonly used analytical solutions. It is shown that the present solution can provide a simple analytical tool for predictions of the peak uplift resistance of strip anchors in sand while a sliding-block failure mechanism dominates

Experimental and numerical investigation of the uplift capacity of plate anchors in geocell-reinforced sand

Plate anchors are frequently used to provide resistance against uplift forces. This paper describes the reinforcing effects of a geocell-reinforced soil layer on uplift behavior of anchor plates. The uplift tests were conducted in a test pit at near full-scale on anchor plates with widths between 150 and 300 mm with embedment depths of 1.5–3 times the anchor width for both unreinforced and geocell-reinforced backfill. A single geocell layer with pocket size 110 mm × 110 mm and height 100 mm, fabricated from non-perforated and nonwoven geotextile, was used. The results show that the peak and residual uplift capacities of anchor models were highest when the geocell layer over the anchor was used, but with increasing anchor size and embedment depth, the benefit of the geocell reinforcement deceases. Peak loads between 130% and 155% of unreinforced conditions were observed when geocell reinforcement was present. Residual loading increased from 75% to 225% that of the unreinforced scenario. The reinforced anchor system could undergo larger upward displacements before peak loading occurred. These improvements may be attributed to the geocell reinforcement distributing stress to a wider area than the unreinforced case during uplift. The breakout factor increases with embedment depth and decreased with increasing anchor width for both unreinforced and reinforced conditions, the latter yielding larger breakout factors. Calibrated numerical modelling demonstrated favorable agreement with experimental observations, providing insight into detailed behavior of the system. For example, surface heave decreased by over 80% when geocell was present because of a much more efficient stress distribution imparted by the presence of the geocell layer

Lateral ground movement effects near connection of medium density polyethylene gas distribution pipes

Buried medium-density polyethylene (MDPE) pipes are extensively used for gas distribution systems. These pipes are sometimes exposed to geotechnical hazards such as ground movement, which may cause significant damage to the pipes. Understanding the behaviour of the pipes subjected to the ground movement is critical for transporting natural gas safely and economically using these pipings. This thesis presents an investigation of MDPE gas distribution pipes subjected to lateral ground movements near a connection. Gas distribution systems include a number of connections and lateral branches to supply gas to communities. When a pipe is subjected to the load from the lateral ground movement, the branch pipes experience axial force, and the pipe itself experiences bending deformation. As a result, excessive strains can develop on the pipes, leading to leakage or breakage. In this study, the bending strain on the pipes and the axial force on branch pipes are investigated using full-scale testing. Tests were conducted using 42.2-mm and 60-mm diameter MDPE pipes buried in the ground in a full-scale test facility. Each type of pipe was tested at two different burial depths in dense sand and loose sand. Test results showed that the axial force on the lateral branch depends on the burial depth, the pipe diameter, and soil density. The pipe under lateral ground movement experienced significant bending strain near the connection. The measured responses of the pipes were reasonably estimated within the linear range of deformations using beam-on-elastic spring idealization. For large deformations, elastoplastic spring parameters were required to simulate the pipe behaviour. The bilinear elastoplastic spring parameters recommended in the pipe design guidelines for steel pipes were modified to simulate the measured responses for the MDPE pipes. Based on the validated spring parameters, a parametric study was conducted using a python script to investigate the effect of burial depth, soil density and landslide magnitude on the pulling force and the bending strain

Uplift response of strip anchors in cohesionless soil

Physical and computational studies investigating the uplift response of 1 m wide strip anchors in sand show that maximum resistances increase with anchor embedment ratio and sand packing. Agreement between uplift capacities from centrifuge and finite element modelling using PLAXIS, based on 0.2 m computed maximum displacements, is excellent for anchors up to embedment ratios of 6. Some divergence occurs for deeper anchors. Pre-peak response is reasonably well reproduced using the Hardening Soil Model available in PLAXIS, although the characteristic post-peak softening in some physical model tests requires a more sophisticated soil model. PLAXIS also produces breakout factors which compare reasonably well with established limit state and finite element based theories. © 2006 Elsevier Ltd. and Civil-Comp Ltd

Experimental and numerical study on pull-out resistance of flip-type earth anchors under different ground conditions, and its application to slope stability

13301甲第5412号博士（工学）金沢大学博士論文本文Ful

FINITE ELEMENT INVESTIGATION INTO THE PERFORMANCE OF EMBEDDED PLATE ANCHORS IN SAND

As offshore energy and other development extend to deeper waters, conventional platforms are increasingly being replaced by floating facilities. Also, up to 60 percent of wind power development is anticipated to be in deeper waters that require floating platforms moored to the seabed by anchors. Seabed soils at sites often contain sandy soil strata; therefore, practical development of offshore wind power requires anchor systems that are suitable for deployment in sands, such as piles, suction caissons and direct embedment plate anchors. Of these options, plate anchors are particularly attractive due to their compact size, light weight, variety of installation techniques, and highly efficient and suitable for a wide range of soil conditions. However, more reliable predictive models for plate anchor performance in cohesionless soils are needed for mooring systems to be securely designed. The limited research focus on plate anchor performance in cohesionless soil, particularly for deep embedment, has triggered a strong motivation for this research. Therefore, extensive small and large deformation finite element simulations were conducted to study the effects of anchor embedment depth, with special emphasis on characterizing the transition in the anchor behavior from a shallow to a deep failure, considering elastic soil behavior (in terms of Rigidity Index Ivr) in evaluating anchor performance. Additionally, there is a significant gap in knowledge concerning the keying behavior of direct embedment plate anchors in sand after installation, and the corresponding irrecoverable loss of embedment. Finally, most previous plate anchors research have focused on either horizontal or vertical anchor orientations while the effect of inclined orientations has received limited attention. The predictions showed that at shallow anchor embedment depths, rigidity index Ivr has negligible influence on anchor capacity while the performance of deeply embedded anchors is strongly influenced by rigidity index Ivr. This study developed an empirical model for predicting anchor pullout capacity as function Nvq (Dvr, γ ', z/D), describing the transition in the breakout factor Nvq from the shallow mode to its maximum value Nvqvmvavx. In regard to the keying process behavior, the large deformation finite element analyses showed that the angle of orientation α at which the maximum pullout capacity occurs increases with increasing e/B ratio, ranging between 75 degrees and 85 degrees. Also, the predictions revealed that as the loading eccentricity ratio e/B increases, the loss in anchor embedment vz/B during rotation decreases. However, once the eccentricity e greater than or equal B, a minimal loss in anchor embedment can be achieved regardless of the plate thickness. A linear relationship was observed between the maximum loss in anchor embedment and anchor pullout angle theta at any e/B ratio. In regard to the pullout capacities of inclined plate anchors in cohesionless soil. An empirical equation was proposed to estimate the breakout factor of an inclined anchor at any inclination angle theta between 0 degrees and 90 degrees. Also, the observations showed a significant sensitivity of the breakout factor Nq to the plate width B

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