The occurrence of large landslides in sensitive clays, such as spreads, can be modelled by progressive development of large inelastic shear deformation zones (shear bands). The main objective of the present study is to perform large deformation finite-element modelling of sensitive clay slopes to simulate progressive failure and dislocation of failed soil mass using the coupled Eulerian Lagrangian (CEL) approach available in Abaqus finite-element software. The degradation of undrained shear strength with plastic shear strain (strain-softening) is implemented in Abaqus CEL, which is then used to simulate the initiation and propagation of shear bands due to river bank erosion. The formation of horsts and grabens and dislocation of soil masses that lead to large-scale landslides are simulated. This finite-element model explains the displacements of different blocks in the failed soil mass and also the remoulding of soil around the shear bands. The main advantages of the present finite-element model over other numerical models available in the literature are that it can simulate the whole process of progressive failure leading to spread. The finite-element results are consistent with previous conceptual models proposed from field observations. The parametric study shows that, depending upon geometry and soil properties, toe erosion could cause three types of shear band formation: (a) only a horizontal shear band without any global failure; (b) global failure of only one block of soil; (c) global failure of multiple blocks of soil in the form of horsts and grabens.

A large-deformation finite-element (FE) modelling technique is presented to model submarine landslides. A strain softening model for undrained shear strength of marine clay is incorporated in the FE modelling. The development of large plastic shear strain concentrated zones (shear bands) and their propagation with displacement of soil mass are simulated. FE simulations show that the existence of a weak layer might result in the initiation and propagation of shear bands leading to large-scale progressive landslides. Such progressive development of failure planes cannot be simulated using the limit equilibrium method of slope stability analysis. Depending upon the geometry and soil properties, a number of failure patterns are identified which are comparable with morphologic features seen in field observations. Based on this type of FE analysis and compared with seabed morphology, the developed failure planes could be identified where the shear strengths are expected to be lower because of pre-shearing than the shear strengths of the soil outside these zones, which could then be implemented in the modelling of seabed for offshore development projects in the areas where failure occurred in the past.

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 — in the plane strain condition using the Mohr–Coulomb (MC) and modified Mohr–Coulomb (MMC) models. The MMC model considers a number of important features and properties 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.

Post-slide investigations suggest that many large-scale submarine landslides occur through marine sensitive clay layers. A nonlinear mathematical model for post-peak degradation of undrained shear strength of sensitive clay is proposed based on experimental results. A method for estimation of model parameters is presented. Incorporating the model, an analytical solution is developed to examine possible mechanisms of large-scale submarine landslides. Analyses are performed for mild infinite slopes where the failure initiates from a "fully weakened zone" of soil having undrained shear strength lower than the shear stress acting parallel to the slope. The driving force, in excess of resistance, generated from the fully weakened zone is then transferred to the surrounding soil elements resulting in shear band formation due to strain-softening behaviour of sensitive clays. When the length of the fully weakened zone is greater than a critical length, catastrophic shear band propagation (self-driven without any additional external force) occurs, which could result in large-scale offshore landslides. A simple design chart is developed to calculate the critical length. Compared with a 2005 study by Puzrin and Germanovich based on a linear post-peak shear strength degradation model, the present study gives a conservative estimation of critical length for catastrophic shear band propagation.Key words: sensitive clay, large deformation, submarine landslides, shear band, undrained shear strength.Résumé : Les enquêtes post-glissement de terrain suggèrent que de nombreux glissements de terrain sous-marin à grande échelle se produisent à travers les couches d'argiles marines sensibles. Un modèle mathématique non linéaire de la dégradation post-pic de la résistance au cisaillement non drainée de l'argile sensible est proposé sur la base de résultats expérimentaux. Une méthode d'estimation de paramètres du modèle est présentée. Incorporant le modèle, une solution analytique est développée pour examiner les mécanismes possibles de glissements de terrain sous-marin à grande échelle. Les analyses sont effectuées pour des pentes infinies douces où l'échec est entamé à partir d'une « zone totalement affaiblie » du sol ayant une résistance au cisaillement non drainé inférieure à la contrainte de cisaillement agissant parallèlement à la pente. La force d'entraînement, au-delà de la résistance, générée à partir de la zone entièrement affaiblie est transférée aux éléments de sol environnant aboutissant en la formation de bandes de cisaillement en raison du comportement d'adoucissement à la déformation des argiles sensibles. Lorsque la longueur de la zone entièrement affaiblie est supérieure à une longueur critique, une propagation d'une bande de cisaillement catastrophique (auto-conduit sans aucune force externe supplémentaire) se produit qui pourrait entraîner des glissements de terrain en mer à grande échelle. Un graphique de conception simple est développé pour calculer la longueur critique. Par rapport à une étude réalisée e...

Large-diameter monopiles are widely used foundations for offshore wind turbines. In the present study, three-dimensional finite element (FE) analyses are performed to estimate the static lateral load-carrying capacity of monopiles in dense sand subjected to eccentric loading. A modified Mohr-Coulomb (MMC) model that considers the pre-peak hardening, post-peak softening and the effects of mean effective stress and relative density on stress-strain behavior of dense sand is adopted in the FE analysis. FE analyses are also performed with the Mohr-Coulomb (MC) model. The load-displacement behavior observed in model tests can be simulated better with the MMC model than the MC model. Based on a parametric study for different length-to-diameter ratio of the pile, a load-moment capacity interaction diagram is developed for different degrees of rotation. A simplified model, based on the concept of lateral pressure distribution on the pile, is also proposed for estimation of its capacity.

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This paper presents the results of a series of physical experiments to quantify the drag force on a submarine pipeline caused by a glide block or an out-runner block impact normal to the pipe axis. The experiments were carried out in a geotechnical centrifuge at C-CORE under submerged conditions at a centrifugal force of 30 times the Earth's gravity (i.e. N = 30) and simulated steady and uniform impact velocities ranging between 0.1 and 1.3 m/s with the soil blocks being approximately 5 m in height in prototype scale. The soil blocks were made of kaolin clay consolidated to have undrained shear strengths ranging between about 4 and 6 kPa. The diameter of the model pipes were 6.35 and 9.5 mm corresponding to about 0.19 and 0.29 m in prototype terms. The shear strain rates, defined as the ratio of impact velocity to pipe diameter, in the centrifuge model are N times higher than that in the prototype. The shear rates simulated ranged from about 10 to 136 reciprocal seconds. The paper presents a method for estimating block impact drag force on submarine pipelines based on the results of the centrifuge experiments. Introduction A submarine pipeline is a system of connected sections of pipe that usually transports crude oil or refined hydrocarbons. The pipe is laid on or buried in the seafloor. It typically ranges from 0.1 m to 1.0 m in diameter. The total length of a pipeline is dictated by the distances between the production platform(s) and the onshore or offshore destination(s) and by the route which poses the least risk in terms of offshore geohazards. Submarine landslides and the associated mass movement can potentially have devastating consequences on seafloor installations such as pipelines, flow lines, well systems, cables, etc. Submarine landslides occur frequently on both passive and active continental margins and slopes, releasing sediment volumes that may travel distances as long as hundreds of kilometres on gentle slopes (0.5 to 3°) over the course of less than an hour to several days [1]. The movement of landslide and the released sediment volumes in general terms are so called ‘density flows’. From the initiation to deposition, density flows undergo complex processes that depend on many factors such as the composition, strength characteristics and properties, terrain topography, etc. Geohazards in an offshore oil and gas perspective can be due to local and/or regional site and soil conditions having the potential to develop into failure events causing loss of life and damage to the environment or field installations. Triggering of these events can be caused by natural geological processes or by man's activities, as outlined in a recent state of-the-art review [2]. Research on understanding the mechanisms behind and the risks posed by submarine slides has intensified in the past decade [e.g. 3, 4-10], mainly because of the increasing number of deep-water petroleum fields that have been discovered and in some cases developed. Production from offshore fields in areas with earlier sliding activity is ongoing in the Norwegian margin, Gulf of Mexico, offshore Brazil, the Caspian Sea and West Africa [11]. Estimating magnitude of the drag forces on pipelines caused by density flow impact is an important design consideration in offshore engineering. For buried pipelines in cohesive soils in slowly moving unstable slopes, the available methods seem to provide more or less similar estimates for the drag force normal to the pipe axis. However, this is not the case for estimates of the drag force parallel to the pipe axis [2]. In cohesive soils, the magnitude of the drag force is a function of the rate at which the soil is sheared during interaction with the pipe. Recent works by Zakeri et al. [1, 12-14] provide a method for estimating drag forces caused by clay-rich debris flow (fully remoulded and fluidized density flow) impacting a pipeline normal to its axis. Later, the work was extended to cover all angles of impact [15].

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