Large Deformation Finite Element Analysis Research Articles

Numerical investigation on reinstallation of jack-up unit with rectangular spudcan close to footprint

This paper presents the jack-up unit reinstallation loads in clay when it has to be reinstalled near an existing footprint due to a previous installation and removal. Large-deformation finite element analyses are performed by using the coupled Eulerian-Lagrangian (CEL) method. A three-leg jack-up unit model is used for the global investigation of the spudcan-footprint interactions. The novel rectangular-shaped flat-based spudcan is considered. A detailed parametric study is carried out with varying influential factors including spudcan shape, offset distance and footprint shape. The effects of these factors are evaluated based on the magnitudes of induced bending moment, horizontal force, lateral displacement and rotation. Compared to the generic conical spudcan, the novel spudcan reduces the peak horizontal force, lateral displacement and rotation by 40%. But it has a larger bending moment at the spudcan level, due to the corner effect. It is found that the bending moment at the hull level position is 800% larger than that at the spudcan level. The flat-based spudcan is effective at decreasing footprint-spudcan interactions, reducing the loads on the leg by about 30%. The critical offset distance is found to be that the spudcan center is at the edge of the footprint.

Research on Optimal Design of Spudcan Structures to Ease Spudcan-Footprint Interactions in Clay and Comparative Analyses with Different Measures

Abstract In order to ease consequences of spudcan-footprint interactions during jack-up rigs reinstalling in the vicinity of an existing seabed footprint, three new types of spudcan shapes, that is, a lotus-shaped spudcan with six circular holes, a flat-bottomed spudcan, and a concave-shaped spudcan, were proposed to perform an optimizing study of the spudcan structures, and the effectiveness of them were analyzed comparatively with other different measures. Firstly, 3D Large Deformation Finite Element (LDFE) analyses were carried out using the Coupled Eulerian-Lagrangian (CEL) method in the commercial finite element package ABAQUS. After calibrating the validity of the numerical calculation model against existing centrifuge test data and LDFE results, the differences in interaction mechanism between the novel spudcans and the generic spindle-shaped spudcan were studied when penetrating near an existing footprint with an eccentric distance of 0.5D, and the horizontal range of plastic deformation of the disturbed soils, the inclination angle of the spudcan and the offset distance of the pile legs were analyzed comparatively as well. The results show that the proposed novel spudcans can mitigate the maximum horizontal sliding force and the maximum bending moment at the top of the pile leg obviously, compared with those of the generic one, which were reduced by 32.59%, 22.47%, 28.18%, and 26.32%, 12.88%, 18.02%, respectively. It also can be seen that all novel structures can ease the adverse consequences of spudcan-footprint interactions effectively, and can improve the in -place stability of the spudcan as well. Finally, three possible measures in mitigating interactions of the spudcan-footprint were contrasted, that is, the novel spudcan (represented by the lotus-shaped spudcan with six holes), stomping method, and perforation drilling method. The results show that all of them can reduce adverse impacts induced by interactions of the spudcan-footprint, and also can improve the in-place stability of the spudcan during reinstallation. In addition, among them, according to the effect of reducing the additional stress of the spudcan, the effectiveness of them can be listed as follows: perforation drilling near an existing footprint > the lotus-shaped spudcan with six holes > stomping method. In terms of the vertical bearing capacity of the spudcan, the lotus-shaped spudcan with six holes can improve it as much as 16.33% compared with the spindle-shaped structure due to the particularity of the structure. While reducing the continuity and strength of soil foundations, the perforation drilling measure leads to the decrease of the vertical bearing capacity of the spudcan by 13.07%. It can be concluded that all the three measures have merits and demerits, so the relevant construction environment conditions and engineering practice should be fully considered when selecting measures to deal with interactions of the spudcan-footprint.

The influence of layered soil on ice-seabed interaction: Soft over stiff clay

Ice gouging is one of the main threats for the structural integrity of the offshore pipelines in the territories that floating icebergs can reach. The safe and cost-effective burial depth of pipelines for protection against the iceberg attack depends on accurate assessment of the subgouge soil deformation and the ice keel-seabed contact forces. Layered seabed soil strata that are found in many geographical locations adds to the complexity of assessing the ice keel-seabed interaction process by signifying the effect of strain rate dependence and strainsoftening in the soil layers interaction areas and ice-seabed contact zone. The existing design codes simplify the seabed condition by assuming a uniform soil strata. This in turn affects the accuracy and cost-effectiveness of the design. In this paper, large deformation finite element (LDFE) analysis was conducted using Coupled Eulerian Lagrangian (CEL) approach to investigate the free-field ice gouging in layered (soft over stiff clay) cohesive seabed. A modified Tresca soil model was coded in a user subroutine to account for the effects of strain rate and strain-softening on the mobilised undrained shear strength. A comprehensive parametric study was conducted to examine the effect of gouging features and soil shear strength profiles on the subgouge soil deformation, developed plastic shear strains and the resultant ice keel reaction forces. The analysis results showed that the seabed response to ice gouging is heavily dependent on the soil strata, where the layers interaction effects can override the usual response expected from a uniform seabed. Practical solutions were proposed to improve the pipeline design in layered seabed soils.

Multilayer-perceptron-based prediction of sand-over-clay bearing capacity during spudcan penetration

A method combining a multilayer perceptron (MLP) and large-deformation finite-element analysis (LDFE) analysis is proposed for addressing large errors in conventional formulations and time-consuming numerical simulations for predicting the bearing capacity of sand over clay during spudcan penetration. The method yielded accurate bearing capacity curves, agreeing well with the LDFE and centrifuge test results. The method can be used for quickly and accurately predicting the bearing capacity curves and for generating many various data samples for theoretical studies of the effects of spudcan penetration into sand over clay on the bearing capacity. The critical depth of the bearing capacity curve was revised based on many several samples generated using the method, and the result was used for fitting the bearing capacity curve. The fitted equations in this study yielded high-accuracy predictions for different scenarios.

A large deformation finite element analysis of uplift behaviour for helical anchor in spatially variable clay

Helical anchors are emerging as an innovative foundation design option for offshore applications to resist uplift loading. Numerous studies have been conducted to investigate the uplift behaviour of helical anchor under different helical bearing plate configurations in clay, using field or model tests and numerical analyses. However, almost all of them are limited to deterministic analyses, ignoring the influence of inherent spatial variability of soil. This work conducts a 3D large deformation random finite element analysis to examine the uplift behaviour of helical anchor in spatially variable clay. Both the stochastic and deterministic trends are reflected in the 3D non-stationary random field adopted. It is found that the spatial variability of soil strength significantly influences the failure mechanism of helical anchor. Parametric studies indicate that the coefficient of variation and vertical scales of fluctuation of soil strength can much influence on the uplift capacity, while the effect of horizontal scales of fluctuation is trivial. The goodness-of-fit test of probability distribution suggests that random uplift capacities follow log-normal distribution, whereby the relationship between factor of safety and probability of failure is established. The findings of this paper might facilitate the safety design of helical anchor.

Installation of caisson in non-uniform clay interbedded with a sand layer

Accurately predict the resistance of caisson penetrating in stratified soil deposits is an engineering challenge for application of such foundation system. This paper performs large deformation finite element analysis to investigate the penetration behavior of caisson foundations in non-uniform clay interbedded with a sand layer. The numerical model is validated by comparing with previously testing data, in which good agreement is obtained. Parametric study is then conducted to examine potential influencing factors, including the penetration depth, the top clay layer thickness, clay properties, and the thickness of the second sand layer. It is found that the soil failure mechanism and corresponding penetration resistance of caisson foundations in clay-sand-clay are significantly different from that in pure clay or sand soil deposits, especially for the part in the middle sand layer, where the peak resistance could be overestimated by 10–40% using the current design codes without considering the influence of the weak underlying layer. In this study, based on intensive numerical modeling results, approximating expressions are proposed to describe the unique trends of the resistance profile for caisson penetration in non-uniform clay interbedded with a sand layer soil deposit, which can provide design guide for engineering application.

Evaluation of ice-seabed interaction mechanism in sand by using self-adaptive evolutionary extreme learning machine

Recent discovered oil and gases in the Arctic area have heightened the need for more attention to ice-seabed interaction during an ice scouring event. The seabed is gouged by these drifting icebergs in warmer months threatening the subsea pipelines transferring the hydrocarbons from offshore to onshore. The simulation of ice scouring seabed needs costly large deformation finite element analysis for the guaranteed operational integrity of the subsea pipelines. In this paper, a cost-effective alternative approach using the self-adaptive evolutionary extreme learning machine (SaE-ELM) algorithm was taken to model the ice-induced seabed scour. Initially, using parameters governing the ice gouging process, 17 SaE-ELM models were developed. Then, a comprehensive dataset was established and properly allocated for training and testing of the developed models. The optimal number of hidden layer neurons and the best activation function were opted for the SaE-ELM network. The premium SaE-ELM models and the most influencing inputs were recognized by conducting a sensitivity analysis. The vertical component of load showed a significant impact on the reaction forces, rather the soil depth and berm height possessed a striking effect for modeling the soil displacements. Ultimately, a set of the SaE-ELM-based equations were presented to estimate the subgouge soil parameters.

Large deformation finite element analyses in TBM tunnel excavation: CEL and auto-remeshing approach

Analyzing tunnel excavation behavior involves large deformation issues. For this reason, large deformation analysis method is required to rigorously investigate the actual effect of excavation on surrounding ground. Two major numerical approaches capable of considering large deformations are applied to investigating the effect of tunnel boring machine excavation: coupled Eulerian-Lagrangian (CEL) and the automatic remeshing after small strain approach. The theoretical basis and application of the methods will be discussed before their relative performance is evaluated through the ground response due to TBM excavation. The ground response will be quantified by estimating the range of the excavation damaged zone (EDZ). By comparing the results, the range of the size of the EDZ will be suggested along the vertical and horizontal direction along the TBM advancement. Based on the computed results, it was found that the size of EDZ around the excavation surface and the tendencies was in good agreement among the two methods. Numerical results clearly show that the size of the EDZ around the tunnel tends to be bigger for harder rocks. The size of the EDZ is found to be direct proportional to the tunnel diameter, whereas the depth of the tunnel is inversely proportional due to higher confinement stress around the excavation surface.

Inner soil heave of stiffened caisson during installation in soft-over-stiff clay

The identification of soil heaves inside a caisson during installation is important for the caisson’s design length, since the inner soil heave during the caisson installation affects the final effective caisson penetration depth, hence caisson capacity. This paper reports an extensive parametric study of stiffened caissons penetrating in double-layer of soft-over-stiff clay using large deformation finite element (LDFE) analyses. The LDFE analysis was first validated against the existing results from centrifuge tests and numerical analyses. Then the parametric study was carried out to explore the inner soil movements during stiffened caisson installations with various influencing factors, such as caisson diameter, caisson stiffener width, top-layer soil strength, double-layer soil strength ratio, and roughness of soil-caisson interface. It is found that the caisson diameter, stiffener width, and top-layer soil strength have significant influences on the inner soil behaviors. Besides, the movements of both soil surface and soil layer interface were also investigated. Based on the LDFE results, design formulas are proposed to estimate the inner soil heaves for soft-over-stiff clay. A design formula to estimate the movement of the soil layer interface for soft-over-stiff clays is also reported.

Energetics of cracks and defects in soft materials: The role of surface stress

In classical fracture theory, the resistance to fracture is governed by the fracture toughness Gc, which is defined as the energy required to create new crack surfaces. To propagate a crack, the energy released to the crack tip (i.e. the energy release rate J) must overcome the fracture toughness Gc. Previous studies have demonstrated that surface stress can significantly modify the crack opening displacement and the stress field in soft solids and hence alter the energy release rate J, assuming small deformation. In this paper, we present a theoretical work on how surface stress affects fracture of soft solids under large deformation. We provide an expression of a modified J-integral for the energy release rate that accounts for surface stress. The expression is verified using large-deformation finite element analysis (FEA) with surface stress elements. Our results show that when the elastocapillary length, defined as the ratio of surface stress σ to the Young’s modulus of the solid E, is comparable to the relevant material size, surface stress noticeably blunts the crack opening profile and reduces the energy release rate.

The influence of pipeline-backfill-trench interaction on the lateral soil resistance: A numerical investigation

Large lateral displacement of trenched subsea pipelines may be induced by ground movement, landslides, ice gouging, etc. Pre-excavated seabed soil is usually used for backfilling as a cost-effective solution. The large difference between the shear strength of the highly remoulded backfill material and the native soil may significantly affect the pipeline-backfill-trench interaction, the failure mechanisms, and consequently the lateral soil resistance. However, this challenging and less-explored aspect has not been covered thoroughly in design codes. In this paper, the influences of pipeline-backfill-trench interaction on the failure mechanism and resultant lateral soil resistance were investigated by large deformation finite element analyses. Two different methods were considered and compared, including the remeshing and interpolation technique with small strain (RITSS) and coupled Eulerian-Lagrangian (CEL) methods, to assess their relative merits for this problem. A modified Tresca model considering strain-softening effects was incorporated and a parametric study was conducted to investigate the influences of key factors including the trench geometry, stiffness of backfill material, native seabed soil properties, burial depth, and intensity of pipeline-trench bed interaction. The study showed that the ignorance of the pipeline-backfill-trench interaction by using uniform soil may result in underestimation and overestimation of the lateral soil resistance against the pipeline moving inside the backfill and into the trench wall, respectively.

Depth integrated modelling of submarine landslide evolution

Submarine landslides are a major geohazard among worldwide continental slopes, posing significant threats to offshore infrastructure, marine animal habitats and coastal urban centres. This study establishes an original numerical package for time-efficient modelling of the entire submarine landslide evolution covering the pre-failure shear band propagation, slab failure and post-failure dynamics. The numerical scheme is based on the conservation of mass and the conservation of momentum and combines the shear band propagation theory and the depth-integrated method, with the consideration of the drag force from the ambient water. Shear band propagation in the weak layer and slab failure in the sliding layer are controlled by the strain softening and rate dependency of the corresponding undrained strength parameters. The post-failure behaviour in the sliding layer, such as retrogression upslope and frontally confined and frontally emergent mechanisms downslope, is also simulated. The numerical results from the proposed method are comparable to the analytical solutions and the large deformation finite element analysis. Application of this method to a back analysis of the St. Niklausen slide in Lake Lucerne reproduced the observed shape of the mass transport deposits, the position of the main scar and the travel distance. Because of its easy implementation and efficiency, the proposed numerical method for modelling of submarine landslides seems promising for practical applications.

Evaluating the Penetration Resistance of Spudcan Foundations in Clay Overlying Sand

The response of spudcan foundations during penetration through a soft clay layer toward a sand layer is investigated through large deformation finite element analyses. Relationships for the penetration resistance taking account of the evolv-ing soil failure mechanisms are explored. The findings show that the substantial increase in the penetration resistance is associated with the squeezing mechanism, as the softer soil that is trapped beneath the spudcan base and the stronger sand layer is squeezed horizontally. A simplified expression is proposed to predict the spudcan penetration resistance between the depth at which the squeezing prevails and the clay-sand interface, with its performance compared with centrifuge test results and conventional design approaches.

Scale effect on bearing capacity of shallow foundations on strain-softening clays

Progressive failure is common in many natural clays of strain-softening behavior and may affect the bearing capacity of foundations placed on such soils. The progressive failure might be insignificant for small structures where the mobilized strains are generally uniform within the failed soil mass. This places a demerit to the applications of knowledge obtained from physical model tests to prototype. The study aims to understand and quantify this scale effect on bearing capacity of shallow foundations on strain-softening clays, with a numerical investigation using a large deformation finite element analysis. The progressive failure is numerically replicated and the controlling parameters for the bearing capacity have been discussed. A correction factor is introduced which is inversely proportional to the foundation size normalized by the proposed characteristic length. An empirical relationship of the correction factor and the normalized foundation size has been obtained via the numerical data, which may improve the design of shallow foundations on strain-softening clays.

The pullout capacity of OMNI-Max anchors in clay

Being an impressive representative of gravity installed anchors (GIAs), the OMNI-Max anchor has many attractive attributes and becomes more and more a conspicuous role in recent deepwater applications. However, the pullout capacity property of OMNI-Max anchors has not been well understood due to their three-dimensional structure and the loading way in variational seabed conditions. Full knowledge of the pullout capacity property is beneficial to understand not only the working performance but also the comprehensive kinematic behaviors of OMNI-Max anchors in the seabed. The present work aims to investigate the complicated pullout capacity property of OMNI-Max anchors in clay under multiple factors, including the anchor embedment depth, the anchor orientation, the bearing area, the loading angle, and the soil strength. Large deformation finite element analyses are performed using the coupled Eulerian-Lagrangian (CEL) technique. Numerous cases of multiple factors are designed and analyzed to acquire the effects of different influential parameters. An explicit expression of the end-bearing capacity factor of OMNI-Max anchors is finally formulated in terms of multiple factors, whose effectiveness and applicability are validated by orthogonal tests and available published data. This expression can be simply utilized to evaluate the pullout capacity of OMNI-Max anchors under complicated conditions.

Capacity of plate anchors in clay under sustained uplift

The use of plate anchors on permanent offshore facilities with taut or semi-taut moorings requires a thorough understanding of their long-term capacity under sustained loading. In this paper, a programme of centrifuge tests and numerical simulations is reported to investigate the capacity of a plate anchor deeply embedded in normally consolidated clay during and after vertical sustained loading. Experimental data show that a sustained load higher than 85% of the ultimate monotonic anchor capacity leads to anchor failure, characterised by acceleration of anchor movement with a large accumulation of vertical displacements towards the soil surface. At sustained loads lower than 85% of the monotonic anchor capacity, the undrained anchor capacity increases due to consolidation and hence strengthening of the soil mobilised by the anchor. The maximum increase in capacity relative to the monotonic capacity is 23% when the sustained load is approximately half the monotonic anchor capacity. A gap-filling technique was proposed and incorporated into large deformation finite element analyses, which are shown to reproduce the experimental observations well.

Numerical modeling of keying of vertically installed plate anchor in sand

Pile driven plate anchors are typically oriented vertically during installation. To function properly in a mooring system they must be rotated, or keyed, to an angle normal to the mooring line load. Keying involves reduction in anchor embedment, with concomitant reduced load capacity, so realistic prediction of embedment loss during keying is essential to design. This study investigates the kinematic behavior and load capacity of a vertically installed strip plate anchor embedded in uniform cohesionless soil using large deformation finite element (LDFE) analysis. Embedment loss during keying is evaluated as a function of load eccentricity, load angle, and plate thickness. The model indicates that the plate angle at which maximum pullout resistance occurs increases with increasing eccentricity. The finite element simulations also show the embedment loss during keying decreases with increasing load eccentricity. Loss of anchor embedment increases with increasing pullout angle. The model also predicts that the loss in anchor embedment increases with decreasing anchor thickness, especially when eccentricity is small. The simulations show anchor during keying was essentially independent of the soil elastic modulus to have virtually no effect on keying behavior for shallow anchors, but elastic effects can be significant for deeply embedded anchors.

Estimation of the excavation damage zone in TBM tunnel using large deformation FE analysis

This paper aims to estimate the range of the excavation damaged zone (EDZ) formation caused by the tunnel boring machine (TBM) advancement through dynamic three-dimensional large deformation finite element analysis. Large deformation analysis based on Coupled Eulerian-Lagrangian (CEL) analysis is used to accurately simulate the behavior during TBM excavation. The analysis model is verified based on numerous test results reported in the literature. The range of the formed EDZ will be suggested as a boundary under various conditions – different tunnel diameter, tunnel depth, and rock type. Moreover, evaluation of the integrity of the tunnel structure during excavation has been carried out. Based on the numerical results, the apparent boundary of the EDZ is shown to within the range of 0.7D (D: tunnel diameter) around the excavation surface. Through series of numerical computation, it is clear that for the rock of with higher rock mass rating (RMR) grade (close to 1st grade), the EDZ around the tunnel tends to increase. The size of the EDZ is found to be direct proportional to the tunnel diameter, whereas the depth of the tunnel is inversely proportional to the magnitude of the EDZ. However, the relationship between the formation of the EDZ and the stability of the tunnel was not found to be consistent. In case where the TBM excavation is carried out in hard rock or rock under high confinement (excavation under greater depth), large range of the EDZ may be formed, but less strain occurs along the excavation surface during excavation and is found to be more stable.

Axial sliding resistance of partially embedded offshore pipelines

This paper presents the results of centrifuge model tests and three-dimensional large-deformation finite-element analysis on axial pipe–soil sliding behaviour of partially embedded subsea pipelines. A three-dimensional re-meshing and interpolation technique with small strain approach was adopted using the Abaqus mesh-to-mesh solution mapping feature with the Cam Clay model. The parameters studied include soil properties, interface friction coefficient, embedment depth and post-lay reduction in vertical load. Embedment depth and post-lay load reduction were found to exercise the strongest influence on the post-consolidation and sliding behaviour. Empirical relationships correlating centrifuge experiments and numerical modelling are proposed for the embedment enhancement factor in terms of the embedment and vertical load ratios, under conditions of very slow sliding, corresponding to the drained condition. For very fast sliding, corresponding to effectively undrained conditions, similar relationships involving embedment and vertical load ratios and interface friction coefficients are also proposed. Finally, the variation of embedment enhancement factor with sliding velocity is expressed in terms of a relative enhancement index, Ψ, which can be adequately described by hyperbolic relationships for the complete range of consolidation history. These relationships provide a basis for the evaluation of the embedment enhancement factor corresponding to an arbitrary sliding velocity, which can be used in routine analysis and design of as-laid underwater pipelines in soft clayey soils.

Modelling rainfall-induced landslides from initiation of instability to post-failure

This paper presents a model for rainfall-induced shallow landslides, revealing the failure mechanism from initiation of instability to final flow failure. The onset of slope failure occurs with a completely formed plastic shear band that is triggered by a rainfall-induced increase in pore pressure. The subsequent flow-like deformation is caused by the strain-softening effect associated with large strains. Due to the complex non-linearity in stress-seepage coupling and large deformation, modelling the entire progressive failure induced by rainfall is always a challenge. Therefore, the model in this paper uses a two-step strategy, i.e., coupled hydro-mechanical finite element (FE) analysis for step 1 and large deformation FE analysis for step 2, to realize the simulation of the entire failure process. An equivalent strength method is used to ensure the physical continuity between step 1 and step 2. A more reasonable evaluation of large deformation during post-failure can be achieved without a predefined slip surface. The model is applied to the analysis of two real rainfall-induced landslide cases. Numerical analysis demonstrates the ability of this model to evaluate the progressive variations in pore pressure, displacement, plastic strain, and factor of safety (FOS) with rainfall infiltration and simulate the final flow-like deformation.