Large Deformation Finite Element Analysis Research Articles

Simulation of controlled modulus column installation using a large deformation finite element analysis

Controlled modulus columns (CMC) are widely used in ground improvement techniques where grout columns are constructed by penetrating an auger into the ground using the displacement approach. Numerical simulation of this problem is challenging owing to the large soil deformations associated with installation. Small strain-based finite element analysis procedures available in commercial finite element modeling software are unsuitable for simulating this problem because they cannot simulate large soil deformations around the auger during column installation. Hence, this study presents a numerical approach based on the finite element method to simulate the installation of CMCs. The numerical modeling technique adopted in this study considers large soil deformations around columns during penetration, and the proposed approach is based on remeshing and interpolation techniques. This method was developed using the Python development environment (PDE) within the ABAQUS/standard finite element program. It can penetrate columns below the ground surface without causing large mesh distortions. In this study, silty sand-type soil was considered; hence, the constitutive behavior was simulated using the Mohr–Coulomb criteria. The load-carrying capacity of a controlled modulus column was predicted and compared with an empirical equation based on the cone penetration resistance. The influence zone and failure mechanism of the soil during installation were determined by considering the stress distribution and soil flow pattern around the column.

The pull-out capacity of torpedo anchor in clay considering the full opening cavity

Torpedo anchors are increasingly being considered for mooring floating facilities in deep waters owing to their efficient and economical installation. The anchor was released from a specified height above the seabed to allow it to gain velocity as it fell freely into the water before penetrating the seabed. A cavity was formed above the anchor shaft after the anchor installation. The formation of this post-installation cavity is largely dependent on the soil strength conditions and affects the anchor capacity after installation. However, there is a lack of understanding regarding the formation mechanisms of the cavity and its effect on the bearing capacity of a torpedo anchor. Thus, this study focuses on the effects of the post-installation cavity on the bearing capacity of the torpedo anchor. The impact of a fully open post-penetration cavity on both the vertical and inclined bearing capacity of a torpedo anchor without fins was investigated using the Coupled Eulerian–Lagrangian (CEL) method in ABAQUS with three-dimensional large deformation finite element (LDFE) analysis. A method for evaluating the vertical bearing capacity of a torpedo anchor, considering the effect of a post-installation cavity, was proposed. This study provides insight into the installation effect on the bearing capacity of torpedo anchors.

Effect of padeye offset ratio on keying behaviours and capacity of square plate anchors in clay

This paper reports the effect of padeye offset ratio on the keying behaviours and capacity of square plate anchors in clay. The investigation is conducted through three-dimensional large deformation finite element analyses using the Remeshing and Interpolation Technique with Small Strain in ABAQUS. Strain rate dependency of the undrained shear strength and soil remoulding are accounted for adopting an extended Tresca model. The results from LDFE analyses are compared with existing analytical solutions, centrifuge test data, and numerical results; with reasonable agreement obtained. Parametric analyses are then carried out varying several critical factors including initial embedment depth, load eccentricity, padeye offset ratio, soil sensitivity, and soil strength heterogeneity. It is seen that the padeye offset has significant influence on the ultimate loss of embedment and capacity under vertical loading. To assess the keying behaviour of square plate anchors in clay, an equivalent soil strength incorporating strain rate dependency and remoulding is adopted. A combined factor defined as a function of the initial embedment depth and equivalent soil strength is used to estimate the ultimate loss of embedment. An equivalent ultimate bearing capacity factor is shown to be a function of the initial embedment depth, padeye offset ratio, and load eccentricity.

Influence of shaft diameter and soil rigidity on uplift capacity factor of circular plate anchor in uniform clay

ABSTRACT The helix plate of the helical anchor can provide large uplift resistance for offshore floating structures. Although numerous studies have been conducted to estimate the uplift capacity factor of circular plate anchors in uniform clay, few studies have considered the effects of soil rigidity and shaft diameter on the uplift capacity factor. This paper studied the effects of soil rigidity and shaft diameter on the uplift capacity factor of shafted circular plate anchors in uniform clay through large deformation finite element analysis. The numerical analysis results showed that the increase in soil rigidity increased the uplift capacity factors. The increase in the shaft diameter ratio decreased the projected bearing area on the top of the circular plate anchor and uplift capacity factors. For the convenience of design engineers, a design process based on a conservative solution was proposed to estimate the uplift capacity factor of circular plate anchors.

A large deformation finite element investigation of SEPLA: Failure mechanism and bearing capacities when loaded differently

The Suction Embedded Plate Anchor (SEPLA) is an innovative deep-sea anchoring measure with precise positioning, easy installation, and high anchoring capacity. Most current research on SEPLA revolves around vertical pullout mechanism and thus its vertical bearing capacity. There are relatively few studies concerning its bearing behavior and failure mechanism when SEPLA is subjected to horizontal and moment loading. The latter is practically relevant when SEPLA is exposed to extreme environmental conditions or keyed under preloading. A more in-depth understanding of its failure mechanism under horizontal and moment loading is thereby equally important. In this work, the Coupled Eulerian-Lagrange (CEL) method is used to conduct large-deformation finite element analyses of SEPLA under vertical, horizontal, and moment loading. Emphasis is placed on ascertaining the dissimilarity of soil flow and failure mechanisms when SEPLA is loaded in different directions. Parametric investigation is done to explore the effect of anchor shape and thickness, embedment depth, and soil-anchor interfacial roughness. Semi-theoretical closed form solutions are developed to estimate the uniaxial bearing capacity factor of plate anchors in three directions. These research outcomes are likely helpful in deepening the understanding of SEPLA failure mechanism when loaded in different directions and obtaining more rational estimates of anchor bearing capacity.

A two-dimensional effective stress framework for modelling whole-life soil strength changes due to pore pressure generation and dissipation, Part 2: Applications

Accurate quantification of the temporal changes in seabed strength allows for more reliable and less conservative geotechnical design. A recently developed effective stress framework, established within a one-dimensional computational domain to quantify changes in soil strength due to pore pressure generation and dissipation, has been extended to a two-dimensional (2D) computational domain to allow for consideration of boundary value problems that are too complex to be simplified to one-dimensional conditions. The work to implement the 2D framework is reported across two companion papers. The first of the two papers utilises large deformation finite element analyses to quantify the spatial distribution of accumulated plastic shear strain. These distributions are encapsulated within a strain influence function that is used within the new 2D framework in this paper to calculate the extent and magnitude of excess pore pressure, and in turn the mobilised soil strength for a number of boundary value problems that represent typical offshore geotechnical processes. The merit of the new 2D framework is explored via retrospective simulations of existing experimental and numerical data. The resulting comparisons demonstrate the potential of the new framework, which is in quantifying the reliability of a range of geotechnical structures under complex loading conditions.

A two-dimensional effective stress framework for modelling whole-life soil strength changes due to pore pressure generation and dissipation, Part 1: Formulation

The undrained shear strength of contractive fine-grained soils changes with time, reducing due to pore pressure generation and increasing during consolidation. There is an increasing appetite to recognise these temporal soil strength changes in offshore geotechnical design, as it provides a basis for potentially less conservative designs. Contributions to this endeavour are reported across two companion papers. This first paper extends an existing effective stress framework that relates the generation of pore pressure to accumulated plastic shear strain, allowing undrained shear strength to be calculated within the context of critical-state soil mechanics. The main development is the extension of the computational domain to two dimensions, allowing calculations to be made for boundary value problems that cannot be satisfactorily simplified to one-dimensional conditions. The magnitude and distribution of accumulated shear strain surrounding objects buried in soil are quantified through a series of large deformation finite element analyses. These spatial distributions are described using a strain influence function in the new 2D framework to calculate the extent and magnitude of excess pore pressure, and in turn the mobilised soil strength around the buried object. The performance of the 2D framework is examined in the companion paper through retrospective simulations of experimental and numerical data.

Numerical analysis of the post-installation consolidated response of skirted foundations

The effect of the installation process on the subsequent foundation consolidation response requires assessment when the preload is initially applied. In the framework of effective stress, the influence of large soil deformations during penetration is analysed in this study using a comparison of results from large deformation finite element (LDFE) analysis based on periodic mesh regeneration (Ritss technique) and a 'wished-in-place' small strain finite element (SSFE) model. The LDFE results demonstrate that considerations of the skirt penetration and soil plug bulge lead to different soil excess pore pressure and void ratio change distributions from that of the SSFE analysis, thereby affecting the foundation settlement and consolidated bearing capacity. The normalised fully consolidated undrained bearing capacity grows linearly with the preload ratio for preloads higher than the penetration resistance, and the capacity obtained from the LDFE is higher. The settlement ratio and the proportion of partially consolidated to fully consolidated capacity gain can be represented as a hyperbolic function of a revised dimensionless time factor. The methods integrated in this study can be used to predict the increase in the vertical undrained capacity of skirted foundations after consolidation, accounting for installation effects in engineering practice.

An exponential strength degradation model for clay: From mathematical model to numerical application

Investigation in strain-softening behavior of clay is of great significance for offshore geotechnical engineering. Although several models could be used to characterize the degradation of undrained shear strength of clay with cumulative shear strain, it is necessary to improve accuracy of them. In this study, based on full-flow cyclic penetration test, an exponential strength degradation model with one parameter α is proposed, where α is the function of the fully remolded resistance factor and the ratio of initial extraction to initial penetration resistance. The reliability of the proposed model is verified by comparing indices (strength and sensitivity) calculated by it and by other existing strain-softening models. In addition, a large deformation finite element (LDFE) analysis based on the proposed model is conducted for pipe-clay interaction, and efficiency of the model is verified by comparing numerical results with centrifuge test results. The influence of α on the normalized penetration resistance were discussed, and demonstrate the necessity of accurately evaluating degradation of undrained shear strength of clay.

Damage mechanism and failure risk analysis of offshore pipelines subjected to impact loads from falling object, considering the soil variability

Trench and burial, as a primary and effective protection measurement for offshore pipelines from impact loads, has received much research attention recently. Previous studies were usually performed based on the assumption that the soil material was homogeneous with deterministic mechanical properties. The soil spatial variability, which is demonstrated to have significant influences on the soil capacity in marine geotechnical analysis, has not been included. This study was motivated to investigate the response of the buried pipelines subjected to the impact loads, with special address on the soil variability. Firstly, a three-dimensional random large deformation finite element analysis model was developed, which was implemented by the field variable (FV) technique to map the non-stationary random field (NSRF) into the verified Coupled Eulerian-Lagrangian (CEL) model (Hereafter referred to as FVRCEL). Then the FVRCEL model was integrated with the Monte-Carlo simulation (MCS) to obtain the statistical characteristics of the pipeline structural response. The failure mechanisms of the pipeline in the random soil with different fluctuation scales were investigated, and a parametric study was performed to identify the influential factors. Finally, the failure probability curves and surfaces were presented, providing clues for the pipeline safety design. The results revealed that in general, more than 50 % of the realized NSRF scenarios in the random analysis yielded more severe dent damage than the deterministic result, indicating that the latter would underestimate the damage degree, which was more pronounced when the increasing gradient of soil strength was high. The horizontal fluctuation scale had a remarkable influence on the pipeline damage behaviours and the corresponding statistical characteristics, of which the inner mechanisms were discussed. From the probabilistic perspective, at most an extra failure probability of 75 % would be suffered if the soil variability was ignored.

Effects of soil remoulding on keying behaviours of strip plate anchors with padeye offset

This paper reports the effect of soil remoulding on embedment loss and pullout capacity of strip plate anchors with padeye offset in normally consolidated clay during keying through large deformation finite element (LDFE) analysis. The effects of strain rate and soil remoulding are considered during keying process of plate anchors. First, the LDFE results are compared with theoretical solutions, centrifuge test results, and LDFE data available in the literature, which shows a good agreement. Then, the influences of initial embedment depth, load eccentricity, padeye offset and soil sensitivity on the embedment loss and pullout capacity of strip plate anchors during keying are investigated. The LDFE results show that the padeye offset and the soil remoulding have significant effects on the ultimate embedment loss and pullout capacity. To assess the keying behaviour of plate anchors, an equivalent soil strength is introduced. A combined factor and an equivalent ultimate pullout capacity are adopted to estimate the embedment loss and ultimate pullout capacity of strip plate anchors in normally consolidated clay, respectively.

Numerical study on keying capacity of gravity installed anchors in sand

Abstract Being the latest representative of GIAs, the OMNI-Max anchor performs comprehensive behaviors in the seabed, like keying and diving, by adjusting orientation and position to derive higher capacity and therefore to avoid anchor failure. Current research of OMNI-Max anchors is concentrated on clay, while there is a notable gap of the work on sand. During the keying process of GIAs, many factors may influence the keying capacity, such as the embedment depth, the anchor orientation, the rotational center, the bearing area and the soil strength. Large deformation finite element analyses combined with a bounding-surface plasticity constitutive model are performed to investigate systematically the keying behavior of GIAs. A series of analytical cases involving multiple factors are designed and analyzed to explore the effects of various factors on the keying capacity of GIAs, defined by the soil resistance coefficient during keying. The soil resistance coefficient increases with increasing soil density, while it tends to be stable with the increase of the embedment depth. The closer the rotational center approaches to the two ends of the anchor, the greater the soil resistance coefficient becomes. An explicit expression of the soil resistance coefficient during keying is derived to quantify the effects of various factors. These findings are helpful to understand further the comprehensive anchor behaviors and to promote the application of GIAs in offshore engineering.

Large deformation finite element analyses of axially loaded helical piles in cohesive soil

Large deformation finite element (LDFE) analysis is suitable for research on soil-pile interaction. The objectives of this study were to develop an LDFE model for axially loaded double-helix piles and to examine the factors that control their failure modes. The model was implemented with the re-meshing and interpolation technique with a small strain approach. Three types of helical piles in clay modeled as a Tresca material were simulated in an axisymmetric setup. The model was calibrated against the results of centrifuge model tests. Analyses show that the failure mode changed gradually from a cylindrical shear mode (CSM) to a transitional failure mode and then an individual bearing mode (IBM) with increasing inter-helix spacing. Current results suggested that there might not be a unique inter-helix spacing ratio that distinguishes CSM from IBM. Further, the effects of helix embedment depth, inter-helix spacing, and undrained soil shear strength on the failure modes were assessed by parametric analyses. Finally, the failure modes and helix-bearing factors, shown to depend on these parameters, were summarized in several charts for potential practical applications. Results suggested that the lower helix's bearing factors remained unchanged, but the upper helix's bearing factors changed with the embedment depth and failure modes.

Numerical investigation of the keying process of a new type of expandable plate anchor

High efficient anchors are increasing demanded to moor floating structures with oil and gas and renewable energy developments move into deeper water. One effective way to increase efficiency is by maximizing the bearing area of the anchor during operation. Along this line of design, a new high efficiency plate anchor composed of two pieces of fluke plates, which are expandable after installation under preloading, is proposed in this paper. The keying process plays a significant role in the anchor's bearing performance. To gain insights into the keying process and optimize the geometry design of this anchor, a series of Large Deformation Finite Element (LDFE) analyses are carried out. From the comprehensive LDFE analysis, this proposed anchor with optimal padeye eccentricity has lower loss of embedment compared with traditional upright anchor and the accompanying higher holding capacity. Besides, the newly proposed expandable anchor also has a much stiffer response, the anchor can bear 3 times the keying load (proof loading) without losing much embedment during operation stage. This has an important implication for saving offshore cost, where the proposed anchor could be keyed using small vessel with limited winch capacity.

Large-Deformation Finite-Element Analysis of Square Foundations in Spatially Variable Sediments

Square foundations often penetrate deeply into seabed sediments so that sufficient bearing capacity is obtained, which inevitably induces large-scale deformation in the surrounding sediments. Most of the previous research on square foundations has assumed the sediment had a uniform or linearly increasing strength profile with depth and has ignored the spatial variability of sediments. This work therefore focused on the influence of sediment spatial variability on the behavior of square foundations by applying a random large-deformation finite-element (LDFE) approach. The LDFE results indicated that the sediment heave effect could increase the bearing capacity by up to 15%. This underscores the advantages of LDFE analysis for bearing capacity predictions. More importantly, the mean bearing capacity from random analyses is smaller than the deterministic solution in uniform sediments. Consequently, deterministic analysis yields an overestimated bearing capacity. Finally, a framework was established for estimating the probability of failure at various depths considering a random sediment strength based on a factor of safety (FOS). This indicated that the partial FOS recommended by the Det Norske Veritas code is equivalent to a probability of failure of 0.1%. Our findings may facilitate the evaluation of the probability of failure for deterministic foundation design based on the FOS.

Interpretation of shear strength of cone penetrating in double layered clays considering the scale effect in centrifuge testing

Cone penetration test (CPT) has been commonly applied as a site investigation tool and a laboratory measurement method in centrifuge tests to study soil properties. In this research, large deformation finite element (LDFE) analysis is utilized to realize the simulation of continuous cone penetration in two-layered clay deposits in centrifuge testing, with the aim of better interpreting the shear strength of cone penetration testing data. The numerical model is validated against previous published data, where good agreement is obtained. From LDFE results, it is concluded that for stiff-over-soft clay, the measured peak resistance in top layer is underestimated and the resistance curve in bottom layer is overestimated at the transitional depth where the stiff soil is trapped below the conical surface of cone. However, for soft-over-stiff clay, owing to the insufficient thickness for cone to reach its full resistance, the effect of top layer thickness needs to be considered. Finally, based on intensive numerical modeling results empirical formulae are proposed to quantify the prediction errors and to obtain a more accurate prediction for the soil profile using CPT data in centrifuge tests.

Applicability analyses on the end-bearing capacity of gravity installed anchors in clay

As an efficient solution for deepwater moorings, gravity installed anchors (GIAs) have many attractive attributes and play a remarkable role in various deepwater applications. The end-bearing capacity factor offers an efficient method to evaluate GIA end bearing under complex conditions involving multiple factors. It can also assess total anchor bearing capacity. Based on numerous large deformation finite element (LDFE) analyses, two formulas have been developed to calculate the end-bearing capacity factors for GIAs in clay under quasi-static or dynamic conditions. Through rigorous statistical analysis, it is demonstrated that the formulas provide accurate predictions of the end-bearing capacity for various GIAs, particularly those with loading characteristics similar to OMNI-Max anchors, i.e., the padeye location and the loading angle. The contribution of the end bearing to the overall bearing capacity of GIAs is analyzed under various loading conditions. Furthermore, a comparative study proves that the formula accounting for the loading rate effect can be degenerated to evaluate the end-bearing capacity factor of the anchors under quasi-static conditions. The results provide a comprehensive understanding of the end bearing and offer an efficient solution for determining the total bearing capacity of various GIAs in clay under complex conditions. These findings contribute to the advancement of design and performance assessment of GIAs, enhancing their reliability and effectiveness in applications.

Torsional installation and vertical tensile capacity of helical piles in clay

Helical piles are being increasingly considered for offshore applications as they avoid the acoustic emissions associated with pile driving, and they provide additional capacity relative to a driven pile. In this paper, the installation and tensile capacity of helical piles is considered through a combination of centrifuge experiments and large-deformation finite-element analyses within a coupled Eulerian–Lagrangian framework. The experiments provide a basis for validating the numerical simulations, but also quantify the expected installation torque and undrained tensile capacity, including the variation with time after installation. The numerical simulations extend the parameter space investigated experimentally, considering the number of helices, their spacing and pitch, in addition to the ratio of pile shaft to helix diameter and the profile of undrained shear strength. Mechanisms revealed through the numerical simulations are reflected in a new analytical model for calculating undrained tensile capacity, which is seen to agree reasonably well with the numerically and experimentally determined capacities.

A particular manner to observe free-edge effects in hybrid elastomer/composites plates

Hybrid elastomer/composite sandwich plate structures are typically used where simultaneously both lateral stiffness and shear flexibility are required. In these hybrid structures, however infinitesimal strains take place in laminated composite layers, but elastomer layer is under hyper-elastic strains. The present work aims to adapt an individual two-step technique to examine three-dimensional stress field in this plate to observe free-edge effects in composite layers. For this purpose, at the first step, employing elastic equivalent properties for the composite layers and nonlinear elastic properties for elastomer layer, large deformation finite element analysis is performed for the hybrid structure by ameliorate an available open source code named FLagSHyp. This analysis provides the load distribution at elastomer-composite interface. Next, using this external load distribution on composite layers, based on Kantorovich technique a detail analysis is proposed to study free edge effect in composite layer. The results are verified numerically and/or experimentally and vital observations involving free-edge effects are reported.

Numerical study on the performance of a novel gravity installed anchor with ring fins in clay

A novel gravity installed anchor with ring fins (RGIA) is proposed in the present work, which combines features of the conventional gravity installed anchor (GIA) and the drag embedment plate anchor, with two soft shanks attached to the anchor body. A systematic study on the performance of the RGIA in clay is performed by large deformation finite element analyses using the coupled Eulerian-Lagrangian technique (CEL), with emphasis on the initial penetration, the behaviors of keying, diving and pulling out, the optimization of the shackle point, and the effect of loading rates on the maximum embedment loss (MEL). The initial penetration of the anchor is simulated under various impact velocities, from which the maximum initial embedment depth in the seabed is acquired. Two factors of the shackle point, en and eh, are defined and the detailed criteria for optimizing the shackle point are put forward. A series of analytical cases are designed to thoroughly explore the effects of the shackle point on the performance of the RGIA in clay, focusing on the keying and diving behaviors and the pullout capacity. The formulas for calculating the diving angle and the MEL are then constructed with high precision, by taking into account the effect of the shackle point, and can be utilized to optimize the position of the shackle point. Comprehensive drag effects under multiple factors, especially the high loading rate, on the performance of the RGIA in clay are quantitatively investigated by performing six groups of cases, from which the effects of drag velocity, initial embedment depth, gradient of shear strength of clay, and cable parameters can be known. The formula for calculating the MEL of the RGIA in clay is also constructed with high precision by taking into account comprehensive drag effects. The numerical study demonstrates that, compared with the OMNI-Max anchor, the RGIA achieves deeper initial penetration depth during installation, requires shallower initial penetration depth that ensures subsequent keying and diving, and possesses stronger ability of resisting dynamic loading with high loading rates.