Large Deformation Finite Element Model Research Articles

Optimization of spudcan penetration and influence on caisson foundation

The penetration of spudcans have adverse effects on pre-existing bucket foundations during the blade lifting process of offshore wind power systems, which brings uncertainties for the design and construction of bucket foundations. This study employs the Euler-Lagrange method to explore the influence of spudcan penetration on the adjacent caissons in homogeneous clay within a large deformation finite element (LDFE) model. The LDFE model is validated against centrifuge tests data, with good agreement obtained. Parametric study is then conducted to explore, (i) the influence of the angle between vessel and caissons on the movements of the caisson, and (ii) potential influence factors on the movements of the caisson with an optimal angle between the vessel and caissons, including soil properties, dimensions of caisson and spudcan, and the distance between spudcan and caissons. The results show that 60° is the optimal angle between the vessel and caissons that results in the smallest movement of the caisson, and the caisson foundation inclination angle ranges between 0° and 0.025°, when the pile shoe penetration progress ranges between 0.1 and 1.0. An empirical formula is proposed to predict the rotational angle of caisson with the optimal angle. The study establishes a quantitative approach to assess the influence of the spudcan penetration on existing three-caisson jacket foundations, which provides guidance for the design and construction of offshore wind turbine bucket foundations.

Determination of the overconsolidation ratio and undrained shear strength of cohesive soils by CPTu measurement

Estimations of the undrained shear strength (Su) and overconsolition ratio (OCR) of cohesive soil are essential for the evaluation of geology investigation. In comparison to time-consuming laboratory tests, the field piezocone penetration test (CPTu) can rapidly evaluate these parameters. However, existing studies have mainly relied on empirical correlations to estimate the undrained shear strength Su and OCR from CPTu measurements. These correlations are mainly empirical, involving fixed coefficients multiplied by CPTu measurement. Based on the coupled Eulerian-Lagrangian method, this study employed large deformation finite element modeling to summarize and extend the relationships between CPTu measurement and the estimation of undrained shear strength Su and OCR in cohesive soils. These relationships allow for predictions based on CPTu measurements without the requirement for laboratory calibration or referencing to field benchmarks. The accuracy and applicability of the proposed prediction formulas are validated through field case studies for comparative analysis.

An efficient approach to tackle the challenges of LDFE modelling of plate anchor-chain system during installation process

This technical note presents an efficient approach to tackle the challenges in large deformation finite element (LDFE) modelling of plate anchor-chain system during installation process. This is realised by using a local coordinate system based on the displacement-control technique, where the chain loading direction is updated compatibly with the anchor behaviour at each calculation increment. The effectiveness of the approach is demonstrated through two analysis examples that model the anchor installation (keying) process. The results agree well with established analytical solution based on plasticity analysis, which was developed based on deeply embedded anchor. The LDFE results numerically testify that plate anchors with well-designed padeye position are able to dive into deeper seabed under chain loading, where this concept was only suggested in previous analytical and plasticity analysis.

Curly beam with programmable bistability

Bistable beams with prebuckled sinusoidal shape are prominently constructed into mechanical metamaterial unit cells with bi- and multi-stability. While the effects of boundary conditions and thickness on their bistability were more studied, few investigations have been focused on the effect of the beam shape. We systematically created new geometries by adding a second sinusoidal term varying in amplitudes and wavelengths, onto the prebuckled bistable beam shape. Nonlinear large deformation finite element modeling showed effective tuning of the forward and backward forces, snapping energy, and the maximum strain, by the wavelength and amplitude. Decent improvements in the overall stability of their second stable state were observed. The enhancement of the relatively weak snap-through behavior of the single beams was demonstrated by pairing a large variety of curly beams into double-beam configurations. Experimental observations on the 3D printed polylactic acid unit cells strongly supported the simulation results, and demonstrated the possibility of building multistable structures. The design of bistable beams with consistent thickness and comparable overall shapes, yet differing mechanical behavior, has opened up new avenues for the development of bi- and multistable structures in programmable mechanical metamaterials, particularly with materials fabricated at the resolution limit of the respective techniques.

High actuation capability and smooth-deformation piezo morphing wing based on multi-layer parallel pre-compressed MFC bimorph

A single macro-fiber composite (MFC) unimorph or bimorph wing can only be applied to micro aerial vehicles (MAV) at speeds lower than 20 m/s owing to its relatively small output work. In this study, a high-actuation MFC pre-compressed bimorph actuator (MFC-PBA) multilayer parallel morphing wing is proposed for application in high-speed miniature unmanned aerial vehicles (mini-UAVs). The morphing wing adopts the proposed ribs with an arc groove and three parallel layers of MFC-PBAs to improve the output work and guarantee smooth morphing wing surface. The large deformation finite element (FE) models of a single MFC-PBA and its multilayer parallel morphing wing are established to evaluate the output deformation and output force. Subsequently, a prototype of the morphing wing is fabricated, and the actuation capabilities of the single MFC-PBAs and morphing wing are tested. The experimental and FE simulation results satisfactorily agree. The results show that at the peak driving voltage, with axial compression forces of 18 and 27 N applied to the side and middle MFC-PBAs, respectively, the deflection angle of the morphing wing reaches ±12°, which is twice that of the morphing wing without compression; and its blocking force is maintained at 1.682 N; thus, its output work has doubled. The multiphysics model of the proposed morphing wing under airflow is established to evaluate the deformation capability. The results show that the present morphing wing achieves a deflection of 5.04° on the side of the incoming flow direction, and a unit lift of 249.1 N/m at an air speed of 40 m/s and angle of attack of 15°. Consequently, the proposed morphing wing exhibits a high deformation capability. We expect the proposed morphing wing will be used for the flight control of mini-UAVs at higher speeds and larger sizes.

Investigation of the installation process of drag-in plate anchors from LDFE modelling

The installation of drag-in plate anchors is investigated using advanced large-deformation finite-element (LDFE) analysis. A new modelling technique that can realise the continuously evolving loading angle on the anchor line is described. The entire anchor trajectory is simulated until the anchor reaches an ultimate state after dragging for a very large distance, typically in the range of 40–70 times the anchor width. An installation in simple uniform and non-sensitive soil is first presented with agreement against well-established plasticity analysis and an analytical solution offering verification of the techniques developed. The effects of anchor padeye position, seabed soil conditions, anchor line properties and the anchor–soil interface on the installation response are then discussed. The findings highlight that plasticity and analytical approaches are not necessarily conservative when the yield surface derived from deep embedded anchors is applied in shallow conditions. The LDFE results also demonstrate the importance of accounting for soil remoulding around both the anchor and anchor installation line. The results of this study provide insights for offshore engineers to consider and incorporate when estimating a drag anchor installation trajectory and holding capacity.

Numerical analyses of energy balance and installation mechanisms of large-diameter tapered monopiles by impact driving

Large-diameter monopiles are widely used as the foundation to support offshore wind turbines (OWTs) in shallow coastal waters. The benefits of small-to-medium diameter tapered piles have been reported in the past. The potential use of large-diameter tapered monopiles installed by impact driving to support OWTs is thus presented, and then comparatively assessed by numerical analyses in terms of energy balance and installation mechanisms. A three-dimensional large deformation finite element (3D-LDFE) model of monopiles driven in clay was developed using a Coupled Eulerian-Lagrangian (CEL) approach. An advanced user-defined hypoplasticity clay (HC) model was employed to model undrained kaolin clay, featuring nonlinear behavior from small strain to large strain. The force-time curve defined by the operating data of a state-of-the-art hammer in the offshore industry was inputted to explicitly model impact driving. Better agreement between the measured and the simulated results was observed to validate the accuracy of the numerical model. The numerical results obtained give greater confidence to the future use of large diameter tapered monopiles for OWTs.

Numerical investigation of caisson with pad-eye stiffener installation into nonhomogeneous clay

Caisson foundations are widely used in offshore engineering. During installation, the soil flow mechanism of stiffened caissons with local thickening at the pad-eye is still not well understood. In this study, large deformation finite element (LDFE) model is established to investigate the penetration behavior of caisson with pad-eye stiffener, where the soil flow mechanism is studied considering soil strain softening effect. The numerical model is validated by comparing with available testing data. Parametric study is then conducted to examine potential influencing factors, including the penetration depth, the geometry of stiffener, the clay shear strength, and strain softening parameters. It is found that the soil failure mechanisms and the corresponding penetration resistances of locally stiffened caisson at the pad-eye are significantly different from that of unstiffened caisson or stiffened caisson with interval rings. Two parameters, i.e. the critical rotational soil flow depth (Hr) and the limiting cavity depth (Hc) are introduced to quantitatively describe the behavior of the observed soil flow mechanisms. Based on the modeling results, a simplified flow mechanism is proposed, and approximating expressions are derived to predict the penetration resistance of the caisson for engineering design application.

Upslope Failure Mechanisms and Criteria in Submarine Landslides: Shear Band Propagation, Slab Failure and Retrogression

AbstractThe volume of a submarine landslide is likely to be amplified by shear band propagation along a basal surface or spreading failure extension in the sliding layer. Although the mechanism of translational submarine landslides has been understood using the interpretation of shear band propagation, assessment of the limits of failure extension, either through shear band propagation or retrogressive spreading, remains a challenge due to the diversity of post failure mechanisms. The paper aims to explore all post failure possibilities and quantify failure initiation and extension accordingly, focusing on the upslope conditions. The different failure mechanisms are explored with the aid of the large deformation finite element modeling. Original criteria for scarp position, shear band length at slab failure, retrogression conditions and ultimate spreading limits are proposed, following the process zone approach considering force equilibrium of the sliding mass. Simplified analytical predictions are shown to agree well with the numerical observations. The findings and criteria presented in the study are expected to help assess the ultimate scale of upslope failure in submarine landslides, which is important in determining geohazard zonation of submarine landslides.

Spudcan-pile interaction in thick soft clay and soft clay overlying sand: A simplified numerical solution

A drilling jack-up is often deployed to perform drilling activities or well intervention at an existing fixed jacket platform. Depending on the clearance distance between the aft spudcan and the nearest pile, the installation of the spudcan may have an adverse effect on the platform pile which in turn could compromise the integrity of the jacket structure. This paper presents a simplified method for assessing the effect of spudcan installation on the pile foundation of a jacket platform in thick soft clay and soft clay overlying sand layers. Adopting a two-stage uncoupled approach, in which predetermined free-field lateral soil displacements are applied to a beam-column model with soil modelled using p-y curves, the soil displacements around a penetrating spudcan is estimated by an analytical solution derived from spherical cavity expansion theory. With appropriate consideration for the volume of soil displaced by the penetrating spudcan for the cases of thick soft clay and consideration for soil squeezing for soft clay overlying sand, the computed free-field lateral soil displacements are in fair agreement with the measured profiles in centrifuge tests and can be used in a beam-column model to estimate the induced bending moment in the pile in practice.The pile response to the free-field lateral soil displacements is assessed using a beam-column model incorporating p–y curves, considering the effect of soil remoulding caused by the installation of the spudcan where appropriate. A numerical model which considers the evolving lateral soil displacements due to the progressive advancement of the penetrating spudcan is also studied. The computed pile bending moment profiles are consistent with the measured profiles in the centrifuge tests and those computed by 3D large deformation finite element (LDFE) model. The proposed practical method is simple but yet effective to assess the influence of the installation of a spudcan foundation on an adjacent pile of a jacket platform in thick soft clay and soft clay overlying sand.

Large Deformation Finite Element Modeling of Rubble Mound Breakwater Built on Soft Seabed Using Coupled Eulerian–Lagrangian Method

For the design of rubble mound breakwaters on soft soil, it is essential to predict the behavior of soft soil and large deformations phenomena occurring in the course of construction of the rubble mound breakwater. Large deformations in various problems can be well simulated using the coupled Eulerian–Lagrangian (CEL) method. In this study, the CEL method has been used to simulate the rubble mounds construction on soft soil and predict the resulting settlements. To validate the numerical model, the results of three experiments conducted in the physical modeling laboratory at Kharazmi University were used. Also, two case studies of real rubble mound breakwaters constructed on soft seabeds were numerically simulated and the results of the numerical model were compared with the recorded data. According to the obtained results, the finite element analysis by the CEL method is able to predict the deformations and settlement occurred during construction of rubble mound breakwaters on soft soils with a reasonable accuracy. In very soft soils with undrained shear strength less than 10 kPa, the CEL simulation outperformed the Lagrangian formulation. Then, parametric study was performed and the effects of height, crest width, breakwater slope and thickness and strength of the soft soil on the amount of rubble mound settlement were investigated.

Scale effects during cone penetration in spatially variable clays

Cone penetration testing (CPT) has become one of the most commonly adopted in situ tests for site investigations, due to its reliability and repeatability of the measurements and the potential for direct use in design. For CPTs in clays, previous studies have paid considerable attention to the selection of penetration resistance factor Nkt considering the effects of soil stiffness, stress and strength anisotropy, soil layering and soil sensitivity, and so on. In contrast, this paper focuses on the effects of spatially variable soil properties on the interpretation and use of CPT data. This has been achieved using large-deformation finite-element modelling with random fields. It is shown that when using CPT data to obtain the true point-to-point statistics of the ground, scale effects could lead to the (unconservative) overestimation of the low estimate and underestimate of the high estimate for variable ground with small scales of fluctuation. Suggestions are made for correcting CPT-measured data to allow for this effect. In contrast, if using CPT data to provide design inputs for (larger) foundations, the same scale effects may be considered to increase beneficially the characteristic values of soil strength for sizing and reduce characteristic soil strengths for installation assessment.

Discussion: A review on the behaviour of helical piles as a potential offshore foundation system

Helical piles have emerged as an attractive foundation system for offshore applications with renewed interest from the offshore community. Significant research gap currently exists in transferring this technology offshore and this paper discusses how existing and emerging knowledge can be successfully used to bridge some of the gaps. We focus on the Coupled Eulerian Lagrangian (CEL) large deformation finite element (LDFE) modelling technique that is commercially available and can be used to model the three-dimensional installation process with consideration of strain rate and softening effects in soft offshore clays. A helical pile of L = 7.5 m long is modelled with one or two large-diameter helices (D = 2 m) attached to a central shaft of d = 0.5 m in diameter.The net effect of strain rate and softening is to increase the installation torque. The measured torque is within the range of 200–400 kN.m for the offshore clay and the pile geometry studied. Additional helices increase the uplift force but to a lesser degree than that of the measured torque. Remoulding induced strength reduction is found to be within the range of 25–33% of the intact clay strength. Issues of extracting and reusing offshore helical pile foundations are discussed.

Implementation of a large deformation finite element modelling technique for seismic slope stability analyses

Post-slide investigations show large displacement of failed soil mass in many earthquake-triggered landslides. An Eulerian-based finite-element modelling (FEM) of large deformation of soil, for pseudostatic and dynamic loadings, is presented in this study. The Eulerian FEM is compared with Lagrangian-based explicit and implicit finite-element (FE) modelling approaches. The dynamic FE modelling of two hypothetical slopes for eight earthquake acceleration–time histories show that the failure surfaces develop progressively, which cannot be modelled using the traditional limit equilibrium method (LEM). A large plastic shear strain concentration (i.e. shear band formation) occurs when the strain-softening behaviour of soil is considered. The similarities and differences between the results of dynamic and pseudostatic FE analyses based on estimated pseudostatic coefficient from acceleration–time records are presented. The duration of an earthquake influences the failure process and displacement of the failed soil mass. The displacement of the toe obtained from FEM is compared with Newmark's simplified approach. The developed Eulerian-based FE modelling technique has been used to simulate large-scale landslides in sensitive clays due to earthquake loading [1].

Vertical Pullout Behaviour of a Torpedo Anchor Vertically in Stiff-Over-Soft Cohesive Soil Bed

This paper reports the results from a large deformation finite element (LDFE) model to provide insights into the vertical pullout behaviors of torpedo anchors in stiff-over-soft soil beds. 10 sets of numerical data were obtained by varying the top layer thickness, soil shear strength of the stiff-over-soft soil bed, tip embedment depth ratio, and pullout speed. The tip embedment depth ratio of the torpedo anchor (the ratio of the embedment depth at anchor tip to the anchor shaft length) varied from 1.48 to 2.91. Soil flow characteristic was discovered by the finite element analysis. The observation of soil flow characteristics provided deep insight with the interaction between the torpedo anchor and its surrounding soil.

Optimising the dimensions of Suction Embedded Anchors by investigating the opening mechanism

This paper aims to optimise the dimensions of Suction Embedded Anchors (SEAs) by investigating the opening mechanism through numerical large deformation finite element (LDFE) modelling. Consisting of two hinged half-cylindrical flukes, an SEA anchor uses a tactical design to allow the flukes to open under preloading to achieve more bearing area and, thus, a higher holding capacity. However, the applicability and practicability of this anchor depends on the opening process, where a critical dimension is the ratio of the anchor height relative to the diameter. A longer anchor increases both the forces prompting the opening process and the forces resisting it. The initial prompting force is the friction along the flukes and pressure on the wall annulus, while the initial resisting force is the soil pressure on the fluke faces. The LDFE results of this study demonstrate that these two components are in different orders of magnitude. Therefore, increasing the anchor height does not necessarily always increase the holding capacity but can have a detrimental effect on the holding capacity. The optimal anchor height is found to be ∼0.75 times the diameter. The detailed opening process and mechanism are demonstrated in the systematic LDFE analyses presented in this paper.

Toward a quick evaluation of the performance of gravity installed anchors in clay: Penetration and keying

Gravity installed anchors (GIAs) are the most recent generation of anchoring solutions to moor floating facilities for deepwater oil and gas developments. Challenges associated with GIAs include predicting the initial embedment depth and evaluating the keying performance of the anchor. The former involves high soil strain rate due to large anchor penetration velocity, while the later influences the subsequent behavior and pullout capacity of the anchor. With the coupled Eulerian–Lagrangian method, three-dimensional large deformation finite element models are established to investigate the penetration and keying of GIAs in non-homogeneous clay. In the penetration model, a modified Tresca soil model is adopted to allow the effects of soil strain rate and strain softening, and user-defined hydrodynamic drag force and frictional resistance are introduced via concentrated forces. In the keying model, the anchor line effects are incorporated through a chain equation, and the keying, diving and pulling out behaviors of the anchor can all be replicated. Parametric studies are undertaken at first to quantify the effects of various factors on the performance of GIAs, especially on the penetration and keying behaviors. Based on the results of parametric studies, fitted formulae are proposed to give a quick evaluation of the anchor embedment depth after the installation, and the shackle horizontal displacement, shackle embedment loss and anchor inclination at the end of the keying. Comparative studies are also performed to verify the effectiveness of the fitted formulae.

Keying behavior of suction embedded plate anchors with flap in clay

Suction embedded plate anchors (SEPLAs) have been applied extensively in deepwater projects due to their high efficiency and low cost. In this paper, a new plasticity model was developed to simulate the keying behavior of SEPLAs. The model was based on three groups of yield envelopes, which considered three different states between the flap and the main plate. Thus, the plasticity model can consider the flap rotation relative to the fluke. The anchor shank was also considered to investigate the three dimensional (3D) effects of the actual SEPLA shape when deriving the yield envelops. A three dimensional large deformation finite element (3D-LDFE) model with flap rotation and shank considered was also set up. Then 3D-LDFE analysis was carried out to verify the plasticity model. The results given by 3D-LDFE analyses agree well with those by plasticity analyses. The effects of the flap on the embedment loss and pullout capacity, and the effects of loading eccentricity and pullout angle on diving behavior are investigated. Optimal padeye offset and pullout angle to drive the anchor diving are proposed.

Effect of surface tension on the adhesion between a rigid flat punch and a semi-infinite neo-Hookean half-space

We study the effect of surface stress on the retraction of a rigid flat circular punch adhering to an incompressible neo-Hookean half space using a large deformation finite element model (FEM). The effect of surface stress is determined by the dimensionless elasto-capillary number, ω=σ/(Ea), where σ is the isotropic solid–air interface surface stress which we shall called surface tension, E is the small strain Young’s modulus and a is the radius of punch. Adhesion of the punch to the half space is characterized by work of adhesion Wad. Within the range of retraction displacement of this work (less or equal to the punch radius), the stiffness of the system is insensitive to retraction displacement and depends only on the elasto-capillary number. In general, surface tension increases the slope of load–displacement curve or the stiffness of the system. For a given work of adhesion, the pull-off force (displacement) increases (decreases) with surface tension. Our results show that the apparent contact angle is not 90 degrees, as predicted by linear elastic theory, but varies from 20 to 180 degrees and is very sensitive to ω when it is near zero. This result can be potentially used to measure solid surface tension.

Interpretation of Layer Boundaries and Shear Strengths for Soft-Stiff-Soft Clays Using CPT Data: LDFE Analyses

AbstractThis paper describes a new approach for interpreting cone penetrometer data in soft-stiff-soft clay deposits. The identification of layer boundaries and interpretation of shear strength profile were of particular interest. The proposed approach was based on an extensive parametric study using large deformation finite-element (LDFE) analyses, with a standard cone penetrometer penetrated continuously from the soil surface. The LDFE model has been validated against existing theoretical solutions and numerical results, with good agreement obtained. Regardless of strength ratio between two successive layers, the interface of soft-stiff layers can be identified at 0.8D (D is the cone diameter) below the kink in penetration resistance curve in the top soft layer. The interface of stiff-soft layers can be demarked at 1.3D above the kink in the penetration resistance profile in the bottom soft layer. The undrained shear strength of a soft clay layer can be interpreted using a single-layer approach and the ...