1,721,046 research outputs found
Implicit implementation of the Prevost model: 25th ALERT Workshop
No full textThe Prevost model for cohesionless soils (Prevost 1985) is currently used for the modelling of their cyclic behaviour, especially in earthquake engineering (Zerfa and Loret 2003) and (Yang and Elgamal 2008). It’s made of conical yield surfaces that allow for plastic deformations in both loading and unloading. Its non-associated volumetric flow rule can roughly take into account pore pressure build-up and cyclic mobility. The method of implementation of a constitutive law is a crucial issue (Montáns and Caminero 2007) and (Mira et al. 2009). On one hand the accuracy of the solution must be ensured, especially when a large number of cycles are considered. But on the other hand, the cost of the computation must be minimized which implies step size as large as possible. In this paper, an implicit implementation of the Prevost model is proposed. The plastic flow rule is computed through a trapezoidal rule, at the time step n + 1/2, in order to increase the accuracy.An iterative process has to be solved, due to the implicit formulation.The Jacobian matrix of the internal Newton-Raphson process is computed analytically to enhance efficiency. The algorithm is implemented in the finite element code LAGAMINE that carries out fully-coupled analysis. Simulations of triaxial tests are compared with “exact” solution
Numerical modelling of transient cyclic vertical loading of suction caissons in sand
Full text linkpeer reviewedThis paper presents numerical investigations of the monotonic and cyclic behaviours of suction caissons upon vertical transient loading. Both drained and partially drained conditions are investigated. Monotonic compression and traction simulations are carried out to qualitatively compare results with the literature and validate the model. They highlight the different modes of reaction of the caisson to both compression and traction loading. A sensitivity analysis points out the strong influence of some parameters on the resistance of the caisson but also on the failure mechanism. The transient behaviour of the caisson upon different kinds of cyclic load signals is analysed. Results reproduce the settlement and pore water pressure accumulations observed during experiments. The influence of the key design parameters on the settlement accumulation is also assessed. Finally a cyclic diagram is proposed to describe the evolution of the final settlement upon different magnitudes of loading
A fully coupled hydro-mechanical model for the modeling of coalbed methane recovery
Full text linkpeer reviewedMost coal seams hold important quantities of methane which is recognized as a valuable energy resource. Coal reservoir is considered not conventional because methane is held adsorbed on the coal surface. Coal is naturally fractured, it is a dual-porosity system made of matrix blocks and cleats (i.e fractures). In general, cleats are initially water saturated with the hydrostatic pressure maintaining the gas adsorbed in the coal matrix. Production of coalbed methane (CBM) first requires the mobilization of water in the cleats to reduce the reservoir pressure. Changes of coal properties during methane production are a critical issue in coalbed methane recovery. Indeed, any change of the cleat network will likely translate into modifications of the reservoir permeability. This work consists in the formulation of a consistent hydro-mechanical model for the CBM production modeling. Due to the particular structure of coal, the model is based on a dual-continuum approach to enrich the macroscale with microscale considerations. Shape factors are employed to take into account the geometry of the matrix blocks in the mass exchange between matrix and fractures. The hydro-mechanical model is fully coupled. For example, it captures the sorption-induced volumetric strain or the dependence of permeability on fracture aperture, which evolves with the stress state. The model is implemented in the finite element code Lagamine and is used for the modeling of one production well. A synthetic reservoir and then a real production case are considered. To date, attention has focused on a series of parametric analyses that can highlight the influence of the production scenario or key parameters related to the reservoir
Specificities of floating offshore wind turbines for risk and safety evaluation of anchoring systems: Spécificités des éoliennes flottantes pour l’évaluation du risque et de la sécurité de leurs systems d’ancrages
No full textFloating offshore wind turbines (FWT) are still in their infancy and represent only 0.2% of currently installed commercial offshore wind capacity (193 MW of 65 GW) but will be critical to achieve net-zero objectives by 2050. The design of mooring systems and anchors for FWTs relies heavily on established Oil and Gas (O&G) practice, although governed by different design requirements. Reassessment and refinement of anchor system design methods and practices is necessary, recognising the different risks from FWT failure and the imperative for mass-production within the next 25 years. This paper identifies first the main differences between floating O&G and FWT and the associated geotechnical challenges and risks; then four key developments relevant to industry needs are selected and the solutions to reduce risk and uncertainty are then detailed: (i) Shared anchors; (ii) Farm-wide reliability assessment; (iii) Whole-life geotechnical design; and (iv) Screw pile installation. These examples show how physical, numerical and theoretical modelling can compensate for the current lack of FWT field experience to reduce design risk and raise project viability. Finally, technical project-wide risk is put into perspective by a comparison with the planet-wide risk resulting from delayed offshore wind farm installation
Hydro‐mechanically Coupled Interface Finite Element for the Modeling of Soil–Structure Interactions: Application to Offshore Constructions
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A consistent calibration process for the Matsuoka-Nakai friction angle under direct simple shear conditions for clay hypoplasticity
No full textIn geotechnical engineering, direct simple shear (DSS) tests are used to determine strength and stiffness parameters of a material. DSS predictions of a constitutive model influence failure mechanisms in plane-strain, finite-element applications. In this article, we introduce a closed-form solution to determine the Matsuoka–Nakai equivalent critical friction angle from direct simple shear stress and normal stress data. In order to apply it to clay hypoplasticity (Mašín, 2013), we carry out DSS simulations to investigate rotations of principal stresses and the principal stress state at critical state for plane-strain conditions.Finally, we interpret DSS predictions of clay hypoplasticity for different overconsolidation ratios and demonstrate that the location of the CSL in the vertical stress–void ratio plane coincides with the location of the CSL in mean effective stress–void ratio plane
Hydromechanical modelling of shaft sealing for CO2 storage
Full text linkpeer reviewedThe geological sequestration of CO2 in abandoned coal mines is a promising option to mitigate climate changes while providing sustainable use of the underground cavities. In order to certify the efficiency of the storage, it is essential to understand the behaviour of the shaft sealing system. The paper presents a numerical analysis of CO2 transfer mechanisms through a mine shaft and its sealing system. Different mechanisms for CO2 leakage are considered, namely multiphase flow through the different materials and flow along the interfaces between the lining and the host rock. The study focuses on the abandoned coal mine of Anderlues, Belgium, which was used for seasonal storage of natural gas. A two-dimensional hydromechanical modelling of the storage site is performed and CO2 injection into the coal mine is simulated. Model predictions for a period of 500 years are presented and discussed with attention. The role and influence of the interface between the host rock and the concrete lining are examined. In addition the impact of some uncertain model parameters on the overall performance of the sealing system is analysed through a sensitivity analysis
Formulation of a 1D finite element of heat exchanger for accurate modelling of the grouting behaviour: Application to cyclic thermal loading
Full text linkpeer reviewedThis paper presents a comprehensive formulation of a finite element for the modelling of borehole heat exchangers. This work focuses on the accurate modelling of the grouting and the field of temperature near a single borehole. Therefore the grouting of the BHE is explicitly modelled. The purpose of this work is to provide tools necessary to the further modelling of thermo-mechanical couplings.
The finite element discretises the classical governing equation of advection-diffusion of heat within a 1D pipe connected to ground nodes. Petrov-Galerkin weighting functions are used to avoid numerical disturbances. The formulation is able to capture highly transient and steady-state phenomena.
The proposed finite element is validated with respect to analytical solutions. An example consisting of a 100 m depth U-pipe is finally simulated. A first continuous heating simulation highlights the nonsymmetric distribution of temperature inside and near the borehole. An estimation of the error on the results as a function of the resolution parameters is also carried out. Finally simulations of cyclic thermal loading exhibit the need to take into account all daily variations if the grouting behaviour must be modelled. This is true especially in case of freeze-thaw damaging risk.Geotherwa
DEM element modelling of silent piling group installation for offshore wind turbine foundations
Full text linkOffshore wind farms are now built in deeper water and bigger foundations are required to stabilise wind turbines of increasing sizes. Pile driving is the most widespread foundation installation method, but more stringent environmental regulations necessitate costly mitigation methods to reduce underwater noise emissions. The silent piling (push-in) concept presented in this work is composed of a cluster of four piles, progressively installed by successive jacking sequences. During one sequence, each pile is moved downward by 0.5m stroke, while the other piles are used as reaction. This paper presents the results of Discrete Element Method (DEM) of the installation process. This work identifies the main features of the push-in installation method, such as pile interaction, progressive plugging and loss of efficiency as a function of depth. It is shown that the cluster capacity can reach six times the weight of the tool necessary to silently install the piles
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