Ouahid Harireche

Acceleration waves, flutter instabilities and stationary discontinuities in inelastic porous media

Abstract coupled constitutive equations for a saturated porous medium are developed in the frame of the theories of mixtures : the fluid constituent is elastic and the solid skeleton behaves as a rate-independent elastic-plastic solid. Then the existence of real acceleration wave-speeds is considered : actually the analysis centers on the modes in which these wave-speeds cease to be real. An explicit criterion indicating the critical value of the plastic modulus at the onset of a stationary discontinuity (one wave-speed is zero) is derived in both cases where the fluid and solid constituents are compressible and where they are not. Furthermore, it is shown that in some circumstances, some wave-speeds cease to be real in the very early stages of the inelastic deformation process due to the incipience of a flutter instability (two wave-speeds are complex conjugate). When it is not excluded, this mode of loss of hyperbolicity of the dynamic equilibrium equations usually precedes the onset of a stationary discontinuity and may occur right at the inception of plastic loading, that is for an infinitely large plastic modulus. Flutter instability is excluded when the plastic behavior of the solid skeleton is associative and its existence depends strongly on the relative positions of the shear and longitudinal elastic wave-speeds. It is not likely to occur if the shear wave-speed is the smallest elastic wave-speed and then a stationary discontinuity prevails.

Modelling the variation of suction pressure during caisson installation in sand using FLAC3D

A suction caisson is an upturned ‘bucket’ of cylindrical shape made from steel. This type of foundation has been very popular in the oil and gas industry and the current trend is to extend its use to offshore wind farms. Seepage conditions play a pivotal role in suction caisson installation process in sand. Pressure gradients generated by imposed suction inside the caisson cavity cause an overall reduction in the soil resistance around the caisson wall and tip. This transient soil loosening around the caisson wall helps caisson penetration into the seabed. In this paper, we present a study of the role of seepage on the suction caisson installation process in homogenous sand. We also investigate the effects of seepage conditions on soil resistance to caisson penetration with a particular focus on how frictional and tip resistances are differently affected. For this purpose, a series of numerical models are developed using FLAC3D. These models are used to investigate the variation of suction pressure during caisson installation in homogenous sand and to predict the amount of suction required to penetrate the caisson to a certain depth. An explicit strategy is used for each embedment depth, which consists of updating current suction based on displacement history available after the previous prescribed displacement increment. The numerical models are developed for different caisson sizes and wall thicknesses to study the effects of caisson geometry on soil resistance during caisson installation. Problem dimensions are normalised with respect to the diameter of the caisson in order to obtain the results that can be applied to any caisson size. The results showed that suction pressure tends to increase with the embedment depth. Additionally, the overall behaviour and the pressure variation with depth are similar for caissons of different sizes and wall thicknesses. Finally, in order to validate the developed numerical models, data from centrifuge tests are investigated and compared with the results obtained from this study. The developed finite difference models are found to be in good agreement with centrifuge tests, in particular for thicker caissons (t/D = 1%).

Geotechnical Performance of Suction Caisson Installation in Multi-layered Seabed Profiles

Suction caissons consist of large cylindrical buckets made from steel. In order to serve as foundations for various offshore structures, suction caissons are pushed into the seabed under pressure differential exerted on their lid by an imposed suction. Despite their wide use in the oil and gas industry, there are still some uncertainties regarding their installation process as a result of changes in seabed profiles such as the existence of low permeability layers as well as the variation in soil properties with depth (e.g. permeability decreasing with depth due to an increase in soil density). It is known that seepage conditions play a pivotal role in the installation process, particularly in sand. Indeed, pressure gradients generated by the imposed suction inside the caisson cavity cause an overall reduction in the soil resistance around the caisson wall and at caisson tip, thereby assisting the penetration into the seabed. Successful installation of caisson foundations relies on accurate prediction of soil conditions, in particular soil shear resistance during the installation. Existing knowledge of the prediction of soil conditions and required suction during caisson installation has some limitations which often resulted into rather conservative design methods. Most design procedures used to control suction during caisson installation assume an isotropic and homogenous seabed profile. Moreover, the actual variation of pressure gradient around the caisson wall at different penetration depths is often ignored, although it significantly affects soil resistance. Natural seabed can possess a heterogeneous property where it may comprise of different layers of soils including the presence of layers with low-permeability i.e. clay or silt. In this paper, the effect of seepage on soil conditions during caisson installation is studied within the frame of the presence of a substratum that consists of silt. Suction induced seepage described throughout the installation process and its effects on frictional and tip resistance are considered. For this purpose, a numerical simulation is conducted on a normalised geometry of the suction caisson and surrounding soil, at different penetration depths. The distribution of pressure gradient on both inside and outside of the caisson wall is taken into consideration in both soil shear and tip resistance. Particular conclusions will be drawn on the implications of the presence of a low permeability silt layer on caisson installation.

Dynamic Strain-Localization in Compressible Fluid-Saturated Porous Media

A stress-point algorithm for coupled inelastic constitutive equations is proposed and analysed. These equations represent the behavior of fluid-saturated elastic-visco-plastic porous media with compressible constituents. The effects of compressibility are further investigated in finite element analyses which display strain-localization.