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

Evaluation of undrained failure envelopes of caisson foundations under combined loading

In this paper, results of a three-dimensional finite element study addressing the effect of embedment ratio (L/D) of caisson foundations on the undrained bearing capacity under uniaxial and combined loadings are discussed. The undrained response of caisson foundations under uniaxial vertical (V), horizontal (H) and moment (M) loading are investigated. A series of equations are proposed to predict the ultimate vertical, moment and maximum horizontal bearing capacity factors. The undrained response of caisson foundations under combined V-H and V-M load space is studied and presented using failure envelopes generated with side-swipe method. The kinematic mechanism accompanying failure under uniaxial loading is addressed and presented for different embedment ratios. Predictions of the uniaxial bearing capacities are compared with other models and it is confirmed that the proposed equations appropriately describe the capacity of caisson foundations under uniaxial vertical, horizontal and moment loading in homogenous undrained soils. The results of this paper can be used as a basis for standard design codes of off-shore skirted shallow foundations which will be the first of its kind

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%)

Installation performance of structurally enhanced caissons in sand

Suction caissons are attractive solutions to support offshore structures. Their capacity, both pull-out and bearing, grows with their embedment depth. However, higher embedment depths increase risks of installation failure due to uncertainty of seabed condition, increased chance of piping and structural buckling. For the first time, this paper investigates installation of structurally enhanced caissons (SECs), in the form of flanges attached to the caisson shaft, through developing a numerical procedure based on finite element analysis. The SEC has the potential to offer additional (over 20%) pull-out and bearing capacity compared with standard caisson and can have substantial positive impacts on torsional capacity. Using the proposed numerical procedure, the impact of adding the flanges on the installation resistance was studied and compared against a standard caisson. A piping criterion was defined which allows tracking the soil region where piping develops and evolves as the installation proceeds. The impacts of flange base sizes and sand compaction on the required suction for installation were studied. The results of this paper can be used as guidance to predict the required suction to install flanged caissons and can facilitate the uptake of the proposed SEC. The proposed numerical framework is applicable to other SEC geometries