Britta Bienen

Undrained Bearing Capacity of Deeply Buried Flat Circular Footings under General Loading

Estimation of a footing’s bearing capacity under general loading is a topic of wide interest, especially for the offshore energy industry. Although there have been several studies in the past few decades on shallow footings under general loading, limited work has been done for deeply buried footings. This paper presents results of a numerical investigation into the undrained bearing capacity of deeply buried flat circular footings in clay of both uniform and linearly increasing shear strength profiles. The footings’ uniaxial bearing capacities and failure surfaces under combined vertical, horizontal, and moment loading are reported. The results are analyzed in the context of application by the mobile jack-up industry, for which understanding of the circular spudcan footing’s bearing capacity under general loading is required for site-specific assessments. The paper also presents a parametric study on the effect of a footing’s aspect ratio (thickness/diameter) on its bearing capacity. To conclude, a closed...

Three-dimensional numerical analysis of centrifuge experiments on a model jack-up drilling rig on sand

Jack-up drilling rigs are usually founded on three shallow footings. Under wind, wave, and current loading offshore, the footings of these tall multi-footing systems transfer large moment loads in ...

Set up and resulting punch-through risk of jack-up spudcans during installation

AbstractThis paper addresses a three-dimensional consolidation problem through experimentation in a geotechnical centrifuge. During a discontinuous footing installation, which is typical for mobile offshore drilling rigs, periods of consolidation are encountered as part of the installation process. While this is acknowledged to expose the platforms to potentially severe consequences due to the risk of rapid uncontrolled leg penetration, to date no method has been established to predict this risk and thus enable its mitigation. The findings of this experimental study show that while the peak penetration resistance postconsolidation is mainly influenced by the duration of the consolidation period, as is expected, the load magnitude held during consolidation also significantly influences the severity of capacity reduction postpeak. The consolidation and postconsolidation behavior is discussed for various penetration depths relevant to typical jack-up installations in the field. The key findings of this study...

Physical modelling of the push-over capacity of a jack-up structure on sand in a geotechnical centrifuge

Offshore jack-up drilling rigs are subjected to loading from wind, waves, and current in addition to their self-weight. This applies combined loading in all six degrees-of-freedom in space on the footings. Although the foundation–soil interaction is crucial to the overall response of a jack-up structure, current state-of-the-art models to predict jack-up footing behaviour, developed using data from single footing experiments, have not been validated for such multi-footing systems under general combined loading. This paper introduces the experimental development of a three-legged model jack-up and loading apparatus designed to investigate the rig’s response — in particular the footing load paths — under combined loading in three dimensions. Push-over experiments were performed in a geotechnical beam centrifuge on silica sand. Experimental results of two tests on dense sand are discussed, highlighting differences in response and mode of failure depending on the loading direction of the jack-up. The importan...

Effect of Installation on the Bearing Capacity of a Spudcan under Combined Loading in Soft Clay

Previous numerical analyses of the bearing capacity of embedded footing under combined vertical, horizontal, and moment loading have been based on the assumption that the foundation is wished-in-place, surrounded by undisturbed soil. Under these conditions, the large displacement and remolding of the soil during the footing installation are not accounted for. This assumption results in an overestimation of the capacity. This paper presents results of the size and shape of the bearing capacity surface of spudcan footing in soft clay that accounts for the effects of installation in the modeling. Results for soil sensitivities between one and five and for embedment depths up to three diameters are provided. The findings have practical application in the site-specific assessment of mobile jack-up platforms in soft clay.

Effects of Consolidation under a Penetrating Footing in Carbonate Silty Clay

AbstractThe effects of consolidation under a footing are generally viewed as beneficial due to the resulting increased capacity. Consolidation may also be actively sought because it minimizes footing embedment, which can be critical for the installation of mobile offshore jack-ups because available leg length is limited. However, it can also set the platform footing up to subsequently punch through the strengthened zone, with potentially serious consequences. The problem is complex due to the three-dimensional nature of consolidation. Further, footing penetration leaves the soil above heavily remolded and generates large excess pore pressures below, such that the soil state even prior to consolidation is significantly altered from its in situ conditions. This study has taken an experimental approach to investigate the effects of consolidation around a footing penetrating into carbonate silty clay and, following detailed discussion of the response, offers a framework to predict the changes to the load-pene...

Three-Dimensional Dynamic Analysis of Jack-Up Structures

As mobile jack-up drilling rigs continue to move into deeper waters and harsher environments there is an increased need to understand their behaviour under storm loading conditions. To improve the assessment of jack-ups for a specific site it has become necessary to analyse these units in three dimensions with models that appropriately reflect the physical processes occurring. This motivated the development of SOS_3D, a computer program that takes a balanced approach to all three interrelated components of the structure, the foundations and the environmental loading in three dimensions. Geometrical structural nonlinearities are incorporated using a path-dependent formulation of beam-column theory to specify an incremental stiffness matrix. A six-degree of freedom strain-hardening plasticity model simulates the complex interaction between the soil and the spudcan foundations. Advanced formulations for environmental loads are also implemented. In this paper, the results of three-dimensional jack-up analyses (i) are compared with two-dimensional simulations; (ii) highlight the importance of dynamic assessments; and (iii) describe the influence of wave spatial dispersion and loading direction on the response.

Uniaxial capacities of skirted circular foundations in clay

AbstractResults of finite-element analyses calculating the undrained capacities of skirted circular foundations under uniaxial vertical, horizontal, and moment loading are presented. Parallel finit...

Centrifuge Experiments to Study Extraction of a Deeply Embedded Spudcan Using Top Jetting

Extracting the spudcan footings of mobile jack-up rigs from the seabed at the end of their operations is challenging when the capacity of the rig to pull is low compared to the extraction resistance of the spudcans. This is particularly the case when the spudcans are deeply embedded in soft clay and subjected to long periods of operation that place load on the foundations and allow for consolidation to occur in the soil. A technical solution used by the offshore industry to overcome spudcan extraction resistance is to use a water jetting system that ejects pressurised water through nozzles on the spudcan face. The aim of using water jetting with nozzles located at the top of the spudcan is to reduce extraction resistance through fracturing and softening of the upper soil. However, the efficiencies of such systems are not known to offshore jack-up operators. Top jetting efficiency is therefore addressed in this paper, which reports a series of physical experiments on jetted spudcan extraction conducted in a geotechnical beam centrifuge. The efficiency of water jetting is reported for extraction from depths of up to three diameters in normally consolidated clay, for different jetting flow rates. The excess pore pressure and maximum breakout force measured reveal insights into the extraction process with top jetting. The maximum extraction resistance is shown to be unaffected by top jetting but relates to the suction developed at the spudcan base, which can be reduced by jetting at the spudcan base (Bienen et al. 2009; Gaudin et al. 2011). Top jetting can, however, reduce the extraction resistance post breakout as indicated by the experimental results of this study.Copyright

Modelling of Spudcan Foundation Penetrations Using an Improved Hypoplastic Model for Soft Clays

In this paper, a new hypoplastic model for K0 consolidated soft clays is presented. The enhanced model is characterized by a small number of parameters, adding two parameters to the original model, whilst significantly improving model predictions of undrained stress paths of K0 consolidated soils. However, the calibration procedure remains simple – model can be calibrated using K0 consolidated triaxial tests and oedometer only. It is demonstrated that the model predictions compare well with element test experimental data. The validation of the model involves large scale boundary value problem simulations incorporating very large deformations, in this case retrospective simulations of centrifuge test of spudcan installation. The enhanced model shows better performance compared to a parent hypoplastic model.