Force-resultant models for shallow foundation systems and their implementation in the analysis of soil-structure interactions

Because jack-up platforms operate at different locations throughout their life, site-specific assessment must be performed to verify that a unit is within its design capacity at each site. As the Foundation-Soil-Structural interaction is an integral component of the assessment of a jack-up under storm loading it must therefore be better understood. The aim of this thesis is to create a comprehensive model for the push-over analysis of jack-up platforms, with a balanced emphasison (i) the nonlinear behaviour of structural elements and (ii) plasticity models for the spudcan-soil interaction.

One of the main uncertainties in assessing jack-up platforms’ in-service performance is the idealised elastic structural models that are used for specific guideline assessment. These models do not adequately capture the nonlinear behaviour of the jack-up as a whole and, hence, cannot provide a realistic estimate of the total capacity of the system. In this thesis, the development of a comprehensive and integrated elasto-plastic jack-up model is presented. This model is representative of a modern jack-up structure and can capture geometrical nonlinearities and plastic behaviour of the structure.

The other uncertainties involve the accurate modelling of the jack-up foundation. With the advent of jack-ups used in harsher conditions for extended durations, the offshore industry has turned to novel types of shallow foundations; however, no guidance is available for the additional capacity provided or how the enhanced alternatives will behave under the combined vertical ( V ), horizontal ( H ) and moment (M ) loading caused by a storm. The results of this thesis addressthis lack of guidance.

In this study, the behaviour of a range of footings on various soil conditions is investigated through a combined approach that is composed of centrifuge experiments, finite element analyses and theoretical derivations. Alongside some unique experimental observations, four independent plasticity-based force resultant models were developed that are able to describe seven foundation types and soil conditions. These are (i) spudcan on loose sand, (ii) spudcan on dense sand, (iii) skirted spudcan on loose sand, (iv) skirted spudcan on dense sand, (v) skirted foundation on loosesand, (vi) skirted foundation on dense sand, (vii) Hybrid (skirted mat and caisson) foundation onsoft clay. The supplementary treatment of the sliding surface makes the existing strain-hardening plasticity theory complete regarding describing the foundation’s behaviour on sand. The yield surface obtained on dense sand in the present study is found to be substantially larger than the results obtained in previous 1- g studies or in centrifuge studies of loose sand. This result has significant implications for site-specific jack-up assessment on dense frictional material. For the skirted novel foundation types, considerably larger bearing capacities under horizontal loading are also confirmed.

Integrated analyses of three-dimensional jack-up structures under quasi-static push-over load areperformed. It was concluded that boundary conditions have a large influence on the global response and the push-over capacity of jack-up units. The overall failure of the system was governed by the failure mechanism of the soil. The outcomes from this study extend the existing strain-hardening plasticity model database, providing an advance in the integrated Foundation-Soil-Structural analysis using plasticity models.