Mobile jack-up rigs are widely used in offshore oil and gas exploration and increasingly in temporary production and maintenance work. There is a steadily increasing demand for their use in deeper water and harsher environments. A typical modem jack-up has three independent legs, each equipped with a footing known as ‘spudcan’. This thesis is concerned with the performance of spudcan foundation subjected to vertical loading correspondent to preloading during its installation into uniform clay. The chief aim of this study is to investigate the bearing behaviour with the corresponding soil failure mechanisms during spudcan penetration. Centrifuge model test and Finite Element (FE) analysis are carried out extensively. In centrifuge modelling, a half-spudcan model and a full spudcan model are used. In the half- spudcan model test, a novel system for revealing soil failure mechanisms and measuring soil deformation has been adopted, in which the half-spudcan model is placed against a transparent window and a subsequent Particle Image Velocimetry (PIV) analysis is performed. The full-spudcan model test is conducted to measure the load-penetration response. In numerical simulation, both small strain and large deformation analyses are carried out with smooth and rough soil-spudcan interfaces considered. At the initial stage of penetration, it is observed that a cavity is formed above the spudcan as it is penetrating into a uniform clay. Meanwhile, soil flows towards the surface and thus soil heave forms close to the spudcan shoulders. With further penetration, the soil underneath the spudcan starts to flow back into the cavity on the exposed top of the spudcan. This backflow causes the spudcan to be embedded while the initially formed cavity remains open. / Eventually, the spudcan becomes fully embedded and the soil flow mechanism reaches a fully localised failure mechanism with deep embedment. The lateral extent of visible distortion due to soil flow is confined well within 1.5-1.6 D (D: spudcan diameter). From both centrifuge and numerical investigations, it is found that in uniform clay, it is inevitable to form a cavity above the spudcan foundation. Thus, the stable cavity depth and soil back flow mechanisms are studied. It is clear that the back flow is caused by a Flow Failure, where it is due to the downward penetration of the spudcan. This is contrary to the Wall Failure that is the mechanism recommended by the current offshore design guidelines to estimate the stable cavity depth. In wall failure, the soil back flow is due to the cavity wall too high to stand. The stable cavity depth is estimated up to 4 times higher by the wall failure mechanism than the one by the flow failure. This explains that the wall failure is never observed in model test. Therefore, a new design chart with design formula is developed for design engineers in the stable cavity depth calculation. The spudcan bearing response is strongly correspondent with the variation of soil failure mechanisms during penetration. At the initial stage of the penetration, the spudcan bearing capacity increases with penetration, which is due to the increase of overburden pressure from cavity formation. At the second stage of the penetration, soil back flow embeds the spudcan, and the spudcan bearing capacity is increasing as the soil flow mechanism transits from its shallow failure mechanism to its deep failure mechanism. / At the final stage of the penetration, the spudcan bearing capacity reaches its ultimate value, where the deep/localised failure mechanism remains. A rough spudcan shows 14 % higher bearing capacity than a smooth spudcan. And a flat-plate shows 8 % higher capacity than a spudcan with a same surface roughness. The ultimate bearing capacity factor N, = 10.5 in uniform soil is recommended as a conservative value when the deep failure mechanism is reached. A correspondent N, = 10.1 in NC clay is suggested for a deeply embedded spudcan.

The lack of knowledge about the behaviour of footings for jack-up rigs under storm loads poses a design problem which can be tackled by model testing. The areas of prime concern are the ultimate loads on footings under combined loading, which affects the safety of the rig, and the rotational stiffness, which affects the interaction between the foundation and the structure. A programme of loading tests was performed on model footings on clay, and was divided into two stages: monotonic loading and cyclic loading. The clay samples were obtained by consolidating Speswhite kaolin slurry in cylindrical tanks 450mm in diameter. The strength and compressibility characteristics of the samples were verified by means of standard laboratory tests. The model footings were 50mm and 100mm in diameter and several shapes were tested: circular flat plate, cones of various angles and model spud-cans. Loads and displacements were monitored using appropriate instrumentation and a data logger. A series of central vertical loading tests provided data for comparison with existing bearing capacity theories. Combined loading tests were performed applying a displacement controlled horizontal load at a fixed height above the footing which was also subjected to a fixed vertical load. The main series of tests involved a parametric study of the relevant variables. Special tests allowed the assessment of the effect of embedment of the footing and the interaction of a flexible leg with the foundation. Cyclic loading tests were carried out using a load controlled system which applied a sinusoidal load simulating wave action. Effects of currents were investigated by introducing an offset to the loading cycle. The influence of amplitude and period of loading as well as the influence of vertical load were also investigated. Special tests were carried out to cover some peculiarities of real loading conditions. Fitting of a three-parameter hyperbola to the test results provided a systematic and accurate method of analysis of monotonic loading tests, leading to valuable information involving stiffness and ultimate loads. Analysis of cyclic loading tests yielded useful qualitative information regarding the progress of settlement and the variation of rotational stiffness and damping ratio with the number of cycles.

El objetivo central del trabajo es realizar un diseño básico de una plataforma tipo Jack Up y el estudio de factibilidad para desarrollar la construcción de esta estructura en Chile, comparando los costos de fabricación con los costos de compra de este equipo.

There is a steadily increasing demand for the use of jack-up units in deeper water and harsher environments. Confidence in their use in these environments requires jack-up analysis techniques to reflect accurately the physical processes occurring. This thesis is concerned with the models appropriate for the dynamic assessment of jack-ups, an important issue in long-term reliability considerations. The motivation is to achieve a balanced approach in considering the non-linearities in the structure, foundations and wave loading. A work hardening plasticity model is outlined for the combined vertical, moment and horizontal loading of spudcan footings on dense sand. Empirical expressions for the yield surface in combined load space and a flow rule for prediction of footing displacements during yield are given. Theoretical lower bound bearing capacity factors for conical footings in sand have been derived and are used in a strain-hardening law to define the variation in size of the yield surface with the plastic component of vertical penetration. The complete incremental numerical model has been implemented into a plane frame analysis program named JAKUP. The spectral content of wave loading is considered using NewWave theory, and the importance of random wave histories shown by constraining the deterministic NewWave into a completely random surface elevation. Using this technique, a method for determining short-term extreme response statistics for a sea-state is demonstrated. A numerical experiment on an example jack-up and central North Sea location is shown to emphasise the difference in long-term extreme response according to various footing assumptions. The role of sea-state severity in the variation of short-term extreme response statistics is also highlighted. Finally, probabilistic methods are used to develop further understanding of the response behaviour of jack-ups. A sensitivity study of influential variables (with probabilistic formulations as opposed to deterministic values) has been conducted using the response surface methodology.

Jack-up units have considerable economic significance because they are used to carry out a large proportion of the world's oil and gas exploration in water depths less than 90.0m. Due to the increase in use of jack-ups in harsher environments, analysis techniques assuming quasi-static and linear structural behaviour have had to be reassessed. This thesis is concerned with non-linear dynamic analysis methods appropriate for a jack-up assessment. Jack-up modelling requires realistic representation of the structure, the foundations and the environmental loading, together with the implementation of appropriate dynamic analysis algorithms. Techniques for each of these aspects of jack-up analysis are reviewed and the implementation of several of the methods in an advanced plane frame analysis program called JAKUP is described. Geometric non-linearity in the structure and work hardening plasticity at the foundations are accounted for in the program. Test cases are presented to verify the implementation of the methods and then some illustrative plane frame quasi-static and dynamic analyses are described. These simple models highlight the importance of accounting for dynamic motions in a jack-up analysis. For the quasi-static analyses, the assumption of pinned footing behaviour is seen to always result in the most conservative displacement and moment predictions. However, the analyses show that this is not always the case when dynamic amplification is accounted for.

Transportation and installation of offshore wind turbines (Tower, Nacelle and Rotor) is a complete process conducted over several phases, usually in sequence. There are several factors that can turn this process into a challenge. These factors can either be due to offshore site conditions or the technical limitations of the installation vessels. Each project has its own characteristic parameters and requires a unique optimum solution. This paper identifies the dynamics of the installation process and analyzes the effects of each phase on the progression of events.The challenges in wind turbine installations due to offshore environment were investigated, the effects of each were explained and their significances were stressed. Special installation vessels were examined and their technical specifications were analyzed in terms of working conditions, dimensions, service performances, and crane capacities as well as projecting future design trends. Several offshore wind farm projects were analyzed; their installation methods were specified, and compared to each other to determine advantages and disadvantages of different pre-assembly concepts. The durations of the sub-phases of the process were defined in terms of different variables such as site conditions and individual vessel performance. These definitions were used for making time estimations, and conducting further analyses regarding the effects of different site specific parameters on the overall project duration.In conclusion, this study considered the main operation parameters in an offshore wind turbine installation context: the benefits and drawbacks of different pre-assembly methods were researched and evaluated resulting in new knowledge and a productive contribution for optimizing “the offshore turbine transportation and installation process”, based on actual time usage.