Suction caissons are a cost-effective alternative to traditional piles in deep to ultradeep waters. No design rule has been available on the axial capacity of suction caissons as part of the mooring system in soft sediments. In this research, a series of centrifuge tests were performed using instrumented model caissons, to investigate the axial capacity and radial stress changes around caissons during installation, consolidation and vertical pullout in normally consolidated, lightly overconsolidated and sensitive clays. Total pressure transducers instrumented on the caisson wall were calibrated for various conditions. The radial total stress acting on the external wall varied almost linearly during penetration and extraction of the caisson, with smaller gradients observed during post-consolidation pullout. Minimum difference was found in the penetration resistance and the radial total stress for caissons installed by jacking or by suction, suggesting that the mode of soil flow at the caisson tip is similar under these two types of installation. Observed soil heave showed that the soil particles at the caisson tip flow about evenly outside and inside the caisson during suction installation. Comparison was made between measurements and various theoretical predictions, on both the radial stress changes during caisson installation, and the radial effective stress after consolidation. Significant under-predictions on excess pore pressure changes, consolidation times and external shaft friction ratios were found for the NGI Method, based on the assumption that the caisson wall is accommodated entirely by inward motion of the clay during suction installation. Obvious over-predictions by the MTD approach were found in both stress changes and shaft capacity of the caissons. A simple form of cavity expansion method was found to give reasonable estimations of stress changes and post-consolidation external shaft friction. A model for predicting the penetration resistance of suction caissons in clay was evaluated. Upper and lower bound values of external shaft friction ratio during uplift loading after consolidation were derived. Uplift capacity of caissons under sustained loading and cyclic loading were investigated, revealing approximately 15 to 30% reduction of the capacity compared to that under monotonic loading. External shaft friction ratios and reverse end-bearing capacity factors were both found to be significantly lower than those under monotonic loading

[Truncated abstract] The demand for offshore wind turbines is increasing in densely populated areas, such as Europe. These constructions are typically founded on a gravity foundation or a large 'mono pile'. Gravity foundations can only be used at locations where strong soils exist and water depths are limited. Costs associated with a 'mono pile' type foundation contribute to a very large percentage of the total investment costs. This research, therefore, focuses upon a different foundation for offshore wind turbines, namely suction caissons beneath a tripod. This foundation can be used in all kinds of soil types and is cheaper than the 'mono pile' foundation, both in the amount of steel used and installation costs. Cheaper foundations can contribute to a more competitive price for offshore wind energy in comparison with other energy resources. To date, there have been relatively few studies to investigate the behaviour of this type of foundation during the installation process and during operational and ultimate loading for seabed conditions comprising dense sand. Two types of investigations were performed during this research to determine the behaviour of suction caissons beneath a tripod. Firstly, an existing computer program was extended to predict the typical loading conditions for a tripod foundation. Secondly, centrifuge tests on small scale suction caissons were performed to investigate the behaviour during the installation and loading phases. The computer program developed helped to quantify the likely ranges of environmental loading on an offshore wind turbine. For a typical 3 MW wind turbine of 90 m height, the vertical load is low at around 7 MN. During storm conditions the horizontal hydrodynamic load can be in the order of 4 MN. During normal working conditions the horizontal aerodynamic loads can reach 0.4 MN, but can increase to 1.2 MN when the pitch system malfunctions and gusts reach 30 m/s. This aerodynamic load will result in a very large contribution to the overturning moment, due to the high action point of this load. When the wind turbine is placed on top of a tripod, these large moments are counteracted by a push-pull system. ... The development of differential pressure was found to depend on the soil permeability, the extraction speed and a consolidation effect. During cyclic loading no obvious signs of a decrease in resistance were observed. During very fast cyclic loading differential pressures developed, which could increase the drained frictional resistance by approximately 40%. All centrifuge tests results were used to develop methods to predict or back calculate the installation process of suction caissons in sand and layered soil, and the behaviour during tensile and cyclic loading. These methods all use the cone resistance as the main input parameter and predict the force (or required suction) as a function of time, for a given rate of pumping or uplift displacement, in addition to the variation of suction with penetration (or force with uplift displacement). These new methods provide a useful tool in designing a reliable foundation for offshore wind turbines consisting of a tripod arrangement of suction caissons embedded in dense sand.