Since the exploitation of wind as a renewable energy resource, jack-up vessels equipped with more than three independent legs are increasingly employed to transport and install the components of offshore wind turbines. By lowering the movable legs the vessel is able to elevate the hull from sea water level. In elevated position the vessel provides a stable platform to perform installation activities. The legs are equipped with spudcans which serve as foundation of the vessel. The elevating process consists of a preload phase to ensure sufficient capacity to withstand operational and possible storm conditions. The preloading of four-legged jack-ups is performed by alternately applying vertical loads on diagonally opposite leg pairs, up to achieving a stable condition in which nearly constant load levels can be held by each leg. The aim of this research is to develop a 3D model to asses the preload duration of the jack-up vessel Aeolus in cohesive soil. The viscous behaviour of cohesive soil, like clay, influence the acting leg load during preload of the jack-up vessel. The shear strength of clay is a function of strain rate meaning that resistance increases due to viscous effects with increasing penetration rate. During spudcan penetration the shearing resistance is high but will reduce significantly when penetration is stopped as the viscoplastic resistance diminishes. Together with the onset of isotach soil behaviour this causes the loads to redistribute between the legs occurs. In this study it is assumed that sufficient preloading is achieved when the leg load reduction is limited to 400 ton / 15 min. To satisfy this criterionmultiple load cycles of each leg pair are performed. Site specific geotechnical data and information on the structural stiffness of the Aeolus have been available for this research and allowed for an accurate analysis of the processes during the preload procedure. The Soft Soil Creep (SSC) model is used as constitutive model and accounts for viscous effects by formulating irreversible strains by means of viscoplasticity. The soil at the project site is classified and the constitutive model is calibrated based upon the available soil test results. The structural behaviour of the vessel is captured via a simplified beam configuration representing the deck structure and legs, the stiffness of the beams is verified using the results from a so-called predrive analysis. The extension of the legs is established bymeans of negatively pre-stressed node-to-node anchors. Simulations of a single spudcan penetrating at various depths and penetration rates are performed to identify the extent of viscous strain rate effects fromthe results. With the developed 3D model Small Deformation Finite Element analyses of the preload procedure are performed. The leg loads and penetrations are monitored and compared to jacking data fromthe actual project site. The processes in the soil and structure are analysed and the influence on the preload procedure and preload duration is identified. For both type of simulations six different case-calculations are performed addressing variation in the initial spudcan depth, the OCR, the penetration rate, the permeability and the type of preload procedure. The simulations indicated that the penetration of a spudcan influences the penetration of an adjacent spudcan, this reciprocal influence of the spudcans emphasizes the importance of onemodel comprising all spudcans in the same 3D soil domain. The developed model slightly overestimates the spudcan penetration and underestimates the total preload duration. Simulations of the overshooting preload procedure and an alternative preload procedure are performed with the FE model. For the soil conditions used in this research, both the overshooting and the alternative procedure are effective in reducing the number of preload cycles to satisfy the preload criterion. Compared to the normal preload procedure, it is expected the overshooting procedure improves the preload duration. For the alternative procedure however, the duration of a preload cycle increases significantly and consequently the procedure does not improve the preload duration. Using a lower spudcan penetration rate during the normal preload procedure is also effective in reducing the number of preload cycles but significantly increases the elapsed time to complete the preload procedure. The above conclusions have been made on the basis of the model results, which is calibrated for the soil conditions at the specific project site.

As the demand for renewable energy is constantly rising, more offshore wind farms are being built at sea to meet European targets towards sustainable energy transition. Van Oord’s offshore installation vessel Aeolus was purposely-built to transport and install foundations of offshore wind turbines. To ensure that the vessel remains competitive in a market that is changing rapidly, Van Oord decided to upgrade this vessel to expand its operational capabilities to work on soft soil seabed conditions. Equipped with moveable legs, the mobile jack-up vessel is able to elevate its hull out of the water and provide a stable work platform. Prior to each installation, the soil is preloaded by penetrating the spudcan footings into the soil under the self-weight of the structure, followed by progressively pumping sea water into the ballast tanks until the target preload is reached. Offshore industry guidelines follow the conventional bearing capacity theory to assess jack-up procedures. However, these conventional procedures are inadequate in incorporating all relevant mechanisms that contribute to the preloading behaviour in cohesive soils. The effect of consolidation around the spudcan footings on the soil strength and the viscous strain rate-dependent soil strength are two important aspects which are poorly described in the current guidelines. Using numerical modelling, a better understanding of these mechanisms is aimed for in order to evaluate spudcan bearing capacity in cohesive soils in a more detailed design. An axisymmetric finite element model of the spudcan foundation is constructed in PLAXIS 2D to assess the effects of consolidation and rate-dependence of soil strength on the use of different preloading strategies. As spudcan penetration is relatively fast and clay permeability is relatively low, consolidation effects are found to be insignificant during the preloading process. Contrary, viscous strain rate effects dominate the soil strength of clay in predominantly undrained conditions. Higher shear strengths are found for higher spudcan penetration rates. During the standard jacking process, when the target preload value is reached and the spudcan is stopped, a reduction of the leg load occurs due to decreasing viscous resistances, which must actively be brought back to the target preload value. In clay soils, this leads to further penetrations and multiple preload cycles are required before the spudcan response is stabilised. In terms of preloading duration, a rather slow spudcan penetration rate is therefore beneficial when using the standard jacking procedure. Alternatively, a different jacking procedure is assessed numerically, which adapts for these viscous strain rate effects by overshooting the target preload value by 10-15%, in order to reach the stable spudcan response more quickly. In this way, the spudcan is brought to the depth, which would have been reached by the creep-like additional penetrations in the iterative preload cycles under the target preload value. Once the load is reduced from the overshoot to the target preload value, it is likely that the preload can be sustained without additional penetrations and preload cycles. The overshooting jacking procedure reduces the required preloading time considerably for both spudcan penetration rates. The largest time savings are gained for the largest penetration rates. However, using this method, larger leg penetration predictions compared to the standard jacking procedure are observed for decreasing spudcan penetration rates. That is because in the standard jacking procedure, less creep-like additional penetrations are present when using a slow spudcan penetration rate. The overshooting jacking procedure can thus be considered beneficial for relatively fast spudcan penetration rates, both in term of preloading duration and spudcan penetrations.