The Offshore wind industry is constantly expanding and one of the key cost drivers is the offshore wind turbine support structure. The foundation accounts for up to 35 % of the total installation cost of the wind turbine. The monopile foundations are the most commonly used support structure with diameters up to 6m and Embedded length/depth (L/D)<7. Proper modeling of the soil-structure interaction (SSI) is important for safe and efficient design. With the introduction of large diameter rigid monopile foundations, the applicability of the current design standard is in question. The current industry standard model for SSI for laterally loaded monopiles is the Winkler foundation model. In this model, the soil is idealized as a row of one dimensional discrete springs spanning along the beam. The stiffness of these springs is obtained using empirically derived p-y curves, which gives the relation between the soil-pressure (p) and deflection (y). The p-y method describes the construction of the p-y curves for any arbitrary set of soil parameters. This method has been applied in the oil and gas industry and is based on field tests conducted on slender piles with diameter 2m. Various studies on the applicability of the model have shown to give erroneous results for large diameter monopiles. Since the soil is idealized as a row of discrete (local) springs, it neglects the continuous (non-local) nature of soil. Additionally, monopiles with L/D <7 exhibit a more rigid behavior and hence industry standard assumptions for laterally loaded piles with respect to vertical tangent criteria and 'zero-toe-kick' do not hold on. A plane strain semi-analytical model is developed which considers the continuous (non-local) nature of soil as well as the soil below the pile toe/tip, which could be significant to the dynamic behavior of the monopile foundations. In this approach, the pile is modeled as an elastic rectangular plate which is discretized using modified Euler-Bernoulli beam elements. and the soil is modeled as an elastic continuum waveguide. The two entities are combined via the discretized interface. Assuming wave motion in soil, semi-analytical solutions satisfying homogeneous and inhomogeneous boundary conditions are obtained for the waveguide, which are used to derive the frequency dependent soil stiffness matrices at the interface. These are then coupled to the plate. For the current study, the L/D ratio of the plate is restricted to the flexible regime, due to the availability of a comparable reference solution in this regime. It is shown that, with L/D ratio of 12.5 already, the dynamic behavior of the plate is sensitive to the soil below the tip. Therefore, with smaller L/D ratios, it is expected that the tip will have a significant influence on the dynamic behavior of the pile.

The monopile is the prevailing foundation concept applied in offshore wind farms, mainly due to construction of offshore wind farms in relatively shallow water. The monopile can provide a technically and financially feasible solution in shallow water environment due to its’ ability to support the wind generator ensuring at the same time the maximum production, since it can provide a foundation concept which ensures that the displacement and rotation thresholds set by the wind turbine manufacturers are being met. However, the trend nowadays is to develop wind farms in deeper water and to use wind turbines of higher capacity in order to increase the electricity production. This can be translated to a demand to increase the dimensions of the monopile, leading to complications in the transportation and installation procedures. This thesis project focuses on the possibility to enhance the structural and dynamic behavior of the monopile by filling it with sand. The effect of the sand-fill on the local buckling, on the static displacement, on the natural frequency and on the damping ratio of the structure have been investigated. In order to estimate the effect of sand-fill on local buckling, its’ effect on the ovalization of the monopile’s cross-section was estimated using the relevant European Standards and Dutch guidelines. The analysis has shown that for an increasing Youngs Modulus (E) of the sand-fill, the ovalization of the monopile at the critical cross-section decreases significantly, which in turn increases the bending resistance of the filled monopile. The effect of the sand-fill on the static displacement has been estimated by modeling the monopile in Plaxis 3D. The effect of sand-fill appears to be more significant for lower static loads. The effect of sand-fill on the fundamental natural frequency of the structure was estimated analytically by using the approximate normal modes obtained by the Euler Bernoulli beam equations and numerically, using the finite difference method to model the structure. The results of the analysis have shown that the total effect of the sand-fill on the natural frequency of the structure is negative, and sand-fill should be applied only in cases when a decrease of the natural frequency of an existing structure is required. The effect of sand-fill on the damping ratio of the structure has been performed through Free Vibration Tests which have been simulated in Plaxis 3D. The analysis has shown that the hysteretic behavior of the sand-fill leads to an increase of the damping ratio of the structure. The beneficial effect of the sand-fill on the damping ratio increases as the initial displacement applied at the top of the structure increases. Finally, a financial analysis was performed in order to identify the influence of sand-fill on the foundation cost. This additional cost sets the minimum threshold to be exceeded by the possible financial benefits achieved due to the higher damping ratio, in order that the sand-fill can be applied as a financially feasible solution.