The optimization of monopiles for offshore wind turbines is closely linked to improved foundation models, particularly for large diameter structures. During the last years significant progress on geotechnical level has been made to assess foundation models of monopiles based on high fidelity finite element models. In contrast, integrated load simulation models for offshore wind turbines commonly incorporate soil-structure interaction on a simplified level. This paper presents an efficient coupling approach, suggesting a practical interface between the offshore wind turbine substructure and sophisticated foundation models on mudline level. The approach is applied for load simulations on fatigue load level. The monopile load spectrum has to be defined in advance. Then, a limited number of static foundation pre-simulations are performed to predict the response on interface level by use of load displacement curves. The integration of these curves into the load simulation model is achieved by an extension to spatial load displacement surfaces. For this purpose an interpolation technique has been particularly developed which works efficient and accurate. The test simulations conducted for this paper are limited to co-directional wind and wave direction and a pre-dominant lateral loaded foundation, but the method offers the flexibility to be evolved to spatially loaded pile or bucket foundations.
In the next decade a large number of offshore wind turbines will be installed in the North and Baltic Sea. Nowadays, the preferred support structure type for locations with water depths up to 40m are monopiles. Besides geotechnical verifications commonly performed at ultimate limit state (ULS) and limiting the pile inclination at serviceability limit state (SLS), the foundation stiffness contributes decisively to the overall design. As key design elements, structural eigenfrequencies as well as fatigue loading are significantly affected. Monopile support structures, also with larger base diameters exceeding 7.50m, are designed following the soft-stiff approach. Accordingly, the first eigenfrequency shall be larger than excitation frequencies from waves or 1P rotor revolution, but less than the 3P blade passing frequency (see Fig. 1). Figure 1 shows the low frequency excitation spectrum of an offshore wind turbine turbine with a rotor speed of 7–12 rpm. The lower limit represents a challenging criterion for larger turbine sizes in practice, whereas eigenfrequencies of smaller turbines may likely reach the upper boundary (3P) (cf. Scharff, Siems (2013)). One should note additionally, fatigue load magnitudes are decreasing with increasing gap between excitation and eigenfrequency. As a result, a prediction of foundation stiffness in a conservative manner is not feasible, because conservative assumptions may lead to uneconomic designs. From that perspective, simply a prediction of the foundation stiffness with maximum accuracy contributes sustainably to safe and optimized monopile designs.
