The foundation stiffness under operational loads is a crucial aspect for the design of monopile foundations, as key design issues, structural natural eigenfrequencies as well as the structural fatigue damage are significantly affected. According to measurement results, the current calculation methods lead to an underestimation of the actual existing foundation stiffness. Concerning this matter, this paper presents numerical simulations to derive foundation stiffnesses. Furthermore, the damping of the system is investigated from the calculated load-displacement and moment-rotation curves in order to determine the hysteretic damping ratios.
Monopile foundations are currently a widely used foundation concept in water depths up to 40 m. For such depths and the current generation of wind turbines with rated energy output > 8 MW, pile diameters of 8 m and more are necessary to fulfill the design requirements. A crucial requirement for monopiles with such diameters regards the stiffness under operational loading, which strongly affects the eigenfrequency of the overall structure. For the usual "soft-stiff" design, it must be ensured that the eigenfrequency lies sufficiently above the 1P excitation frequency resulting from the rotational frequency of the wind turbine in order to avoid resonance effects and associated high fatigue loads.
It is common practice in the design of monopiles to calculate the bearing behavior with a subgrade reaction model, in which the soil is represented by springs with non-linear load-displacement relationships (Fig. 1). This approach is called p-y method and stated in the current Offshore Guidelines (API 2014, DNVGL 2018). For piles in sandy soils, based on investigations of Reese et al. (1974) and Murchison et O'Neill (1984), a hyperbolic tangent function is used to describe the p-y relationship. The pu-value and the initial stiffness of the p-y curve Epy characterize the p-y curve. These values are depth-dependent and calculated from the angle of internal friction φ' and the buoyant unit weight γ' of the sandy soil. The method was originally developed and calibrated for flexible and small diameter piles. For the application to large-diameter monopiles, a modification of this method is necessary in order to account for the bearing behavior which is more similar to the behavior of a rigid pile.
