Coupled simulation of a monitored monopile under high-intensity loading

抄録

The offshore wind turbine structure is designed to have the first natural frequency between the rotor frequency (1P) and blade passing frequency (3P), a so-called soft-stiff design. Degradation of the pile-soil interaction stiffness can lead to a reduction of the first natural frequency, bringing it closer to the 1P and wave frequency. This leads to dynamic amplification of stresses in the structural steel. An analysis of the degradation of the pile-soil interaction stiffness and the associated reduction of the first natural frequency is therefore warranted. While 1D beam-column models are used for routine analysis of monopile-soil interaction, the modelling of the stress-strain response of the soil in non-linear 3D finite elements analysis with pore pressure generation/dissipation can offer improved insight into the cyclic degradation of soil stiffness and its effect on the monopile stiffness. In this work, a coupled dynamic analysis of a laterally loaded monopile located in the Belgian North Sea is presented. A 3D Finite Element (FE) soil-monopile model is developed in ABAQUS. The soil is modeled using the hypoplastic constitutive model, calibrated to advanced static and cyclic laboratory tests. The model makes use of the dynamic u-p formulation that allows for the generation and dissipation of pore pressures. The monopile is subjected to high-intensity loads (storm conditions) and the degradation of monopile lateral stiffness due to storm loading is investigated. The reduced stiffness is introduced in an integrated model of the wind turbine structure to quantify the reduction of the first natural frequency. It is found that cyclic loading leads to the degradation of the pile-soil stiffness, which in turn leads to the degradation of the wind turbine's frequencies. The effects are more pronounced on the frequency of the 2^nd mode of vibration.