ABSTRACT

Offshore wind turbines are required to help transition the power supply in Germany from fossil fuels to renewable energies. To achieve this ambitious goal, ever larger wind turbines with a total height of over 300 meters are intended to be used in future. The wind turbines as well as the foundation structure are particularly exposed to cyclic wind and wave loads. Therefore, the design has to focus on the accumulation of deformations due to cyclic loads. The objective of this paper is to present the behavior of a gravity-based foundation under drained cyclic loads. A new small-scale model stand for experimental investigations was built. In this study the method for the sand preparation before each test is described in detail. It is shown that a sufficiently homogeneous state exists before starting the monotonic and cyclic test series, so that tests with the same conditions can be reproduced. The test stand allows to investigate drained as well as partially undrained loading conditions. In this study, the movement of the foundation due to drained cyclic loading is investigated and interpreted. The results from the new stand will in future be used to validate developed numerical methods.

INTRODUCTION

In addition to monopiles, jackets and suction buckets, gravity-based or shortly gravity foundations are a possible foundation type for offshore wind turbines (Muhammed et al., 2018). All of these offshore structures are subjected to cyclic loads from wind and waves (Figure 1). Cyclic loads can lead to an accumulation of deformations in saturated soils as well as to an accumulation of excess pore water pressures with corresponding reductions in strength or bearing capacity (Glasenapp 2016; Safinus et al. 2011, Saathoff and Achmus 2020). The load-deflection curve shown in Fig. 1 elucidates that cyclic loading is expected to lead to an increase of the horizontal deformation with ongoing cyclic loading. Simultaneously, an increase of the permanent rotation of the foundation occurs. In particular for foundation structures of offshore wind turbines, strict criteria regarding the permanent inclination have to be kept. Hence, the accumulation of displacements as well as excess pore pressure and permanent rotation needs to be estimated within the design process.

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