Subproject B4 – Collaborative Research Centre 1463

Things are often stormy on the open sea. Wherever a strong wind blows, high waves are not far away. Offshore structures such as wind turbines have to permanently withstand all kinds of load events. Loads caused by wind and waves are also referred to as cyclic actions, i.e. actions that occur recurrently and proceed so slowly that inertial forces can be neglected.

In case of a wind energy converter, the loads from self-weight, wind and waves are transferred via the structure into the foundation elements and then into the soil. In the German North Sea, for example, the seabed consists mainly of sand of different grain sizes. In addition to the grains, the sand also has small pores that are filled with water. The seabed is therefore completely saturated with water.

Although a wind turbine looks rigid and stable, it still sways back and forth a little due to wind and waves loads. These deformations are also transferred to the surrounding ground. The storm event can therefore have a major impact on the entire structure. On the one hand, there may be an accumulation of deformations, i.e., a permanent tilting. On the other hand, there may also be changes in stress ratios. In addition, an accumulation of excess pore water pressures can also result, which in turn leads to reductions in strength or load-bearing capacity. For offshore structures, relatively tight tolerances apply with respect to permanent deformation, so an accurate deformation prediction should be made during design. Therefore, during foundation design, a strong focus must be placed on capturing the effects of cyclic actions.

However, neither validated calculation methods nor a uniform procedure exists for the static and geotechnical verifications required as part of the design. In order to investigate the behavior of the in-situ soil under cyclic loads, i.e. during a storm event, different laboratory tests are carried out in practice (e.g. cyclic simple shear device in Figure 1 or cyclic triaxial tests, drained or undrained, with constant load or constant volume). At best, there are vague concepts but no validated methods to transfer the soil behavior determined in the element test to the behavior of the system, i.e., the foundation elements. There is a lack of basic knowledge about which cyclic tests are reasonable to perform and how to consider the results of these tests in the design.

Fig. 2: Schematic representation of a cyclic simple shear device

The objective of the subproject is to create a basic knowledge required for this purpose in order to be able to realistically describe the load-bearing behavior of a cyclically loaded foundation and its changes over the operating time. This is absolutely necessary for the realization of a digital twin of a megastructure planned in the CRC, since otherwise neither the behavior in the operating phase can be described nor the effects of changed operating concepts can be investigated. In order to be able to investigate and describe the behaviour of the soil under cyclic loads, a new model stand for experimental investigations was planned by IGtH (Figure 2). The investigations are carried out on a model of a gravity foundation under drained as well as partially drained conditions. In possible later funding periods, however, other foundation variants shall also be considered.

Fig. 2: Video of cyclic loading of a small-scale gravity base foundation.

The subproject B04 deals with cyclic element tests, experimental system tests and numerical simulations (Fig. 3). The long-term goal is the development of a generic methodology for the prediction of the load-bearing and operational behavior of intensively cyclically loaded foundations using high-quality cyclic laboratory tests.