Subproject B4 – Collaborative Research Centre 1463

Out at sea, conditions often are stormy. Where strong winds blow, high waves are never far away. Offshore structures such as wind turbines must withstand all types of weather permanently. Loads caused by wind and waves are also referred to as cyclic effects, meaning effects that occur repeatedly and progress so slowly that inertial forces can be neglected.

In a wind turbine, the loads from its own weight, wind, and waves are transferred through the tower shaft into the foundation elements and then into the ground. The seabed, for example, in the German North Sea, predominantly consists of sand with varying grain sizes. In addition to the grains, the sand also contains small pores filled with water. Therefore, the seabed is fully saturated with water.

Although a wind turbine is stable, it still sways slightly back and forth under the influence of wind and waves. These deformations also affect the surrounding soil. As a result, a storm event can have a significant impact on the entire structure. On the one hand, this can lead to an accumulation of deformations, resulting in a permanent tilt. On the other hand, changes in stress conditions may also occur. Additionally, there can be an accumulation of excess pore water pressure, which in turn can lead to reductions in strength or load-bearing capacity.

Fig. 1: Cyclic loaded Offshore Windturbine (schematic)

For offshore structures, relatively tight tolerances apply to permanent deformation, so a highly accurate deformation prediction should be made during the planning phase. Therefore, the foundation design must place a strong emphasis on capturing the effects of cyclic loading.

For the static and geotechnical verifications required as part of the design process, neither validated calculation methods nor a standardized approach currently exist. To study the behavior of the soil under cyclic loads, particularly during a storm event, various laboratory tests are conducted in practice (e.g. cyclic direct shear tests as shown in Fig. 1, or cyclic triaxial tests, drained or undrained, with constant load or constant volume). At best, there are rough concepts but no validated methods for translating the soil behavior determined in element tests to the behavior of the system, i.e. the foundation elements. There is a lack of fundamental knowledge about which cyclic tests are meaningful to conduct and how the results of these tests should be incorporated into the design process.

The aim of this subproject is to create the fundamental knowledge required to realistically describe the load-bearing behavior of a foundation subjected to cyclic loading and its changes over the operational lifetime. This is essential for the planned realization of a Digital Twin of a megastructure within the Collaborative Research Center (CRC), as without it, neither the behavior during the operational phase can be described, nor the effects of altered operational concepts can be investigated.

To study the behavior of soil under cyclic loading at the element level, further tests using the direct shear device acquired during the first funding period are planned for the second funding period. The goal is to describe the effects of preconditioning in the form of preloading or preshearing on the cyclic response of the soil sample under drained and undrained loading. The results of the planned several hundred tests will then be transformed into a contour plot and serve as a basis for numerical calculations in AP3 (Fig. 2).

Fig. 2: Deformed sample in a direct simple shear test (DSS) and the evaluation of a test under constant volume (cv condition).

To study and describe the behavior of soil under cyclic loading at the system level, a new model setup for experimental investigations was planned and completed by IGtH during the first funding period. The investigations in the first funding period were conducted on a model of a gravity-based foundation under drained and partially drained conditions. In the current funding period, the focus is instead on investigations of a monopile model foundation, also under monotonic and cyclic loading, which can be implemented with simple, minor adjustments to the existing model setup.

Fig. 3: Adapted test setup with a monopile model foundation

In this context, in addition to the overall deformation of the monopile (settlement and rotation), the pore water pressures and their accumulation over time are of particular interest (Fig. 3).

With the investigations at the element and system levels, the prediction of the behavior of various foundation structures can be validated and further developed using the explicit numerical model (EPPE) developed by IGtH. One goal is the development of a generic methodology for predicting the load-bearing and operational behavior of foundations subjected to intensive cyclic loading.

In close collaboration with subproject A4, experiments with a monopile model (D ~ 60 cm) are also planned in the large wave flume (GWK+). The behavior of the pile under wave loading, as well as under additional cyclic horizontal loading applied via a test cylinder, is to be measured, with a particular focus on the magnitude of pore water pressures in the sand bed. Furthermore, the effects of scour and scour protection layers on the pile’s load-bearing behavior will be investigated. These experiments provide an excellent complement to the experimental investigations planned on a significantly smaller scale in AP 2 and can be used to validate the explicit and implicit numerical simulation models of AP 3 (Fig. 4).