Research Project for the Geotechnical Behaviour of Cyclically Loaded Gravity Base Foundations for Offshore Wind Turbines, Cuxhaven, Germany
Currently there is little empirical information available regarding the design of gravity foundations for offshore wind turbines. Therefore the STRABAG group decided to build a full scale test model of their proposed gravity foundation onshore at Cuxhaven, on the North Sea coast. The test foundation was subjected to a series of tests to simulate the loading conditions offshore, which are dominated by high cyclic wave loads during storms. The location of the test bed was chosen due to soil conditions, which are similar to those encountered in the North Sea.
The cruciform pre-stressed concrete box girder base measures 40m x 40m, with a height of 8m. At each end of the box girders, single bearing pads, each with an area of approx. 100m², are located to transfer the loads to the subsoil. On this base an approx. 23m high shaft, which tapers to a dia. of 5.80m at the top, was constructed. The offshore 65 m high foundation will then be topped with a steel tower and the turbine. To increase the dead weight of the base, the box girders and concrete shaft are filled with sand after installation.
The complete test foundation was built within a 7m deep sheet-piled excavation, which was subsequently flooded to a depth of 4m during the tests. Approximately 150 sensors were installed to measure the stresses, pore water pressures and displacements during the test loadings. Two steel cables were connected from the top of the tower to hydraulic jacks mounted on a reaction frame. Forces of between 1000 and 12000kN were applied with a load frequency of 0.05 to 2.0 Hz, corresponding to wave periods of 20 to 5 seconds. The maximum loads which could be modelled correspond to a 1 year storm event with max. wave heights up to 15 m.
After approximately 6 months of testing, during which some 1,500,000 load cycles were applied to the prototype, the tests are concluded and the results are now being evaluated.
The research project is funded by the Federal Ministry for the Environment, Nature Conservation and Reactor Safety on the basis of a resolution of the German Parliament of the Federal Republic of Germany.
Summary of a paper from Sedlacek, G., Miehe, A., Libreros, A., and Heider, Y., ED. Zueblin AG, Renewable Energy, Central Technical Division, Stuttgart, Germany.
Michael Paul EngTech TMICE, Member in Germany