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Serviceability of foundations for offshore wind power plants

During their lifetime offshore wind power plants (OWPP) are subjected to a high-cyclic loading caused by wind and waves, leading to an accumulation of permanent deformations in the soil. The tilting of the foundation of an OWPP has to be kept in small tolerances in order to guarantee the serviceability of the turbine. However, no validated models for the prediction of the long-term deformations caused by a high-cyclic loading exist so far. For that purpose the IBF has developed a special strategy, using finite element calculations and a high-cycle accumulation (HCA) model. Only a few cycles are calculated with a conventional constitutive model, while larger packages of cycles are treated with the HCA model. The HCA model predicts the permanent strains in sand due to a high-cyclic loading in an analogy to viscoplastic models formulated for creep deformations in cohesive soils. The parameters of the HCA model can be determined from cyclic triaxial tests or estimated from correlations based on granulometry. In a first step the HCA model has been validated on model tests with different scales. These model tests were performed on gravity base and monopile foundations with different soil densities, loading amplitudes and directions of cyclic loading. The model tests have been simulated with the HCA model, showing a relatively good agreement between the measurements and the prediction. In a second step the HCA model has been applied to a large-scale (1:1) test performed by the Ed. Züblin AG on a shallow foundation under high-cyclic loading. This foundation is a prototype of real OWPP foundations. The good agreement between the FE prediction and the measurements in the large-scale test, regarding settlement, tilting, stress redistribution and pore water pressure development in the subsoil, confirms the applicability of the HCA model for predictions of the serviceability of OWPP foundations.

Author(s):

Torsten Wichtmann    
Institute for Soil Mechanics and Rock Mechanics, KIT
Germany

Hauke Zachert    
Institute for Soil Mechanics and Rock Mechanics, KIT
Germany

Theodoros Triantafyllidis    
Institute for Soil Mechanics and Rock Mechanics, KIT
Germany