Cyclic testing of North Sea sands subject to loads from offshore wind turbine foundations

This thesis examines the cyclic behaviour of North Sea sands under dynamic loads from offshore wind turbine foundations, a crucial aspect of renewable energy development. It focuses on geotechnical factors, especially soil-pile system degradation, vital for offshore wind structures' stability and sustainability. The methodology involved undrained static and cyclic simple shear tests on various North Sea sands, considering both dry and saturated states, and variables like consolidation stresses, relative densities, and cyclic shear stresses. This simulated real marine conditions, aiding a comparative analysis with established models like Andersen's (2015). Key factors such as fine content and drying methods were also analyzed, essential for predicting sands' mechanical behaviour under cyclic stresses. Replicating cyclic DSS test conditions as per the Fugro report for a Belgian offshore wind farm at Ghent University, the study validated the consistency of test results. Key findings show an inverse relationship between Cyclic Stress Ratio (CSR) and failure cycles, influenced by sands' relative density. Higher CSR levels indicated more pronounced effects on soil deformation and failure mechanisms. Denser sands exhibited increased resistance to cyclic loading. Deviations from Andersen's models, particularly at lower CSR and shear strain ranges suggest a diverse response in North Sea sands, indicating a need for model recalibration. These findings emphasize the need for considering CSR and local soil characteristics in offshore wind turbine foundation design, advocating for a tailored approach for a comprehensive understanding of soil behaviour under varying conditions.