Soil-Anchor Interaction Effects on Dominant Frequencies in Offshore Wind Turbines Supported by Tension Leg Platforms

This study presents an experimental investigation into the influence of anchor-soil interaction on the dynamic behaviour of tension leg platform (TLP)-type floating wind turbines. A 1/100 scale model of the NREL 5-MW reference wind turbine was fabricated using a 3D printer, with scaling parameters determined based on Froude scaling laws to ensure dynamic similarity between the model and prototype. A comprehensive discussion of the applied scaling principles is provided, along with a detailed description of the calibration procedures for the custom-developed six-axis sensors used in the experiments. Free vibration tests were performed on the scaled model to evaluate the influence of different anchoring systems-suction caissons, triple-suction caissons and gravity anchors-under varying seabed conditions. Throughout the experiments, six-axis sensors installed on both the nacelle and within the floating platform captured time-dependent accelerations along the x, y and z directions, as well as rotational responses about the same axes. To ensure the robustness and repeatability of the results, each test was conducted a minimum of three times, mitigating potential experimental uncertainties. The experimental findings demonstrated a pronounced influence of anchoring systems and seabed conditions on the dominant surge and pitch frequencies of the floating wind turbine model. Specifically, relative to the fixed-bottom case, the first peak frequency was reduced by 20.3% with suction caissons, 8.5% with triple-suction caissons and 28.1% with gravity anchors. Additionally, an increase in seabed relative density from 30% to 68% led to a 19.6% rise in the dominant frequency, attributed to the increased soil stiffness and lateral resistance. These results highlight the critical role of anchor-soil interaction in shaping the dynamic behaviour of TLP-type floating wind turbines, emphasizing the necessity of integrating these effects into their design and analysis to enhance predictive accuracy and ensure structural reliability.