STATIC AND DYNAMIC RESPONSE OF MONOPILES FOR OFFSHORE WIND TURBINES
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Current design recommendations for monopile foundations for offshore wind turbines were designed and tested for offshore oil and gas structures. Large-diameter (3.5 to 7 m) monopiles, like the ones used for offshore wind turbines do not follow the same elastic deformation relationships as those developed for small-diameter (0.5 to 1.5 m) monopiles, like the ones used for offshore oil and gas industry. Design recommendations and other publications differ by up to 90% in the prediction of displacement and rotation influence factors used in the design of rigid monopiles. Furthermore, offshore wind turbine structures are sensitive to rotations and dynamic changes in the pile-soil system. Accurate prediction of rotation is a major design problem. A numerical analysis is presented for the study of static loading of monopiles for offshore wind turbines. Different foundation geometries and loading conditions were studied and used to develop a functional form of design equations. Studied variables include (i) pile diameter, (ii) pile length to diameter ratio, (iii) load eccentricity to pile diameter ratio, and (iv) soil stiffness distribution profile. An experimental study of the dynamic response of model offshore wind turbines founded on monopiles is presented. These dynamic experiments were analyzed by comparing the physical model test results of natural frequency to an analytical solution, and two methods were compared for the damping calculation. Studied parameters include (i) soil condition, (ii) pile foundation material, (iii) rotor and nacelle mass, and (iv) loading condition. The response to free vibration and its decay was monitored using a vertical array of MEMS accelerometers, and the collected data were processed using auto-power spectra to assess the natural frequency of the structures. Physical model results were compared to closed-form approximations using Rayleigh?s energy method. Good prediction of natural frequency was achieved for turbines founded in rock and stiff sandy soil conditions, however, the natural frequency of turbines founded in soft clay was overpredicted by up to 40%. Damping results were achieved in all cases using the logarithmic decrement method. Relative pile-soil stiffness effect on damping was also studied. For a given impulse loading, damping ratios slightly increase as the pile becomes more rigid. These results highlight the importance of structure-sediment interaction in the evaluation of the dynamic performance of offshore wind turbines.