| Composite bucket foundation(CBF)is a new type of foundation for offshore wind turbines(OWTs).When it is installed,it will exist in the marine environment for a long time and will resist long-term repetitive loading from wind,wave and current,and may be impacted by a ship or objects.In order to study the dynamic characteristics of CBF,the small-scale(1:150)foundation model with and without bulkheads were designed and experimental studies on the dynamic characteristics of a small scale foundation model in sand under variable amplitude cycle loading,constant amplitude cycle loading and variable frequency cyclic loading were carried out.The influence and function of the bulkheads on the stiffness degradation,accumulative deformation and frequency-domain impedance of the CBF were revealed by the experimental study and numerical simulation method.Then the dynamic characteristics of the large-scale(1:10)foundation model were studied,and the reliability of the stiffness degradation,accumulative deformation and variation of frequency-domain impedance for the CBF under cyclic loading were further clarified.Finally,aiming at the dynamic response of CBF under ship collision,a three-dimensional explicit dynamic collision finite element model of large-scale ships and CBF was established.A series of numerical simulations were carried out and the dynamic response and crashworthiness of CBF were revealed.The main content of the innovation and contribution in this thesis are listed as follow.(1)Through the experimental study on the dynamic characteristics of small-scale CBF under variable amplitude cycle loading,it is revealed that one of the functions for the bulkheads is to enhance the dynamic bearing capacity of CBF,increase the energy consumption capacity,delay the stiffness degradation and change the damping.(2)Through the experimental study on the dynamic characteristics of small-scale CBF under constant amplitude cycle loading,it is revealed that another role of the bulkheads was not only to delay the appearance of plastic deformation,but also to effectively reduce the accumulative deformation final value and the long-term settlement of CBF.The bulkheads could also effectively reduce the incremental deformation and increase the unload stiffness.The natural frequency of the whole structural system became larger after suffering long-term cycle loading,and was 8% greater than the initial natural frequency,and the natural frequency is still far less than the 3P frequency.(3)Through the experimental study on the dynamic characteristics of small-scale CBF under variable frequency cycle loading,it is revealed that the stiffness and inertia effect of the CBF with and without bulkheads reduced with the increase of the load frequency.And the damping increased with the increase of the load frequency.The stiffness and inertia effect of the foundation with bulkheads were smaller than that of the foundation without bulkheads,so the damping was.(4)Through the experimental study on the dynamic characteristics of the prototype CBF under cyclic loading,it is found that the accumulative deformation values produced by the large-scale foundation were very small and had little effect on the bearing capacity of the foundation.The accumulative deformation caused the foundation to tilt;however,the amount of tilt was tolerable for a wind turbine support structure,so it did not have effect on the stability of CBF.It also plays a guiding role in predicting the characteristics of foundation prototype.In addition,the adsorption force appears in the middle room of CBF under cyclic loading,which is beneficial to the bucket-soil combined bearing mode.(5)Through composite cylinder foundation prototype and large-scale ship dynamic simulation,the dynamic response of CBF was obtained.When the radius of curvature of the transition segment got smaller,the structural strength became weaker.When 48 bundles pre-stressed tendons were added in the transition segment,it could improve the crashworthiness performance of the entire structure and meanwhile it was economic due to the save of a large amount of materials.The prestressed tendons in the arc transition segment play a major role in the energy consumption during the ship collision. |
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