Research On The Monitoring Method Of Foundation Support Con-dition For Monopile Offshore Wind Turbines Considering Soil-structure Interaction

At present,the world is under the background of‘carbon peak’and‘carbon neu-tralization’.Offshore wind power generation technology has great advantages in reduc-ing carbon emissions and solving the problem of energy shortage.The dynamic char-acteristics of offshore wind turbines（OWT）are very sensitive,and the fundamental frequency of the structure is very close to the excitation frequency of external loads.In addition,OWT has obvious soil-structure interaction（SSI）,which may cause the change of support condition and leads to fundamental frequency shift or even unpre-dictable resonance and catastrophic damage.For the current large number of offshore wind farms,how to efficiently and accurately monitor the foundation support condition has become an urgent problem to be solved.For this reason,a model updating frame-work based on measured vibration characteristics and implicit objective function is pro-posed to monitor the changes of stiffness and damping of OWT foundation,which is further verified by laboratory experiments.In the experiment,the layered soil composed of saturated sand and clay is specially considered to simulate the actual soil conditions in the Bohai Sea of China.At the same time,different pile lengths are introduced,and the long-term SSI effects of rigid piles and flexible piles are considered respectively.The specific work is as follows:The second chapter introduces the theoretical basis of complex frequency and complex modal mode shape,gives a detailed theoretical introduction to the EFDD iden-tification method of modal parameters,and gives relevant modal testing suggestions.In addition,the basic idea of model updating is expounded.In chapter three,the simplified calculation model of three springs and rotational dampers is used to simulate soil-structure interaction,and the variation law of founda-tion stiffness of OWT system under different pile length is studied by static loading test.By comparing the foundation stiffness obtained from the test with the existing theoret-ical formula of foundation stiffness,it is found that the foundation stiffness of rigid pile is mainly affected by the length of pile,and the foundation stiffness of flexible pile is mainly affected by the elastic modulus of soil.the variation law of foundation stiffness observed in the test is consistent with the theoretical formula.In chapter four,a series of dynamic cyclic loading tests were carried out to observe the soil-structure interaction law of OWT system under the combined action of wind load and wave load.It is found that for the OWT system in layered foundation,the change of foundation stiffness under cyclic load is affected by both the hardening effect of sand layer and the softening effect of clay layer.Compared with the pure sand con-dition,the change of the fundamental frequency of the OWT system is obviously re-duced in the layered foundation condition.At the same time,it is found that the change of pile length leads to a significant difference in the fundamental frequency of OWT system.Under the same other test conditions,the maximum fundamental frequency changes of OWT systems with pile lengths of 42cm,47cm and 52cm are 8.1%,-4.8%and-4.5%,respectively.In chapter five,a model updating framework considering both foundation stiffness and damping parameters is proposed,and the foundation condition monitoring of OWT system in layered foundation is realized.The comparison between the test results and the model updating results proves the reliability of the model updating framework.Af-ter the model is updated,the variation amplitude of foundation stiffness is about 40%and 15%respectively when the buried depth of pile foundation is 35cm and 45cm,in-dicating that the amplitude of foundation stiffness change is negatively related to the buried depth of pile foundation.The variation law of the rotating stiffnessis basi-cally consistent with the fundamental frequency,which indicates that it has the greatest influence on the fundamental frequency of the OWT system.The initial value of rota-tional dampingbetween 0.01 and 10 has little effect on the model updating results and convergence rate,indicating that it has little effect on the structural fundamental frequency,but considering damping can effectively solve the problem of multiple so-lutions in model updating.