Behavior Of Offshore Wind Turbine Jacket Foundation Under Cyclic Loadings And Calculation Methods

Offshore wind power development is one of the key ways to achieve China’s carbon neutrality goal.For offshore wind turbines(OWTs),the embedded foundation in seabed is the ultimate undertaker of all the external environmental loads.The OWT’s safety and stability is directly determined by the embedded foundation.At present,"large-scale" and "deep-water" are the main trends for the development of offshore wind power.Due to the advantages of large stiffness and small loads from wave and current,jacket foundation has become the most suitable type of OWT in the water depth range of 50～100m.For the OWT’s foundation design,it is often controlled by two core factors: the one is natural frequency,which must be located in the narrow safety band to prevent resonance;the other is accumulated deformation.It is necessary to ensure that the accumulated rotation at jacket foundation top does not exceed 0.25° during the 25-year service life.However,most of the wind farms in the coastal area of Southeast China are affected by typhoons.Under the extreme loads from typhoon and huge wave,how to ensure the safety of OWT is the most severe challenge in the foundation design.In the past few decades,many studies have been conducted on the responses of jacket supported OWT under cyclic loadings and related calculation methods,but there are still many key problems to be solved.OWT is a kind of high-rise structure which is sensitive to foundation deformation,the pile-soil interface behavior plays an important role in its deformation characteristic.Therefore,the main existing problems are summarized as follows: in terms of the shear characteristics of pile-soil interface,there is a lack of research on the continuous shear process during pile installation and cyclic loading process in service period.And the effects from the installation shear displacement and shear path on the subsequent cyclic degradation characteristics of interface are not clear;for the simulating of cyclic degradation process of pile-soil interface,the existing cyclic t-z model has many parameters and some of them are difficult to be calibrated through simple geotechnical tests;in terms of jacket foundation OWT with multi-piles,the analysis method coupling pile-soil cyclic interaction and the dynamic response of supported structure is lacked.Also,the convenient evaluation method for the responses of OWT under cyclic loadings with large number is also not available;for the jacket foundation OWT with multi-caissons,the suction caisson models which can reflect the responses under axial cyclic loadings are basically finite element models with complex constitutive and many parameters.Some parameters are difficult to be calibrated.In the engineering design,the convenient evaluation method for the cyclic responses of jacket foundation OWT with multi-caissons under cyclic loadings is lacked.Aiming at the above problems,systematic studies are carried out in this thesis through combining theoretical analysis,element test and numerical simulation.The research contents and innovations are summarized as follows:1.Systematically carry out the pile-soil interface shear tests,including cyclic shear tests under constant normal stiffness condition,and cyclic shear tests considering previous shear distance and shear path of pile installation.The study reveals the cyclic degradation characteristics and internal mechanism of interface strength,and reveals the effects of particle type,previous shear displacement,shear path and other factors on the subsequent cyclic degradation characteristics of interface.Based on the theoretical framework of boundary surface model,a cyclic degradation t-z model is proposed and verified by experiments.The degradation of interface strength,stiffness and the development of accumulated deformation under cyclic shearing can be effectively considered by this model.It contains 7 parameters and can be calibrated by interface cyclic shear test.2.The analysis method of jacket foundation OWT with milti-piles under cyclic loadings is established.Based on the cyclic degradation t-z model,the time history response analysis of jacket foundation OWT under extreme cyclic loadings is realized through the secondary-development of finite element software;based on the test data of cyclic interface shear with large number,a simplified interface strength degradation formula is established,and two simplified cyclic t-z model and corresponding calculation methods are formed.The evolution of natural frequency and accumulated deformation of OWT under cyclic loadings with large number can be evaluated conveniently by using the proposed method.At the same time,a series of numerical simulations are carried out to analyze the evolutions of natural frequency,accumulated deformation,load sharing characteristics and hot spot stress for jacket foundation OWT under extreme cyclic loadings of typhoon.The proposed method can provide technical support for the selection and geometry determination of OWT’s foundation.3.The spring-damper model of suction caisson under axial cyclic loadings is established,and subsequently,the analysis method to evaluate the dynamic responses of jacket foundation OWT with multi-caissons under cyclic loadings is formed.Firstly,the elastoplastic t-z and Q-z springs reflecting cyclic degradation effects are introduced to reflect the axial cyclic responses of suction caisson,including the displacement accumulation,pressure accumulation and degradation of loading stiffness.Furtherly,the analysis method for jacket foundation OWT with multicaissons under extreme cyclic loadings are established,which offers an effective method to evaluate the cyclic responses of OWT.And latter,the proposed method is verified by centrifuge model tests,which can provide a useful reference for the preliminary design of jacket foundation with multi-caissons in typhoon area.