Lateral Bearing Characteristics And Evaluation Method For Large-diameter Monopile Foundation Of Offshore Wind Turbine In Completely Weathered Rock Seabed

With the proposal of the "Carbon Peak,Carbon Neutrality" national strategic goal,developing new clean energy represented by offshore wind power has become an effective way to solve energy shortages and environmental pollution problems.China has a long coastline and abundant offshore wind resources,and the offshore wind power project is developing rapidly in China in the past decades.At present,the cumulative installed capacity of offshore wind power in China is 26.39GW,ranking first in the world,and its newly installed capacity has been ranked first in the world for many years.The south-east coastal area is one of the most abundant offshore wind energy resources in China.With the gradual planning and construction of offshore wind farms in Fujian,Guangdong and Shandong Province,the geological conditions of offshore wind power construction have gradually shifted from soil seabed to weathered granite seabed.There is abundant research on the safety analysis and design methods of large-diameter monopile foundation of offshore wind turbine in soil seabed at domestic and foreign,however,there are few reports on the studies of large-diameter monopile foundation in weathered granite seabed.The geologic design parameters and p-y curve design method of offshore wind turbine foundation in the existing specifications and research are basically only for soft clay and sandy soil,however,dynamic soil properties of weathered granite under cyclic loading,bearing characteristics of monopile foundation in weathered granite seabed and evaluation of long-term service performance need to be systematically studied.Completely weathered granite,as a kind of special soil formed by weathering of granite,which contains a lot of viscosity mineral,also has a certain amount of weathering quartz mineral,moreover.its physical and mechanical properties is similar to soil,and retains some granite rock characteristics,leading to significantly different from clay and sand soil materials in engineering geological properties.Weathered granite seabed is the the research feature,bearing performance is the main study line,and the large-diameter monopile foundation is the research object in this study.Aiming at the dynamic characteristics of completely weathered granite under cyclic loading,bearing characteristics and bearing capacity evaluation method during operation of large-diameter monopile foundation of offshore wind turbine in completely weathered granite seabed,research methods are the dynamic triaxial test,theoretical analysis,model test,numerical analysis and field test.At the soil unit scale,the dynamic response of completely weathered granite under cyclic loading is analyzed,and its bearing characteristics are revealed,and the cyclic failure criterions are established.On the model scale,the horizontal loading characteristics of the large diameter monopile foundation in completely weathered granite seabed are studied,and the p-y design method is also established.In the engineering scale,the vibration characteristics of monopile foundation are investigated,and evaluation method of bearing capacity of monopile foundation is established based on vibration characteristics in the operation period.The primary research work and main research results are as follows:(1)Dynamic responses and mesoscopic bearing characteristics of completely weathered granite under cyclic loading.Dynamic triaxial tests under different stress paths are carried out basted on the GDS dynamic test system.The evolution of stressstrain hysteresis curve,effective stress path change,pore water pressure accumulation and stiffness attenuation of completely weathered granite samples under long-term cyclic loading are analyzed,and the mesoscopic bearing characteristics of completely weathered granite samples at soil unit scale are revealed.Under different cyclic stress ratios(CSR),the dynamic responses of completely weathered granite specimens under cyclic loadings can be divided into two different development modes,i.e.,the stable mode and the destructive mode.According to the two different development modes,two cyclic failure criteria were proposed for completely weathered granite based on accumulated strain and dissipated energy respectively.By comparing with the mechanical properties of silty sand and clay,the unique mechanical properties of completely weathered granite are comprehensively analyzed.It provides property parameters of seabed materials for foundation design of offshore wind turbine in rockbased sea areas,and fills in the research lack of mechanical properties of completely weathered granite seabed materials in the field of offshore wind power.(2)Hypoplastic cyclic constitutive model of completely weathered granite.Based on testing results of necessary geotechnical tests,static triaxial tests and dynamic triaxial tests of completely weathered granite,the parameters of the hypoplastic cyclic constitutive model of completely weathered granite are calibrated.Moreover,according to testing results of the static triaxial tests,dynamic triaxial tests and monopile foundation model tests the applicability of hypoplastic cyclic constitutive model of completely weathered granite and the accuracy of numerical analysis method are verified.Finally,the numerical analysis model of the interaction between completely weathered granite seabed and monopile foundation is established,which overcomes the defect of the traditional elastoplastic constitutive model in reflecting the hysteresis and nonlinearity of soil materials.(3)Bearing characteristics and failure modes of large-diameter monopile foundation of offshore wind turbine in completely weathered granite seabed.Numerical analysis and model test are carried out to investigate the horizontal loading characteristics of large-diameter monopile foundation in completely weathered granite seabed.The load-displacement response,pile deformation,bending moment and p-y curve of monopile foundation are investigated under static and cyclic loadings,and the ultimate bearing prediction model of rigid monopile foundation is established.Based on the full-scale pile-soil interaction numerical model,the influence of cyclic loading with different strength on the post-cyclic bearing capacity of large diameter monopile foundation is analyzed,and cyclic loadings are divided into three different stages,i.e.,small cyclic response stage,significant cyclic response stage,ratchet effect stage.According to calculation results of numerical analysis,the p-y curve design method for large diameter monopile foundation of offshore wind turbine in completely weathered granite seabed is established.The strength reduction model of moderately weathered granite is established,and failure modes of different types of rock-socketed monopiles are investigated by considering the imbedment effect of weakly weathered granite layer on the bottom of rock-socketed monopiles.Above studies fill in blank of design method and failure mode of monopile foundation of offshore wind turbine in weathered rock seabed.(4)Evaluation method for bearing capacity of monopile foundation of offshore wind turbine in completely weathered granite seabed during operation.The vibration characteristic model test is carried out for the supporting structure and monopile foundation of offshore wind turbine.A new cyclic loading device is designed for the vibration model test of wind turbine structure.The acceleration signal characteristics in time domain and frequency domain under long-term cyclic loading are analyzed,and the natural frequency variation law of foundation structure is revealed under cyclic loading.The cumulative displacement variation law of monopile foundation under cyclic loading is investigated,and the internal relationship between natural frequency and bearing capacity of monopile foundation is revealed.Based on vibration characteristics,the evaluation method for bearing capacity during operation of offshore wind turbines is established.On the basis of above research,the field vibration tests of monopile foundation and offshore wind turbine are carried out in Pinghaiwan offshore wind farm in Putian City,Fujian Province,the natural frequency of wind turbine structure is analyzed,and the operation status of monopile foundation is assessed.