Study On Mechanical Properties And Bearing Capacity Of Typical Seabed Soil In North And South Offshore Wind Farms In China

Offshore wind power has become a reliable means for countries to solve energy shortage and climate problems because of its unique advantages such as cleanness,sustainability and abundant resources.Although China’s offshore wind power industry started late,it has developed rapidly in recent years with the advantages of terrain and policy.There are many offshore wind farms in China,and the geological conditions of each region are very different,showing significant regional concentration.Typical areas include Shandong and Jiangsu sea areas dominated by soft clay seabed and Fujian and Guangdong sea areas dominated by weathered rock seabed.There are great differences in the physical and mechanical properties of seabed materials in these two typical areas,resulting in great differences in the bearing capacity of offshore wind power foundation.Therefore,in this paper,typical seabed rock and soil materials(mucky clay(soil A)in Bohai Sea area and weathered granite residual soil(soil B)in Fujian sea area)are taken from two regional offshore wind farms in North and South of China.The mechanical properties and bearing characteristics of the two kinds of marine rock and soil are systematically compared by means of theoretical analysis,laboratory test and finite element analysis,so as to provide guarantee for the design of offshore wind power foundation.In this paper,the basic mechanical properties and static characteristics of two kinds of marine soils are determined through indoor foundation geotechnical test and static triaxial test;Then a series of dynamic triaxial tests with different confining pressures and different dynamic stress ratios are carried out to compare the development laws of the dynamic characteristics of the two kinds of marine soils.Based on the test results and pore pressure failure criteria,the critical failure criteria of the two kinds of soils are proposed;Finally,ABAQUS software is used to establish the numerical model of offshore wind power single pile based on marine soil,study the bearing capacity of pile-soil foundation under horizontal static load and dynamic load considering wind and wave action,and compare and analyze the applicability of API code to p-y curve in various working conditions.The research conclusions are as follows:(1)Through basic physical and mechanical laboratory tests,it is found that silty clay in Bohai sea has high moisture content,low density,high liquid limit and high compressibility,while weathered granite residual soil in Fujian sea has low moisture content and high density,medium liquid limit and medium compressibility.Through static triaxial test,the effective shear strength indexes of two kinds of soil are determined as follows:soil A c’=11.88 kPa、φ’=30.94°;soil B c’=6.57kPa、φ’=34.77°.(2)The dynamic triaxial test is carried out by stress control method,and the development laws of dynamic characteristics of two kinds of marine soils are discussed.The research shows that under cyclic dynamic load,the mucky clay in Bohai Sea area is easier to destroy and its dynamic strength is lower than that of weathered granite residual soil in Fujian sea area;Based on the test data,the logarithmic function σd=m*lnNf+n fitted the relationship between the critical failure vibration times and dynamic strength in the three confining pressures of dynamic triaxial test,extended to any confining pressure level,and constructed the soil critical failure model(σd=a*lnNf+b*σc/lnNf+c),which shows that the critical failure surface of muddy clay in Bohai Sea area is located below the weathered granite residual soil in Fujian sea area,indicating that the dynamic bearing capacity of residual soil is stronger under the same conditions.(3)The numerical model of offshore wind turbine single pile under static and dynamic load is established.The results show that the pile bending moment,shear force and soil resistance of silty clay foundation under horizontal static load are smaller than those of residual soil foundation,but the pile displacement is larger.The comparison between the p-y curves under static and dynamic loads and the p-y curves calculated by API specifications shows that dynamic loads reduce the bearing capacity of seabed foundation,and there is a deviation in the applicability of API specifications for the two kinds of soil foundations,especially for residual soil.