Comparative Study On Liquefaction Resistance Of Sand Foundation Of Three-Bucket Foundation And Four-Bucket Foundation For Offshore Wind Turbines

With the increasing global demand for renewable and clean energy and the maturity of the offshore wind power industry chain,the development of offshore wind power will maintain a competitive advantage in the future energy market.As the supporting structure of Offshore Wind Turbines(OWTs),the bucket foundation has the characteristics of fast installation and high bearing capacity,especially the multi-bucket foundation,which has the advantages of dispersing load and improving antioverturning capability,attracts more and more attention in the marine engineering field.The foundations for OWTs are mostly located in the offshore wind farm with saturated sand and silt.When sand foundation liquefies,its bearing capacity will be greatly reduced or even totally incapacitated,that may seriously affect the service life and engineering benefit of the foundation structure.The seismic response characteristics of the multi-bucket foundation for OWTs and the liquefaction analysis of the surrounding soil are particularly important for the development and application of foundation structures and engineering safety.The main contents and conclusions of this paper are as follows:(1)Based on the shaking table tests of three-bucket and four-bucket jacket foundation,the excess pore water pressure,excess pore pressure ratio and acceleration response of the sandy soil for two types of bucket foundations under earthquake are compared and analyzed,and the improvement effect of multi-bucket jacket foundation on anti-liquefaction performance of sandy soil is clarified.The growth of the excess pore water pressure and acceleration response of the sandy soil are restained due to the additional load effect and hoop effect,that help reduce the potential liquefaction risk.Besides,it is found that there is a high risk area for liquefaction near the bottom of the bucket wall.(2)According to the test results,the influences of two types of multi-bucket foundations on liquefaction and acceleration responses of sandy soil are compared and analyzed.The advantages of four-bucket jacket foundation in seismic performance are confirmed.The sand foundation of the four-bucket jacket foundation liquefies later,and the acceleration response is smaller.It is found that the improvement effect of multibucket jacket foundation on anti-liquefaction performance of sandy soil weakens with the increase of the distance between the soil and the foundation structure.(3)The three-dimensional models of three-bucket and four-bucket jacket foundation and their foundation soil are established with finite element(FE)software ADINA.The distribution characteristics and variation law of the excess pore pressure ratio and acceleration amplification factor of the sandy soil along the depth direction are analyzed.The reliability of the FE method is verified by comparing the FE results with the test results.The influence of the existence of multi-bucket foundation on the improvement of the stability for the soil inside and around the foundation is clarified.Among them,the influence of the four-bucket jacket foundation on the foundation soil is more significant because of its greater self-weight and symmetrical advantage.The liquefaction occours later.When the peak acceleration of the input earthquake wave is0.4 g,the excess pore pressure ratio of the sand at the center of the three-bucket jacket foundation reaches 1.0 at a depth of 0.1 m,which indicates that the shallow foundation soil has lost its bearing capacity.While the excess pore pressure ratio of the soil at the center of the four-bucket jacket foundation is about 0.9.At the same time,the amplification effect of foundation soil on seismic wave is clarified.The acceleration response of foundation soil in free field shows the phenomenon,that is,the acceleration response is amplified under small earthquakes while attenuated in the case of large earthquakes.The restraint effect of multi-bucket foundation on sandy soil can effectively reduce its seismic response.With the increase of vibration amplitude,the liquefaction of the shallow soil leads to the increase of soil damping ratio,hindering the upward transmission of shear wave,and the attenuation of acceleration amplification effect of foundation soil.