Scouring Characteristics Around Suction Multi-bucket Foundation And Its Influence On Bearing Capacity In Sandy Soil

As a key developing area of renewable energy in China,offshore wind power will enter a new development period during the"14th Five-Year Plan"period.Suction bucket foundation is expected to become an important type of foundations for offshore wind turbines in the future due to its advantages such as:convenient construction and installation,low noise,reusability,and easy dismantlement.In the marine environment,vortices generated by ocean currents and waves around the suction bucket foundation will cause the local scouring around the bucket.Local scouring will reduce the bearing capacity of the foundation,thereby causing negative influence on the structural stability and dynamic response characteristics of offshore wind turbines.At present,research on local scour of offshore wind power foundations focuses on pile foundations,and the research on the impact of local scour on the bearing capacity of foundations is also mainly carried out around pile foundation and single-bucket foundations.In order to study the local scour around suction bucket foundation in the marine environment and the impact of scouring on the bearing characteristics,research method combining physical model test and numerical simulation will be used.The main research contents and conclusions are as follows:The indoor wave flume scouring model test is designed to study the scouring characteristics of suction bucket foundation under different wave and current conditions and different foundation layout forms.The influencing factors of scouring process,scouring pit shape and maximum scouring depth were analyzed according to the scouring depth around the bucket and the three-dimensional scouring terrain.The test results show that the scouring process is divided into three stages:rapid scouring-slow scouring-scouring balance.The form of scouring is a semi annular scouring pit at the front and herringbone sedimentation at the rear.Under the same current conditions,the maximum scouring depth of tripod bucket foundation is greatly affected by the height diameter ratio,the bucket spacing and the current angle.The maximum scouring depth increases with the increase of height diameter ratio;Within the optimum bucket spacing range considering the balance of steel consumption growth rate and bearing capacity growth rate,the scouring depth around the inner side of the front bucket increases with the reduction of bucket spacing,but the scouring depth around the outer side of the bucket remains constant;When the current direction is parallel to the center line of the front and rear bucket,the front bucket will hide the rear bucket.When angle exists between the current direction and the center line of the front and rear bucket,the vortex shedding of the front bucket will enhance the scouring of the rear bucket.The wave-current condition also has great influence on the maximum scouring depth around the bucket:under current condition,the scouring depth around the bucket increases with the increase of current velocity,and the growth rate gradually becomes faster;Under the combined of wave and current,with the same current velocity,the scouring depth around the bucket increases and the growth rate gradually decreases with the increase of KC number and Fr number,.Based on the research on the local scouring of suction bucket foundation,the bearing capacity model test of multi-bucket foundation under scouring condition is designed to study the failure mode and ultimate bearing capacity of tripod-bucket foundation under horizontal and bending moment.According to the movement phenomenon of the bucket foundation model in the test process and the displacement data calculated by the finite element method,the movement and failure mode of the tripod-bucket foundation under the action of horizontal and bending moment and the influence of scouring on it are analyzed.The results of model test are verified and extended through finite element simulation,in order to analyze the effect of scouring rate e and bucket spacing S/D on ultimate bearing capacity under horizontal load and bending moment.The calculation method of scouring correction coefficients d_he and d_me of the ultimate bearing capacity of tripod-bucket foundation is also put forward.The failure envelope of tripod-bucket foundation under M-H loading is obtained through finite element numerical calculation using the fixed load ratio method.The effects of scouring rate e and tube spacing S/D on the failure envelope are analyzed.Based on previous studies,the calculation method of M-H failure envelope of three tube foundation under scouring condition is proposed.