| With the adjustment of China’s energy strategy,wind power has become the third largest energy source after coal power and hydropower.Bucket foundation is an important basic type of offshore wind turbine.Bucket foundation has many advantages,such as short installation period,saving steel,and reusable.It has obvious advantages in reducing project cost and recycling.In the application of the bucket foundation,the evaluation penetration resistance,the control of suction during penetration,and the prediction of the height of the excess soil plug are directly related to whether the bucket foundation can be successfully settled to the specified depth.In this paper,the theoretical analysis,numerical simulation,model tests and field tests are used to study the stability of seepage flow,the calculation method for penetration resistance,the prediction method of excess soil plug height and the drag reduction measures.The specific research contents and results are as follows:(1)Model tests and numerical calculations were carried out to study the characteristics of the seepage field and seepage stability during the installation of bucket foundation by suction.The influence of soil permeability,bucket foundation structure and pressure of bucket foundation with inner compartments on the seepage field is revealed.Considering the actual conditions of the foundation that may be encountered,the calculation method of the critical suction of the impervious layer under the sand is proposed,and the calculation method of the critical suction of bucket foundation with inner compartments when the inner compartments is different is proposed.(2)Model tests of bucket foundation installation in sand were carried out.The change laws of the earth pressure and pore pressure in bucket wall and bucket tip during the installation by suction were revealed.The laws of the influence of the two installation methods on the side friction resistance and the tip resistance were analyzed.The calculation method of the penetration resistance and the required suction of the bucket foundation are established.(3)Model tests of bucket foundation installation in clay were carried out.The change laws of the earth pressure and pore pressure in bucket wall and bucket tip during the installation by suction were revealed.The laws of the influence of the two installation methods on the side friction resistance and the tip resistance were analyzed.Based on the API specification,the calculation methods of penetration resistance in the clay by positive pressure and suction are proposed,respectively.(4)Experimental research on the phenomenon of excess soil plug by suction installation of bucket foundation in sand and clay were carried out.The influence of the penetration rate(v)and thickness-to-diameter ratio(t/Do)of the bucket foundation on the phenomenon of excess soil plug were studied.The mechanism which the penetration rate(v)and thickness-to-diameter ratio(t/Do)affect the excess soil plug rate was revealed by combining the measured pore pressures in the tests.The calculation methods of the excess soil plug height considering the penetration rate(v)and thickness-to-diameter ratio(t/Do)of the bucket foundation is proposed.(5)Aiming at the installation of bucket foundation in low permeability soil,the application of drag reducing ring is proposed.Combining experiments and numerical calculation methods considering soil strain softening,the drag reduction mechanism of the drag reduction ring is revealed.The influence of the layout of the drag reduction ring on the degree of drag reduction is studied.The law of the influence of the drag reduction ring on the strength of the soil around the bucket wall is analyzed.The calculation method of penetration resistance in silt considering suction and drag reduction structure is proposed. |
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