| Offshore jack-up drilling platforms are currently the most widely used drilling platforms at home and abroad.Most drilling activities with water depths below 150 meters are carried out by mobile jack-up drilling platforms.The spudcans are the foudations of jack-up platforms.The spudcan generally has a diameter of 10-20 m,and is shaped as a disc-shaped foundation with a tapered angle.It usually penetrates into the soil to a depth of about several diameters during installation.During the installation process,the soil above the spudcan will flow back to fill the cavity above,and the soil around the spudcan will reduce its strength due to soil disturbance and remodeling,resulting in a decrease in the shear modulus of the soil,which will affect the stiffness and bearing capacity of the spudcan.In the design of jack-up platform,the initial stiffness of the spudcan will be integrated into the entire structural analysis as a boundary condition,thus influence the internal force of the entire jack-up drilling platform.The ground may be in elasto-plastic state or plastic yield state under storm loading,so secant rotational stiffness reduction formula is used for spudcan rotational stiffness reduction,which requires the combination of stiffness and bearing capacity yield surface.The study of secant rotational stiffness,yield surface and the initial stiffness of spudcan is to study the mechanical behavior of spudcan in elastic to elastoplastic and then to plastic soil.Although there have been some studies on the stiffness and bearing capacity of the spudcan in the past few decades,the spudcan is almost always assumed to be buried in undisturbed soil,and the installation effect of the spudcan is not considered,which is obviously contrary to the reality.In addition,the research on the envelope of the spudcan foundation has been further developed,and the research on the secant rotation stiffness of the spudcan foundation related to the envelope surface needs to be improved.The above problems are further discussed in this paper.A dual-stage Eulerian–Lagrangian analysis technique is used to simulate the whole process from the installation stage to the load-displacement behavior after installation,and the effectiveness of the model and the numerical method is verified by comparing with the centrifuge test results,the available analytical solutions and numerical solutions.This research can be divided into three parts: the initial stiffness,the secant rotation stiffness and the envelope of spudcan.In the part of initial stiffness,without considering the installation effect,the influence of non-homogeneity of strength,embedding depth and size of spudcan on the dimensionless initial stiffness coefficient of spudcan is explored,and the expression of dimensionless initial stiffness coefficient is proposed.When the installation effect is taken into account,the reduction coefficient is used to reflect the initial stiffness reduction,and the expression of the reduction coefficient is proposed.In the part of secant rotation stiffness and yield surface,the installation effect on them is emphatically studied.Combined with the verification and analysis of the envelope surface considering installation effect,the failure ratio form which can take installation effect and the eccentricity of the envelope surface into account is put forward,and on this basis,the formula of secant rotation stiffness reduction factor is updated. |
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