Study On The Bearing Mechanism And Optimization Design Of The Embedded Large-diameter Cylinder Structure

The bearing mechanism and failure mode of the thin-walled embedded large-diametercylinder structure ( or bucket foundation ) in offshore engineering subjected to lateral loadssuch as waves, currents, and icebergs is studied based on the theory of granular material'sstructural mechanics, plastic limit analysis and elasto-plastic finite element method. Themajor work can be summarized as follows.1. A simplified analytic method to calculate the large-diameter cylinder's globaldisplacement considering the interaction between the cylinder and soil foundation is putforward based on the subgrade reaction method of elastic foundation and the theory of limitequilibrium, in which the complex force-displacement mechanism of the cylinder structureis analyzed into the superimpose of three simple displacement modes namely the verticaldisplacement w , lateral displacement u and rotational angular displacement α .2. A plastic limit analysis model of the laterally loaded large-diameter cylinder structureembedded in soft clay is established for the first time, in which the failure mechanism ofthe cylinder is illustrated as the combined failure modes of the pushed-up soil's failurewedge, lateral pressing of the cylindrical cavity expansion and the spherical slip shearingfailure surface at the bottom of the cylinder. The critical failure mechanism andupper-bound of lateral load are obtained by Nelder-Mead's modified simplex method andgenetic algorithm. Through this model, the cylinder's ultimate lateral load can be obtainedavoiding complex and time-consuming nonlinear elastic-plastic FEM analysis on theall-stage loading history, and some important achievements are got with initiatingsignificance, such as the relationship between the embeddment and the rotation center, thefailure wedge's depth, the ultimate lateral load and lateral resistance.3. Based on the upper-bound limit analysis model, a simplified calculation method withthe form of piecewise interpolating function is brought forward to calculate the ultimatelateral load on the large-diameter cylinder embedded in soft clay.4. The nonlinear two-directional spring element numerical analysis model of embeddedlarge-diameter cylinder structure is established, in which the interaction between thethree-dimensional cylindrical shell structure and soil foundation is simulated by coupling ofshell element and spring element, and nonlinear soil resistance-displacement model in theform of p-y curve is introduced. The involving nonlinear spring element model and relevantnumerical processing are great breakthrough from the traditional linear spring elementmodel, which better reflects the failure displacement mechanism of large-diameter cylinderstructure under step loading process.5. Optimization mathematical model of embedded large-diameter cylinder structureunder lateral loads is brought forward for the first time, in which nonlinear spring elementnumerical analysis model of embedded large diameter cylinder structure is taken as thekernel of structure analysis, and optimization process adapting to the characteristic oflarge-diameter cylinder is constituted with secondary development in ANSYS based on theoptimization method of random walk with direction exploration.