Numerical Analysis Of Cyclic Bearing Capacity Of Embedded Structures In Sand Based On Boundary Surface Model

Embedded offshore structures are designed to resist the extremely complex marine environment.Inevitably,such structures and the soil around are directly or indirectly suffering from the continuous cyclic loads caused by wind,tides and waves.The nonlinear dynamic characteristics of ocean soil generate a complex load-displacement response of embedded offshore structures under cyclic loads and difficulties in the design and maintenance of the structure.Presently,the research on the cyclic bearing capacity of embedded offshore structures is usually limited to clay.Due to the highly complicated stress-strain response of sand,the research on the cyclic dynamic characteristics of the embedded offshore structure in sand is still in the preliminary stage.In this paper,based on a bounding-surface plasticity model for drained cyclic behaviors of sand and an error-controlled explicit integration algorithm,a numerical model for analysis of the cyclic bearing capacity of embedded offshore structures in saturated sand is established.Firstly,a summary of existing researches on cyclic bearing characteristics of embedded offshore structures is conducted.It is found that the main research means for the dynamic response of embedded offshore structures is model tests and field tests.Although different kinds of numerical analysis methods have seen a booming development,still no well-performed numerical analysis model has been introduced yet.The dynamic constitutive numerical analysis which is equipped with a reasonable constitutive model theory is considered to be one of the most important methods to evaluate the cyclic bearing characteristics of embedded offshore structures.However,the simulation of anchor-soil interaction under long-term and low-frequency cyclic loads caused by wind,wave and currently cannot rely on the existing dynamic constitutive models of sand,because these models derive from seismic loads featuring with high-frequency and instantaneity.Secondly,by using an explicit integral algorithm with error control,a boundingsurface plasticity model for cyclic loading analysis of sand is programmed into a userdefined material subroutine which can be called by a finite element software.A finite element model of soil element is developed to simulate triaxial tests of Toyoura sand under monotonic and cyclic loading.The simulation results agree well with the model tests,proving that the built model can reasonably describe the mechanical response of sand under different loading conditions.A finite element model of embedded offshore structure(the plate anchor)is established to simulate the anchor response under monotonic loading.The obtained load-displacement response law consists of the model tests,verifying the feasibility of the numerical analysis model in solving the boundary value problem of geotechnical engineering.Finally,a series of numerical analyses are conducted on the cyclic bearing characteristics of plate anchors in the Ottawa sand.The numerical results indicate that with the application of cyclic load,the permanent displacement of the plate anchor gradually accumulates,and the cyclic load will cause the plate anchor to move continuously.By investigating the influence of cyclic loading factors on the cyclic bearing characteristics of plate anchor,it is found that the larger the cyclic amplitude is,the larger the initial displacement and displacement change rate are.Larger cyclic mean produces larger initial displacement,but smaller displacement accumulating rate.