Finite Element Analysis Of Bearing And Deformation Behaviors For Marine Anchor Foundations Under Cyclic Loads

Large-diameter monopile,deep-water suction anchors and other marine anchor foundations,are subjected to working loads(static loads)from superstructures as well as cyclic loads from low frequency long-term and continuous waves in Marine environment.Analysing the deformation process and bearing capacity of marine anchor foundations under combined static loads and cyclic loads is very important for evaluating the stability of suction anchor foundations.Therefore,a finite element analysis method based on cyclic elastic-plastic constitutive model is developed to analyze the bearing and deformation behaviors for large-diameter monopile foundation,a finite element analysis method based on stiffness degradation model is developed to describe the cyclic degradation process of suction anchor foundation in soft clays.The numerical simulation of the existing centrifugal model tests for large-diameter monopile is carried out by establishing a three-dimensional finite element model of pile-soil interaction based on the ABAQUS software.A cyclic elastic-plastic bounding surface model is used to describe the cyclic stress-strain responses of soils around pile during numerical calculation.The finite element analysis method capable of tracking the displacement-time history of large-diameter monopile in dry sands under lateral cyclic loads is established by calling the UMAT subroutine of the model and setting appropriate pile-soil contact conditions,analysis step and calculation parameters.By comparing with the centrifugal model test results,the established finite element analysis method can describe the deformation process of large-diameter monopile under lateral monotonous and cyclic loads in dry sands,and reflect the nonlinearity,hysteresis and displacement accumulation characteristics of cyclic load-displacement curves.Besides,the distribution characteristics and variation law for the bending moment and displacement of large-diameter monopile with the increase of the number of cycles can be predicted.The proposed stiffness degradation model for soft clays was embedded in the ABAQUS software package by encoding the USDFLD subroutine for soft clays with uniform strength and the linear change strength along the depth respectively,and defining relevant field variables.A three-dimensional finite element model of the suction anchor foundation in soft clays was established.The model tests of suction anchors under combined static loads and cyclic loads in soft clays were simulated by using the ideal elastoplastic model based on the Tresca yield criterion and combine with the coded USDFLD subroutine.Numerical simulation methods are developed to describe the cyclic degradation process of suction anchor for the uniform strength in soft clays and the linear change strength along the depth in soft clays,respectively.The comparison between numerical simulation and model test results shows that the finite element method based on the stiffness degradation model can predict the accumulative displacement of suction anchor in soft clays under different cyclic load levels,simulate the cyclic deformation process to some extent,and can describe the nonlinearity,hysteresis and cyclic accumulation characteristics of the load-displacement responses of suction anchors.The cyclic bearing capacity of anchors can also be predicted well based on an appropriate displacement failure criteria.In addition,the method has relative high computational efficiency because of using static constitutive model rather than complex dynamic constitutive model to calculate the soil deformation.