Numerical study of effect of fluke inclination on behavior of drag anchor in clay with linearly increasing shear strength under unidirectional and combined loading

Drag anchor is a widely used anchor type in offshore engineering for the mooring system. The prediction of the anchor trajectory installation and the final position is important for anchor selection in design. The existing method using yield envelope method for trajectory prediction ignored the shallow anchor behavior but applied the deep yield envelope from a deeply embedded horizontal fluke in uniform clay for the whole drag-in installation process. However, the anchor fluke embedment depth and inclination angle change continually during installation in clay with linearly increasing shear strength soil profile in practice. Studies on the effect of fluke inclination angle on the anchor behavior in clay with such non-uniform soil profile under unidirectional and combined loading are important and necessary for the improvement of the yield envelope method to ensure a reasonable prediction. The current 2D finite element studies investigate the anchor behavior for inclined fluke in clay with linearly increasing shear strength under unidirectional vertical, horizontal and rotational loadings first. Then the effects of the fluke inclination angle, soil non-homogeneity and embedment depth ratio on the shallow yield envelopes are investigated. It is found that the effect of fluke inclination angle on the vertical capacity factors for anchor in clay with non-uniform and uniform soil profile is largely different. The resultant large impact on the yield envelopes shown here illustrates the importance of considering the fluke inclination angle and soil non-homogeneity in the prediction of anchor trajectory using yield envelope method.     

Behavior of a shallow plate anchor in clay under sustained loading

Laboratory model test results for the uplift of a shallow circular plate anchor embedded in a soft saturated clay are presented. For all tests the bottom of the anchor plate was vented to eliminate the mud suction force. The tests were divided into two categories: (1) short‐term tests to determine the variation of the net ultimate uplift capacity and hence the breakout factor with embedment ratio, and (2) creep tests with sustained uplift loads at varying embedment ratios. Based on the model test results, the variation with time, has been determined for the rate of strain of the soil located above the plate anchor. Empirical relationships for obtaining the rate of anchor uplift have been proposed.     

A method analyzing deformation of anchor foundations in soft clay under static and cyclic loads

The paper presents a constitutive model to describe undrained cyclic stress-strain responses of soft clays based on the equivalent visco-elastic and creep theories. The hysteretic and nonlinear stress-strain responses of soft clays are described using the equivalent visco-elastic relationship and variations of the cyclic modulus and the damping ratio with the octahedral shear strain, respectively in the model. The cyclic accumulative strain is described using the Mises creeping potential function and the associated flow rule. The method determining the model parameters is given by static and cyclic triaxial tests. The finite element method to analyze deformation of anchor foundation in soft clay under static and cyclic loads is developed based on the model. For the method, a cyclic loading time history is divided into a series of incremental loading sub-processes which include one load cycle at least. The cyclic stress-strain responses of soil elements at any time are not tracked in detail and determined by the equivalent visco-elastic calculations for every loading sub-process. The accumulative deformation of anchor foundations is calculated using the initial strain algorithm. The method has been implemented in ABAQUS Software by developing interface programs. Model tests of the suction anchors are conducted and predicted using the method. Comparisons of predicted and model test results show that the method can be used to evaluate cyclic stability and reveal the failure process and mechanism of anchor foundations by analyzing deformation time-histories.