Predicting the cyclic behaviour of suction anchors based on a stiffness degradation model for soft clays

Abstract

The behaviour of suction anchors subjected to combined average and cyclic loads in soft clays is an essential consideration in their design. A new numerical computation method was developed based on a stiffness degradation model for soft clay that was proposed by performing a series of undrained cyclic triaxial tests and embedded in the ABAQUS software package by encoding the USDFLD subroutine and defining relevant field variables. The numerical method predicts the cyclic behaviour of suction anchors by coupling the elastic-perfectly plastic model with the Mohr-Coulomb yield criterion and the proposed stiffness degradation model that can reflect the stiffness degradation and the accumulation of plastic deformation of soils around the anchor during cyclic loading. The numerical method was verified by a comparison with the model test results of suction anchors subjected to combined average and cyclic loads in soft clays. The proposed method can predict the cyclic deformation as well as the bearing capacity and capture the nonlinearity, hysteresis and cyclic accumulation characteristics of the load-displacement responses of suction anchors in soft clays.

Introduction

Suction anchors have been widely used in taut mooring systems for floating facilities, such as floating production storage and offloading (FPSO) units. In many cases, the suction anchor needs to be installed in soft clay foundations, which are subjected to average loads caused by the floatage of the superstructure as well as cyclic loads induced from various environmental factors such as winds, waves and flows in the marine environment [1], [2], [3], [4]. The strength and stiffness of the soil around the suction anchor will be degraded under long-term cyclic loading, which will induce the cumulative displacement of the anchor along the mooring direction. The failure of suction anchors occurs when the cumulative displacement reaches a certain failure standard. Therefore, proposing an appropriate method to analyse the cyclic deformation process and evaluate the cumulative displacement is of great significance for the design and safe service of suction anchor foundations in soft clays.

The finite element method has been widely used for the study of suction anchor behaviours in soils. Some studies have also been reported on the finite element analysis of suction anchors under static or cyclic loads. Many attentions have been focused on the simulation of the behaviours of suction anchors under static loads. The selection of a soil constitutive model is very important for finite element analysis. Sukumaran et al. [5] determined the capacity of a suction anchor subjected to lateral loads in soft clays under undrained conditions using a linear elastic model with the Von Mises strength criterion. Cao et al. [6] simulated the behaviour of suction anchors subjected to vertical loading in normally consolidated clays based on the modified Cam-Clay model of porous soil materials. A computational procedure was proposed by Maniar [7] to simulate the installation process and the load-displacement response of anchors subjected to axial and inclined loads, and a bounding surface model was used to describe the nonlinear behaviour of the clayey soil. Monajemi and Razak [8] investigated the failure mechanism and ultimate capacity of an anchor under combined V-H-M loading by conducting nonlinear analysis using a simplified elastic-perfectly plastic model for saturated clays. Ahn et al. [9] estimated the holding capacities of optimally loaded suction caisson anchors embedded in cohesive soils with a linear strength distribution by the total stress analysis based on the Von Mises model of soil. Kim et al. [10] investigated the performance of suction anchor groups subjected to horizontal pull-out loading by performing centrifuge model tests and numerical simulations, and the soil was modelled using the Mohr-Coulomb failure criterion with the non-associated flow rule. Wang and Guo et al. proposed a novel two-dimensional static model for a mooring cable during pretensioning and a three-dimensional quasi-static model for a cable in service [11]. In addition, Guo et al. performed model tests with different loading angles and cyclic parameters to investigate the failure mode and capacity of suction caissons under inclined short-term static and one-way cyclic loadings [12].

A series of model tests were performed by Chen and Randolph [1] and Zhu et al. [13] to investigate the responses of suction anchors subjected to cyclic loading. However, research on numerical simulations of the cyclic behaviours of suction anchors is limited. The main difficulty is the lack of constitutive models that can not only reasonably describe the nonlinear cyclic stress-strain responses of soft clays but also be easily implemented in 3D nonlinear finite element analysis codes. These models, such as the elastic-perfectly plastic model, modified Cam-Clay model, and Mohr-Coulomb model encoded into commercial finite element software, are incapable of calculating the cyclic responses of soil. Therefore, researchers have performed numerical simulations of the cyclic behaviours of suction anchors using advanced constitutive models. Gelagoti et al. [14] proposed a simplified equivalent linear iterative approach to account for the nonlinear lateral stiffness and bearing capacity of suction caissons by a simplified kinematic hardening model in the context of Von Mises associative plasticity. Kourkoulis et al. [15] simulated the behaviour of suction caissons subjected to wave and earthquake loading using a simple kinematic hardening model. Zhang et al. [16] predicted the cyclic behaviour of suction caissons using a finite element method based on a thermodynamics-based constitutive model. Cheng et al. [17], [18] simulated the cyclic behaviours of suction anchors by developing an elastoplastic bounding surface model and encoding it into ABAQUS software.

Although the advanced constitutive model with different kinematic hardening rules can describe the cyclic responses of soil to some extent, the complexity of the model leads to inefficient numerical calculation, especially for long-term cyclic loads. The cyclic loading can lead to the degradation of stiffness for soft clays, and the stress-strain curve shows significant nonlinear and hysteresis properties. In this paper, the stiffness degradation model for soft clays was proposed by performing a series of cyclic triaxial tests, and then the model was embedded in the ABAQUS software package by encoding the USDFLD subroutine and defining relevant field variables. The behaviours of suction anchors subjected to combined average and cyclic loads in soft clays were simulated by coupling the elastic-perfectly plastic model with the Mohr-Coulomb yield criterion and the proposed stiffness degradation model. This numerical method has higher computational efficiency than previous methods using advanced constitutive models.