A Study On Bearing Capacity Behavior Of Suction Bucket Foundation In Beach-Shallow Sea

As a novel type of foundation in beach-shallow sea, suction bucket foundation emerges in the need of exploration of boundary oil fields. The new kind of foundation makes up the defects of traditional pile platform and gravity platform, such as great gravity and high engineering cost. Bucket foundation is circular plate with skirts and has some advantages, for example, saving investment and iterative use. It is especially suitable for complex condition such as marine soft soil ground and worse sea environments, which has been praised as the dawn of skirted foundation. With the development of our national marine exploitations in Bohai Bay, suction bucket foundation is possible to construct platform in soft clay area. However, bucket foundation is not only subjected to the long-term action of vertical load induced by all weights of platform and equipment, but also is imposed by both horizontal load and moment caused by wind and wave as well as current. So far, the studies of bearing capacity behavior of bucket foundation in soft clay subjected to combined loads have been not well clarified and there are a lot of work to be done for our engineering use. Therefore it will be theoretically important and practically significant to examine the working mechanism and failure pattern and to work out effective methods for evaluating bearing capacity behavior of suction bucket foundation in soft ground under combined loads. In this dissertation, the studies are emphasized on numerical methods for evaluating the bearing capacity behavior and failure mechanism of suction bucket foundation in soft ground under combined loads. The main investigations consist of the following parts.1. To investigate the feature of seepage fields during the suction penetration of bucket foundation, the seepage field correspondent to different suction pressure can be simlutated by changing the boundary conditions based on the general-purpose FEM analysis package ABAQUS. It is addressed that the regulations of seepage field at one time are uniform during installation and the fluid fields are influenced by soil parameters and permeability.2. To evaluate the bearing capacity behavior of ocean foundation in thick soft ground, the rational analysis method is addressed. Based on general-purpose finite element software ABAQUS, the three-dimensional elasto-plastic finite element model for single suction bucket foundation and the surrounding soil is established. Moreover, the ultimate bearing capacity of suction bucket foundation subjected to monotonic load such as horizontal, vertical and moment load is evaluated by displacement-controlled method. The failure mechanisms of bucket foundation subjected to vertical load, horizontal load or moment load are investigated. Finially, the effect factors of ultimate bearing capacity of bucket foundation are confirmed through comparative study.3. Based on the failure mechanisms of FEM, the failure mechanism of bucket foundation under vertical load and horizontal load are assumed by the upper-bound limit analysis respectively. Comparison with results from FEM analyses based on ABAQUS shows that the proposed method is applicable. The upper-bound limit analysis of plasticity results can agree with that from the FEM within an acceptable accuracy.4. Based on the loading procedure of Swipe test, the bearing capacity behaviors of single bucket foundation under combined loads are investigated by FEM.Firstly, failure mechanisms of suction bucket foundation subjected to combined loads on homogeneous soft ground are evaluated by a lot of numerical analysis. Based on the failure envelopes in different load spaces and the experimental calculated equations about failure envelopes by Taiebat and Carter, the experimental calculated equations about failure envelopes are presented, which can be suitablitiy for any ratios of suction bucket foundations. With the failure envelope in V-M-H load space by FEM, the stability of bucket foundation is evaluated. Moreover, the bearing behavior of suctin bucket foundation under inclined loading is lack of studies in the nation and foreign. Considering M-0, a finite element method for assessing bearing capacity of offshore foundations or structures under eccentric and inclined loads is numerically implemented in the framework of the general-purpose FEM software package ABAQUS.Secondly, a series of numerical calculations are applied to investigate the bearing capacity behavior of bucket foundation in anisotropic or inhomogeneous soft clay and embedded into the finite element software ABAQUS through second-phase development.Thirdly, Based on the finite element software ABAQUS through second-phase development, a three-dimensional elasto-plastic quasi-static finite element model for assessing cyclic bearing capacity behavior of suction bucket foundation subjected to varied combined loads is developed, which is combining the concept of cyclic shear strength proposed by Andersen to consider cyclic softening behavior of soft clay seabed. The variation of stresses and failure mechanisms of bucket foundation under varied combined loads are investaged and the rules of cyclic ultimate strength are obtained. Moreover, based on the relationship of bearing capacity of foundation under combined loads and varied combined loads, the failure envelope under varied combined loads can be extrapolated according to that under combined loads.5. Based on studying the bearing behavior of double bucket foundation and four bucket foundation, the effection of distance between two foundations on double bucket foundations is investigated. Moreover, according to some actual work conditions of CB20B platform, the stability of four bucket foundations is evaluated and failure envelopes are plotted to assess the work conditions of CB20B platform.