【作者】 武科；

【导师】 栾茂田；

【作者基本信息】 大连理工大学 ， 岩土工程， 2007， 博士

【摘要】 吸力式桶形基础是一种新型的海洋平台结构基础型式，它是伴随着边际油开发的需要应运而生的。这种新型的基础型式弥补了传统的导管架平台和重力式平台等固定式浅海海洋工程结构自重大、工程造价随水深大幅度地增加等不足；比较适宜于软土地基和恶劣的海洋环境，而且具有造价低、可重复使用等优势，被海洋石油专家誉为“导管架平台基础工程技术新时代的曙光”。随着我国在渤海以及近海海洋油气田的开发，桶形基础结构为软黏土地区海洋平台的建设提供了一条新路。然而，吸力式桶形基础在正常工作中，不仅受到上部海洋平台结构巨大自重及其设备所引起的竖向荷载的长期作用，而且一般往往遭受波浪与地震等所引起的水平荷载、力矩荷载的共同作用。目前，针对多种荷载分量共同作用的复合加载模式下吸力式桶形基础承载力特性的研究，尚缺乏被广泛认可的理论体系与计算方法，在我国进行实际工程应用还需要做大量的工作。因此，有必要针对复合加载模式下吸力式桶形基础结构的变形机理及其承载力特性，开展深入而系统的理论分析与数值计算等方面的综合研究，发展和完善深厚软黏土地基上吸力式桶形基础结构的有关设计理论体系与计算方法，为我国海洋平台吸力式桶形基础结构的设计与应用提供理论依据和技术支持。为此，本文着重围绕复合加载模式下软基上吸力式桶形基础结构的失稳破坏机制及其承载力特性等方面进行了比较系统而深入的探索，论文的主要研究工作包括下列方面：1．针对吸力式桶形基础负压沉贯安装过程形成的桶体内外渗流场，采用改变桶内海床泥面水头边界条件可以模拟不同的负压条件，从而建立了吸力式桶形基础负压沉贯渗流场数值分析模型，确定了沉贯过程中地基土性参数对渗流场水头分布及渗流水头损失等渗流场特性的作用。2．针对深厚软黏土等不良地基，提出了符合海洋地基稳定性分析的计算分析方法；进而，基于大型通用有限元分析软件ABAQUS，建立了软黏土地基上单桶结构及周围土体共同作用体系的三维弹塑性有限元数值分析模型，采用位移控制方法，通过系统地有限元数值计算，研究了不同荷载分量单独作用下吸力式桶形基础的失稳破坏模式及极限承载力特性，阐述了能够反映吸力式桶形基础地基极限承载力特性的经验计算关系式，为桶形基础的设计提供了重要的参考依据。3．根据有限元分析所得到的竖向荷载、水平荷载作用下软黏土地基上吸力式桶形基础失稳破坏模式，基于塑性极限分析原理，分别提出了竖向荷载、水平荷载作用下的三维极限分析上限解法。以此为基础，将本文所提出的极限分析上限解法所得到的极限承载与有限元分析所得到的相应结果进行了对比。根据对比结果的一致性，验证了本文所建议的极限分析上限解法的合理性。4．针对多种荷载分量共同作用的复合加载模式下吸力式桶形基础的承载性能，采用Swipe试验加载方法分别探讨了软黏土地基上单桶基础的承载力特性。首先，在大型通用有限元分析软件ABAQUS上，通过大量的三维弹塑性有限元数值分析，确定了不同的复合加载模式下地基的各种失稳破坏机制，绘制了不同荷载组合空间内地基的破坏包络面，进而基于Taiebat与Carter所提出的地基三维破坏包络面的经验数学表达形式，对其进行修正，提出了适合不同长径比的桶形基础在不同荷载空间内的地基破坏包络面数学表达式，结合有限元分析所得到的V-H-M荷载空间内单桶基础地基三维破坏包络面，以此评价复合加载模式下单桶基础的稳定性。进一步，针对国内外缺乏考虑倾斜荷载作用下桶形基础承载力特性的现状，通过考虑力矩荷载M=0情况下倾斜荷载作用对单桶基础承载力的影响，探讨了竖向倾斜荷载与水平倾斜荷载共同作用下偏心距e与倾斜荷载的相互关系，以此绘制了偏心距e与倾斜荷载V-H荷载空间内的三维破坏包络面。其次，基于ABAQUS二次开发，通过考虑软黏土不排水抗剪强度的横观各向异性和非均质性，分别探讨了不排水抗剪强度的横观各向异性和非均质性对吸力式桶形基础承载力特性的影响。最后，基于ABAQUS二次开发，通过考虑软黏土地基的循环累积变形以及荷载的循环特征，结合Andersen等所提出的软黏土循环强度概念，建立了一种变值(循环)复合加载模式下吸力式桶形基础的三维弹塑性拟静力循环有限元计算模型，针对不同循环次数，通过大量的的变值(循环)复合加载有限元数值计算，阐明了单桶基础在循环荷载作用下地基中应力状态变化特点和地基破坏模式，得到了单桶基础受不同循环荷载作用下循环极限承载力的变化规律；进而把变值(循环)复合加载模式与复合加载模式下桶形基础的不同荷载空间内的破坏包络面进行了对比分析。5．通过研究双桶、四桶组合基础在复合加载模式下的地基承载力特性，探讨了桶间距对双桶基础地基承载力的影响；进一步，结合我国第一座吸力式桶形基础采油平台CB20B实际工况，探讨了四桶基础结构的稳定性，通过绘制地基破坏包络面，对实际工况中的CB20B采油平台稳定性进行了评价。更多还原

【Abstract】 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.更多还原