【作者】 孙曦源；

【导师】 栾茂田；

【作者基本信息】 大连理工大学 ， 岩土工程， 2009， 博士

【摘要】 伴随着海洋油气资源开发向边际油田的进军,桶形基础作为一种新型的海洋平台结构基础形式应运而生。其外形上多为底端开口、顶端封闭的倒扣大直径圆桶。安装时,首先依靠桶体自重使其部分地插入土中以形成密闭空间,然后抽出桶内的空气,利用内外压力差,将桶基逐步压入至海床内预定深度完成安装。施以其相反的过程,桶基便可从土中拔出,运送到其他海域进行循环再利用。这种新型的基础形式弥补了传统的导管架平台、重力式平台等固定式浅海结构自重大、工程造价随水深大幅增加等不足,较适用于软土地基及恶劣的海洋环境,被挪威专家誉为“导管架平台基础工程技术新时代的曙光”。近年来,我国针对渤海、东海、南海等近海海域油气资源开发的力度不断加大,桶形基础由于自身所独有的特点必将在软黏土地区海洋平台的建设中起到不可替代的重要作用。桶形基础在服役过程中,不仅受到上部平台结构自重及其设备所引起的竖向荷载的长期作用,还将承受风、波浪、潮流、冰等所引起的水平荷载、力矩荷载的共同作用。而根据已有的工程经验可知,其中水平荷载的承受能力对于桶型基础的稳定性而言则显得尤为重要。目前,针对水平荷载作用下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,在我国进行实际的工程应用还需要做大量的准备工作。因此,有必要针对水平加载模式下桶形基础的变形机理及其承载力特性,开展系统而深入的理论分析与数值计算等方面的综合研究,发展并完善深厚软黏土地基上桶形基础的有关设计理论体系与计算方法,为我国海洋平台桶形基础的设计与应用提供理论依据和技术支持。为此,本论文着重围绕水平荷载作用下软基上桶形基础的失稳破坏机制及承载力特性等方面进行了探索,主要研究工作包括以下诸方面:1.基于大型通用有限元分析软件ABAQUS,对单调水平加载条件下软土地基中单桶形基础进行了系统地三维有限元建模分析。本文采用较为先进的主从接触对算法来模拟桶体与土体界面之间复杂的力学行为,建立了考虑桶体外表面与土体之间产生裂缝的有限元计算模型,同时考虑了桶体内表面、桶体顶面及桶体底面与土体之间的接触特性,避免了传统有限元分析中较少考虑桶体外表面与土体之间产生裂缝现象或者较少考虑桶体内表面与土体之间的摩擦接触特性的不足,从而较为全面且合理地反映了单桶形基础与土体之间的相互作用机制。根据有限元分析结果可知,处于软土地基中的单桶形基础在水平荷载作用下的失稳破坏机制为桶基绕泥面以下、基底以上某一点发生整体转动倾覆破坏,且在此过程中,桶前出现被动破坏楔体,桶后出现桶土脱离现象,这一切均明显有别于传统的重力式基础或桩基础。通过二次开发及变动参数比较研究,分别探讨了土体不排水抗剪强度S_u、土体有效重度γ′、桶体长径比L/D、加载点距泥面高度h等因素对软土地基中单桶形基础水平承载能力的影响。并将有限元分析所得到的承载力结果及桶壁表面土压力分布规律与已有的一些模型实验进行了定量或定性的比较,从而验证了本论文所建议的有限元模型及有限元计算结果的合理性。同时也表明,根据本文有限元分析所得到的软基中单桶形基础的失稳破坏机制与模型试验是较为一致的。2.根据三维有限元分析所得到的水平荷载作用下软黏土地基中单桶形基础失稳破坏模式及作用于桶壁上的土压力分布规律,基于三维极限平衡原理,本文提出了一种适用于工程中估算软黏土地基中单桶形基础水平承载力的三维极限平衡方法。利用本文提出的极限平衡方法,可以很方便地求得软黏土地基中单桶形基础的水平承载力值,将之与已有的现场模型试验结果进行对比,两者结果吻合较好,从而验证了本文所建议的三维极限平衡方法的合理性与可行性,为指导工程实践提供了一定的理论依据。3.考虑到波浪等循环荷载的作用可能导致海床软黏土地基发生强度弱化与刚度退化的循环软化效应,基于Andersen等所提出的软黏土循环强度概念,建议了非线性弹塑性循环强度模型,并在大型通用有限元软件ABAQUS平台上进行二次开发,实现了软黏土地基中单桶形基础循环承载力的拟静力计算。通过计算表明:单桶形基础在循环荷载作用下的失稳破坏形态及地基中的等效塑性应变分布与单调荷载作用下的极限破坏状态是存在一定差异的。在循环失稳状态下,单桶形基础的转动中心明显偏离了桶体的中轴线,而向桶体受荷侧靠近,同时桶体底部的滑动面不再是以单桶形基础静载倾覆失稳时的转动中心为球心的球面,而可能是一种更为复杂的空间曲面。通过变动参数比较研究初步表明,单桶形基础的循环承载力与埋深、荷载作用点高度及荷载循环破坏次数等诸多因素密切相关,而当埋深及荷载循环破坏次数等因素一定时,在不同荷载作用点高度条件下所得到的循环承载力与对应的静载极限承载力相比降低程度基本上一致。4.针对目前对于桶形基础的探讨多集中于单桶形基础,而较少涉猎需考虑桶间效应的多桶形基础的研究现状,本论文以单桶形基础三维有限元建模分析经验为基础,对更具有工程背景的四桶形基础的水平承载能力进行了三维有限元变动参数比较分析。主要参数包括:桶间距离B(小桶间距、小桶间距)、桶间连接刚度(无连接、弹性连接、刚性连接)。由分析可知,同等条件下,四桶形基础的水平承载能力随桶间距离B的增大表现出先增大后近似恒定的变化趋势:而当桶间距离B一定时,四桶形基础的水平承载能力则随桶间连接刚度的增大而增大,即无连接＜弹性连接＜刚性连接。5.根据三维有限元分析所得到的水平荷载作用下软黏土地基中四桶形基础的失稳破坏模式,基于塑性极限分析原理,分别提出了适用于计算刚性桶间连接条件下小桶间距与大桶间距四桶形基础水平承载力的三维极限分析上限解法。利用本文提出的三维极限分析上限解法,可以很顺利地求得软黏土地基中四桶形基础的水平承载力值,将之与三维有限元分析所得到的结果进行对比,两者一致性较好,从而验证了本文所建议的三维极限分析上限解法的合理性与可行性。更多还原

【Abstract】 As a new type of foundation in shallow water, bucket foundation emerges in the need of exploration of boundary oil fields. The bucket is a large diameter cylinder, opened at the bottom and closed at the top. It is usually installed by applying underpressure to its interior after it is allowed to penetrate under its own weight. Taking the reverse process, the bucket will be pulled out and transferred to other sea areas for recycle. This new kind of foundation makes up the defects of traditional gravity foundation and pile foundation, which has great gravity and causes high engineering cost. It is especially suitable for complex conditions such as soft clay ground and worse sea environments. Therefore, bucket foundation is praised as the dawn of skirted foundation by Norway experts. With the development of our national marine exploitations in Bohai, Donghai and Nanhai, bucket foundation must have an extensive prospect in the future because of its own advantages. During working process, bucket foundation is not only subjected to the long-term action of vertical load induced by all weights of platform and equipment, but is also imposed by lateral loads and moments caused by wind, wave, current and ice. According to engineering experience, the lateral bearing capacity is more important to the stability of bucket foundations. So far, the studies of bearing capacity behavior of bucket foundation in soft ground subjected to lateral loads have been not clarified well and there are a lot of works needed to be done for engineering use. Therefore, it is much necessary to examine the working and failure mechanism of bucket foundation under lateral loads in order to find out effective methods for evaluating the lateral bearing capacity of it in soft ground. In this dissertation, studies are emphasized on numerical methods for evaluating the bearing capacity and failure mechanism of bucket foundation in soft ground under lateral loads. The main investigations consist of the following parts:1. Based on the general-purpose finite element analysis package ABAQUS, the finite element computational model for single-bucket foundation in soft ground under lateral loads is established.In order to examine the mechanical behavior on the interfaces and establish a finite element model that considering the potential crack at active zone in soft ground under lateral loads, the advanced master-slaver contact pair algorithm is utilized. At the same time, friction contact between the inner surface, the top surface as well as the bottom surface of bucket and neighboring soil surfaces is considered. Therefore, the deficiency of conventional finite element analysis which seldom considers the potential crack between the outer surface and soil surface or less takes account of the friction contact between the inner surface and soil surface is avoided and the detailed soil-structure interaction mechanism can be simulated in a more rational way. On the basis of finite element analysis, it is concluded that the structure tends to rotate around a certain point which is located below the ground surface and above the bucket bottom. In addition, there is an individual wedge in passive zone and a crack in active zone as failure happening. This failure mechanism is remarkably different from that of traditional gravity structure and pile foundation.By second-phase development of ABAQUS and parametric computations, the effects of undrained shear strength of the soil S_u, effective unit weightγ’ slenderness ratio of the bucket LID and height of the load point above the mudline h on the lateral bearing capacity of single-bucket foundation are discussed. Based on the numerical results, it indicates that the failure mechanism and earth pressure distribution obtained by finite element analyses show good agreement with that got from existing model tests, from which the rationality and validity of finite element method are verified.2. Based on the failure mechanism of single-bucket foundation in soft clay ground under lateral load and earth pressure distribution observed by FEM, a limit equilibrium method for evaluating the lateral bearing capacity of single-bucket foundation is proposed. Comparison with finite element results obtained by ABAQUS shows that the results of limit equilibrium method can agree with that from FEM within an acceptable accuracy and the suggested method is applicable for directing engineering practice.3. On the base of the cyclic shear strength concept proposed by Andersen et al for considering cyclic softening behavior of soft clay seabed, which is characterized by strength weakening and stiffness degradation induced possibly by wave cyclic loading, a nonlinear elasto-plastic-cyclic strength model is suggested and embedded into the finite element analysis pack ABAQUS through second-phase development, then a quasi-static finite element method for assessing the cyclic bearing capacity of single-bucket foundation is developed. The computational results indicate that the failure mode of cyclic instability and distribution of equivalent plastic strain in soft ground are somewhat different from those of ultimate failure state under static loading. In the failure mode of cyclic instability, the rotation center deviates obviously from the symmetry axis of the bucket and near to the loading side of the structure and the sliding surface at the bucket bottom turns to be a more complex spacial surface instead of a spherical surface which center is located on the rotation center of the structure. Parametric studies show that the lateral cyclic bearing capacity of single-bucket foundation is influenced by some factors such as the embedded depth, height of load point and cyclic loading number to failure etc. Moreover, variating the height of load point only, decreased degree of the lateral cyclic bearing capacities of single-bucket foundation in soft ground compared with those under static loading are approximately same.4. According to the actuality that most studies pay their attention on single-bucket foundation but little on multi-bucket foundation for which the influence of bucket group effect on lateral bearing capacity should be not overlooked, parametric computations and comparative analyses on the lateral bearing capacity of multi-bucket foundation (four buckets) are carried out using ABAQUS based on the research experience of single-bucket foundation in this paper. It is found that the lateral bearing capacity of multi-bucket foundation increases at first, and then remains constant mostly with the increase of B. Keeping B invariable, it indicates that the lateral bearing capacity of multi-bucket foundation increases with the increase of connection stiffness, specifically, no connection<elastic connection<rigid connection.5. Based on the failure mechanisms of multi-bucket foundation in soft clay ground under lateral load with rigid connection condition observed by FEM, the upper bound method based on limit analysis for evaluating the lateral bearing capacity of small-spacing and large-spacing multi-bucket foundation is suggested respectively. Comparison with finite element results obtained by ABAQUS shows that the proposed method is applicable. The results of upper bound method can agree with that from the FEM within an acceptable accuracy.更多还原