复合加载条件下吸力式沉箱基础承载特性数值分析

吸力式沉箱基础的承载特性是海洋工程结构设施建造与设计中的一个关键问题。这种新型的深水海洋基础型式,通常承受竖向上拔荷载与水平荷载的共同作用,其工作性能与设计理论远远不能满足工程实践的需要。本文采用有限元分析方法对吸力式沉箱基础的极限承载特性进行数值计算。以大型通用有限元分析软件ABAQUS为平台,通过二次开发,数值实现了Swipe试验加载方法和固定位移比分析方法,针对不同的沉箱长径比、土的强度折减系数,探讨了沉箱基础在垂直上拔荷载和水平荷载单调联合作用下的极限承载力,通过对不同荷载组合的数值计算构造了复合加载条件下沉箱基础破坏包络面。     

基于泥沙流变特性的吸力式沉箱减阻技术

针对吸力式沉箱在黏性底床中沉贯安装的减阻问题,基于黏性泥沙的流变特性,开展了一系列不同振动荷载作用下的室内沉贯模型试验,分析了振动荷载对吸力式沉箱沉贯过程的影响和沉贯减阻效果。试验结果表明：在压力沉贯阶段或吸力沉贯阶段施加高频振动荷载,均能促使吸力式沉箱侧壁周围的土体发生流化,有效降低沉箱的沉贯阻力;减阻效果与振动频率和沉箱的长径比有关,与振动频率成正比,与沉箱的长径比成反比;存在一个临界频率,当振动频率大于该频率后,沉贯阻力随频率的减小不明显;施加高频振动荷载有助于减小吸力沉贯阶段沉箱内的土塞高度,促进沉箱沉贯到位。研究成果可为黏性泥沙流变减阻技术在吸力式沉箱中的应用提供理论依据和技术参考。     

循环荷载下饱和粉土中伞式吸力锚水平承载性能研究

利用ABAQUS有限元软件构建模型,对伞式吸力锚施加水平循环荷载,研究伞式吸力锚的承载性能。在固定荷载大小的情况下,研究锚体位移以及不同位置土体的应力应变。分析可得,在水平循环荷载的作用下,土体的最大塑性应变出现在筒裙位置,筒裙和锚支的结合发挥了主要的承载作用。改变荷载工况,研究不同荷载大小对伞式吸力锚的影响。研究发现,随着循环波浪荷载作用强度的增大,筒裙的稳定作用愈加明显,且这种作用在循环波浪荷载的后期阶段相对更加突出。     

V⁃H荷载作用下海洋平台吸力式桩桶桩土承载特性研究

用于海洋平台的吸力式桩桶基础作为一种新型平台基础正逐渐成为人们研究的重点。为研究吸力式桩桶单桩基础的受力特性,对V-H（竖向—水平）联合荷载作用下的吸力式单桩基础桩土的承载特性进行了数值模拟,并将数值模拟的有限元解通过与API规范中对p-y曲线的计算方法进行对比来验证有限元模型的可行性,最后采用分级作用力的加载方式对其破坏包络曲线进行绘制,并推导出相应的函数表达式。研究表明,采用ABAQUS有限元分析软件对吸力式桩桶进行数值模拟是可行的,随着对吸力式桩桶所施加V-H联合荷载的不断增大,吸力式桩桶所能体现的应力和弯矩极限值也在随之增大,其位移变化主要在施加荷载的区域附近,最后在联合荷载作用下所体现的极限承载状态,即包络曲线大致呈四分之一的椭圆形状。     

深水平台吸力沉箱基础工程地质问题分析

针对深水平台吸力沉箱基础,讨论了与沉箱安装有关的分析方法及涉及的工程地质参数,分析了承受竖向拉拔荷载、倾斜与水平荷载作用的吸力沉箱极限承载力的分析方法及涉及的工程地质参数,对与吸力沉箱设计有关的其它问题也进行了分析.在此基础上,阐明了与吸力沉箱设计有关的工程场地调查内容及需要确定的工程地质参数.其目的是为开发深水平台吸...     

离岸裙式吸力基础在砂土地基中沉贯性研究

吸力基础是海洋工程中新型的一种基础型式,广泛应用于海洋平台、海洋浮动式结构等,近年来,也被作为浅海离岸风力发电工程的基础。吸力基础易遭受较大的水平动力荷载和弯矩,从而可能产生较大水平位移和转角;同时,由于海床冲刷,会降低其承载能力。为克服这些不足,提出了一种新型吸力基础———裙式吸力基础,把分析传统吸力基础砂土中的沉贯方法,拓广到裙式吸力基础中,研究该基础型式在砂土中的可沉贯性以及所需的吸力;并与同情况下的传统吸力基础进行了比较,证明了所提出的裙式吸力基础具有较好的沉贯性能,具有工程实践推广价值。     

水平荷载作用下裙式吸力基础承载性能研究

传统吸力基础是一个单桶结构,被广泛作为海洋平台、漂浮结构的基础,近年来也被推广到海上风电塔架。作为风电塔架基础,要充分提高其水平承载能力。为此,提出一种改进的基础形式—裙式吸力基础。采用Z_SOIL有限元软件,针对砂土地基,从水平单调加载和循环加载两个方面,对传统单桶吸力基础和裙式吸力基础进行了承载性能对比研究,得到了相应的荷载-位移曲线。研究结果表明,裙式吸力基础由于设置了"裙"结构,显著提高了其抵抗水平静载和循环水平动力荷载的能力,并能有效控制基础的水平位移,是值得推广应用的一种新型海洋工程基础形式。     

流体-单腿平台结构-土相互作用体系的地震响应分析

本文主要对流体-重力式单腿平台结构-土相互作用体系在地震荷载作用下的动力响应问题进行了研究。分析了考虑土-结构相互作用及不考虑相互作用效应对体系动力响应的影响,同时研究了沉箱基础半径和海水深度变化对体系动力响应的影响,并结合一个简单实例进行了数值分析。     

海床表层软土对宽浅式筒型基础承载特性影响研究

近海海床表层多为软黏土或淤泥质土,为探究海床表层软土对海上风电宽浅式筒型基础承载特性的影响,以中国广东某海域风电场为背景,通过有限元分析的方法,研究竖向、水平、弯矩荷载作用下软土层厚度和土体强度对基础极限承载力、破坏模式以及筒基土压力分布的影响。研究结果表明:当软土层厚度小于H/2(H为筒裙高度)时,单向荷载作用下宽浅式筒型基础极限承载力随软土层厚度的增加呈线性减小的趋势;当软土层厚度大于H/2后,承载力降低速率逐渐增大。表层软土的存在,使得塑性区范围缩小,软土层内土体塑性破坏更加明显。竖向荷载作用下,随软土层厚度的增大,筒顶承载先减小后增大,筒内侧摩阻力先增大后减小;水平荷载和弯矩作用下,筒侧被动土压力的降低是引起软土覆盖地基中基础承载能力降低的主要因素。     

粉土中吸力桶沉贯阻力与土塞失稳机理研究

针对粉土中吸力桶在吸力沉贯中的两个关键性问题——沉贯阻力和内部土体稳定性,在自行研制的试验平台上进行了一系列吸力桶沉贯模型试验。试验结果表明,粉土中吸力沉贯时,Andersen所提供的基于CPT锥尖强度的阻力计算公式能较好预测阻力发展趋势,但计算结果偏小;而修正的承载力公式预测结果在沉贯前期与实测值较吻合,沉贯后期预测偏大。吸力沉贯前期,贯入阻力随深度稳步增长,直到贯入深度达到某个临界值后,阻力增长缓慢甚至保持不变。桶内粉土在吸力作用下的失稳机理与黏土或砂土不同,在渗透力作用下表现为从土层表面开始的自上而下管涌或渗流侵蚀。沉贯后期,端部土体在较高水头差下发生失稳并向桶内流动,造成了桶内土体密度降低,端阻力与内壁摩阻力降低,总贯入阻力主要由外壁摩阻力提供。并结合有限元模拟对上述实验现象的内在机理进行了解释。     

软土地基沉箱防波堤失稳模式及稳定性分析方法

针对现阶段深水软黏土地基防波堤建设的设计理论和稳定性分析方法尚不成熟,结合实际工程,采用三维弹塑性有限元数值分析方法,研究在水平或竖直单一方向荷载以及复合加载条件下软黏土地基上沉箱防波堤的失稳模式,提出破坏包络线的稳定性判别方法。在波浪水平荷载作用下,深水软基上沉箱防波堤发生倾覆失稳破坏,失稳转动点为沉箱底面以下中轴线偏右的某点,不同于规范中规定的岩石或砂质地基沉箱倾覆转动点为其后踵点;在重力等竖向荷载作用下,沉箱的失稳模式为结构整体下陷,抛石基床及地基形成连贯的塑性区域,呈现较明显地冲剪破坏形式;在水平、竖向复合荷载作用下,软基上沉箱防波堤的破坏包络线由结构倾覆破坏线和地基承载力破坏线组成,包络线将荷载组合区分成稳定区、仅发生水平承载力不足倾覆破坏区、仅发生地基竖向承载力不足破坏区、同时发生水平承载力和地基竖向承载力不足破坏区4个区域。研究成果为深水软基沉箱防波堤建设提供参考和借鉴。     

Evaluation of pullout load capacity of suction caissons in sand using a three-dimensional displacement approach

An investigation was conducted to obtain analytical solutions for the pullout behavior of a suction caisson undergoing inclined loads in sand. The inclined load is transformed into an equivalent load system in which the vertical, horizontal, and moment loads are applied on the center of the lid of the suction caisson. The vertical and lateral stiffness coefficients along the skirt of the suction caisson in sands are presented using the new three-dimensional elastic solutions taking into account the nonhomogeneous and nonlinear properties of the sand. The vertical, lateral, and rocking stiffness coefficients on the base of the suction caisson are presented considering the solutions of a hollow rigid cylindrical punch acting on the surface of a soil. The yield, pullout, and failure for sands with the nonhomogeneous and nonlinear characteristics are taken into consideration. The effects of the load inclination, the loading depth, and the aspect ratio on the pullout load capacity of the suction caisson are presented. Behaviour of the suction caisson in sand prior to failure is clarified from the relationship between tensile load, displacement, and rotation and that between depth, vertical pressure, and lateral pressure.     

Analysis of pullout load capacity of suction caissons in clay by a three-dimensional displacement approach

A study was made to present analytical solutions of pullout load capacity for a suction caisson subjected to inclined tension in clay. The inclined tension on the skirt of the suction caisson is transformed into an equivalent system comprised of the vertical, horizontal, and moment load applied on the center of the lid. The vertical and horizontal stiffness coefficients along the skirt of the suction caisson in clay are presented by three-dimensional elastic solutions considering the nonhomogeneous and nonlinear property of clay. The vertical, horizontal, and rocking stiffness coefficient of the suction caisson on the base are presented considering the solutions of a hollow rigid cylindrical punch acting on the surface of clay. The envelopes of the horizontal and vertical ultimate load capacity for clay are presented. The yield, pullout, and failure for clay are taken into consideration. The effects of load inclination, loading depth, and aspect ratio on the pullout load capacity are shown. Behavior of the suction caisson in clay up to failure is investigated using the relationship between tensile load and displacement and that between depth, vertical, and horizontal pressure.     

Characteristic Test Study on Bearing Capacity of Suction Caisson Foundation Under Vertical Load

Suction caisson foundations are often subjected to vertical uplift loads, but there are still no wide and spread engineering specifications on design and calculation method for uplift bearing capacity of suction caisson foundation.So it is important to establish an uplift failure criterion. In order to study the uplift bearing mechanism and failure mode of suction caisson foundation, a series of model tests were carried out considering the effects of aspect ratio,soil permeability and loading mode. Test results indicate that the residual negative pressure at the top of caisson is beneficial to enhance uplift bearing capacity. The smaller the permeability coefficient is, the higher the residual negative pressure will be. And the residual negative pressure is approximately equal to the water head that causes seepage in the caisson. When the load reaches the ultimate bearing capacity, both the top and bottom negative pressures are smaller than Su and both the top and bottom reverse bearing capacity factors are smaller than 1.0 in soft clay. Combined the uplift bearing characteristics of caisson in sandy soil and soft clay, the bearing capacity composition and the calculation method are proposed. It can provide a reference for the engineering design of suction caisson foundation under vertical load.     

Assessment of cyclic response to suction caisson in clay using a three-dimensional displacement approach

An investigation was made to present analytical solutions of cyclic response to suction caisson subjected to inclined cyclic loadings in clay using a three-dimensional displacement approach. A model representing the relationship between vertical load and vertical displacement and that between lateral load and lateral displacement along the skirt of suction caisson subjected to cyclic loadings is proposed for overconsolidated clay. For the effect of vertical load on cyclic load capacity of suction caisson, using the Mindlin solution in the case of a vertical point load, the vertical stress of soil under the base of suction caisson is presented. For the stress state of soil beneath the base of suction caisson subjected to cyclic loading, the Mohr–Coulomb failure line and critical state line are presented and the relationship between total stress, effective mean principal stress, stress difference, and pore-pressure is elucidated. The comparison of results predicted by the present method for a suction caisson subjected to cyclic loadings in clay has shown good agreement with those obtained from field tests. Cyclic behavior of clay up to failure is made clear from the relationship between cyclic tensile load, vertical and lateral displacements, and rotation and that between depth, vertical, and lateral pressures.     

圆形沉箱防波堤波压力的试验分析

通过断面物理模型试验,获得了圆形沉箱结构直立堤在各种工况组合下,承受水平波压力、底部浮托力的大量数据信息。对这些数据进行了分析总结,得出了有关圆形沉箱直立堤波浪力的分布规律。     

Dynamic response and sliding distance of composite breakwaters under breaking and non-breaking wave attack

Over the last 15 years improved awareness of wave impact induced failures has focused attention on the need to account for the dynamic response of maritime structures to wave impact load. In this work a non-linear model is introduced that allows evaluating the effective design load and the potential sliding of caisson breakwater subject to both pulsating and impulsive wave loads. The caisson dynamics is modelled using a time-step numerical method to solve numerically the equations of motion for a rigid body founded on multiple non-linear springs having both horizontal and vertical stiffness. The model is first shown to correctly describe the dynamics of caisson breakwaters subject to wave attack, including nonlinear features of wave–structure–soil interaction. Predictions of sliding distances by the new method are then compared with measurements from physical model tests, showing very good agreement with observations. The model succeeds in describing the physics that stands behind the process and is fast, accurate and flexible enough to be suitable for performance design of caisson breakwaters.     

Simplified limit solutions for the capacity of suction anchors under undrained conditions

Upper bound plastic limit analyses (PLA) can provide a useful framework for estimating the load capacity of suction caisson anchors in purely cohesive soils. Since arbitrary assumptions regarding the soil stress state are not required in the PLA formulation, it may be used with greater consistency compared to other simplified approaches such as limit equilibrium methods. While PLA methods do not attempt to include all of the complexities of anchor behavior, they can provide a relatively simple framework for visualizing anchor kinematics leading to an understanding of the relative importance of various parameters on suction anchor load capacity. The most rigorous PLA formulations involve postulating a three-dimensional anchor-soil failure mechanism and deriving expressions for internal energy dissipation throughout the mechanism. This approach can involve extensive numerical integrations and a relatively complex scheme for optimizing the failure mechanism to obtain a least upper bound collapse load. Considerable simplification is possible if the problem is formulated in terms of ultimate unit resistances (lateral, axial, and their interaction) that can be exerted by the soil on the caisson. In this case, the caisson failure mechanism can be characterized in terms of one or two optimization variables. Simple expressions for the ultimate unit resistances acting on the caisson can be obtained from several sources including rigorous PLA solutions, finite element techniques, or experimental measurements. General expressions are possible by limiting consideration to common, idealized strength profiles such as uniform or constant gradient. Such simplified formulations are particularly valuable for providing an analysis tool accessible to practicing engineers. Suction caisson anchors can be subjected to a variety of load orientations including nearly vertical uplift forces imposed by the vertical tendons of tension leg platforms, horizontal loads imposed by catenary mooring systems, and inclined loads imposed by taut moorings. Recently, PLA methods have been applied to the analysis of suction caissons subjected to this range of loading conditions. This paper reviews the formulation of these analyses and summarizes the most significant findings.     

Numerical Simulation on Oscillation-Sliding-Uplift Rock Coupled Motion of Caisson Breakwater Under Wave Excitation

Corresponding to the sliding and the overturning failure, the elementary motion modes of caisson breakwater include the horizontal-rotational oscillation coupled motion, the horizontal sliding-rotational oscillation coupled motion, the horizontal vibrating-uplift rocking coupled motion, and the horizontal sliding- uplift rocking coupled motion. The motion mode of a caisson will transform from one to another depending on the wave forces and the motion behaviors of the caisson. The numerical models of four motion modes of caisson are developed, and the numerical simulation procedure for joint motion process of various modes of caisson breakwater under wave excitation is presented and tested by a physical model experiment. It is concluded that the simulation procedure is reliable and can be applied to the dynamic stability analysis of caisson breakwaters.     

Numerical Simulation on Oscillation-Sliding-Uplift Rock Coupled Motion of Caisson Breakwater Under Wave Excitation

Corresponding to the sliding and the overturning failure, the elementary motion modes of caisson breakwater include the horizontal-rotational oscillation coupled motion, the horizontal sliding-rotational oscillation coupled motion, the horizontal vibrating-uplift rocking coupled motion, and the horizontal sliding-uplift rocking coupled motion. The motion mode of a caisson will transform from one to another depending on the wave forces and the motion behaviors of the caisson. The numerical models of four motion modes of caisson are developed, and the numerical simulation procedure for joint motion process of various modes of caisson breakwater under wave excitation is presented and tested by a physical model experiment . It is concluded that the simulation procedure is reliable and can be applied to the dynamic stability analysis of caisson breakwaters.