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。     

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。     

地震荷载沉箱码头大变形分析的离散元法初探

探讨了应用离散元分析方法求解地震荷载下沉箱和背后填土大变形问题的可能性,开发了能描述沉箱的平面形状及沉箱与土体摩擦特性的矩形单元。并通过简单实例分析了地震时沉箱码头的变形,再现了沉箱码头地震灾害的发生过程,为沉箱在地震情况下的大变形分析提供了一种新途径。比较了两种沉箱模型对分析结果的影响,提出了相应的方法。     

吸力式沉箱基础长期水平荷载作用下承载特性试验研究

通过两组不同水平荷载作用下吸力式沉箱基础长期模型试验,对吸力式沉箱基础随时间的位移变化规律以及土压力分布规律进行了研究。试验结果表明:在长期模型试验中位移发展主要集中在试验前期,后期位移稳定需要更长时间,土体流变效应较为明显。土压力沿深度分布曲线呈抛物形状,表明沉箱基础在水平荷载作用下为转动模式,随时间增加被动区土压力变化呈增大趋势,主动区土压力呈减小趋势。土压力发展主要集中在试验前期,后期土压力变化相对较小,但土压力稳定所需时间较长,同时荷载值越大土压力稳定所需时间越长。     

复合加载下桶形基础循环承载性能数值分析

作为一种新型基础形式,吸力式桶形基础除了承受海洋平台结构及自身重量等竖向荷载的长期作用之外,往往还遭受波浪等所产生的水平荷载及其力矩等其它荷载分量的瞬时或循环作用。对复合加载模式下软土地基中桶形基础及其结构的循环承载性能尚缺乏合理的分析与计算方法。应用Andersen等对重力式平台基础及地基所建议的分析方法,基于软黏土的循环强度概念,在大型通用有限元分析软件ABAQUS平台上,通过二次开发,将软土的循环强度与Mises屈服准则结合,针对吸力式桶形基础,基于拟静力分析建立了复合加载模式下循环承载性能的计算模型,并与复合加载作用下极限承载性能进行了对比。由此表明,与极限承载力相比,桶形基础的循环承载力显著降低。     

重力式海洋平台沉箱的断裂分析

研究重力式海洋平台沉箱的表面裂纹最大深度,通过考虑沉箱有一个内表面裂纹,将问题简化为无裂纹重力式平台问题和带裂纹矩形板问题的叠加,应用有限元方法对重力式海洋平台无裂纹情形进行了静力分析,得到了与裂纹位置对应处的环向拉应力。计算裂纹矩形板的应力强度因子,得到海洋环境荷载下沉箱内表面裂纹的最大深度为0.066 8m,计算结果可供海洋平台的设计参考。     

半潜式平台结构整体可靠性分析方法

针对海洋环境中的半潜式平台多方面不确定性,基于半潜式平台结构整体强度分析方法,考虑多种波浪荷载作用的结构响应,采用非线性逐步垮塌分析方法,研究平台在不同控制工况下的极限承载力模型,建立相应的功能函数.根据结构体系可靠度理论和失效模式之间的关系,采用串联体系的可靠度计算方法.最后,对一座具体半潜式平台在100年重现期波浪荷载作用下进行了体系可靠性计算.计算结果表明平台整体可靠度水平由波浪引起的主要结构响应形式决定.通过计算,验证可靠度计算方法具有高效性和可行性,可以为大型深水浮式平台结构的设计、维护和评估提供理论依据.     

深水系泊新型拖曳锚研发关键技术

作为深海工程应用中一种新型的拖曳嵌入式系泊基础,法向承力锚与目前新型的深水绷紧索系泊方式结合,在深水条件下的优势非常明显.综合比较了新型拖曳锚、吸力锚以及桩锚在施工、性能以及经济性等多方面的特点.提出了开展新型拖曳锚研发的若干关键技术.在对国外的实验研究现状进行综合评述的基础上,重点介绍了在构建新型拖曳锚模型实验平台方面取得的成果,涉及模型水槽、拖曳与回收系统、测量系统、模型锚板设计以及拖曳-系泊转换机构等关键技术.     

深水浮式系统耦合分析方法研究

深水浮式系统由系泊系统、立管系统和浮式结构(船体)构成,船体与柔性结构的耦合效应是非常显著的,文章介绍了深水平台耦合效应产生的原因和定义.耦合分析方法的步骤及其原理,并分析了传统非耦舍分析方法的不足.实例表明,耦合分析方法更接近于实际.     

Model tests on installation and extraction of suction caissons in dense sand

A series of model tests were performed on steel- and Perspex-made suction caissons in saturated dense marine sand to explore installation and extraction behaviors. The extractions of the caisson were conducted by applying monotonic loading or by pumping water into the caisson. Responses of suction caissons to pullout rates, aspect ratios, and extraction manners were examined. Test results show that a cone-shaped subsidence region occurs around the suction caisson during the suction-assisted installation. The pullout bearing capacity of the suction caisson in sand is dominated by the loading rate and the loading manner. For the suction caisson subjected to monotonic loading, the maximum bearing capacity is reached at the pullout rate of about 20.0?mm/s. The mobilized vertical displacement corresponding to the pullout capacity increases with increasing the pullout rate. The passive suction beneath the suction caisson lid reaches the maximum value when the pullout bearing capacity is mobilized. In addition, during the suction caisson extracted by pumping water into the caisson, the maximum pore water pressure in the caisson is obtained under the displacement of approximately 0.04 times the caisson diameter. The absolute values of the maximum pore water pressures for the suction caissons approximately equal those of the maximum vertical resistances at the monotonic pullout rate of 5 mm/s. When the vertical displacements of the suction caissons with the aspect ratio of 1.0 and 2.0 reach 0.92 and 1.77 times the caisson diameter, respectively, the seepage failure occurs around the caissons. Using a scaling method, the test results can be used to predict the time length required for the prototype suction caisson to be extracted from the seabed.     

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.     

Experimental Studies on Extraction of Modified Suction Caisson (MSC) in Sand by Reverse Pumping Water

A suction caisson can be extracted by applying reverse pumping water,which cannot be regarded as the reverse process of installation because of the dramatically different soil?structure interaction behavior.Model tests were first carried out in this study to investigate the extraction behavior of the modified suction caisson(MSC)and the regular suction caisson(RSC)in sand by reverse pumping water.The effects of the installation ways(suction-assisted or jacking installation)and the reverse pumping rate on the variations of the over-pressure resulting form reverse pumping water were investigated.It was found that neither the RSC nor the MSC can be fully extracted from sand.When the maximum extraction displacement is obtained,the hydraulic gradient of the sand in the suction caisson reaches the critical value,leading to seepage failure.In addition,the maximum extraction displacement decreases with the increasing reverse pumping rate.Under the same reverse pumping rate,the final extraction displacements for the RSC and MSC installed by suction are lower than those for the RSC and MSC installed by jacking.The final extraction displacement of MSC is almost equal to that of the RSC with the same internal compartment length.Based on the force equilibrium,a method of estimating the maximum extraction displacement is proposed.It has been proved that the proposed method can rationally predict the maximum extraction displacement and the corresponding over-pressure.     

Upper Bound Solutions for Uplift Ultimate Bearing Capacity of Suction Caisson Foundation

Suction caisson foundation derives most of their uplift resistance from passive suction developed during the pullout movement. It was observed that the passive suction generated in soil at the bottom of the caisson and the failure mode of suction caisson foundation subjecting pullout loading behaves as a reverse compression failure mechanism.The upper bound theorems have been proved to be a powerful method to find the critical failure mechanism and critical load associated with foundations, buried caissons and other geotechnical structures. However, limited attempts have been reported to estimate the uplift bearing capacity of the suction caisson foundation using the upper bound solution. In this paper, both reverse failure mechanisms from Prandtl and Hill were adopted as the failure mechanisms for the computation of the uplift bearing capacity of the suction caisson. New equations were proposed based on both failure mechanisms to estimate the pullout capacity of the suction caisson. The proposed equations were verified by the test results and experimental data from published literature. And the two solutions agree reasonably well with the other test results. It can be proved that both failure mechanisms are reasonably and more consistent with the actual force condition.     

海上风机吸力基础的水平受荷研究综述

吸力基础具有施工速度快、安装过程中受海况天气影响小且易于回收重复利用等优点,被广泛应用于海洋工程。当吸力基础作为海上风电塔架的基础时,常常承受较大的水平荷载,因此其水平承载力是设计的主控因素。介绍了海上风机基础的设计要求,分析了影响基础水平承载性状的因素,总结了吸力基础受水平单调荷载、水平循环荷载和不同荷载组合三个方面的研究现状。讨论了水平荷载的大小、水平加载的高度(偏心率)、循环荷载的频率、循环荷载的次数、循环荷载的幅值、循环荷载的方向性、竖向荷载对吸力基础水平承载性状的影响,考虑了水平荷载的非共线性,指出了目前研究的不足,明确了吸力基础水平承载性状进一步研究的方向,提出了供工程实践参考的建议。     

Analytical and experimental studies on installation of a suction caisson with tampered tip in clay

Concrete suction caissons have been successfully used as breakwaters or seawalls in recent years. The relative large wall thickness-to-diameter ratio of a concrete caisson can lead to the formation of a full soil heave plug that may cause difficulties in the installation of concrete caisson in clay. One way to overcome this limitation is to use a tampered tip for the caisson wall. An analytical method is proposed in this article to calculate the minimum suction pressure required to penetrate a caisson and the maximum allowable suction pressure that can be applied to avoid too much soil heave plug during the installation of the suction caisson. Four model tests were conducted in normally consolidated clay to study the installation process of a concrete suction caisson with tampered tip and to verify the proposed analytical method. The height of the soil heave plug in the caisson with a tampered tip is observed to be about half of that in the caisson with a flat tip.     

Experimental studies on sand plug formation in suction caisson during extraction

A series of model tests were conducted on Perspex-made suction caissons in saturated dense marine sand to study the sand plug formation during extraction. Suction caissons were extracted by pullout loading or by pumping air into the suction caisson. Effects of the pullout rates, aspect ratios and loading ways (monotonic or sustained) on the pullout capacity, and plug formation were investigated. It was found that the ultimate pullout capacity of the suction caisson increases with increasing the pullout rate. The sand plug formation under the pullout loading is significantly influenced by the pullout rate and the loading way. When the suction caisson is extracted at a relatively slow rate, the general sand boiling through the sand plug along the inner caisson wall occurs. On the contrary, the local sand boiling will occur at the bottom of the suction caisson subjected to a rapid monotonic loading or a sustained loading. Test results of the suction caisson extracted by pumping air into the caisson show that the pressure in the suction caisson almost follows a linear relationship with the upward displacement. The maximum pressures for suction caissons with aspect ratios of 1.0 and 2.0 during extraction by pumping air into the caisson are 1.70 and 2.27 times the maximum suction required to penetrate the suction caisson into sand. It was found that the sand plug moves downward during extraction by pumping air into the caisson and the variation in the sand plug height is mainly caused by the outflow of the sand particles from the inside of the suction caisson to the outside. When the suction caisson model is extracted under the pullout rate of 2?mm/s (0.28?mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases to the maximum value with increasing the penetration depth and then reduces to zero. On the contrary, when extracted under the pullout rate of 10?mm/s (1.4?mm/s for the prototype), the hydraulic gradient along the suction caisson wall increases with increasing the pullout displacement. When extracted by pumping air into the caisson, the hydraulic gradient reaches the critical value, and at the same time, the seepage failure occurs around the suction caisson tip.     

Experimental studies on the anti-uplift behavior of the suction caissons in sand

A series of model tests was conducted in sand to explore the anti-uplift behavior of suction caissons, considering the effects of aspect ratios, load inclination angles and loading positions. This paper emphasizes on analyzing the deformation characteristic and the mechanism of the suction caissons under various loading conditions. The movement modes of the suction caisson are different when the load inclination angle increases from 0° to 90° corresponding to various mooring positions. The pull-out bearing capacity decreases with load inclination angles increasing. When the load inclination angle changes from 0° to 60°, the bearing capacity reduces more significantly than that between inclination angle of 60° and 90°. While the load inclination angle is relatively small, the pull-out capacity of the suction caisson decreases after reaching the peak as the loading position moves downwards. Moreover, the optimum loading position locates between 2/3 and 3/4 of the caisson length. The optimum loading position is at the bottom of the caisson when the load inclination angle exceeds 60°. However, the influence of the loading position on the pull-out capacity of the caisson can be ignored while the load inclination angle equals to 90°. The pull-out bearing capacity increases as the aspect ratio increases but the aspect ratio has no effect on the deformation characteristic of the suction caisson.     

Investigation of scour effects on lateral behaviors of suction caisson

ABSTRACT

The use of suction caissons can reduce the development costs of offshore wind energy and has broad application prospects. However scour around marine foundations is inevitable, it gravely affects the stability of marine engineering. Therefore, there is an urgent need to study the weakening effects of scour on suction caisson. In this study, the variation trends of remaining soil parameters (the effective unit weight and the peak effective friction angle) after scour are examined with consideration of the dilatancy and stress history of sandy silt. It is found that the parameters of shallow soil change considerably after scour, and the larger the scour depth, the greater is the change in the parameters. However, the deep soil is less affected. On the basis of these findings, scour effects on the ultimate moment capacity of suction caisson are studied. The ultimate moment capacity is found to greatly reduce under scour, and its calculated value is larger than the actual value when the effects of dilatancy and stress history are ignored. To simplify calculation, it is feasible to replace the ultimate moment capacity when both dilatancy and stress history are considered with that when only dilatancy is considered.