深海嵌入式板锚循环拉拔过程模拟及承载特性研究

As a new-type mooring foundation for large deepwater floating offshore structures, the embedded plate anchor often needs to withstand the complicated sea conditions and marine geologic environments. Inevitably, the cyclic bearing performance of the embedded plate anchor is closely related with the cyclic behaviors of the marine soft clay. The studies on the embedded plate anchor so far are still mainly focused on its embedded depth, orientation, and bearing capacity under the monotonic loading. And the few available analyses on the cyclic bearing capacity were based on quasi-static methods, which could not capture the evolution of cyclic behaviors of the marine soft clay reasonably and accurately. With regarding to this, based on descirbing the cyclic shakedown and degradation behaviors of the marine soft clay under different cyclic loading levels, this project aims to explore the mechanisms of the cyclic bearing behaviors of the embedded plate anchor-seabed coupling system. The evolvement in the strength and stiffness of the marine soft clay subjected to the cyclic loading with sustained shear stress are investigated. Taking into account the anisotropic consolidation effect, a three-dimensional bounding-surface plasticity model is established for the cyclic shakedown and degradation of the marine soft clay. Then, both the numerical simulation and physical modeling on the cyclic pull-out process of the embedded plate anchor-seabed coupling system are thoroughly carried out to reveal the evolution of its cyclic bearing performance. The cyclic failure criterion of the embedded plate anchor is proposed and a new approach for assessing its cyclic bearing capacity is presented. It is believed that this research could provide a scientific reference for the design of embedded plate anchors and other similar types of embedded offshore structures.

嵌入式板锚是深海大型浮式结构的一种新型系泊基础，常处于复杂海况和海床地质环境条件，其循环载荷下的承载性能与海洋软黏土复杂动力特性密切相关。目前嵌入式板锚的研究多集中于对其嵌入深度、姿态及静承载力的确定；已有的循环承载力分析主要基于拟静力法，不能较合理地反映海洋土循环动力特性演变的本质。本项目旨从揭示海洋软黏土不同应力水平下呈现的循环稳定和循环软化特性出发，探究嵌入式板锚-海床耦合系统的循环承载性能的演变机理。拟开展偏压循环载荷下海洋软黏土强度、刚度等动力特性试验研究，提出能考虑偏压固结效应的土体循环稳定和循环软化三维边界面塑性模型；进行数值模拟并结合模型试验验证，分析嵌入式板锚-海床耦合系统循环拉拔过程的动力响应，揭示其循环承载特性演化规律；建立嵌入式板锚循环失稳判据并提出循环承载力分析方法。本项目获得的研究成果能够为嵌入式板锚及类似嵌入式海洋工程结构的设计提供科学依据。

深海嵌入式板锚因需深嵌入海床土中，循环承载性能与海洋土体特性关系密切。在长期复杂的海洋环境中，海洋土体动力特性将会发生一系列演变，甚至出现强度软化和刚度退化特性。本项目旨在从建立揭示土骨架-孔隙水循环耦合机理的土体本构模型出发，开展板锚长期循环承载响应分析，合理评估板锚长期循环承载力。.本项目的研究内容包括.（一）考虑海洋软黏土偏压固结效应的循环稳定和循环软化三维边界面塑性模型.（1）提出了广义各向同性硬化准则，推广了Masing准则在弹塑性理论中（尤其是边界面塑性模型）的应用。基于该硬化准则，建立了考虑固结各向异性的三维边界面塑性模型，可以反映土体的循环稳定和循环软化特性。.（二）长期循环载荷下嵌入式板锚-海床耦合系统相互作用及承载特性演变机理.（1）循环载荷作用下嵌入式板锚表现出三种典型循环承载特性：循环稳定、循环次稳定和循环退化；对于循环退化工况，载荷幅值对其位移响应具有决定性作用；对于循环次稳定状态，载荷均值和载荷幅值之和决定了其位移；对于循环稳定状态的板锚，载荷均值的增大可以有效地减小其循环位移。.（2）针对饱和黏土板锚长期承载特性的离心机实验表明：当长期载荷小于0.8倍的静承载力时，板锚的长期承载特性不会发生变化，即土体的固结效应弥补了因吸力消散而降低的承载力；但若大于该值，板锚将会发生破坏。.（3）1g模型试验下，沙土中板锚的循环承载力研究率先确定：当循环载荷低于临界值时，板锚在3个小时（典型风暴）下，并不会产生显著位移变形，进而承载力不变；对于高于临界值的工况，板锚循环承载力将退化为静承载力的60%。.（三）基于数值和试验结果，确定了板锚的循环承载力评估模型。.将边界面塑性模型与流体连续性和Darcy渗流结合起来，建立了土体骨架-孔隙水耦合分析模型，提出了板锚循环承载力的评估方法。该评估方法不限于特定基础结构形式，因此也可用于对相似海洋工程基础如吸力锚、动力锚等的循环承载性能分析。