深海粘性土中滑动防沉板复合承载力的确定方法

Sliding mudmats are a new type of shallow footings in deep water, used to support subsea facilities such as pipe manifolds or pipeline end terminations on clayey soil. The sliding mudmat is allowed to undergo sliding cycles with finite magnitudes, followed by termination of the sliding cycles and partial or full consolidation of soil. The capacity of the mudmat under extreme combined loadings is thus much larger than that at the initial settlement of the mudmat. The consolidation mechanism of the sliding mudmats is not interpreted clearly, and there has been no appropriate approach to determine the combined load-carrying capacity. In this project, an elasto-plastic soil model is developed to consider the role of soil sensitivity, and the laboratory tests and cyclic ball penetrometer tests in a centrifuge are conducted together to obtain the soil properties. A three-dimensional large deformation finite element (LDFE) approach in effective stress formula is developed, with the soil model established being incorporated. The entire process of ‘sliding-consolidation-combined loading’ is investigated through the centrifuge tests and LDFE simulations. The influences of the self-weight of the structure supported, magnitude and number of sliding cycles, consolidation time and soil sensitivity are qualified. The histories of the excess pore pressures and undrained strengths in soil are investigated, and the envelope under combined loadings is established to provide theoretical and technical supports for the infrastructure in deep water.

滑动防沉板是新近出现的一种深海粘性土浅基础形式，用于支撑与油气输送管道相连的水下生产系统。其特点是经历一定幅度的循环滑动后停止运动，地基部分或完全固结，当后期极端复合加载出现时地基承载力已经远高于防沉板刚就位时。目前滑动防沉板加固地基的工作机制尚不清晰，承载力的确定没有可靠的方法。本项目旨在构建考虑防沉板滑动与固结效应的复合承载力确定方法。首先发展包含粘性土灵敏度影响的弹塑性本构模型，联合运用土工试验和离心机内球形仪的循环“压-拔”试验，标定本构参数；然后建立有效应力形式的三维大变形有限元方法，并纳入构建的土体模型；通过离心试验与大变形分析，考察防沉板“滑动-固结-复合加载”完整过程，定量评估上部结构自重、循环滑动幅值和次数、固结时间和灵敏度等因素的耦合影响。揭示防沉板地基内超静孔压和不排水强度的演变规律，建立用于工程设计的复合承载力包络面，为深海基础设施建设提供理论指导与技术支持。

滑动防沉板是深海油气开发中新近采用的一种新型浅基础形式，防沉板的循环滑动有利于减小几何尺寸，显著降低制造和安装成本。然而，已有的滑动防沉板研究不仅数量有限，且多针对防沉板的某个特定工作阶段。本项目发展了有效应力形式的大变形有限元模拟方法与离心机中的模型试验技术，追踪粘性土中滑动防沉板“滑动-固结-复合加载”的完整工作过程，建立防沉板滑动阻力与滑动后复合承载力评估方法。提出了防沉板安装就位后地基部分固结引起的复合承载力包络面表达式，揭示了循环滑动过程中滑动摩擦系数随循环次数和固结时间的变化规律，发现安装后固结与开采固结的耦合作用可能导致防沉板的抗滑力增加一倍以上，进而给出了滑动摩擦系数随服役时间增长的归一化曲线。考虑循环滑动效应，建立了极端竖向力和水平力复合作用下的承载力包络面。项目成果的科学意义在于厘清了不同排水条件下防沉板地基破坏机制的渐进演变，应用价值在于直接支撑滑动防沉板的实际设计与优化选型。在本项目支持下，发表和录用SCI论文11篇、中文核心论文7篇；培养博士生3人、硕士生6人，其中5人已获得博士或硕士学位；项目负责人2020年获得国家杰出青年科学基金资助。