海底软黏土斜坡的失稳机制及灾变演化规律

The stability of subsea soft clay slope is a key problem that is commonly encountered by offshore oil and gas exploration. There are not many slope stability researches that considered the dynamic characteristics of subsea soft clay, its large deformation softening behaviour and strain rate dependency. There are even less that considered the initialisation of slope instability and its catastrophic evolution. This proposal is going to use triaxial experiments and penertration experiments to perform a detailed research into the mechanical characteristics of deep sea soft clay found in the South Sea. Based on the MSS two surface material model, large deformation calculation will be developed to reasonably reflect above characteristics. With this constitutive model incorporated into dynamic finite element simulation, the process of fracturing of slope and the development of the sliding surface will be modelled. Then using the initiation of the sliding body and the hardening parameters as input values, SPH depth averaged integration method will be used to model the sliding body evolution from mud flow all the way to final sedimentation. From this a complete analysing method that encompasses all the stages of slope instability, catastrophic failure up to sedimentation, is to be developed and this will be validated using geotechnical centrifuge tests. The project will also focus on the continental shelf on the North side of South sea and explore the effect of, for instance, earthquake loading, engineering disturbance, excess pore water pressure and soft layers as well as their interactions and combined effects in order to reveal the mechanisms of slope instability and catastrophic failure. This proposal has very important contribution to the scientific area of the analysis of slope stability and this would have important value of application and contribute significantly to the safety evaluation of deep sea oil and gas exploration in the South Sea.

海底软黏土斜坡的失稳和灾变是海洋油气工程中经常面临的关键问题。已有研究中，考虑海底软黏土动力特性、大变形软化特性和应变速率影响的成果还不多，涵盖斜坡失稳触发和灾变演化全过程的研究更为少见。本项目拟联合采用三轴试验和触探试验对南海海底软黏土的力学特性进行细致研究，并基于MSS双边界面模型开发大变形算法合理反映上述特性，采用动力有限元模拟斜坡渐进破坏、滑移体形成的过程，再将滑移体初始形态、强度等指标作为输入条件，采用SPH平均深度积分法模拟由滑移体到泥屑流直至最终沉积的演化过程，从而建立一套涵盖斜坡失稳、灾变和再沉积全过程的分析方法，并开展离心模型试验进行验证。针对我国南海北部陆坡，探讨地震荷载、工程扰动、超孔隙水压力和软弱层等多种因素的相互影响规律，揭示斜坡失稳和灾变机制。本项目在斜坡稳定分析领域具有重要科学意义，在南海深水油气开采工程安全评价方面具有重要应用价值。

重点针对南海北部陆坡的海底软黏土斜坡进行稳定性评价和海管路由选址，评价海底滑坡的致灾范围以及不同阶段的滑坡对海底管线的冲击作用。项目从海洋土的软化特性入手，重点讨论了定向剪切、主应力轴旋转条件下海底土的模量软化，并提出了考虑状态参量和主应力轴旋转效应的弹塑性本构模型。通过改变触探头形状和加压等方式改进了传统T-bar触探仪，研发了LRMP触探仪，针对南海软黏土和商品高岭土开展了不同剪切应变速率、大变形的循环剪切，提出海底土强度评价公式。采用极限平衡分析、有限元分析和动力时程分析相结合的办法对南海北部陆坡的陡坡区进行了评价；研究了敏感性黏土海床的续发性失稳机制，讨论了软弱夹层、摩擦效应、滑移体静/动态下滑过程对海床稳定性的影响；完善了前期工作基础的SPH平均深度积分法，实现了海底滑坡的全物理过程模拟。调整技术路线，重点研究滑坡初始阶段，即土体强度较高、流速较慢时对海底管线的冲击作用，通过一系列的海底滑坡-管线相互作用的离心模型试验和CFD数值模拟分析，讨论了海底滑坡在泥屑流阶段的运移状态，以及不同冲击角度、不同流速、不同土强度等状态下的海底滑坡对置于海床表面或者悬跨状态的海底管线的冲击作用力，提出了法向和水平向冲击力的量化表达式，较为创新地对作为导致海管整体屈曲重要因素的竖向抬升力进行了系统量化。提出临界悬跨高度以及管线受力的包络线，讨论了已有评价管线受滑坡冲击力评价方法的适用范围以及管线受滑坡冲击力的主要影响因素。.本项目发表学术论文8篇，其中SCI/EI收录6篇，EI收录2篇；授权专利3项；课题负责人晋升教授，团队成员中有3位老师晋升为副教授，培养毕业博士1名、硕士7名。成果应用于我国XX气田陡坡及海管路由稳定评价和海底管线路由选址、南海岛礁筑岛稳定性评价等工程，项目负责人作为第二完成人获2020年度辽宁省科技进步一等奖1项。