深海浅层地基土触变性及其对海底管线工作特性影响研究

The seabed sediments in deep water are normally soft clay, with high water content and low soil strength. Due to the special deposition history and the consequent characteristic structure, the soil presents thixotropy during and after disturbance. It is presented in the soil mechanical behaviour that the deposits is softened during disturbance (with decreasing strength and deformation modulus); and hardened after disturbance (the strength and deformation modulus increases over the timescales). As developments of oil and gas industry moves into deep water from shore, the as-laid pipeline represents an increasing fraction of the capital expenditure. The adoption of subsea wellhead and other relevant processing facilities leads to the design methods that rely on the mechanical behaviour of surficial sediments. Considering the pipeline laying and successive shut-down and start-up cycles, the soil thixotropy has a significant influence on the design for pipeline buckles, and on the associated problem of incremental axial walking, and also on the uneven settlement of pipeline. Correct estimation of these soil properties is still in its infant stage, and currently there is no straightforward method for allowing reliable measurement. In order to advance the design practice, this research sets out a framework for various aspects of pipeline-soil interaction, linked to the fundamental soil thixotropy properties, by ways of numerical, and physical modelling. The feasible technique of the novel shallow toroidal and hemiball penetrometers, which are developed during the candidate’s PhD and patented recently, is utilised in this project, representing a time-efficient simplification of the same behaviours of pipeline operation. The achievements from this project are capable to capture the effect of thixotropy on the pipeline-soil interaction and fill the gap of the reliable assessment of the surficial seabed sediments.

深海浅层土体一般为软黏土，含水量较高且强度较低。其生成和沉积特性使其具有一定结构性，受扰动或扰动停止后结构性或土颗粒排列改变的现象称为触变性。宏观上表现为土体受扰动时具有软化特性(强度和变形模量降低)，扰动后具有硬化特性（强度和变形模量都随时间增加）。随着深海油气的开发，海底管线需长距离直接铺设于深海浅层地基土上，纵观管道的铺设安装及运行过程，浅层土体的扰动软化及固结硬化特性显著，其对管线的承载力及约束力具有时变性及阶段性，对解决工程中管线屈曲、轴向整体移动及不均匀沉降问题带来了极大挑战，严重危害管线及连接管道部件的工作性能。目前触变性的机理尚未清晰，亦缺乏相应手段测定其特性。本课题拟基于申请人研发的深海浅层土体原位探测技术，运用数值模拟和模型实验的手段，研究和发展可用于评估海洋浅层土体的触变性对管道竖向承载力及轴向有效摩阻力的影响的原位探测方法，填补可靠探测海洋浅层地基土力学特性的空缺。

海底管道的安全可靠运营是海上油气田生产运输的根本保证。管道运行期间需要承受高温高压的工作环境和恶劣的波流荷载，其全周期内的在位稳定性的评估显得尤为重要。而海床土是海管服役期在位稳定抗力来源，因此，识别、理解深海浅层土体的触变性及量化深海管道-海床土相互作用是海管设计中不可或缺的环节。本课题基于研究者开发的深海土体原位探测技术，基于大变形有限元计算方法和模型试验，模拟铺设过程中海管周围土体形态及强度演变，随后将海管周围土体的应力应变状态映射到后续模型，作为初始状态开展后续在位稳定性分析，实现管道安装及在位稳定性的仿真。本项目主要从以下三个方面展开研究：.1).开展有限元分析方法研究海底管道贯入过程中深海浅层地基土触变性的宏观扰动软化及固结硬化特性的机理。研究考虑了深海管道安装过程会影响管道的初始嵌入海床深度、管道周围土体和管道真实的接触边界性质以及周围土体的强度演变及分布规律。研究首先验证了大变形分析方法的可靠性，随后考量了管道贯入过程造成的管土接触边界随着埋置深度的非线性增加，重点量化了在管道以不同应变率贯入条件及不同排水条件下海床重塑形态及强度演化的规律。.2).对管道在位稳定性进行研究。利用自由边界面跟踪和场变量插值映射方法，基于管道铺设结果，构建了海底管道轴向运动三维有限元管土作用单元模型，对单位管道的轴向摩阻力激发机理进行了深入研究。考虑轴向剪切速度、累计塑性应变、剪切时间等轴向管土作用控制因素，系统量化了单位管道轴向摩阻力与海床土典型触变性力学指标（土体灵敏度、土体延展性因子、应变率因子）的关联特性。.3).提出了海底管道对海床土体触变（应变软化和固结硬化）激励下的工作特性的评估方法研究。研究基于有限元模拟和离心机模型试验，整合了原位探测设备与海底管道的测试结果，并进行对比分析，建立了探测设备课直接测得工程参数及表征深海浅层土、管道工作特性的无量纲参数之间的联系。