【作者】 吴钰骅；

【导师】 金伟良；

【作者基本信息】 浙江大学 ， 结构工程， 2007， 博士

【摘要】 海底管道被称为海洋油气田开发的生命线工程，它的安全可靠运行是海上油气田正常生产的根本保证。本文以国家“863计划”子课题“长距离海底管道分布式光纤传感技术”(2001AA602022-1，2001AA602210-1)为依托，在海床液化、海底管道局部冲刷、悬跨海底管道涡激振动、海底管道结构—流体—土共同作用等方面进行了深入地研究；针对海底管道破坏因素，在开发长距离光纤传感系统的基础上，进行了静态和动态应变监测的标定试验。本文的主要研究内容有：采用Biot动力固结有限元法对波浪荷载下海床土体的响应规律进行研究，分析了波浪荷载下超静孔隙水压力、有效应力的分布规律，提出了波浪荷载下均质弹性海床液化条件和液化深度计算方法。同时，研究了液化土体粘滞系数估算方法，探讨了海床液化对海底管道的影响。设计了一种新型的海底输油管道水槽冲刷试验模型，研究海底管道与砂床之间存在不同间隙时砂床起动流速的变化规律，探讨了海底管道底砂床砂粒起动的机理，提出了微小间隙时海底管道局部冲刷起动流速的拟合公式。基于κ-ω模型提出了综合考虑近壁流速和湍动能的组合流速和修正Shields数，并据此分析了局部冲刷起动状态和发展过程，提出了相应的局部冲刷起动和局部冲刷平衡判断标准。基于κ-ω模型分析了静止圆柱体在不同间隙比时的涡激力变化规律。认为间隙比的大小对涡激力频率影响相对较小，但对拖拽力和上举力的平均值和波动幅值影响较大。提出了拖拽力和上举力波动幅值随间隙比变化的拟合公式。建立海底管道流固耦合计算模型，分析了考虑流固耦合时海底管道结构涡激振动响应规律，提出了考虑流固耦合的涡激振动基频的计算方法。分析表明，海底管道振动各参量在横流向主要响应频率为流固耦合涡激力基频，而顺流向为基频的两倍。基于Fluent软件进行二次开发，建立了海底管道结构-流体-土耦合振动的数值计算模型。根据数值模拟分析可知，海底管道相对于海水属于轻柔结构，结构固有阻尼较小时可能发生颤振，海底管道固有阻尼对抑制海底管道涡激振动有重要作用。海底管道固有频率与涡激振动基频越接近，从海流中吸收的振动能量就越大，管道振幅也越大。当海底管道固有频率与顺流向涡流激振频率相近时，管道涡激振动基频将发生迁移。当土体刚度较小时，土体刚度减小将使海底管道涡激振动幅值增大。当液化土体粘滞系数较高时，土体液化对管道涡激振动影响较小；当液化土体粘滞系数在10kPa／(m／s)左右时，土体液化对管道涡激振动具有抑制作用；当液化土体粘滞系数很小时，土体液化将使海底管道涡激振动幅值增大。针对海底管道破坏因素，进行了长距离海底管道分布式光纤传感监测系统的静态试验和动态试验，探讨了其监测可行性和监测精度，并对不同铺设工艺下的测试值进行了标定，为该系统在实际工程中的应用做准备。更多还原

【Abstract】 Submarine pipeline system is a main pattern in transportation of offshore oil and gas, and plays an important role in the offshore oil and gas development. To improve the security of submarine pipeline, Zhejiang University and CNOOC applied for the project of distributed optical fiber sensing technology for long-distance submarine pipeline, which sponsored by National Hi-Tech Research and Development Program of China （863 Program）. As a part of this project, this paper lucubrated in the state of submarine pipeline in severe ocean environmental condition such as liquefaction of seabed, local scour of submarine pipeline, vortex-induced vibration of suspended span submarine pipeline. Furthermore, the static and dynamic testing of the long-distance distributed optical fiber sensing system has been done to establish an efficient monitor system for submarine pipeline. The major contents are summarized as follows:The Biot-dynamic-consolidation-finite-element method was introduced to analyze.the effect of wave condition and seabed soil characteristic on seabed liquefaction. It’s found that the amplitude distributions of pore pressure and effective stress in elastic homogeneous seabed are mainly determined by the wave condition and boundary condition. The fit curves of amplitude distribution were applied to discuss the liquefaction criteria and liquefaction depth of seabed. Furthermore, the calculation method for the viscosity coefficient of liquefied soil was studied. Then, the response of submarine pipeline in liquefied soil was discussed.The tests for the physical model of local scouring to submarine pipeline under the action of sea current were carried out to study the critical states of the onset of local scour. The mechanism of seabed scouring was discussed. The empirical formula of threshold velocity for small gap between submarine pipeline and seabed was found. To study in detail, the low-Reynolds-number Wilcox k-ε model was applied to analyze the experimental results. A new concept of the combined velocity, which considered the flow velocity and turbulence near wall, was proposed to define the critical onset condition of local scour. On this concept, the Shields’ parameter was revised to consider the influence of turbulence for local scour. And then, the critical onset condition and the developing process of local scour were analyzed by the revised Shields’ parameter to determine corresponding criteria of onset condition and. equilibrium condition of local scour.The vortex-induced force on the immovable cylinder with different gap ratio was studied by the low-Reynolds-number k-ω model. Based on Fluent software, the submarine pipeline was simplified as a cylinder supported by two springs to establish a vibration analysis model, which can consider fluid-structure interaction. The fluid state and structural response for vortex-induced vibration was discussed by this method. It was found that the base frequency of vortex-induced vibration shifted when the fluid-structure interaction was considered, and the modified calculation method for the base frequency (f_w0) was proposed. The structure vibration frequency is f_w0 in vertical direction, and 2 f_w0 in current direction.The model considering submarine pipeline-fluid-seabed interaction was established by the Newmark method and k-ω model with Fluent software to discuss the influence of structural damping, length of span, soil stiffness and liquefaction. It’s found that submarine pipeline is light flexible structure in contrast with water. If the damping of pipeline were little, the pipeline would flutter in the flow. Therefore, the damping of pipeline is efficient in controlling the vortex-induced vibration of submarine pipeline. The vibration energy absorbed from the fluid is determined by the difference between the base frequency (f_w0) and natural frequency of submarine pipeline. The vibration frequency would shift, when the natural frequency of submarine pipeline near to 2 f_w0. The decrease of soil stiffness will increase the vibration amplitude of submarine pipeline. The vortex-induced vibration of submarine pipeline would be restrained at the liquefied soil with viscosity coefficient about 10 kPa/（m/s）, and would be enlarged when the viscosity coefficient of liquefied soil was very little.The static and dynamic testing for the long-distance distributed optical fiber sensing system has been carried out. The monitoring capability for static deformation caused by seabed liquefaction and local scour of submarine pipeline was feasible. The monitoring system can detect the vortex-induced vibration. These capabilities can provide information for judging the health condition of submarine pipeline.更多还原