Study On Interaction Mechanism Between Submarine Pipeline-Fluid-Seabed And Its Monitoring Technology

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.