The Study On The Response Of Silt Seabed To VIV Of Pipeline Free Span

Due to the safety and reliability, submarine pipeline is a high effective and continuous transferring mode, and it has been served as an important role in the offshore oil and gas exploitation which expands day by day. According to the support of the National Science Foundation of China (NSFC.41006024)“The impact mechanism of the strong wind weather on the reliability of the submarine pipelines”, this work was focused on the following points: the cause of pipeline span; the vortex‐induced vibration of these pipelines during suspension status and the response of the supporting subsurface seabed soil and the coupling effect of the submarine pipelines, external current and seabed soil. Based on the typical bottom sedimentary setting, the safety of the submarine pipelines in the Chengdao, which located at the Yellow River Delta, was examined in the thesis according to the field observed data and the simulation result. The main topics of the thesis are divided into four aspects and list as following:1. Based on analyzing and comparing with numerous pipeline routes investigation data of different areas, the cause and developing trend of pipeline span under the characteristics of the marine dynamic environment was studied.2. VIV simulation model of pipeline‐fluid‐seabed system was established based on the geography data.3. In order to obtain the key parameters of the model, such as damping ratio of the liquefied soil, the physical simulation experiment between the pipe and seabed was designed. According to the analysis of the data, the system vibration damping ratios of the pipeline in different depth were obtained.4. The measured geotechnical data and physical simulation parameters are usedto verify the numerical simulation model, and then to calculate.By analysis of numericalsimulation model results, the conclusions are following:The rigidity of pipeline is relative larger than sea bed soil, when vibration occurred and energy would spread a certain distance along the pipeline. Due to different displacement of the pipeline during the vibration propagation, the berried pipeline would be destroyed by the effect of shear stress.For the seabed, the sediment along the pipe was disturbed leading to the declined rigidity coefficient, and sediment particles were more easily to be stirred, so that the seabed was eroded. Because of the pipeline belongs to light structure, vortex‐induced vibration leads to a pipeline flutter.Base on numerical models result, when the sediment of supporting section liquefied, it could absorb vibration energy of pipeline, and reducing the vibration propagation distance. However, in the real environment, the flow velocity gradually increases over time and then decreases gradually. If resonance of span pipeline happened in low flow velocity, although active section increasing led to increase of inherent frequency which then jumped out of resonance interval and restore stationary, when the velocity of flow increase，span pipeline could resonate again.Because of the liquefied sediment could not provide effective support and constraint for pipeline, the initial point failed to support the pipes, it can be regarded as the fulcrum of the pipeline span gradually moved to both ends and the deflection increase, midpoint or low structure strength joint might produce yield deformation or fracture.The49m was the safe span length of508mm half buried pipeline in the north of Cheng Dao obtained by calculations. At this length, the inherent frequency of the system of pipeline span appeared greatly raised, which concerned with the change rule of total mass coefficient and total rigidity coefficient resulted from the length change of pipeline span system. The method proposed in this paper can be used to calculate safe span length of different types of pipeline in other sea areas and provide foundation for pipeline safety evaluation and treatment method.There are some innovative points in this study and as followed:Firstly, we present real sediment and pipeline status of Chengdao in the Yellow River submarine delta, and establish the numerical model for regional pipeline safety, according to the numerical simulation methods of the tube cross vortex‐induced vibration during the liquefaction of the support seabed sediment.Secondly, the safety spanning distance of the typical pipeline in this study area are obtained by the numerical models. This work could provide information for the spanning case in this study area. If considering about the liquefaction of the seabed sediment, the question about deflection increases and damage by the gravity could be answered to a certain extent in the traditional method. And the traditional method is limited to the effect of the Vortex‐induced vibration. Once the process is not finished in a suitable way, the natural frequency of the pipeline spanning system is decreased, and increases the frequency of the vortex‐induced vibration leading to the risk of pipeline damage.