CFD Simulation Of Submarine Pipeline And Pile Behavior

Submarine landslide is one main reason triggering the damage of the submarine structures (such as submarine pipelines, piles). Pipelines, which are used to transport oil and gas. which is usually founded directed on the seabed of the deep sea. Due to the long transport distance and rugged terrain, local suspended span of the pipeline will come out. These rugged terrains, such as trough and trench, are the main routes of submarine landslide which seriously threats to the stability of submarine pipeline. Submarine slide has relatively large velocity and is likely to change the behavior of its adjacent flows. These two facts could have significantly impacted the marine structures. At the same time, the existence of marine structures also changes the flow behavior, such as horseshoe vortex and wake vortex around the pile. This phenomenon causes local scour around pile and may reduce the bearing capacity of the foundation.To date, model test and numerical simulation (CFD) are the major means for investigation of the impact of geological hazards to the marine structures, under the theoretical framework of soil mechanics and fluid mechanics. Numerical simulation (CFD) is a commonly adopted tool to verify the results of model test, and explore the behavior of field fluid-structure interaction further. This thesis aims at investigating the fluid-structure interaction, based on a series of CFD analysis. The computed results are verified against experimental results reported in the literatures.The main contributions of this work include:1. Identifying key factors affecting the drag coefficient of subsea pipeline and proposing an improved calculation method for the drag coefficient.These were achieved by numerical analysis (based on Herschel-Bulkley model) simulating the interaction between a submarine landside and a suspended pipeline. The computed results were verified by comparing to published experimental data. Based on the numerical analyses, it is revealed that the drag coefficient of submarine pipeline decreases when non-Newtonian Reynolds increases. There is a critical suspended height, corresponding to which non-Newtonian Reynolds equals a constant. The drag coefficient increases with the suspended height when the suspended height is smaller than the critical height. When the suspended height exceeds the critical value, however, the drag coefficient remains the same. An improved calculation method of drag coefficient is proposed and the relationship between the normal drag coefficient of subsea pipeline and suspended height is revealed.2. Revealing the flow mechanism around piles in a comprehensive manner.Three turbulence models are adopted to simulate the change of flow field around the pile. The computed results agree well with the results reported in the literatures, justifying the validity of the numerical model and the turbulence models. Various factors including pile spacing, turbulence models, roughness of the bed surface, change of water velocity and variation of water depth are taken into consideration. It is found that when the spacing between piles (S/D) is larger than4, the interactions between piles becomes negligible. The turbulence models have larger influence on the flow field around piles. The flow velocities around piles on smooth seabed are larger than rough seabed. The smaller the seabed roughness is, the larger the influences are, which change the pile distribution of flow field near the seabed surface. The down flow velocity in front of piles increases with the free upstream velocity, leading to an increasing pressure gradient zone in front of the piles. At the same time, the separation angle moves backward on both the side of the pile and the flow velocity is larger, and flow separation occurs. Then the flow velocity at the back of piles increases, and adverse pressure gradient zone is developed. Horseshoe vortex has been formed in front of the pile near seabed surface, with an influence area of about0.5D ahead of the pile.