Study On The Erosion And Protecting Technique For Submarine Pipeline

The submarine pipeline is the most important oil transport tool in ocean oil exploitation. It is an indispensable component in offshore oil production systems. Because the ocean environment is very complicated, the influences of many uncertain factors make it very difficult to lay and manage underwater pipelines. Along with the exploitation of deep ocean oil, the application of submarine pipeline has become increasingly widespread. Hence more and more attention is being paid to the erosion and destruction problems caused by ocean dynamics.When a pipeline is laid on a sand bed, and subjected to a current, the pressure difference between the upstream and the downstream of the pipe will induce a seepage flow underneath the pipe. When the seepage flow velocity is increased, piping will appear which means the scour under the pipe starts. So the pressure difference is a main dynamic factor for the scour under pipelines, at the same time, the wake eddy produced by pipeline is an affixation dynamic environment factor.Based on the basic equations of boundary layer, this paper used the perturbation approach to derive the pressure distribution around the pipeline, and established the seepage pressure difference acted on the sediment particle in the erosion hole and its critical value. Considered the equilibrium of forces acted on the sediment particle, some formula such as incipient velocity and erosion-stopping velocity and erosion hole depth were set up.By the research for the scour under the submarine pipeline, two new methods to protect submarine pipelines using flexible spoilers were proposed. The first method is to install a flexible spoiler on the top of the pipe. It can disturb the flow field, increase the local scour and the scour hole depth, and makes the pipe sinking and self-burying. The second method is to install two flexible spoilers under the pipe. They can decrease the pressure gradient between both sides of the pipeline, decreases or holds back the seepage flow underneath the pipe, so that use it to decrease or prevent the scour.Physical experiments for the scour under the pipe with flexible spoilers, rigid spoilers and without spoilers were carried out. The pressure on the surface of the pipe, pressure gradient between both sides of the pipeline and scour hole depth under the pipelines under unidirectional current and reversing current were measured and compared. Experiment results indicated that rigid spoilers and flexible spoilers on the top of the pipe both increase the pressure gradient between both sides of the pipeline, increase the scour depth and scope, and make the pipe suffer a down pressure, accordingly make pipelines self-burying under the sandy seabed. By the compare of the self-burying depth between the pipe with rigid spoilers and flexible spoilers, the result was found that when the two end of the pipe were fixed, the self-burying depth under the pipe with rigid spoilers and flexible spoilers were very close to same value. However, when the two end of the pipe can freely go up and down freely, the self-burying depth under the pipe with flexible spoilers is larger then that with rigid spoilers. By the compare between the pipe with flexible spoilers under it and without spoilers, the result was found that flexible spoilers under the pipe can decrease the pressure gradient between both sides of the pipe and the scour depth under the pipe, and increase the lift force so that to keep the pipe stable. When the flexible spoilers reach a certain length, the pressure gradient is equal to or less than the critical pressure gradient for the scour happen, so the bottom scour almost disappears. That is said this kind of effective protection for submarine pipeline can be obtained.Besides theoretical research and experimental investigation above-mentioned, a vertical two-dimensional numerical model was used to simulate the time dependent local scour below offshore pipelines subject to unidirectional steady flow. It is found that standard k-εturbulence model is more preferable than the SGS model in the 2D scour model developed in this study. The flow field and the time dependent local scour were computed by the model agree well with the measure results, that shown us the parameter choice and grid dispersed and mobile boundary process technique as well as the solving approach to sediment control equations were reasonable and feasible.