油气田水下生产系统基础的冲刷研究

发布时间：2018-03-23 23:00

本文选题：水下生产系统　切入点：CFD　出处：《中国海洋大学》2015年博士论文

【摘要】：随着能源需求的与日俱增,人类对海洋油气田开发的重视度也逐步提高,可达到的水深随着技术的不断成熟也越来越深。面对水深导致的开发成本的大幅提高,一种新的开采系统---油气田水下生产系统得到了普遍关注。水下生产系统的适用范围从几十米至几千米,且建造成本不随水深的增加而增加。水下生产系统主要由水下井口和采油树、管汇、跨接管、脐带缆、海底管线、立管、控制系统及水下处理系统等组成,通过控制系统的远程操控,将油气井采出的油气水从海底输送到依托设备或陆上终端。水下生产系统的基础多数采用防沉板,且通过裙板入泥进行固定。如果基础处于渔业区,为了防止防沉板上裸露放置的管汇对渔网产生拖挂,需在管汇上部放置保护罩。但保护罩的存在会改变周围流场,从而产生冲刷,有可能引起防沉板的不均匀沉降甚至滑移,从而对生产系统的其余部分产生附加荷载,对其正常运作产生安全隐患。水下生产系统基础保护罩的迎流面、背流面皆不垂直于床面,且前后都存在可透水的预留口,这种结构前人并未做过相关研究,而且针对工程结构实际尺寸所进行的冲刷研究也很少。为了分析带有保护罩结构的水下生产系统基础周围的冲刷情况,本文采用了物理模型试验和数值模拟相结合的方法。首先对正常工况下基础的局部冲刷进行模型试验,然后通过耦合求解水流运动、悬移质扩散、推移质输运以及床面高程变化的冲刷模型对物理模型试验进行数值分析,其中水流运动通过RNG k-ε湍流模型对雷诺时均方程进行封闭求解,并利用VOF追踪自由液面。模拟结果充分证明了该数值模拟方法的可行性。鉴于实验室比尺模型试验的结果难以反推至水下生产系统基础的原型,论文又对水下生产系统基础的原型在正常、极端两种工况下的冲刷进行了数值模拟。建立三维数值模型时,如果模拟真实水深,计算量会非常大,同时鉴于海底水质点速度引起的海床剪切应力是影响泥沙运动的主要因素,因此在保证近底水质点速度的前提下,将水深减小,以使数值模拟大范围流场成为可能。一年一遇潮流、一年一遇波浪的正常工况下,波浪产生的海底水质点流速相对较小,可以忽略不计,因此正常工况可看作纯水流作用下。小尺寸基础的物理模型试验、数模验证皆表明,迎流面、背流面的冲刷皆起始于流速放大处。首先,背流面预留口附近产生冲刷,但由于模型高出沙面的高度较小,尾涡强度不大故而背流面的冲刷发展很慢；而迎流面的冲刷则开始于尖角处,在预留口两侧下潜流的辅助下,冲刷坑向中间迅速发展,致使尖角处冲刷深度最大,而迎流面中间预留口位置的冲刷深度较小。对于该工况下大尺寸原型的冲刷模拟则表明,由于保护罩高出海底高度的大幅增加,尾涡的发展使得背流面的冲刷范围加大。背流面预留口位置首先出现的冲刷坑在尾涡的加速下迅速向周围扩展,个别位置的冲刷坑深度已超过第一层底沙的厚度。而迎流面的冲刷虽然还是起始于尖角位置,但在向中间发展的过程中,由于预留口位置防沉板高度造成的下潜流使得中间位置的冲刷坑更深。百年一遇潮流、百年一遇波浪的极端工况下,波浪、潮流产生的水质点速度皆很大,再加上波浪作用下又导致边界层变薄,致使整个沙面都出现了冲刷现象。初始在迎流面尖角处形成的冲刷坑在波浪的振荡过程中被弱化,最终在前后两预留口附近形成了明显的冲刷坑,尤其是背流面出水口附近的冲刷坑深度已达到两层底沙的厚度。此外,不同比尺的物理模型在同一水深下的冲刷试验表明,基础周围的冲刷随比尺的增大而严重,这表明水深与没水基础高度的相对比值对冲刷有重要影响。为了研究该比值的影响,本文对水下生产系统基础模型在不同水深下的冲刷进行了数值模拟研究,结果表明,当水深与基础高度的比值超过7.5时,基础周围的冲刷将不再受水深的影响而相对稳定。水下生产系统基础的冲刷研究需要综合考虑水、沙、基础之间的相互作用,与出水的基础结构相比,没水基础流场的尾涡中增加了一项,该项由流经基础上方的水流发展而来,其强度与基础离开沙面的高度有关。就本文研究的基础而言,除没水外,基础内部还可透水,这就需要考虑透水口的出水对基础背流面的影响,以上因素皆导致背流面的冲刷甚为复杂。以物理模型试验为依据,以数值模拟方法为手段,本文得到了水下生产系统的基础在纯流和浪流共同作用下的冲刷范围及冲刷深度,从而对后期的设计及安装提供指导。此外,对其他形状大尺寸模型的冲刷数值模拟也提供了参考。
[Abstract]:With the energy needs of mankind to grow with each passing day, the offshore oil and gas development importance also gradually improve, can reach the depth with the maturity of the technology is more and more deep. In the face of water depth caused a substantial increase in the cost of development, a production system, the new system of oil and gas field water has been widespread concern. The scope of underwater production system from tens of meters to several thousand meters, and the construction cost increases with the increase of water depth. The water is mainly composed of an underwater wellhead and Christmas tree, manifold, cross over, umbilical cable, submarine pipeline, pipe production system, control system and water treatment system composed by remote control the control system, the oil and gas production of oil and gas conveying water from the seabed to rely on equipment or the onshore terminal. Most basic underwater production system using mudmat, and through the apron into the mud is fixed. If in the fishing area, for The prevention of the placement of mudmat bare pipe produced on the net in the trailer, the manifold is placed on the upper part of the protection cover. But the protection will change the flow field around the existing cover, resulting in erosion, may cause uneven settlement of mudmat even slip, resulting in additional load on the rest of the production system. The normal operation of security risks. Underwater production system based protective cover flow, back flow surface is not perpendicular to the bed surface, and the existence of both before and after the reserved port water permeable, this structure is not done previous related research, and according to the engineering structure of the actual size of the scour are few. In order to analysis of scour protection cover structure with underwater production system around the base, this paper uses the method of physical model test and numerical simulation. The combination of the first test model of local scour on the basis of the normal conditions Then, by solving the coupling flow, suspended sediment diffusion, bedload transport and erosion model change of bed elevation in numerical analysis of physical model experiment, the flow through the RNG k- turbulence model of Reynolds averaged equations are solved and closed, using VOF to track the free surface. The simulation results prove the feasibility the numerical simulation method. In view of the laboratory scale model test results difficult to backstepping underwater production system based on the prototype, and underwater production system based on the normal prototype, the extreme two conditions of erosion were simulated. The three-dimensional numerical model is established to simulate the real, if the water depth, the computation will be very large, at the same time as the sea water seabed shear velocity stress is the main factor affecting the sediment movement, so under the premise of ensuring the near bottom velocity of water particle under water Deep decreases, which makes the numerical simulation the flow field becomes possible. Once a year the trend, a normal operating year wave under the sea water velocity wave is relatively small, can be ignored, so the normal condition can be regarded as pure water. Under the action of the physical model test of small size based on the model validation showed that welcome back, flow, flow erosion are starting in velocity enlargement. First of all, the dorsal stream surface kept scouring near the mouth, but due to a smaller height model of high sand, vortex strength and back flow erosion has very slow development; while the upstream erosion was started in the corner, auxiliary undercurrent in the reserved hole on both sides of the rapid development, to the middle of the scour pit, resulting in the maximum scour depth at the corners, and the upstream face middle reserved port position of scour depth is smaller. For the large size of the prototype scour under the condition simulation shows that, by In the protection cover above the seabed height increased sharply. The development of vortex flow erosion makes the back range increase. The scour downstream reserved port position appears first accelerating in the wake of rapid expansion to the surrounding and the depth of scour hole in the position of the individual has more than the first layer of sediment thickness and flow. The scour though starting at corner position, but in the process of development to the middle, because the flow reservation mouth position mudmat height caused by making the middle position of the scour hole deeper. The hundred years trend, the extreme conditions of hundred years of waves, waves, water particle velocity flow generated are large, then and under the action of wave and the boundary layer becomes thinner, resulting in the sand have been weakened in the erosion phenomenon. The initial wave oscillation process in the upstream scour hole forming surface at the corners of the final two before and after the reservation near the mouth of the formation The scour pit is obvious, especially the depth of the pit back flow near the outlet has reached two of the thickness of bottom sediment. In addition, proved that the physical model test of different scales in the same depth of scour around foundations with scale increasing serious, which indicates that the water depth and the relative ratio of water based on the height of erosion has an important influence. In order to study the influence of the ratio, the water production system under the basic model in different depth of erosion is studied by numerical simulation. The results show that when the ratio of depth and height of foundation of more than 7.5, the foundation scour around will not be affected by the effect of water depth and relative stability water erosion research production system. Basic needs comprehensive consideration of water, sediment, the interaction between the foundation, compared with the basic structure of water, no water flow in the wake of an increase, the flow through the foundation above The current development, the strength and height of the sand foundation left. The basis of this study, in addition to no water, but also the basis of internal flooding, which need to be considered on the basis of the water permeable mouth back flow impact, the above factors lead to back flow surface erosion on the physical model is very complex. According to the test, by means of numerical simulation, this paper obtained the basic water production system under the flow erosion range of interaction and the scour depth in pure flow and waves, and for the later design and installation to provide guidance. In addition, the simulation also provides a reference for other large size shape erosion numerical model.

【学位授予单位】：中国海洋大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TE95

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