EXPERIMENTS ON TWO-PHASE FLOW CHARACTERISTICS IN A CATENARY RISER SYSTEM WITH THE GAS AND LIQUID MIXTURE TRANSPORTATION

严重段塞流是海洋工程气液混输管线-立管系统中常见的一种特殊有害流动现象,采用水平-下倾-悬链线立管气液混输组合管道系统,通过系列实验在悬链线立管中获得了严重段塞流、间歇流和震荡流等流型,阐述了这些流动现象的形成机理,提出了能够产生严重段塞流的判定准则.结果表明,悬链线立管严重段塞流具有明显周期性,在一个周期内的流动特征可分为液塞形成、液体出流、液气喷发及液体回流等4个阶段,进而给出了各阶段中相关流动参数的变化规律.在实验中同时还对悬链线与垂直立管中严重段塞流形成机理进行了比较分析,发现两者在液塞形成阶段有显著差别.其中,在悬链线立管中液塞形成之前首先需要经历一个气液混合液塞形成过程,而垂直立管则没有这个过程.     

下倾管-立管水气严重段塞流数值模拟

针对海洋油气传输中常见的下倾管-立管系统, 采用Brackbill模型模拟气液相界面间表面张力, VOF方法追踪气液两相运动界面, 提出了管内气液两相流数值模拟方法. 在低气液相进口折算速度下, 数值模拟了该种管型下的严重段塞流动现象, 分析了相关物理参数的变化特性. 结果表明, 在严重段塞流下, 管内流型流态、压力、液塞运动速度、立管出口气液相平均速度、下倾管及立管内含气率等均具有明显周期性特征, 而且一个周期内严重段塞流可分为4个阶段, 进而给出了各阶段中相关参数的变化特性. 数值模拟结果与相关文献中的实验结果吻合良好,表明了该数值模拟方法的有效性.      

正弦振荡来流下柔性立管涡激振动发展过程

在风浪流的作用下,海洋浮式结构物将带动悬链线立管在水中作周期性往复运动,从而在立管运动方向上产生相对振荡来流,这种振荡来流将激励立管悬垂段发生“间歇性” 的涡激振动. 在海洋工程水池中对一个4m 长的立管微段进行模型试验研究,以探索相对振荡来流作用下立管涡激振动产生的机理及其发展的物理过程. 试验通过振荡装置带动模型作正弦运动来模拟不同最大约化速度U_Rmax、不同KC（Keulegan-Carpenternumber）的相对振荡来流,利用光纤应变片测量立管涡激振动响应. 结合模态分析方法处理试验数据得到位移响应时历,继而提出相对振荡来流下柔性立管涡激振动发展的3 个阶段:建立阶段、锁定阶段以及衰减阶段. 并进一步总结了最大约化速度U_Rmax,KC 对涡激振动发展过程的影响规律. 最终获得不同最大约化速度U_Rmax下,涡激振动各发展阶段随KC 所占时间分布比例图.      

气液两相段塞流作用下管道流固耦合动力学分析

气液两相流的不稳定流动和管道动力响应之间的耦合作用是引起管道振动问题的直接原因。本文通过轻烃回收重沸器管道的流固耦合动力学数值仿真和气液两相流理论分析,深入研究了管道振动内在机理,发现了垂直管段中段塞流是引起管道振动的原因。通过研究现场工况中不同气液组分下段塞流的流动规律,进而进行了管道流固耦合动力学分析,获得了不同工况下管道振动响应。数值结果表明:稳定段塞流和不稳定段塞流冲击作用下管道振动显著;在强烈段塞流作用下,管道振动峰值更加显著,振动峰值超过10mm,振幅峰峰值也超过8mm。研究揭示了气液两相段塞流作用下管道的振动特性,为这类典型管道振动问题的治理提供了理论和方法依据。     

深水钢悬链线立管疲劳寿命计算方法

基于悬链线理论及立管与海床接触模型,建立了一种估算立管疲劳寿命的方法,模型中考虑了深水浮式平台运动过程中立管悬链线形状的变化及触地区域海床接触刚度大小对疲劳寿命的影响。分析表明:浮式平台的运动会引起立管悬链线形状的改变以及立管内部应力的重新分布,这是引起立管疲劳损坏的主要原因;随着海床土体刚度的增大,立管疲劳寿命逐渐降低,为了获得准确的立管寿命,有必要对海底土体进行更为全面细致的研究。通过与现有分析方法比较,本方法具有计算方便快捷,结果可靠等优点。研究可为钢悬链线立管的设计分析提供一定的参考。     

气液二相流研究概述

气液二相流研究液体和气体(或蒸气)两相介质共存条件下的流动特性。二相体系可以是液体中含有气体或(蒸气)泡或者气体,(或蒸气)中含有液体微滴,也可能因其中气泡或液滴的聚并,两相间形成更复杂的分布状态。气液二相流是自然界、日常生活中常见的现象,在许     

基于Level Set方法的气-液-固三相流动模型与模拟

采用基于Level Set方法与离散颗粒模型相结合的方法,建立了一个用于描述气液固三相流动的新模型.模型耦合了颗粒与气泡、颗粒与液相以及气泡与液相之间的相互作用.应用该模型对液固悬浮液中的典型现象--气泡的单孔及多孔形成过程以及颗粒夹带现象进行了三维模拟,检验了其可行性.并进一步研究了颗粒的存在对气泡的形成与上升过程的影响以及气泡诱导的液相流动对颗粒行为的影响.研究结果表明,所提出的模型能够真实地预测三相流中气泡与颗粒分散相的特征,为研究多尺度的三相流动提供了一种新途径.     

粘土悬浮液明渠流的减阻现象

水和粘土悬浮液在水槽中的流动试验证明，在粗糙边界上的粘土悬浮液明渠流阻力明显小于清水流．在同样水力条件下粘土悬液的时均流速比清水流速大得多．当悬液浓度较高时，流动阻力系数比清水流阻力系数的一半还小．实验表明粘土悬液的高粘性和屈服应力抑制了紊动的发展，减小了紊动剪力，导致阻力降低．对于光滑边界湍流，粘土悬液的阻力系数与清水流的相近，不发生减阻现象     

深水钢悬线立管触地区力学特性数值模拟分析

通过将钢悬线立管与海床土体的相互作用模拟成为一根置于一系列非线性弹簧上的梁单元,根据海底土体P-y曲线,对钢悬线立管与海底土体之间的相互作用进行了数值模拟分析,研究了海底形成的沟槽形状、沟槽的最大深度及触地点附近立管的弯矩变化。研究结果表明:钢悬线立管在循环位移作用下,在靠近立管加载端的下部海床上形成沟槽;土体刚度越大,形成的沟槽最大深度越小;循环位移载荷幅值越大,海床上形成沟槽的最大深度也越大;随着位移载荷的逐渐增大,沟槽最大深度点的位置向远离加载端的方向移动;立管触地点附近的弯矩变化幅度从大到小排列依次为线性硬土、线性软土、非线性黏土。     

框架结构悬链线效应研究新进展

本文主要研究框架结构悬链线效应方面的新进展,并着重对钢筋混凝土框架结构与钢框架结构悬链线效应的区别,以及强震作用下框架结构连续倒塌过程中的悬链线效应进行了探讨.本文首先介绍悬链线效应的概念,即处于结构整体中的水平构件在失去抗弯能力后,通过轴力和很大挠度形成的力矩来抵抗外载荷产生的弯矩的现象.在火灾、强震等作用下,框架结构局部梁构件会依次进入弹性状态、弹塑性状态,进而屈服产生塑性铰,成为瞬变机构,此时梁构件的挠度会迅速增大,形成悬链线效应,当塑性铰失效时,则会产生机械铰,当悬索拉力超过极限承载能力时,机械铰断裂.其次通过算例来研究两端铰接约束梁在小变形状态下轴力与挠度形成的力矩与弯曲抵抗力矩的比值随载荷、跨度以及截面高宽比变化的关系.接着对国内外学者的框架结构悬链线效应研究进行系统的分析和总结,重点研究钢筋混凝土梁构件由压拱效应向悬链线效应的转化过程.最后对框架结构悬链线效应研究需要进一步开展的工作进行探讨和展望,通过利用悬链线效应的强度储备来增强结构的潜在强度和抗倒塌能力.      

Planar dynamics of inclined curved flexible riser carrying slug liquid–gas flows

Flexible risers transporting hydrocarbon liquid–gas flows may be subject to internal dynamic fluctuations of multiphase densities, velocities and pressure changes. Previous studies have mostly focused on single-phase flows in oscillating pipes or multiphase flows in static pipes whereas understanding of multiphase flow effects on oscillating pipes with variable curvatures is still lacking. The present study aims to numerically investigate fundamental planar dynamics of a long flexible catenary riser carrying slug liquid–gas flows and to analyse the mechanical effects of slug flow characteristics including the slug unit length, translational velocity and fluctuation frequencies leading to resonances. A two-dimensional continuum model, describing the coupled horizontal and vertical motions of an inclined flexible/extensible curved riser subject to the space–time varying fluid weights, flow centrifugal momenta and Coriolis effects, is presented. Steady slug flows are considered and modelled by accounting for the mass–momentum balances of liquid–gas phases within an idealized slug unit cell comprising the slug liquid (containing small gas bubbles) and elongated gas bubble (interfacing with the liquid film) parts. A nonlinear hydrodynamic film profile is described, depending on the pipe diameter, inclination, liquid–gas phase properties, superficial velocities and empirical correlations. These enable the approximation of phase fractions, local velocities and pressure variations which are employed as the time-varying, distributed parameters leading to the slug flow-induced vibration (SIV) of catenary riser. Several key SIV features are numerically investigated, highlighting the slug flow-induced transient drifts due to the travelling masses, amplified mean displacements due to the combined slug weights and flow momenta, extensibility or tension changes due to a reconfiguration of pipe equilibrium, oscillation amplitudes and resonant frequencies. Single- and multi-modal patterns of riser dynamic profiles are determined, enabling the evaluation of associated bending/axial stresses. Parametric studies reveal the individual effect of the slug unit length and the translational velocity on SIV response regardless of the slug characteristic frequency being a function of these two parameters. This key observation is practically useful for the identification of critical maximum response.     

Modeling and simulation of severe slugging in air–water pipeline–riser systems

A mathematical model, numerical simulations and stability and flow regime maps corresponding to severe slugging in pipeline–riser systems, are presented. In the simulations air and water were used as flowing fluids. The mathematical model considers continuity equations for liquid and gas phases, with a simplified momentum equation for the mixture, neglecting inertia. A drift-flux model, evaluated for the local conditions in the riser, is used as a closure law. The developed model predicts the location of the liquid accumulation front in the pipeline and the liquid level in the riser, so it is possible to determine which type of severe slugging occurs in the system. The numerical procedure is convergent for different nodalizations. A comparison is made with experimental results corresponding to a catenary riser, showing very good results for slugging cycle and stability and flow regime maps.     

An appraisal of liquid–liquid slug flow in different pipe orientations

Gas–liquid slug flow occurs over a wide range of phase flow rates and in a variety of practical applications during gas–liquid two-phase flows. The range of slug flow increases further in narrow pipes (<0.0254 m), undulated pipelines, riser tube, etc. On the other hand, the past literature shows that slug flow is rarely observed for liquid–liquid cases. In the present study, an interest was felt to investigate whether liquid–liquid slug flow occurs in situations known for excessive slugging in gas–liquid cases. For this, experiments have been performed in narrow (0.012 m ID) vertical and horizontal pipes and an undulated pipeline of 0.0254 m internal diameter where the V-shaped undulation comprises of an uphill and a downhill section between two horizontal pipes. The studies have been performed for both peak and valley orientation of the undulation. Kerosene and water have been selected as the test fluids and the optical probe technique has been used to supplement visual observations especially at higher flow rates. The studies have revealed the existence of the slug flow pattern over a wide range of phase flow rates in all the three geometries. Interestingly, it has been noted that the introduction of an undulation induces flow patterns which bear a closer resemblance to gas–liquid flows as compared to liquid–liquid flows through a horizontal pipe of 0.0254 m diameter.     

Effect of gas release and condensation on characteristics of two-phase gas-and vapor-liquid slug flow

A model of gas-liquid and vapor-liquid slug flow in vertical channels is suggested. The effect of gas release from a gas-saturated liquid and vapor condensation in a cooling channel on characteristics of two-phase slug flow is investigated. It is shown that gas release and condensation affect significantly the structure of the two-phase gas-liquid and vapor-liquid slug flow.     

Challenges in slug modeling and control for offshore oil and gas productions: A review study

The upstream offshore multi-phase well-pipeline-riser installations are facing huge challenges related to slugging flow: An unstable flow regime where the flow rates, pressures and temperatures oscillate in the multi-phase pipelines. One typical severe slug is induced by vertical wells or risers causing the pressure to build up and hence originates the oscillating pressure and flow. There exist many negative consequences related to the severe slugging flow and thus lots of investments and effort have been put into reducing or completely eliminating the severe slug. This paper reviews in details the state-of-the-art related to analysis, detection, dynamical modeling and elimination of the slug within the offshore oil & gas Exploration and Production (E&P) processes. Modeling of slugging flow has been used to investigate the slug characteristics and for design of anti-slug control as well, however most models require specific facility and operating data which, unfortunately, often is not available from most offshore installations. Anti-slug control have been investigated for several decades in oil & gas industry, but many of these existing methods suffer the consequent risk of simultaneously reducing the oil & gas production. This paper concludes that slug is a well defined phenomenon, but even though it has been investigated for several decades the current anti-slug control methods still have problems related to robustness. It is predicted that slug-induced challenges will be even more severe as a consequence of the longer vertical risers caused by deep-water E&P in the future.     

The elimination of severe slugging-experiments and modeling

Severe slugging can occur in a pipeline-riser system operating at low liquid and gas rates. The flow of gas into the riser can be blocked by liquid accumulation at the base of the riser. This can cause formation of liquid slugs of a length equal to or longer than the height of the riser. A cyclic process results in which a period of no liquid production into the separator occurs, followed by a period of very high liquid production. This study is an experimental and theoretical investigation of two methods for eliminating this undesirable phenomenon, using choking and gas lift. Choking was found to effectively eliminate or reduce the severity of the slugging. However, the system pressure might increase to some extent. Gas lift can also eliminate severe slugging. While choking reduces the velocities in the riser, gas lift increases the velocities, approaching annular flow. It was found that a relatively large amount of gas was needed before gas injection would completely stabilize the flow through the riser. However, gas injection reduces the slug length and cycle time, causing a more continuous production and a lower system pressure. Theoretical models for the elimination of severe slugging by gas lift and choking have been developed. The models enable the prediction of the flow behavior in the riser. One model is capable of predicting the unstable flow conditions for severe slugging based on a static force balance. The second method is a simplified transient model based on the assumption of a quasi-equilibrium force balance. This model can be used to estimate the characteristics of the flow, such as slug length and cycle time. The models were tested against new severe slugging data acquired in this study. An excellent agreement between the experimental data and the theoretical models was found.     

T型微通道反应器内气液两相流动机制及影响因素

基于液滴或气泡的多相微流控是近年来微流控技术中快速发展的重要分支之一.本文利用高速显微摄影技术和数字图像处理技术对T型微通道反应器内气液两相流动机制及影响因素进行实验研究.实验采用添加表面活性剂的海藻酸钠水溶液作为液相,空气作为气相.研究T型微通道反应器内气液两相流型的转变过程,并根据微通道内气泡的生成频率和生成气泡的长径比对气泡流进行分类.研究发现当前的进料方式下,可以观测到气泡流和分层流2种流型,且依据气泡生成频率和微通道内气泡的长径比可将气泡流划分为分散气泡流、短弹状气泡流和长弹状气泡流3种类型,并基于受力分析确定3种气泡流的形成机制分别为剪切机制、剪切-挤压机制和挤压机制.考察不同液相黏度和表面张力系数对不同类型气泡流范围的影响规律.结果表明:液相黏度相较于表面张力系数而言,对气泡流生成范围影响更大.给出不同类型气泡流流型转变条件的无量纲关系式,实现微通道生成微气泡过程的可控操作.      

Heat transfer during gas hydrate formation in gas-liquid slug flow

A model of heat transfer during gas hydrate formation at a gas-liquid interface in gas-liquid slug flow is suggested. Under the assumption of perfect mixing in liquid plugs, the recurrent relations for temperature in then-th liquid plug and heat and mass fluxes from then-th gas slug are derived. Total mass and heat fluxes in gas-liquid slug flow during gas hydrate formation are determined.     

The characteristics of gas/liquid flow in large risers at high pressures

Although most of the work reported on two-phase flows are limited to small pipe diameters, two-phase flow in large risers are increasingly being encountered in the petroleum and nuclear industries. In the present work, a wire mesh sensor was employed to obtain void fraction and bubble size distribution data and visualizations of steam/water flow in a large vertical pipe (194 mm in diameter) at 46 bar. For comparison purposes, measurements were made at similar phase velocities and physical properties to a dataset for nitrogen/naphtha flow in a similar-sized riser. There exist significant differences between both sets of data. Churn-turbulent flow is observed in the present work instead of slug flow, and this differs from the intermittent and semi-annular flow patterns reported for nitrogen/naphtha data. The mean void fraction of the nitrogen/naphtha data is higher than that of the present steam/water data due to the differences in purity in the liquid phases. Furthermore, core peak distributions are observed for the present work in contrast to the flatter profiles deduced for the nitrogen/naphtha using a power law relationship.