Near-horizontal Pipes Research Articles

Velocity profiles description and shape factors inclusion in a hyperbolic, one-dimensional, transient two-fluid model for stratified and slug flow simulations in pipes

Abstract In a previous work it has been shown that a one-dimensional, hyperbolic, transient five equations two-fluid model is able to numerically describe stratified, wavy, and slug flow in horizontal and near-horizontal pipes. Slug statistical characteristics can be numerically predicted with results in good agreement with experimental data and well-known empirical relations. In this model some approximated and simplified assumptions are adopted to describe shear stresses at wall and at phase interface. In this paper, we focus on the possibility to account for the cross sectional flow by inserting shape factors into the momentum balance equations of the aforementioned model. Velocity profiles are obtained by a pre-integrated model and they are computed at each time step and at each computational cell. Once that the velocity profiles are known, the obtained shape factors are inserted in the numerical resolution. In this way it is possible to recover part of the information lost due to the one-dimensional flow description. Velocity profiles computed in stratified conditions are compared against experimental profiles measured by PIV technique; a method to compute the velocity profile during slug initiation and growth has been developed and the computed velocity distribution in the liquid phase was compared against the one-seventh power law.

Effects of monoethylene glycol (MEG) on three-phase flow characteristics in near-horizontal pipes

An experimental study is conducted using a 0.152-m ID facility to investigate the effects of the presence of monoethylene glycol (MEG) on three-phase stratified wavy flow in horizontal pipelines. The experiments are conducted under low liquid loading condition, which is very commonly observed in wet gas pipelines. The analyzed flow characteristics include wave pattern, liquid holdup, aqueous phase fraction and pressure gradient.The experimental range of this study covers superficial gas velocity, vSg, values from 8 to 23m/s, superficial liquid velocity, vSL, values of 0.01–0.02m/s, and inlet liquid stream aqueous phase fraction, WCMEG, values of 0–100%. Experiments are conducted with 51wt% of MEG in the aqueous phase, and the results are compared to the case with no MEG, presented by Karami et al. (2016). Differential pressure transmitters, a quick closing valve and pigging system, and a high speed camera are used to acquire the measurements. The trends in the resulting data with respect to input parameters are investigated. The performances of common models and predictive tools are compared to liquid holdup, pressure gradient and aqueous phase fraction experimental results.The observed wave patterns include stratified smooth and stratified wavy with 2-D waves, 3-D waves, roll waves, and atomization flow, with transitions varying by changing the liquid phase. The trends of pressure gradient, liquid holdup, and aqueous phase fraction with respect to vSg, vSL, WCMEG, and wt% of MEG are observed and physical justifications are provided. The predictions of OLGAS 7.1, TUFFP unified model v. 2012, Beggs and Brill (1973), Taitel and Dukler (1976), and Xiao et al. (1990) are compared to the acquired experimental data. The liquid holdup experimental data are under-predicted by all the models, especially for higher WCMEG values. However, the results from OLGAS 7.1 and Xiao et al. (1990) model are in better agreement with experimental data. The three-phase aqueous phase fraction trends are not predicted well. The complicated nature of liquid-liquid interactions causes higher prediction uncertainties. In addition, a statistical analysis is conducted to find the best applicable closure relationships in the modeling of two-phase low liquid loading flow.MEG is used continuously in deep water gas production systems as a hydrate inhibitor. However, MEG mixing in multiphase flow and its effects on flow parameters are not well understood. This paper provides with comprehensive data for three-phase stratified flow for a 0.152-m ID pipe, with MEG in the aqueous phase. In addition, the prediction performance of the commonly used predictive tools in the industry is provided.

Measurement of liquid film distribution in near-horizontal pipes with an array of wire probes

A test section consisting of a circumferential array of conductance probes has been developed to measure the thickness distribution around the pipe wall of a liquid layer flowing in near horizontal pipes. When the film thickness is known, the array can be employed to measure the local film flow rate by injecting a high conductivity tracer into the liquid flowing at pipe wall.The test section consists of a short pipe made of a non-conducting material installed in a flow rig designed to operate at an appreciable pressure (40bar). The flow loop is made of metallic pipes connected to the electrical earth. The conductance probes are made of three parallel, rigid wires spaced along the flow direction and are used to measure the height or the electrical conductivity of the liquid layer. The three-electrode geometry is aimed at minimizing current losses toward earth. The simultaneous operation of all the probes of the array, without multiplexing, allows a substantial reduction of current dispersion and a good circumferential resolution of film thickness or conductivity measurements. The probe geometry may generate an appreciable disturbance to the gas–liquid interface. This aspect of the proposed method has been studied with an experimental and numerical investigation relative to free falling liquid layers.

Incomplete fluid–fluid displacement of yield stress fluids in near-horizontal pipes: Experiments and theory

We present results of a primarily experimental study of buoyant miscible displacement flows of a yield stress fluid by a higher density Newtonian fluid along a long pipe, inclined at angles close to horizontal. We focus on the industrially interesting case where the yield stress is significantly larger than a typical viscous stress in the displacing fluid, but where buoyancy forces may be significant. We identify two distinct flow regimes: a central-type displacement regime and a slump-type regime for higher density ratios. In the central-type displacement flows, we find non-uniform static residual layers all around the pipe wall with long-wave variation along the pipe. In the slump-type displacement we generally detect two propagating displacement fronts. A fast front propagates in a thin layer near the bottom of the pipe. A much slower second front follows, displacing a thicker layer of the pipe but sometimes stopping altogether when buoyancy effects are reduced by spreading of the front. In the thin lower layer the flow rate is focused which results in large effective Reynolds numbers, moving into transitional regimes. These flows are frequently unsteady and the displacing fluid can channel through the yield stress fluid in an erratic fashion. We show that the two regimes are delineated by the value of the Archimedes numbers (equivalently, the Reynolds number divided by the densimetric Froude number), a parameter which is independent of the imposed flow rate. We present the phenomenology of the two flow regimes. In simplified configurations, we compare computational and analytical predictions of the flow behaviour (e.g. static layer thickness, axial velocity) with our experimental observations.

Effect of Fluid Properties on Flow Patterns in Two-Phase Gas−Liquid Flow in Horizontal and Downward Pipes†

This paper investigates the effect of gas density and surface tension on flow pattern transitions in horizontal and near-horizontal pipes. Experiments were conducted at atmospheric conditions in a 12.75-m long pipe with a diameter of 0.024 m and downwardly pipe inclinations of 0°, 0.25°, and 1°. The effect of gas density was examined using CO2 and He gases and the effect of surface tension was examined using aqueous solutions of normal butanol. The various transitions were identified visually and by statistical analysis of film height measurements. Gas density strongly affects the transition to 2-D and K−H waves, whereas the transition from stratified to slug flow remains rather unchanged. Both wave transitions can be described satisfactorily by existing models in the literature with some modifications. A reduction in surface tension causes the transitions to 2-D waves to be shifted to much lower gas rates. For downward flows, as previously reported in the literature, even a small inclination can cause an...

Hydrodynamic Modeling of Three-Phase Flow in Production and Gathering Pipelines

The transport of unprocessed gas streams in production and gathering pipelines is becoming more attractive for new developments, particularly those in less friendly environments such as deep offshore locations. Transporting gas, oil, and water together from wells in satellite fields to existing processing facilities reduces the investments required for expanding production. However, engineers often face several problems when designing these systems. These problems include reduced flow capacity, corrosion, emulsion, asphaltene or wax deposition, and hydrate formation. Engineers need a tool to understand how the fluids travel together, to quantify the flow reduction in the pipe, and to determine where, how much, and what type of liquid that would form in a pipe. The present work provides a fundamental understanding of the thermodynamics and hydrodynamic mechanisms of this type of flow. We present a model that couples complex hydrodynamic and thermodynamic models for describing the behavior of fluids traveling in near-horizontal pipes. The model presented herein focuses on gas transmission exhibiting low-liquid loading conditions. The model incorporates a hydrodynamic formulation for three-phase flow in pipes, a thermodynamic model capable of performing two-phase and three-phase flash calculations in an accurate, fast, and reliable manner, and a theoretical approach for determining flow pattern transitions in three-phase (gas-oil-water) flow and closure models that effectively handle different three-phase flow patterns and their transitions. The unified two-fluid model developed herein is demonstrated to be capable of handling three-phase systems exhibiting low-liquid loading. Model predictions were compared against field data with good agreement. The hydrodynamic model allows (1) the determination of flow reduction due to the condensation of liquid(s) in the pipe, (2) the assessment of the potential for forming substances that might affect the integrity of the pipe, and (3) the evaluation of the possible measures for improving the deliverability of the pipeline.

Flow regime independent, high resolution multi-field modelling of near-horizontal gas–liquid flows in pipelines

For fully-developed two-phase flows, maps that correlate experimental and semi-empirical expressions for flow regimes are widely used. For calculations of the various important two-phase flow parameters, this in turn requires correlations for various interfacial and wall interaction effects that are flow regime dependent. For many systems of practical interest, however, the evolution of flow regimes (such as slug flow in oil–gas pipelines) is of interest because the development lengths are long and flow regimes may change in regions where pipeline inclination changes due to the terrain. It is shown here that for slow transients in near-horizontal pipes, the one-dimensional multi-field model, when solved with sufficient resolution, does not require flow regimes to be specified or flow regime dependent closure relationships. The formulation predicts the development of flow regimes and various flow parameters without the need for maps, or the need to change closure relationships. To accomplish this, the model includes four fields, i.e. continuous and dispersed liquid, continuous and dispersed gas, as well as a set of appropriate closure relationships from the literature. For the main application considered here, i.e. slow transients in oil–gas pipelines, order of magnitude analyses indicate that certain inertial terms in the model are very small and can be neglected in comparison to the others. Advantage is taken of this to simplify both the structure of the mathematical problem and the solution procedure, which is sufficiently accurate that mass is conserved for each of the four fields. Furthermore, the calculations require high spatial resolution, so a fast, easily-parallelizable numerical procedure has been applied. The results indicate that the development of certain flow regimes, including transitions from bubbly to stratified flow and vice versa, slug flow including slug frequency and length, and the evolution of these parameters along a pipeline are well predicted by the model when compared to experimental data. As part of the validation it is also shown that the model predicts, without need to change closure relationships, flow regimes in fully-developed near-horizontal two-phase flows in good agreement with existing flow regime maps. This suggests that for slow transients in flows for which one-dimensional effects dominate, predictions can be made without requirements for flow regime maps and closure relationships that depend on them.

Experimental Study of Low Liquid Loading Gas-Liquid Flow in Near-Horizontal Pipes

Gas-liquid two-phase flow exists extensively in the transportation of hydrocarbon fluids. A more precise prediction of liquid holdup in near-horizontal, wet-gas pipelines is needed in order to better predict pressure drop and size downstream processing facilities. The most important parameters are pipe geometry (pipe diameter and orientation), physical properties of the gas and liquid (density, viscosity and surface tension) and flow conditions (velocity, temperature and pressure). Stratified flow and annular flow are the two flow patterns observed most often in near-horizontal pipelines under low liquid loading conditions. Low liquid loading is commonly referred to as cases in which liquid loading is less than 1,100m3/MMm3 (200 bbl/MMscf). Low liquid loading gas-liquid two-phase flow at −1° downward pipe was studied for air-water flow in the present study. The measured parameters included gas flow rate, liquid flow rate, pressure, differential pressure, temperature, liquid holdup, pipe wetted perimeter, liquid film flow rate, droplet entrainment fraction and droplet deposition rate. A new phenomenon was observed with air-water flow at low superficial velocities and with a liquid loading larger than 600m3/MMm3. The liquid holdup increased as gas superficial velocity increased. In order to investigate the effects of the liquid properties on flow characteristics, the experimental results for air-water flow are compared with the results for air-oil flow provided by Meng. (1999, “Low Liquid Loading Gas-Liquid Two-Phase Flow In Near-Horizontal Pipes,” Ph.D. Dissertation, U. of Tulsa.)

Stability of stratified gas–liquid flow in horizontal annular channels

The stability of gas–liquid stratified flow regime in horizontal annular channels is investigated. Based on the available air–water experimental data, a simple criterion for the prediction of conditions that lead to flow regime transition out of the stratified wavy flow pattern is proposed. The proposed method is an extension of the instability criterion of Taitel and Dukler [AIChE J. 22 (1976) 47] for gas–liquid flow in near-horizontal pipes.

Solids Transport in Multiphase Flows—Application to High-Viscosity Systems

Solids transport in multiphase systems falls under the umbrella of “flow assurance.” Unlike issues such as waxes and hydrates, solids transport has received relatively little attention to date. This is especially true for solids transport in high-viscosity fluids such as Venezuelan crude, where viscosities around the 300–400-cP mark are commonly encountered. This paper describes some experiments performed on the BP Amoco 6-in. multiphase flow test facility located at Sunbury. These looked at the transport of field representative sand through a pipeline dip. Several fluids were selected for these experiments to examine the influence of liquid viscosity on the results. These were water, oil, and two different carboxymethylcellulose solutions (150 and 300 cP). These experiments showed that, in slug flow, water and low-viscosity oil were able to transport the sand uphill, whereas neither high-viscosity solution was able to transport the solids. This feature was examined in comparison to the model for solids transport in near-horizontal pipes discussed in this paper. Three-phase flow experiments (water-oil-air) were also performed to investigate the effect of oil or water prewetting of the solids on solids transport. If prewetted by water, the sand could not be moved by oil slugs. Once water was added to the system, the sand became increasingly mobile.

The transport of particles at low loading in near-horizontal pipes by intermittent flow

The transport of sand particles at low concentration in horizontal and near horizontal pipes in air/water slug flow has been studied. Low concentration means less than 1 in 1000 by volume, a level of concentration of interest in the transport of sand by oil and gas in subsea flowlines (Stevenson & Thorpe, Proceedings of the Ninth International Conference on Multiphase Flow, Cannes, France, 1999) but much lower than the range of interest in all previous work on hydraulic conveying which may be thought to be a related field of study. The velocity of sands of sieving diameter of 1.1 and 0.51 mm have been measured for various gas and liquid flowrates and in liquids of viscosity in the range 1.0– 7.1 mPa s in two 12 m long pipes of internal diameter 40 and 70 mm . Video recordings have been analysed to determine the mechanisms involved in transport of particles in slug flow and are used to explain the observed decrease in particle propagation velocity with increased liquid viscosity. There being no totally satisfactory mechanistic model for intermittent flow in the literature, a semi-theoretical model for sand transport is not currently facilitated. Therefore the approach taken is that of dimensional analysis. Particle velocity, V P , is successfully correlated using dimensionless groups which include liquid viscosity, particle size and liquid and gas flow rates. The preferred correlation for sand in liquids of viscosity less than about 5 mPa s is V P j f =0.95 1+ j g j f − 1.38 j g j f +0.88 Fr f Re f Fr f d D 1.5 −0.180. All the parameters, except gravitational acceleration, have been varied in the experimental programme. It is found that particle velocity, V P is independent of pipe inclination in slug flow for pipe inclinations of up to three degrees. The above correlation is used to produce an equation that predicts the point of incipient-sand deposition. This equation can be used to estimate the maximum turndown of two-phase offshore oil production from satellite wells before sand will begin to deposit at the bottom of the pipe.

Two-phase flow of brine in long pipelines: analysis of field experiments

The design of two-phase flow lines for geothermal applications requires reliable information about main flow parameters. In the present work a one-dimensional, steady-state model for the computation of hold-up and pressure losses in horizontal and near-horizontal pipes is presented. The model is based on a mechanistic analysis of Stratified, Intermittent and Bubbly flow. The model has been implemented in the two-phase flow simulator, HORF, which has been developed in order to predict pressure losses relative to pipeline flow of geothermal brines. It is shown that the predictions are in very good agreement with field data obtained at Latera geothermal field, in Italy, relative to a flow line 2400 m long, and 18“ nominal diameter.

An investigation of void fraction in liquid slugs for horizontal and inclined gas—liquid pipe flow

The void fraction in liquid slugs has been determined for air—water fiow in horizontal and near-horizontal pipes by a newly-developed conductance probe technique. A semi-empirical correlation has been developed and compared with the present measurements and available data. This correlation predicts reasonably well the observed effects of diameter, inclination and physical properties.