Slug Frequency Research Articles

Assessment of two‐phase slug frequency correlations in horizontal pipes under different operational conditions

AbstractAccurate prediction of the slug frequency in horizontal pipe flow is essential for appropriate design and operation in various industrial processes. This study provides a comprehensive review of existing empirical correlations for slug frequency in horizontal pipes, highlighting their limitations and applicability. A total of 36 correlations were examined, and 1083 data points were collected from experiments using pipes with inner diameters ranging from 3.7 to 150 mm. The correlations were categorized based on pipe diameter and gas–liquid working fluids. The correlations based on the Froude number had the best performance for superficial liquid velocities within the range of vSL = 0.502–1.505 m/s, with a maximum mean relative difference (MRD) of ±30% and a mean absolute relative difference (MARD) of 40% for superficial gas velocities vSG greater than 1 m/s. The Strouhal number produced the most effective correlations for most of the datasets examined in slug frequency testing. In contrast, the air–oil slug frequency correlations were unable to accurately predict the air‐water experimental data, except for that of Al‐Safran (2016), which was limited to small‐diameter pipes. 909 data points were used to evaluate slug frequency for high viscous fluids; the results showed that the correlation of Baba et al. (2017) provided the best prediction performance, with a maximum MRD of 30%.

A Review of the Measurement of the Multiphase Slug Frequency

The slug frequency (SF), which refers to the number of liquid slugs passing through a pipe during a specific time, is an important parameter for characterizing the multiphase intermittent flows and monitoring some process involving this kind of flow. The simplicity of the definition of SF contrasts with the difficulty of correctly measuring it. This manuscript aims to review and discuss the various techniques and methods developed to determine the slug frequency experimentally. This review significantly reveals the absence of a universal measurement method applicable to a wide range of operating conditions. Thus, the recourse to recording videos with high-speed cameras, which can be used only at a laboratory scale, remains often necessary. From the summarized state-of-the-art, it appears that correctly defining the threshold values for detecting the liquid slugs/elongated bubbles interface from physical parameters time series, increasing the applicability of instrumentations at industrial scales, and properly estimating the uncertainties are the challenges that have to be faced to advance in the measurement of SF.

Monitoring Slug Flow Using Distributed Acoustic Sensing Technology with Different Sensing Cable Configurations

Summary Monitoring slug flow in real time over long distances is essential for facility design, operation, and flow assurance management. In this study, we investigated the use of distributed acoustic sensing (DAS) technology for slug flow monitoring, exploring various cable designs and deployment methods. The experimentation encompassed the use of five distinctive fiber-optic cable deployment methods: three internal cables of varying designs (thin, flat, and thick), supplemented by two external cables—one placed straight atop the pipeline while the other was helically wrapped around it. All cables were connected consecutively and successfully captured the dynamic variation of the slugging phenomenon along the pipe. We introduce different data processing algorithms for slug flow characterization, including standard deviation (SD) downsampling, slug frequency determination, and semblance for velocity extraction. The experimental results indicate a higher slug frequency but smaller slug sizes near the inlet of upward inclined pipe, where most slug structures originate. The structure velocity shows a positive correlation with DAS amplitude or the maximum strain rate, which could be related to the slug size. The flat cable exhibited a heightened amplitude response to slugs, while the internal thin and the external straight cables provided the most distinct delineation of slug patterns. The internal thick cable provided the least sensitivity among all due to its design. Although the linear deployment of fiber-optic cables is more practical compared to helically wrapped ones, their resolution limits detailed analysis of more intricate slug characteristics for short pipelines, such as precise velocity measurements of slugs. Data from the helically wrapped cable can address these limitations, providing more comprehensive insights.

Practical aspects of multiphase slug frequency: An overview

AbstractSlug frequency, the number of liquid slugs passing through a point per unit of time, is an intrinsic parameter that is used to characterize multiphase slug flows in pipes. In this review we discuss practical aspects of slug frequency for industrial purposes from an examination of published literature and available experimental data. The review shows that slug frequency appears to play a key role in the modelling of intermittent flow using 1‐D mechanistic slug models and 3‐D computational fluid dynamics tools. In addition, various global parameters and phenomena used to design, optimize, and control industrial pipelines are directly impacted by slug frequency. This manuscript highlights the importance of slug frequency not only for petroleum engineering but also for chemical, nuclear, and mechanical engineering.

Characterizing the development of gravity-driven slug flows using high-speed imaging and PIV-PLIF techniques

Although developing gravity-driven slug flow frequently occurs in oil and gas, and energy systems, its development dynamics remain underexplored; this gap, in turn, has left the underlying relationships between flow evolution and transport phenomena in these applications inadequately characterized as well. The present study experimentally investigates the spatiotemporal-spectral development of gravity-driven air–water and CO2-water slug flows in a vertical 25.4 mm ID pipe. Enhanced flow visualization techniques, utilizing high-speed imaging and particle image velocimetry-planar laser induced fluorescence (PIV-PLIF), were employed to determine the behaviors of gas and liquid phases and interactions at four positions along the pipe axis. A machine vision-based algorithm was employed to extract slug unit cell characteristics and instantaneous void fraction signals, allowing for a comprehensive statistical analysis of gas phase behavior across the flow domain. A novel algorithm was also developed to preprocess raw PIV-PLIF images, facilitating phase discrimination and noise reduction before PIV cross-correlation analyses are conducted. The results showed a logarithmic growth in the lengths of Taylor bubbles, liquid slugs, and slug unit cells along the pipe, with liquid slugs constituting nearly 60 % of slug units downstream. Taylor bubble length distributions correlated well with log-normal fits, while liquid slug and unit cell lengths transitioned from log-normal patterns upstream to near-normal distributions downstream with broader and less peaked shapes. Taylor bubble velocities and appearance frequencies of the flow structures declined exponentially along the pipe, with Taylor bubble velocities showing narrower and more peaked near-normal distributions downstream. Instantaneous void fraction signals exhibited fewer, wider peaks and troughs with reduced small-amplitude oscillations downstream. The analysis of the signals indicated a complete bubbly-to-slug transition at Z/D=30. Gas-liquid phase interactions, classified as almost-zero, weak, and strong, impacted liquid phase velocity profiles and the behavior of Taylor bubbles, with minimum stable liquid slug lengths of 8–9 D and a wake length of 1.8 D observed. Empirical correlations were developed to represent the spatiotemporal-spectral aspects of flow development, with spectral parameters, particularly liquid slug frequency, identified as the most reliable indicators of the fully developed region, predicting entrance lengths of 114.0 D and 113.4 D for air–water and CO2-water, respectively. Gas density was found to strongly influence flow characteristics and transition, accelerating the approach to the fully developed region.

Investigation on intermittent flow characteristics in horizontal pipe by visualization measurement method

Accurate measurement and prediction of the intermittent flow characteristics in horizontal pipes is important for constructing multiphase flow models and ensuring pipe flow safety. In this paper, a quantitative image post-processing technique for intermittent flow characteristics based on gray histogram image similarity is proposed, which can realize the measurement of slug frequency. In addition, the technique also has the ability to classify a large number of images, and can quickly find the elongated bubble head, liquid film area and liquid slug area of intermittent flow. On the basis of this technique, combined with image post-processing methods such as gas–liquid interface feature analysis, a set of intermittent flow image processing technique with perfect route is formed. Based on this post-processing technique, the similarity image oscillation trajectories of plug flow and slug flow are obtained. There are differences in the similarity image oscillation trajectories of the two intermittent sub-flow patterns, and the similarity image of the plug flow has an obvious platform period and trailing rising line, which can be used as a basis for the classification of the two intermittent sub-flow patterns. A correlation for predicting the slug frequency of intermittent sub-flow patterns is developed. The accuracy of this slug frequency prediction correlation can be improved by about 10 % compared to not dividing the sub-flow patterns. When the mixture Froude number Frm is less than 5.0, the radial position of the elongated bubble head decreases linearly as the Frm increases. When the Frm is greater than 5.0, the elongated bubble head oscillates near the middle of the pipe. Prediction correlations for the radial position of the elongated bubble head and the slug velocity are established separately, and the maximum error is ± 10 %. The modified mixed Froude number is proposed, and based on this, a new prediction model for the transition from plug flow to slug flow is established.

An investigation on hydrodynamic behavior of horizontal gas–liquid intermittent flow by S-PLIF and parallel-wire conductance sensors

Horizontal gas–liquid two-phase flows widely exist in a variety of industrial fields such as the environment, chemical industry and energy. Intermittent flow, characterized by the quasi-periodic alternation motion of liquid slugs and large-scale Taylor bubbles, is the most prevalent flow pattern in horizontal gas–liquid two-phase flows. Comprehensive research on the hydrodynamic behavior of gas–liquid intermittent flow is essential for revealing the mass and heat transfer characteristics between phases, thereby establishing physical models of flow parameters. In this study, a structured planar laser-induced fluorescence (S-PLIF) system and paired parallel-wire conductance sensors (PWCSs) are designed to detect the gas–liquid interface structures and the hydrodynamic parameters. The S-PLIF system consists of a laser, a visualization box, a Ronchi ruling-plate and a pair of high-speed cameras. An optical linear prism forms a laser sheet in the transverse direction and the middle of the test section, and the cameras are positioned at 70° and 90° respectively to the plane of the laser sheet, referred to as S-PLIF70 and S-PLIF90. Under the laser excitation, bright and dark stripes generated by the Ronchi ruling-plate are deflected at the gas–liquid interface. The influence of total reflection can be effectively avoided by identifying the deflection position of the stripes, allowing for accurate detection of the gas–liquid interfacial structures. The effects of the interface fluctuations and detached bubbles on the S-PLIF70&90 results are investigated. Fluctuation and evolution characteristics of the gas–liquid interface are analyzed by probability density function (PDF) and power spectral density (PSD). Meanwhile, hydrodynamic parameters, i.e., Taylor bubble nose and tail velocity, Taylor bubble and slug length, and slug frequency, are detected by the PWCSs, and compared with previous correlations. The evolution characteristics of the interfacial structures and hydrodynamic parameters are uncovered as the flow condition changes.

Characterization of Gas-Liquid Two-Phase Slug Flow Using Distributed Acoustic Sensing in Horizontal Pipes.

This article discusses the use of distributed acoustic sensing (DAS) for monitoring gas-liquid two-phase slug flow in horizontal pipes, using standard telecommunication fiber optics connected to a DAS integrator for data acquisition. The experiments were performed in a 14 m long, 5 cm diameter transparent PVC pipe with a fiber cable helically wrapped around the pipe. Using mineral oil and compressed air, the system captured various flow rates and gas-oil ratios. New algorithms were developed to characterize slug flow using DAS data, including slug frequency, translational velocity, and the lengths of slug body, slug unit, and the liquid film region that had never been discussed previously. This study employed a high-speed camera next to the fiber cable sensing section for validation purposes and achieved a good correlation among the measurements under all conditions tested. Compared to traditional multiphase flow sensors, this technology is non-intrusive and offers continuous, real-time measurement across long distances and in harsh environments, such as subsurface or downhole conditions. It is cost-effective, particularly where multiple measurement points are required. Characterizing slug flow in real time is crucial to many industries that suffer slug-flow-related issues. This research demonstrated the DAS's potential to characterize slug flow quantitively. It will offer the industry a more optimal solution for facility design and operation and ensure safer operational practices.

Investigations of the Formation Mechanism and Pressure Pulsation Characteristics of Pipeline Gas-Liquid Slug Flows

The pipeline system is widely used in marine engineering, and the formation mechanism and flow patterns of two-phase slug flows are of great significance for the optimal design of and vibration prevention in a complex pipeline system. Aiming at the above problems, this paper proposes a modeling and solving method for gas-liquid slug flows. First, a VOF-PLIC-based coupling gas-liquid slug flow transport model is conducted. Second, to reduce the fuzzy boundary between the gas-liquid coupling interfaces, an artificial compression term is added to the transport equations, and the formation and evolution mechanism of severe slugging flow in piping systems is investigated. The pressure pulsation and gas content characteristics of the gas-liquid coupling process are explored. Research results found that the slugging phenomenon occurs at the gas-liquid interface, where liquid slugging frequency reaches its peak. The pipeline system has prominent periodic characteristics of the slugging phenomenon, and the period decreases when the gas-phase converted speed rises; pressure fluctuation amplitude increases, and the gas-phase velocity change is the inducing factor for the drastic change of pressure fluctuation. The research results can offer theoretical references for optimal designs of and vibration prevention in marine pipeline systems.

Effects of riser height and pipeline length on the transition boundary and frequency of severe slugging in offshore pipelines

Investigating the transition boundary and frequency characteristics of severe slugging under different riser heights and pipeline lengths provides important support for the expansion of offshore oil and gas to deeper waters. The visualization research on air-water two-phase flow is carried out in a system with a horizontal pipeline length of 1687 m and riser heights of 16 m, 22 m, and 29 m, and flow pattern maps are plotted. Based on existing experimental data, the database of the flow pattern and slugging frequency in pipeline-riser systems with riser heights of 3–29 m, pipeline lengths of 10–1687 m, and pipe diameters of 25.4–50.8 mm is established. The long pipeline makes the gas pressure growth rate slower than that of the liquid pressure in the riser, which causes severe slugging occurring at higher superficial gas velocity. On the contrary, increasing the riser height causes the boundary of severe slugging to shift to a lower superficial gas velocity. The severe slugging length is greater in the higher riser and longer pipeline system, causing a smaller severe slugging frequency. A correlation for predicting severe slugging frequency at various riser heights, pipe lengths, and pipe diameters is established by employing gas and liquid Froude numbers.

Two-phase Upward Flow-Induced Vibrations in U-bend under Different Orientations

U-bends are frequently employed in industries to change the direction of fluid flow. Flow Induced Vibrations (FIV) are very prominent due to internal two-phase flow and vary with U-bend orientation. Potential possibilities of spatial orientations of a U-bend make it impractical to predict these vibrations using an experimental approach. This study investigates the two-phase flow-induced vibrations for 0°, 30°, 60° and 90° upward flow in a U-bend using CFD. In-plane and out-of-plane directional vibrational data was gathered for these four orientations of U-bend. For constant superficial velocities with void fraction of 0.5, impact of orientation on slug frequencies was observed with Fast Fourier Transformation. In-plane (x-axis) forces applied at the outlet bend significantly increase 39% compared to the corresponding forces at the inlet bend. Forces at the outlet bend along the y-axis (out-of-plane) were reduced by 26%, but the forces along the z-axis show a more moderate decrease of 7%. This study provides significant data essential to forming a correlation of two-phase flow-induced vibrations with multiple upward flow U-bend orientations.

A Self‐Powered Multiphase Flow Detection Through Triboelectric Nanogenerator‐Based Displacement Current

AbstractAccurate measurement of complicated multiphase flow is crucial to the safety and efficiency of petroleum and chemical industrial facilities. However, the existing multiphase flow detection techniques are not applicable to pipelines in remote regions including deserts or deep seas, due to the high cost of providing a stable power supply. Herein, a self‐powered multiphase flow sensor, composed of a liquid‐driven triboelectric nanogenerator (TENG) ‐based signal generator, a ring‐type transmitter, and a string‐type receiver, is proposed. Theoretical modeling of displacement current between transmitter and receiver implies that the received current signal can accurately reflect the wetting state of the receiver, validated by a combined experimental (accuracy above 97%) and simulation study. Coupling with a quantitative analysis algorithm, a multiphase flow detection system with numerous receiver measurement points is developed to precisely monitor various flow parameters, including slug frequency (one point), slug length (two points), and flow pattern (four points), which is verified by spontaneous high‐speed camera recordings of water–air flow. The present work provides a paradigm‐shift way to develop a self‐powered, inexpensive, and accurate technique to detect multiphase flow at remote industrial facilities.

Numerical simulation of two-phase slug flows in horizontal pipelines: A 3-D smoothed particle hydrodynamics application

A fundamental difficulty of studying gas–liquid pipe flows is the prediction of the occurrence and characteristics of the slug flow regime, which plays a crucial role in the safety design of oil pipelines. Current empirical methods and one-dimensional computational models only achieve limited success. While 3-D numerical simulations are highly recommended, they have been very seldom used. We perform 3-D Lagrangian numerical simulations of gas–liquid pipe flows by adapting an existing multi-phase smoothed particle hydrodynamics (SPH) method based on a Riemann solver to achieve an efficient solver. To realize the high inlet velocities of gas and liquid an in- and outlet boundary condition is presented. The results are validated against existing experimental data, numerical simulations and analytical solutions. Multiple gas–liquid pipe flow patterns are predicted, namely, smooth stratified, stratified wavy, bubble flow, slug flow and bubble flow. Several principle characteristics of slug flows, e.g., pressure gradient, slug development and slug frequency are analyzed in a 3-D fully Lagrangian framework.

Effects of flow characteristics on the heat transfer mechanism in Taylor flow

The capability of Taylor slugs to enhance heat transfer rate compared to that obtained from single-phase flow is well-documented. This study numerically investigated the hydrodynamics characteristics and heat transfer mechanism in liquid-liquid Taylor flow. A novel analysis method was introduced by comparing fluid flow and heat transfer parameters distributed at the axial and radial planes of the channel. In order to investigate transport phenomena, the unit cell length and frequency of slug generation over a wide range of phase superficial velocity ratio were examined. The simulations showed that the viscosity difference between the phases is a critical parameter of the slug frequency; a higher amount increases the frequency much greater. Conversely, a lower viscosity difference between the phases allows water slugs to expand axially more. The higher temperature gradient and a stronger recirculation in the shorter liquid plug region enhance the heat transfer rate leading to the highest cooling performance over the channel wall within a unit cell. The results also showed the significant importance of establishing shorter slugs, which improves the cooling performance in the slugs and enhances the heat transfer rate in the liquid slug region due to the greater radial flow mixing. The developed numerical model was verified by the results found in the literature showing good agreement.

Experimental investigation on long hydrodynamic slugs in offshore pipeline

In the process of liquid-dominated offshore oil and gas transportation, long hydrodynamic slugs are likely to occur in subsea pipelines at high liquid holdup. A large number of long hydrodynamic slug data at high liquid velocity and low gas velocity were obtained by using visual images in a 46 mm ID, 1657 m long pipeline. The flow pattern map close to the industrial parameters is plotted, and slug flow can take place at a lower superficial gas velocity (VSG = 0.045 m/s) in the long pipeline. The maximum slug length is 2.5 times the average slug length, and the longest liquid slug is 969 D near the transition boundary of slug flow. When VSG < 1.00 m/s, slug length increases rapidly and slug frequency decreases significantly with the decrease of superficial gas velocity at the same superficial liquid velocity. The prediction correlations of slug velocity, length, and frequency are developed for long-distance offshore pipelines at high liquid velocity and low gas velocity, and the maximum error is less than 25%.

Investigating the Impact of Undulation Amplitude of Unconventional Oil Well Laterals on Transient Multiphase Flow Behavior: Experimental and Numerical Study

The growing popularity of unconventional wells has led to increased interest in assessing and predicting their production performance. These wells, with their extended-reach structures, are able to generate and access larger reservoir volumes. Therefore, understanding the impact of a well’s lateral trajectory on its transient production performance is crucial. This study investigates the effect of lateral-trajectory undulation amplitude on flow behavior based on the experimental results obtained at the University of North Dakota using an undulated two-phase (UTP) flow loop. The experiments involved injecting an air-and-water mixture through a section with variable undulation amplitude followed by a vertical section. The results revealed that the increasing undulation amplitude resulted in lower translational velocity, frequency, and length, with consistent slug acceleration along the system profile. Additionally, the frequency of slugs decreased as they traveled through the vertical section. The measured data indicated that higher undulation amplitudes led to increased horizontal pressure losses and variability, suggesting larger instabilities. The numerical simulations predicted lower translational velocity and frequency, longer slug length, and similar vertical pressure losses when compared to the experimental results.

Influence of pipeline diameters and fluid properties on slug frequency in horizontal pipelines

The accurate prediction of slug frequency is critical for the construction of multiphase flow models and the flow assurance of oil and gas pipelines. In this paper, the slug frequency data are obtained by visualization method in short (65 mm ID, 215 D) and long (46 mm ID, 35957 D) pipelines. The slug frequency of horizontal pipeline is inversely proportional to pipeline diameter (26–300 mm) and proportional to the liquid viscosity (1.01–7500 mPa·s). The quasi-developed distance for the air–water slug flow at VSG ≤ 1.00 m/s is about 1200 D, and the frequency of the quasi-developed slug flow is less affected by the pipeline length. Based on the gas Strouhal number and the liquid viscosity ratio, the quantitative relation between the slug frequency data at different viscosities is constructed, and a new correlation is developed to predict both the slug frequency of water and viscous liquids.

Transition from bubbling to jetting in submerged gas-liquid jets through a minichannel

This paper provides a first insight into the transition from bubbling to jetting of submerged jets generated by the ejection of gas-liquid mixing flows through a minichannel into water. The ejections of rivulet, annular, churn, and Taylor-annular flows are found to generate jet behaviors of aperiodic bubbling, impact bubbling, chaotic bubbling, jetting-like jets, unstable jetting, and stable jetting. The correspondence relationship between the in-channel flows and submerged jets is revealed by establishing their flow regime maps, as well as the prediction models of transition boundaries. Compared to single-phase gas jets, the participation of liquid phase in mixing flows significantly promotes the transition from bubbling to jetting. Specifically, the increase of liquid phase velocity decreases the critical gas phase velocity for the transition from bubbling to jetting-like jets, as well as the transition from unstable jetting to stable jetting, characterized by the increase of liquid slug frequency and jet penetration length, as well as the decrease of liquid slug length, jet pitch-off frequency, and jet cone angles. As a result, via the in-channel mixing with a liquid phase of a Weber number of 110 prior to orifice ejection, the critical Mach number for the onset of stable jetting is significantly reduced from 3.76 to 1.25.

Experimental investigation of induced vibrations in horizontal Gas-Liquid flow

One of the governing contributors to offshore structures' (e.g., pipelines and deep-water risers) accumulative long-term fatigue damage is the internal flow-induced vibrations caused by slug flow. Due to the complexity of the numerical modeling of the internal flow and fluid–structure interaction, experimental campaigns are often performed to predict the asset response and provide a benchmark for numerical model validation. In this study, an experimental campaign was conducted which included a unique computer vision data acquisition system and a large-scale facility designed to mimic industry field conditions. This study aims to relate flow parameters to structural response and explain the mechanisms that govern the relationship between both domains.The presented results reveal that the gravitational forces of the fluid dominate the transverse deflections of the pipe and, therefore, the slug liquid holdup, film liquid holdup, and slug length showed strong correlations to pipe deflections. However, centrifugal forces were observed to have significant contributions to pipe deflection for high translational velocity slugs. Additionally, it was found that for most of the test conditions, the primary pipe excitation frequency correlates to the slug frequency. It was discovered that no-slip liquid holdup has a positive correlation to both the magnitude and range of pipe deflection and that increases in superficial gas and liquid velocities led to decreases and increases in pipe deflection, respectively. Furthermore, it was observed that the measured frequency responses of the structure tended to increase with increasing gas velocity until the flow pattern in the pipe began to transition to annular flow.

Experimental and Numerical Study on the Slug Characteristics and Flow-Induced Vibration of a Subsea Rigid M-Shaped Jumper

The subsea jumper has become an essential part of subsea production systems as a gas–liquid mixing pipeline connecting the pipeline end manifold (PLEM) to the Christmas tree. During oil and gas transportation, as a common flow pattern, the alternating flow characteristics of the slug flow easily cause pipeline vibration, resulting in pipeline instability or fatigue damage. The present study investigates experimentally and numerically the slug flow characteristics in the subsea M-shaped jumper and its induced vibrations of the jumper. The flow pattern evolution and slug characteristics of the inner slug flow under different gas–liquid velocities are obtained: the slug frequency and slug velocity, as well as the pressure fluctuation and vibration characteristics caused by the slug flow. The results show that the pressure fluctuations in the front and rear parts of the M-type jumper are obviously different. With the increase in the air–water mixing, the two characteristics, the slug frequency, and the slug velocity also increase. The gas velocity has a greater influence on the slug frequency than the liquid velocity. The slug length decreases as the slug frequency increases. Furthermore, numerical simulations under various experimental conditions are carried out. The results show that the simulation results of the pressure data, the slug characteristics, and the induced vibration amplitude are in good agreement with the experimental data.