Summary During slug flow, knowledge of the slug frequency is essential for the gas-liquid receiving facility design as well as for predicting various slug flow characteristics such as slug length and pressure drop. Various methods proposed in the literature for predicting the slugging frequency in horizontal and inclined pipes were examined. These included both empirical correlations as well as mechanistic models. Slug flow frequency data taken with an air-water system in a laboratory flow loop together with data from the published literature were compared to the predictions of the various methods. A total of 399 data points were collected covering pipe diameters from 1 to 8 in. and inclinations from 0 to 11° above the horizontal. A total of eight published methods were compared to the data but none was found satisfactory. For this reason, the mechanistic slug frequency model of Taitel and Dukler1 was investigated in detail as an alternative unbiased prediction method. This model required the solution of the unsteady-state equations for mass and momentum by a finite difference technique. This numerical model gave satisfactory results at the expense of considerable computer CPU time. For faster slug frequency calculations a new correlation was developed utilizing all 399 data points. This resulted in 0% average error (bias) and 60% average absolute error. This correlation represents a significant improvement in slug frequency prediction accuracy over the other methods studied. Introduction One of the most prevalent flow patterns encountered during production and transportation of oil and gas is slug flow. Knowledge of the slugging frequency is central to the prediction of the slug flow characteristics. Slug frequency is required as an input quantity in mechanistic models2 which predict the fluid mechanical features of slug flow. In addition, proper sizing of separation facilities depends on reliable prediction of slug frequency. Slug flow may cause undesirable process upsets if the separation and processing facilities are not designed to handle the gas and liquid flow rate variations that occur during slug flow. Because of the multitude of factors that affect slug frequency, researchers in the past have been cautious to suggest that use of their results be limited to the range of experimental conditions for which data were taken. Most available empirical correlations have been derived from pipes of diameter smaller than 2 in. (see Refs. 3 through 7). According to these correlations, slug frequency is given as a function of a few flow parameters such as the Froude number and flowing liquid fraction. However, the list of variables that may have an influence on the slug flow characteristics should also include pipe diameter, length and topography, gas and liquid flow rates, densities, and viscosities. Slug frequency has been considered as an entrance phenomenon, particularly so for horizontal and slightly inclined pipes.1 Generally short (high-frequency) slugs are formed at the entrance of the pipe. However, rapid merging of the short slugs occurs further downstream that results in longer (low-frequency) slugs. Finally, the slug length becomes long enough for the slug to be stable. This work like most other correlations and published data on slug frequency and slug length pertains to the downstream developed slugs and not to the entrance slugs. Hill and Wood8 identified seven variables that they thought affect slug frequency the most. Thus they indicated the need for five dimensionless groups. Although a large database was available to these authors, the wide range of parameters made their analysis difficult. No conclusive relationships were found among the five dimensionless numbers, although some trends became evident. Following the lack of significant progress with the dimensionless analysis route, they identified a new approach for correlating slug frequency based on observations of slug formation in test rigs. These observations indicated that the two parameters that had the predominant influence on slug formation were the depth of the stratified liquid film in the region of slug formation and the gas-liquid slip velocity. Then they proposed a correlation expressing slug frequency as a function of these two parameters. Tronconi9 proposed a correlation between slug frequency and the properties of the waves that are responsible for slug generation. He observed that the characteristic frequencies of slugs during intermittent horizontal flow were related to the periods of unstable waves responsible for generation of slugs in the inlet region of the pipe. He investigated the onset of slugging by estimating the wave properties in accordance with the theory of finite amplitude waves originally developed by Kordyban and Ranov10 and by Mishima and Ishii.11 The resultant correlation was successfully tested with a limited amount of data from small diameter horizontal pipes. No comparisons with inclined slug flow data were presented. Taitel and Dukler12 developed a mechanistic model for predicting slug frequency. Although their approach is consistent with many of the observed features of slug formation, solution of the two partial differential equations describing their model poses numerical challenges. In this article, the various correlations and models that have been proposed for slug frequency prediction will be described in detail. Comparison of these prediction methods with slug frequency data is subsequently presented. Conclusions will also be summarized.
Upward cocurrent gas-liquid annular flow was investigated in a 50.8 mm ID cylindrical vertical pipe. The film flow was studied by measuring instantaneous local film thickness, wall shear stress, and pressure gradient. Analysis of these data revealed that at low gas flow rates the film motion is controlled by a switching mechanism, as speculated by Moalem-Maron and Dukler (1984). In the region of high gas flow rates the switching process is suppressed and traveling roll waves characterize the film motion. G. Zabaras and A. E. Dukler University of HoustonHouston. TX 77004 D. Moalem-MaronFaculty of Engineering Tel-Aviv University Tel-Aviv, Israel SCOPEVertical upward cocurrent gas-liquid annular flow exists in a wide variety of industrial processes such as production and pipeline systems for offshore delivery of oil and gas; emergency core cooling facilities for protection of nuclear reactors in the event of a loss-ofcoolant accident; process equipment for gas-liquid contacting and reaction; and high-capacity geothermal wells with associated surface equipment.A variety of empirical or semiempirical correlations have been proposed for predicting such characteristics as pressure drop (Orkiszewski, 1967;Wallis, 1969) and equilibrium rates of entrainment (Whalley and Hewitt, 1978). However, the predictions are not reliable at conditions different from those of the experiments used to construct the correlations. The pressure-gradient in annular flow is much higher than for turbulent gas flow in a smooth pipe. A number of relationships have been proposed for interfacial shear, but these correlations do not give reliable predictions outside the range for which they were originally derived. Thus the need exists for an understanding of the basic physical processes taking place in order to be able to construct sound, mechanistically based models. In a recent theoretical study, Moalem-Maron and Dukler (1984) suggested that the variation of the thickness of the liquid film resulted from a process of switching between unstable steady state solutions to the equation of motion rather than to the motion of roll and capillary waves along the surface. This paper reports on new measurements and an interpretation to test this idea.
The structure of the wavy interface on a falling liquid film is studied for conditions of countercurrent gas flow in order to investigate mechanisms for flooding. Measurements taken just below the liquid feed and at 1.7 m down the tube show that under all conditions, including flooding, the waves propagate only downward and are never of such amplitude as to bridge the tube. These observations are in contrast to speculations in the literature that upward flow of waves or bridging of liquid due to waves cause flooding. In the mechanism suggested, flooding is due to flow reversal in the film just at the liquid entry.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractDuring slug flow, knowledge of the slug frequency is essential for the gas -liquid receiving facility design as well as for predicting various slug flow characteristics such as slug length and pressure drop.Various methods proposed in the literature for predicting the slugging frequency in horizontal and inclined pipes were examined. These included both empirical correlations as well as mechanistic models. Slug flow frequency data taken with an air-water system in a laboratory flow loop together with data from the published literature were compared to the predictions of the various methods. A total of 399 data points were collected covering pipe diameters from 1 to 8 inches and inclinations from 0 to 11 degrees above the horizontal. A total of eight published methods were compared to the data but none was found satisfactory. For this reason, the mechanistic slug frequency model of Taitel and Dukler (1976) was investigated in detail as an alternative unbiased prediction method. This model required the solution of the unsteady-state equations for mass and momentum by a finite difference technique. This numerical model gave satisfactory results at the expense of considerable computer CPU time. For faster slug frequency calculations a new correlation was developed utilizing all 399 data points. This resulted in 0% average error (bias) and 60% average absolute error. This correlation represents a significant improvement in slug frequency prediction accuracy over the other methods studied.
As the industry's deepwater developments continue to mature, newer discoveries in the ultra deepwater demonstrate a trend towards more difficult and heavier hydrocarbons that are far removed from existing infrastructure. Since heavy oils represent a significant reserve-base, there is a strong economic incentive within the industry to develop technologies to profitably produce these hydrocarbon reserves. Heavy oils are often characterized by their high viscosity, low API gravity and low reservoir energy. Heavy oils are also prone to the formation of emulsions. The combination of these factors makes the production and transportation of heavy oils a major challenge from a flow assurance perspective. Development of a robust flow assurance strategy will play a central role in the system selection, detailed design, and operation of deepwater heavy oil fields. In this paper, we identify some of the most significant flow assurance challenges associated with heavy oil production and discuss technology developments needed to overcome them. In particular, we have focused our attention on viscosity management techniques and emulsion formation tendencies of heavy oils and also assessed the risk posed by solids such as hydrates, wax and asphaltenes. We also present a brief analysis of the operability aspects for producing deepwater heavy oils, describe major differences from conventional lighter oils, and evaluate its impact on the topsides infrastructure and subsea system selection and design.
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