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Multiphase Flows

Dual-Frequency Severe Slugging in Horizontal Pipeline-Riser Systems

[+] Author and Article Information
Reza Malekzadeh

e-mail: rezamalekzadeh@gmail.com

Ruud A. W. M. Henkes

Department of Multi-Scale Physics,
Delft University of Technology,
P.O. Box 5,
2600 AA Delft, The Netherlands

1Corresponding author.

Contributed by the Fluids Engineering Division of ASME for publication in the JOURNAL OF FLUIDS ENGINEERING. Manuscript received February 27, 2012; final manuscript received October 12, 2012; published online November 20, 2012. Assoc. Editor: Mark R. Duignan.

J. Fluids Eng 134(12), 121301 (Nov 20, 2012) (9 pages) doi:10.1115/1.4007898 History: Received February 27, 2012; Revised October 12, 2012

A new type of severe slugging is found that can occur in two-phase flow of gas and liquid in pipeline-riser systems. This instability, which will be referred to as dual-frequency severe slugging (DFSS), generates a different class of flow oscillations compared to the classical severe slugging cycle, having a dominant single frequency that is commonly found in a pipe downward inclined by a few degrees from the horizontal connected to a vertical riser. The DFSS flow pattern was found in laboratory experiments carried out in a 100 m long, 50.8 mm diameter horizontal pipeline followed by a 15 m high, 50.8 mm diameter vertical riser operating at atmospheric end pressure. The experimental facility also included a 400 liter gas buffer vessel, placed upstream of the pipeline, to obtain extra pipeline compressibility. Air and water were used as the experimental fluids. At constant inflow conditions, we observed a type of severe slugging exhibiting a dual-frequency behavior. The relatively high-frequency fluctuations, which are in the order of 0.01 Hz, are related to the classical severe slugging cycle or to an unstable oscillatory process. The relatively low-frequency fluctuations, which are in the order of 0.001 Hz, are associated with the gradual cyclic transition of the system between two metastable states, i.e., severe slugging and unstable oscillations. Numerical simulations were performed using OLGA, a one-dimensional two-fluid flow model. The numerical model predicts the relatively low-frequency fluctuations associated with the DFSS flow regime. The laboratory experiments and the numerical simulations showed that the evolution of the DFSS is proportional to the length of the pipeline.

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Figures

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Fig. 1

Schematic of the experimental facility. The test loop comprises both the horizontal pipeline and the vertical riser.

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Fig. 2

Experimental riser ΔP traces of the four mentioned flow patterns corresponding to (a) STB (USL = 0.61 m s−1 and USG0 = 2.01 m s−1), (b) SS3 (USL = 0.40 m s−1 and USG0 = 1.01 m s−1), (c) USO (USL = 0.10 m s−1 and USG0 = 3.02 m s−1) and (d) DFSS (USL = 0.20 m s−1 and USG0 = 0.51 m s−1)

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Fig. 3

A graphical illustration of (a) severe slugging of type 3 in the horizontal pipeline-riser experiment and (b) severe slugging of type 1 in a downward inclined pipeline-riser experiment

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Fig. 4

Stages marked on a cycle of an experimental riser ΔP trace of (a) severe slugging of type 3 in the horizontal pipeline-riser experiment (USL = 0.40 m s−1 and USG0 = 1.01 m s−1) and (b) severe slugging of type 1 in a downward inclined pipeline-riser experiment

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Fig. 5

(a) Experimental flow map of the system, indicating unstable oscillations, severe slugging of type 3, dual-frequency severe slugging, and stable flow and (b) experimental riser ΔP traces of DFSS, SS3, and USO (USG0 = 0.81 m s−1 and USL = 0.2 m s−1, 0.3 m s−1, and 0.05 m s−1, respectively) showing DFSS fluctuates between SS3 and USO

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Fig. 6

Experimental riser ΔP traces corresponding to (a) 0.4 m3 air buffer volume (USL = 0.20 m s−1 and USG0 = 0.51 m s−1), (b) 0.2 m3 air buffer volume (USL = 0.21 m s−1 and USG0 = 0.51 m s−1), (c) 0.0 m3 air buffer volume (USL = 0.20 m s−1 and USG0 = 0.51 m s−1), and (d) FFT of the experimental riser ΔP traces of the three mentioned gas buffer volumes

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Fig. 7

Experimental water outflow rates corresponding to (a) 0.4 m3 air buffer volume (USL = 0.20 m s−1 and USG0 = 0.51 m s−1), (b) 0.2 m3 air buffer volume (USL = 0.21 m s−1 and USG0 = 0.51 m s−1), and (c) 0.0 m3 air buffer volume (USL = 0.20 m s−1 and USG0=0.51 m s−1)

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Fig. 8

Predicted riser ΔP with three different cell sizes of 100 cm, 25 cm, and 5.08 cm for case 2 in Table 2 (USL = 0.10 m s−1 and USG0 = 0.51 m s−1)

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Fig. 9

Experimental and predicted riser ΔP for all experimental cases of DFSS mentioned in Table 2. The air buffer volume is 0.4 m3.

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Fig. 10

predicted riser ΔP for 100 m, 200 m, 300 m, and 400 m long horizontal pipeline followed by the riser (USL = 0.20 m s−1 and USG0 = 0.51 m s−1). The air buffer volume is 0.0 m3. The simulated 100, 200, and 300 m long horizontal pipeline are equivalent to Figs. 6(c), 6(b), and 6(a), respectively.

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