This document outlines a case study on estimating slug volume and duration in a natural gas pipeline, focusing on the impact of heavy fractions and different slugging types. Using various methodologies, including Aspen HYSYS hydraulic analysis, the project aimed to predict slug behavior based on gas composition and operating conditions. Sensitivity analysis indicated that the design size for a slug catcher should be conservative, potentially limiting vessel size to around 1,000 bbl.

1. Process Simulation
Flow Assurance Case Study
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2. Estimation of slug volume using
Aspen Hydraulics

3. Background
• Heavy fraction in natural gas condenses may
result in slugs reaching facilities
• Sizing the slug catcher requires estimation of
slug volume and duration
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4. Project description
• Much richer gas was expected in the client
pipeline.
• The objective of this project was to estimate
the anticipated slug volume and duration
• Initial estimates indicated a 3,000 bbl vessel
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5. Slugging types
• Terrain –induced slugging
– Caused by change of elevation in pipeline.
• Hydrodynamic slugging
– Caused by formation of wave on liquid surface as
a result of faster moving gas phase.
• Pig-Induced slugging
– Caused by pigging operations in pipeline. Pig is
designed to push most of the liquid contents to
the outlet.
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6. Slug volume prediction methods
• Based on the composition and operating
conditions at slug catcher
• Using Aspen HYSYS Pipe segment slug analysis
tool (Hydrodynamic slug)
• Estimation using liquid hold-up profile
(Pig-Induced)
• Transient analysis using Aspen Hydraulics
(Terrain-Induced, Pig-Induced)
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Elevation profile
73 and 47 points were selected (as a balance between accuracy and
model speed) to represent Pipe1 and Pipe2 respectively.

8. Validation Case
Location Pressure (kPa)
Reported HYSYS Pipe
Pipe1-60 KM from Inlet 5,099 5,130
Compressor1 Suction 4,998 5,011
Compressor1 Discharge 6,486 6,657
Pipe2-40 KM from
Compressor1 Station
5,901 5,911
Compressor2 Suction 5,026 5,026 (spec)
Pipe1 pressure drop 101 119
Pipe2 pressure drop 1,460 1,631
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Pipe2a
30 KM
Pipe 1
140 KM
Compressor
Station 1
Pipe2b
50 KM
Compressor
Station 2
Assumptions:
The ground temperature was assumed to be 4 C.
The pipe depth was assumed to be 2 m.

9. Slug Prediction
• Recovered liquid volume flow rate from flash
calculation
• Slug volume based on the liquid holdup profile
• Slug Volume based on transient analysis using
HYSYS Hydraulics
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10. Slug Prediction
• Recovered liquid volume flow rate from flash
calculation
• Slug volume based on the liquid holdup profile
• Slug Volume based on transient analysis using
HYSYS Hydraulics
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11. Gas composition
• Client provided 3 compositions for current
case, One year prospect and Ultimate.
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Analysis Methane content (mol%)
Current 90.59
One year Prospect 86.91
Ultimate 79.50

12. Flash calculation results
Analysis Recovered Liquid (bbl/day) @ 29.5 F and 530 psig
Unadjusted composition Adjusted heavy composition
Current 0 366
One year Prospect 101 338
Ultimate 0 345
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• The values in this table represent the anticipated steady-
state liquid production.
• Recovered liquid is strong function of C6+ composition.
Client was advised to get an extended analysis for the
current production which may provide some guidance
regarding the expected C6+ representation for future
production.

13. Sensitivity Analysis – C6+/C7+ distribution
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• The provided composition for the “One year prospect” contained 500 ppm of
C7+, which was assigned to C9 and C10 respectively

14. Slug Prediction
• Recovered liquid volume flow rate from flash
calculation
• Slug volume based on the liquid holdup profile
• Slug Volume based on transient analysis using
HYSYS Hydraulics
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15. Liquid holdup profile
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0
0.0005
0.001
0.0015
0.002
0.0025
0 10000 20000 30000 40000 50000 60000 70000 80000
Liquid holdup
Distance from Compressor1 discharge (m)
Pipe2 (A/B) liquid holdup profile
𝐸" = 𝐴 % 𝐿'() − 𝐿' ℎ𝑜𝑙𝑑𝑢𝑝 '
2
'3)

16. Slug volume based on liquid holdup profile
• 𝑉5678 = 𝐸" − 𝑄" ∗ 𝑃𝑖𝑔 𝑇𝑟𝑎𝑛𝑠𝑖𝑡 𝑇𝑖𝑚𝑒
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Pipe1 Pipe2
Pig Transit Time (hrs) 20 2
Method Slug Volume (bbl)
HYSYS-HTFS-Liquid Slip 102 9
HYSYS-Hydraulic Pipe (SS) 98 8
Fan and Danielson
(ConocoPhilips)
103 25
Some comments for why we looked at Pipe1
Pipe1 liquid-gas production is 1/3 of Pipe2 ---> Lower phase velocities
---> Higher liquid accumulation ---> Higher pig-induced slug

17. Slug Prediction
• Recovered liquid volume flow rate from flash
calculation
• Slug volume based on steady state liquid
holdup profile
• Slug Volume based on transient analysis using
HYSYS Hydraulics
– Aspen Hydraulics allows you to simulate the
pigging rigorously in dynamic mode.
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18. Model setup
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- Steady-state values for initialization of dynamics mode
- Used steady-state temperature profile and disabled heat transfer calculations in the
dynamics mode
- Used Event Scheduler to run the pigs
- High Performance Pig

19. Slug Volume- Transient modelling
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- Peak of ~ 550 bbl/day
- Most of the liquid is recovered within 1-2 hours

20. Pressure drop – Transient modelling
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500
550
600
650
700
750
1440 2440 3440 4440 5440 6440 7440 8440
Pressure Drop (kPa)
Elapsed Time (min)
Pressure drop in Pipe 2b

21. Pigging frequency
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Pigging Interval (hrs) Pigging Slug Size (bbl)
Pipe1
1 1.7
5 8.7
12 20.8
24 41.6
48 83.1
72 100.3
78 101.5
Pigging Interval (hrs) Pigging Slug Size (bbl)
Pipe2
1 10.1
3 30.2
5 44.3
7 50.0
10 50.1

22. Effect of Pipe1 slug on slug catcher
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0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0 10000 20000 30000 40000 50000 60000 70000 80000
Liquid holdup
Distance from Compressor1 Discharge(m)
Steady-state holdup profile in Pipe2
Holdup profile 5hours after Pipe1 slug sent
to Pipe2

23. Conclusions
• Results are highly sensitive to the composition
of heavy components
• A conservative design would limit the size of
the vessel to around 1,000 bbl
• Aspen HYSYS enables the estimation of slug
volumes with various levels of detail
• HYSYS Upstream Hydraulics allows the
evaluation of transient slug formation
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24. Thank you for your attention
• QUESTIONS?
• Alberto@processecology.com
• Ahad@processecology.com
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25. Gas composition
Gas Composition (mol %)
Current Case One year prospect Ultimate
As
Given
Heavy As Given Heavy As Given Heavy
Methane 90.59 90.59 86.91 86.91 79.50 79.50
Ethane 5.84 5.84 8.41 8.41 15.55 15.55
Propane 1.95 1.95 2.47 2.47 3.23 3.23
i-Butane 0.26 0.26 0.38 0.38 0.27 0.27
n-Butane 0.37 0.37 0.52 0.52 0.43 0.43
i-Pentane 0.05 0.05 0.12 0.12 0.03 0.03
n-Pentane 0.05 0.05 0.09 0.09 0.03 0.03
n-Hexane 0.04 0.04 0.07 0.07 0.05 0.05
n-Heptane N/A 0.00 0.04 0.03 N/A 0.00
n-Octane N/A 0.00 0.01 0.01 N/A 0.00
C9 N/A 0.00 0.00 0.00 N/A 0.00
C10+ N/A 0.04 N/A 0.01 N/A 0.03
Nitrogen 0.52 0.52 0.43 0.43 0.44 0.44
CO2 0.29 0.29 0.55 0.55 0.44 0.44
Sum 99.96 100.00 100.00 100.00 99.97 100.00
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