1. Stabilization of severe slugging based on a low-dimensional nonlinear model
Presented at AIChE Annual Meeting in Indianapolis, USA
November 7th, 2002
Espen Storkaas and Sigurd Skogestad
Dep. of Chemical Engineering
Norwegian University of Science and Technology
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2. Outline Introduction / Problem Description Model Description Case
Controllability analysis
Controller configurations and design
Summary and conclusion
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3. Slug cycle 1 2 4 3 Liquid blocking Slug growth/pressure buildup
Liquid Production
Gas production/Liquid fallback 4 3
Go through the stages
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4. Slug cycle (2)
Experiments performed by the Multiphase Laboratory, NTNU
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5. Objectives Avoid slug behavior (open loop stable)
Obtain stationary behavior (open loop unstable)
Issues
Impossible/existence?
Modeling
Control
Previous Work
Hedne & Linga (1990)
Henriot et al. (1999)
Havre et al. (2000)
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6. Modeling basis Three states Continuous
Description of both slug- and ”stationary” flow regime
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7. Model Upstream gas volume (1 state) Riser (2 states) Choke valve
Holdup gas
Constant volume
Ideal gas law
No liquid dynamics
Riser (2 states)
Holdup of liquid and gas
Stationary pressure balance
Pressure-flow relationship for gas
Entrainment of liquid
Choke valve
Simplified gas valve
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8. Case Test case for slug-flow in OLGA Constant feed
Valve closing time : 1min
Downstream pressure: 50Bar
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9. Model tuning Based on bifurcation chart from OLGA simulations
Tuning based on Hopf point (purely imaginary poles)
Amplitude/frequency
Focus on stabilizable area
Stable slug
Unstable stationary
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10. Poles and Zeros Unstable (RHP) poles need feedback for stabilization
Imposes lower limit on bandwidth
RHP zeros limit performance
Imposes upper limit on bandwidth
RHP poles combined with RHP zeros may render stabilizing control impossible if the zero is close to the pole
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11. Poles and zeros FT Operation points: ρT DP P1 Zeros: z P1 DP Poles
0.175
70.05
1.94
-6.11
0.0008±0.0067i
0.25 69
0.96
-6.21
0.0027±0.0092i P1
Zeros: y z
P1 [Bar]
DP[Bar]
ρT [kg/m3]
FQ [m3/s]
FW [kg/s]
0.175
3.2473
0.0142
0.0048
0.25
3.4828
0.0131
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12. Stabilization using P1 Easy Gain K = 0.5 Bar-1 Integral time τI=500sPT PC
ref
Easy
Gain K = 0.5 Bar-1
Integral time τI=500s
MS = 1.02, MT = 1.04
Bandwidth = 0.15 s-1
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13. Stabilization using P1 (2)
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14. Stabilization using downstream measurements (1)
Difficult
Stabilizing controller: Volumetric flow in slave controller
K = 8 s/m3, τI = 40 s
Cascade: Valve position reset
K = 0.01 m3/s, τI = 750 s
MS = 1.37, MT = 1.47
Bandwidth (outer loop) = s-1 ZT ZC
z ref FT FC
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15. Stabilization using downstream measurements (2)
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16. Summary and conclusions
Stabilization of desired flow regime
Simple, physically motivated model for control purposes
Useful for measurement selection
Choice of control configuration
Stabilization using upstream pressure simple
Stabilization using downstream measurements possible
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17. Acknowledgements References
Norwegian Research Council for financial support
ABB and Statoil for supervision and collaboration
References
Havre, K., Stornes, K. and Stray, H. Taming Slug Flow in pipelines, ABB Review 4 (2000), p
Hedne, P. And Linga, H. Suppression of terrain slugging with automatic and manual riser choking, riser choking, Advances in Gas-Liquid Flows (1990), p
Henriot, V., Courbot, A., Heintze, E. And Moyeux, L. Simulation of process to control severe slugging: Application to Dunbar pipeline, SPE Annual Conference and Exhibition, Huston, Texas(1999). SPE56461
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