1. Stabilization of Desired Flow Regimes in Pipelines
Presented at AIChE Annual meeting in Reno, USA
November 9th, 2001
E. Storkaas ,S. Skogestad and V. Alstad
Department of Chemical Engineering
Norwegian University of Science and Technology
Introduction
Spesify that it is multiphase flow
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2. Outline Introduction / Problem description Model description
Previous work
Case descriptions
Simulation results
Controllability analysis
Closed loop example
Go briefly through the points
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3. Introduction Introduce Petronics Describe main figure
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4. Slug cycle 1 2 Liquid blocking Slug growth/pressure buildup
Liquid Production
Gas production/Liquid fallback 4 3
Go through the stages
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5. Flow regime map: Horizontal flow
Brief comments
Mark that this is open loop stability regions
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6. Flow Regime map: Pipeline-Riser
Again open loop stability regions
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7. Problem description
Multiphase transport of oil and gas in pipelines and wells with elevation changes can give rise to unstable flow known as slug flow
Causes operational problems for the downstream processing units
Idea: Avoid slug flow by using feedback control to extend the stability region of the desired (non-slug) flow regime
This presentation: Analysis and simulation of simple case study
Present and give background for the idea
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8. Previous work on avoiding slug flow
Most people in this field regard it as a design problem (e.g. increase pressure, design slug catcher, …)
Use of feedback control to stabilize desired non-slug flow regime:
Hedne and Linga (1990) : Implementation on test rig
Henriot et al. (1999): Simulations with TACITE and (probably) implementation on Dunbar pipleline
Havre et al. (2000): Simulations with OLGA and implementation on Hod-Valhall pipeline.
Brief
Focus on the inability to regard this as an contol probelm
Credit Kjetil
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9. Model description Mass and momentum balance for each phase
Slip through interphase friction
Pure phases; bubbles in liquid and droplet field neglected
Mass transfer neclected
Isothermal
Boundary conditions
Mass flow of each phase into pipe
Multiphase valve with constant pressure downstream
Common way of doing it,
but motive and thus different model characteristics prioritized
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10. Case description Simple Case Feed 1 kg/sec Downstream pressure 20 bar
Exibits severe slugging in OLGA simulations
Mass conservation grid location indicated 1 –13
Brief,
say something about discretization
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11. Open loop simulation
Describe
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12. Open loop simulation (2)
Describe
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13. Open-loop stability – Pressure levels
Describe
Hopf bifurcation
Why shape of limit cycle curves
Comment flattening nature of fixedpoint curve
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14. Open Loop Stability – RHP-poles
Crossing of imaginary axis coincides with bifurcation point
Worse as choke is opened
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15. Tool for selecting input and output for stabilization: Pole Vectors
Largest element in pole vector minimizes input usage (H2 and H-norm of KS (Havre et al., 1998)
Output: Use pressure measurement before riser.
Expalin Pole vectors
Comment on the difference between operating points
Tie up to RHPZ (next slide)
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16. Reason for problem: RHP-Zeros
RHP-zeros limit achivable bandwith of the system
Bandwith must be higher than 2p (real poles) for stabilization
Thus stabilization is impossible with pressure sensors in the riser (inclining section)
Describe
Tie up to previous slide
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17. Closed loop – Stabilizing the flow
Simple PI-controller
Gain:1 Bar-1
I : 2000 s
Pressure sensor in bottom of riser
Describe
Explain
Tie up to next slide
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18. Havre et al. (2000): Implementation on Hod-Vallhall pipeline
Mention that this is the system is the one shown on the first drawing
Describe, actual implementation
Point out gain in average pressure drop
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19. Summary and Conclusion
A ”simple” model for severe slugging has been developed
Simple case study shows severe slugging
Stability properties investigated
Control implications
Break limit cycle; nonlinear aspects important
Keep system stable; Linear controller sufficient
Sum up and conclude
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20. References http://www.chembio.ntnu.no/users/espensto/Publications/
Havre, K. And Skogestad, S. (1998), Selection of varibles for regulatory control using pole vectors, Proc. IFAC symposium DYCOPS-5,Corfu, Greece (1998), p
Havre, K., Stornes, K. and Stray, H. Taming Slug Flow in pipelines, ABB Review 4 (2000), p.55-63
Hedne, P. And Linga, H. Supression of terrein 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 Conferance and Exibition , Huston, Texas(1999). SPE56461
For full paper see:
Refer to paper
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