Trondheim, 2001 Internet: www.ntnu.no/

Stabilization of Desired Flow Regimes in Pipelines

Presented at AIChE Annual meeting in Reno, USANovember 9th, 2001

E. Storkaas ,S. Skogestad and V. Alstad

Department of Chemical EngineeringNorwegian University of Science and Tecnology

Trondheim, 2001 Internet: www.ntnu.no/

Outline

• Introduction / Problem description• Model description • Previous work• Case descriptions• Simulation results• Controllability analysis• Closed loop example

Trondheim, 2001 Internet: www.ntnu.no/

Introduction

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Slug cycle

1. Liquid blocking2. Slug growth/pressure

buildup3. Liqiud Production4. Gas production/Liquid

fallback

1 2

34

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Flow regime map: Horizontal flow

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Flow Regime map: Pipeline-Riser

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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

Trondheim, 2001 Internet: www.ntnu.no/

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.

Trondheim, 2001 Internet: www.ntnu.no/

Model descriptionMass and momentum balance for each phase

LGkx

uAt

A kkkkk , , 0)()(

LG,k , )sin()()()( 2

kikwkk

kk

kkkkkk gAxPA

xuA

tuA

•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

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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

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Open loop simulation

Pipe length

Flat section Declining section Inclining sec.

Vol

ume

f ra c

t ion

l iqui

d

0

1

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Open-loop stability – Pressure levels

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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.

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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)

Trondheim, 2001 Internet: www.ntnu.no/

Closed loop – Stabilizing the flow

• Simple PI-controller• Gain:1 Bar-1

• I : 2000 s• Pressure sensor inbottom of riser

Trondheim, 2001 Internet: www.ntnu.no/

Havre et al. (2000): Implementation on Hod-Vallhall pipeline

Trondheim, 2001 Internet: www.ntnu.no/

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

Trondheim, 2001 Internet: www.ntnu.no/

References

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), p453-469

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