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 Trondheim, 2001 Internet: www.ntnu.no/

Outline Introduction / Problem description Model description Previous work Case descriptions Simulation results Controllability analysis Closed loop example Go briefly through the points Trondheim, 2001 Internet: www.ntnu.no/

Introduction Introduce Petronics Describe main figure Trondheim, 2001 Internet: www.ntnu.no/

Slug cycle 1 2 Liquid blocking Slug growth/pressure buildup Liquid Production Gas production/Liquid fallback 4 3 Go through the stages Trondheim, 2001 Internet: www.ntnu.no/

Flow regime map: Horizontal flow Brief comments Mark that this is open loop stability regions Trondheim, 2001 Internet: www.ntnu.no/

Flow Regime map: Pipeline-Riser Again open loop stability regions Trondheim, 2001 Internet: www.ntnu.no/

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 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. Brief Focus on the inability to regard this as an contol probelm Credit Kjetil Trondheim, 2001 Internet: www.ntnu.no/

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 Trondheim, 2001 Internet: www.ntnu.no/

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 Trondheim, 2001 Internet: www.ntnu.no/

Open loop simulation Describe Trondheim, 2001 Internet: www.ntnu.no/

Open loop simulation (2) Describe Trondheim, 2001 Internet: www.ntnu.no/

Open-loop stability – Pressure levels Describe Hopf bifurcation Why shape of limit cycle curves Comment flattening nature of fixedpoint curve Trondheim, 2001 Internet: www.ntnu.no/

Open Loop Stability – RHP-poles Crossing of imaginary axis coincides with bifurcation point Worse as choke is opened Trondheim, 2001 Internet: www.ntnu.no/

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) Trondheim, 2001 Internet: www.ntnu.no/

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 Trondheim, 2001 Internet: www.ntnu.no/

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 Trondheim, 2001 Internet: www.ntnu.no/

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 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 Sum up and conclude Trondheim, 2001 Internet: www.ntnu.no/

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. 614-619 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.453-469 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 For full paper see: http://www.chembio.ntnu.no/users/espensto/Publications/ Refer to paper Trondheim, 2001 Internet: www.ntnu.no/