Presented at AIChE Annual Meeting in Indianapolis, USA

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Presentation transcript:

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 www.chemeng.ntnu.no/~espensto/ 2/27/2019

Outline Introduction / Problem Description Model Description Case Controllability analysis Controller configurations and design Summary and conclusion www.chemeng.ntnu.no/~espensto/ 2/27/2019

Slug cycle 1 2 4 3 Liquid blocking Slug growth/pressure buildup Liquid Production Gas production/Liquid fallback 4 3 Go through the stages www.chemeng.ntnu.no/~espensto/ 2/27/2019

Slug cycle (2) Experiments performed by the Multiphase Laboratory, NTNU www.chemeng.ntnu.no/~espensto/ 2/27/2019

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) www.chemeng.ntnu.no/~espensto/ 2/27/2019

Modeling basis Three states Continuous Description of both slug- and ”stationary” flow regime www.chemeng.ntnu.no/~espensto/ 2/27/2019

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 www.chemeng.ntnu.no/~espensto/ 2/27/2019

Case Test case for slug-flow in OLGA Constant feed Valve closing time : 1min Downstream pressure: 50Bar www.chemeng.ntnu.no/~espensto/ 2/27/2019

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 www.chemeng.ntnu.no/~espensto/ 2/27/2019

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 www.chemeng.ntnu.no/~espensto/ 2/27/2019

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 -0.0034 3.2473 0.0142 -0.0004 0.0048 -4.5722 -0.0032 -7.6315 0.25 3.4828 0.0131 -4.6276 -7.7528 www.chemeng.ntnu.no/~espensto/ 2/27/2019

Stabilization using P1 Easy Gain K = 0.5 Bar-1 Integral time τI=500s PT PC ref Easy Gain K = 0.5 Bar-1 Integral time τI=500s MS = 1.02, MT = 1.04 Bandwidth = 0.15 s-1 www.chemeng.ntnu.no/~espensto/ 2/27/2019

Stabilization using P1 (2) www.chemeng.ntnu.no/~espensto/ 2/27/2019

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) = 0.001 s-1 ZT ZC z ref FT FC www.chemeng.ntnu.no/~espensto/ 2/27/2019

Stabilization using downstream measurements (2) www.chemeng.ntnu.no/~espensto/ 2/27/2019

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 www.chemeng.ntnu.no/~espensto/ 2/27/2019

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.55-63. Hedne, P. And Linga, H. Suppression of terrain 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 Conference and Exhibition, Huston, Texas(1999). SPE56461 www.chemeng.ntnu.no/~espensto/ 2/27/2019