1 Anti-slug control on a small-scale two-phase loop Heidi Sivertsen and Sigurd Skogestad Departement of Chemical Engineering, Norwegian University of Science and Technology

2 Outline Introduction Experimental set up Results anti-slug control Controllability analysis

3 Slug cycle 1.Liquid build-up 2.Pressure buildup 3.Mass acceleration 4.Liquid fallback

4 Slug cycle Experiments performed by the Multiphase Flow Technology Laboratory, NTNU (prof. Nydal)

5 Why is riser slugging undesired? Slugging leads to: Oscillation of separator level, bad separation and in some cases separator flooding. Pressure oscillations cause wear and tear on equipment. Unnecessary flaring. Pressure variations can damage the reservoir. Reduced production rate.

6 How to avoid riser slugging The traditional solutions have been to change the process either by changing the design: Larger separator Slug catchers Other design changes Or by changing the operational conditions: Increase separator pressure Choking to increase riser pressure Slug catcher

7 How to avoid riser slugging Now: control has become the way to handle the problem. Several tests ans implementations have been carried out: Courbot (1996) Havre, Stornes and Stray (2000) Hedne and Linga (1990) Sarica and Tengesdal (2000)

8 Experimental mini-loop

9 Valve opening 100%

10 Experimental mini-loop Valve opening 25%

11 Experimental mini-loop Valve opening 15%

12 Experimental mini-loop: Storkaas 3-state model Bifurcation diagram Slugging No slug Predicted smooth flow: Desirable but open-loop unstable

13 Use of feedback control to stabilize riser slugging With the use of a feedback loop we can change the dynamics of the system, making it stable where it would not be otherwise!

14 Feedback control using PI controller PC SP PI controller Gain= -2.5 bara -1 τ I = 10s

15 Experimental vs theoretical results Controller on

16 Feedback control with topside measurements? PC SP Not reported

17 Controllability analysis using model Valve opening Z ± i Open loop poles of the system RHP poles! Skogestad and Postlethwaite: For complex RHP- poles, p RHP, each real RHP-zero, z RHP, of the system must approx. obey : z RHP > 2.3 * | p RHP | If not, the system is not stabilizable using feedback control.

18 Fig: Measurements and estimates available P1P1 P2P2 ρFQFQ FwFw ± 0.192i ± 0.076i Zeros of the system for z = 0.25 Controllability analysis using model P1P1 P2P2 FQFQ FWFW ρ Requirement for stabilization: z RHP > 0.4 Requirement for performance: z >> 0 (both RHP and LHP zeros)

19 Conclusion Experimental loop analyzed using simple model Simple PI controller using inlet pressure P 1 stabilizes the flow. Not possible using single topside measurement Future work: Using only topside measurements - Cascade control configuration - H ∞ controller

20 Experiments: M.Sc. student Ingvald Baardsen Model: Ph.D. student Espen Storkaas Finance: Statoil The Norwegian Research Council Acknowledgments

21 References Courbot (1996). Prevention of Severe Slugging in the Dunbar 16” Myultiphase Pipeline. Offshore Technology conference, May 6-9, Houston, Texas Havre, Stornes and Stray (2000). Taming slug flow in pipelines. ABB review 4, Hedne and Linga (1990). Suppression of Terrein Sugging with Automatic and manual Riser Choking. Advances in Gas-Liquid Flows, pp Sarica and Tengesdal (2000). A new technique to eliminating severe slugging in pipeline/riser systems. SPE Annual Technical Conference and Exhibition, Dallas, Texas. SPE Storkaas, Godhavn and Skogestad (2003). A low-dimentional model of severe slugging for controller design and analysis. MultiPhase’03, San Remo, Italy, June 2003 Skogestad and Posthletwaite (1996). Multivariable Feedback Control. John Wiley & Sons