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

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

1 2

34

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

Experimental mini-loop

Valve opening 100%

10

Experimental mini-loop

Valve opening 25%

11

Experimental mini-loop

Valve opening 15%

12

Experimental mini-loop: Storkaas 3-state modelBifurcation 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

PCSP

PI controllerGain= -2.5 bara-1

τI = 10s

15

Experimental vs theoretical results

Controller on

16

Feedback control with topside measurements?

PCSP

Not reported

17

Controllability analysis using model

Valve opening Z

0.12 0.25

-20.3411 -35.2145

-0.0197 0.0071 ± 0.1732i

Open loop poles of the system

RHP poles!

Skogestad and Postlethwaite:

For complex RHP- poles, pRHP, each real RHP-zero, zRHP, of the system must approx. obey : zRHP > 2.3 * | pRHP | If not, the system is not stabilizable using feedback control.

18

Fig: Measurements andestimates available

P1 P2 ρ FQ Fw

-1.285 46.984 0.092 -3.958 -65.587

0.212 -0.0547 -0.369 ± 0.192i -0.007 ± 0.076i

Zeros of the system for z = 0.25

Controllability analysis using model

P1

P2

FQ

FW

ρ

Requirement for stabilization:

zRHP > 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 P1 stabilizes the flow.

• Not possible using single topside measurement

• Future work: Using only topside measurements

- Cascade control configuration

- ∞ 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, 55-63.

• Hedne and Linga (1990). Suppression of Terrein Sugging with Automatic and manual Riser Choking. Advances in Gas-Liquid Flows, pp 453-469.

• Sarica and Tengesdal (2000). A new technique to eliminating severe slugging in pipeline/riser systems. SPE Annual Technical Conference and Exhibition, Dallas, Texas. SPE 63185.

• Storkaas, Godhavn and Skogestad (2003). A low-dimentional model of severe slugging for controller design and analysis. MultiPhase’03, San Remo, Italy, 11-13 June 2003

• Skogestad and Posthletwaite (1996). Multivariable Feedback Control. John Wiley & Sons