Example stabilizing control Antislug control Twophase pipe flow

Example stabilizing control: Anti-slug control Two-phase pipe flow (liquid and vapor) 1 Slug (liquid) buildup

Slug cycle (stable limit cycle) Experiments performed by the Multiphase Laboratory, NTNU 2

Experimental mini-loop 3

z p 2 Experimental mini-loop Valve opening (z) = 100% p 1 4

z p 2 Experimental mini-loop Valve opening (z) = 25% p 1 5

z p 2 Experimental mini-loop Valve opening (z) = 15% p 1 6

z Experimental mini-loop: Bifurcation diagram No slug p 2 p 1 Valve opening z % 7 Slugging

z Avoid slugging: 1. Close valve (but increases pressure) No slugging when valve is closed p 1 Valve opening z % 8 p 2

Avoid slugging: 2. Build large slug-catcher z p 2 p 1 • Most common strategy in practice 9

Avoid slugging: 3. Other design changes to avoid slugging z p 2 p 1 10

Avoid slugging: 4. Control? Comparison with simple 3 -state model: Valve opening z % Predicted smooth flow: Desirable but open-loop unstable 11

Avoid slugging: 4. ”Active” feedback control PC PT p 1 Simple PI-controller 12 ref z

Anti slug control: Mini-loop experiments p 1 [bar] z [%] 13 Controller ON Controller OFF

Anti slug control: Full-scale offshore experiments at Hod-Vallhall field (Havre, 1999) 14

Poles and zeros Topside ρT Operation points: P 1 z FT DP Poles DP 0. 175 70. 05 1. 94 -6. 11 0. 0008± 0. 0067 i 0. 25 69 0. 96 -6. 21 0. 0027± 0. 0092 i P 1 Zeros: y z P 1 [Bar] DP[Bar] ρT [kg/m 3] FQ [m 3/s] FW [kg/s] -0. 0034 3. 2473 0. 0142 -0. 0004 0. 0048 -4. 5722 -0. 0032 -0. 0004 -7. 6315 -0. 0004 0 -0. 0034 3. 4828 0. 0131 -0. 0004 0. 0048 -4. 6276 -0. 0032 -0. 0004 -7. 7528 -0. 0004 0 0. 175 0. 25 15 Topside measurements: Ooops. . RHP-zeros or zeros close to origin

Stabilization with topside measurements: Avoid “RHP-zeros by using 2 measurements • Model based control (LQG) with 2 top measurements: DP and density ρT 16

Summary anti slug control • • Stabilization of smooth flow regime = $$$$! (or Rand!) Stabilization using downhole pressure simple Stabilization using topside measurements possible Control can make a difference! Thanks to: Espen Storkaas + Heidi Sivertsen and Ingvald Bårdsen 17