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Stabilizing control and controllability:

Control solutions to avoid slug flow in pipeline-riser systems

Espen Storkaas

Trondheim 7.6.2005

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Thesis summary

1. Introduction2. Controllability analysis of a two-phase pipeline-riser

systems at riser slugging conditions3. A low-dimensional dynamic model of severe slugging

for control design and analysis4. Implications of input rate limitations on controllability

and controller design5. Stabilization of multiphase flow in pipelines with single-

loop and cascade controllers6. Model-based anti-slug controllers7. Extended slug control – An industrial application8. Conclusions and further work

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Outline

• Introduction• Slug flow in pipeline-riser systems• Modelling of pipeline-riser systems for control

applications• Controllability analysis• Effect of input rate limitations• Controller design• Extended slug control• Conclusions

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Introduction

• Oil producing wells also produce gas and water• Longer multiphase tie-in lines in offshore oil production

from increases flow-related challenges• Flow assurance technology plays an increasingly

important role– Hydrates– Wax– Corrosion– Flow regimes

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Slug flow in pipeline-riser systems

• Riser slugging

*Pictures from SINTEF Multiphase flow laboratory

• Hydrodynamic slugging

• Terrain slugging• Transient slugging

.......

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Riser slugging and control - History

• From design challenge to control objective• First relevant publication : Schmidt et al (1979)• Experimental work by Hedne & Linga (1990)• Simulations studies and experimental work from several

sources (Total, Shell, ABB, Statoil)• First industrial application: Hod-Valhall (Havre et al

2000), more has followed in later years• Included in design of new projects (riser slugging

potensial at design conditions)

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Outline

• Introduction• Slug flow in pipeline-riser systems

• Modelling of pipeline-riser systems for control applications

• Controllability analysis• Effect of input rate limitations• Controller design• Extended slug control• Conclusions

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Main case study

• Test case for riser slugging in OLGA– Simplified geometry– Two-phase flow– Constant feed– Constant pressure behind choke

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Bifuracation diagram for riser slugging

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Modelling (1) – Lessons learned from two-fluid model

• Two-fluid model used to investigate system caracteristics

• Transition to instability through Hopf bifurbation

• Complex unstable poles• Controllability analysis

gives information about measurement selection

• Simpler model should be used

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• The model must:– Describe the dominant dynamic behavior of the system for the

time scales for which control is to be effective

• The model should :– be continuous– be simple (low state dimension)– contain few empirical coefficients

Modelling (2)- Design specs for simplified model

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Modelling (3) – Simplified 3-state model

Three dynamical states

Given by valve equation:From entrainment model

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Modelling (4) – Entrainment model

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Modelling (5) –Properties of 3-state model

• Phenomenological model with 3 dynamical states• Based on bulk properties• Describes both riser slugging and unstable stationary

operating points• 4 empirical parameters – easy to tune• Hopf bifurcation, complex unstable poles• Very useful for controllability analysis and controller

design

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Modelling (6) – Model comparison

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Outline

• Introduction• Slug flow in pipeline-riser systems• Modelling of pipeline-riser systems for control

applications

• Controllability analysis• Effect of input rate limitations• Controller design• Extended slug control• Conclusions

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Controllability analysis

• Investigation into a plants achievable control performance

• Independent of controller

• Step in valve opening:

Inverse response

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Measurement evaluation

• Achievable performace can be represented by lower bounds on closed-loop transfer functions such as– Sensitivity function S– Complementary sensitivity function T– Input usage KS

• Bound computed from 3-state model• Small numerical value for lower bounds on closed loop

transfer functions indicate a good measurement candidate

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Measurement evaluation

y RHPZ MS=MT

PI - 1 0.11

DP 0.016 1.9 0.25

Q - 1 0.09

Unstable system at 30% valve opening pi=0.0007±0.0073

Low steady-state gain

Similar results from two-fluid model

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Conclusions from controllability analysis

• Inlet or riserbase pressure well suited for stabilizing control– Time delay may prevent the use of inlet pressure for long

pipelines

• Pressure at top of riser not suitable for stabilizing control due to unstable zero dynamics

• Flow measurement at riser outlet can be used for stabilization but has lacking low-frequency gain– Best used as a secondary measurement in a cascade or in

combination with another measurement

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Outline

• Introduction• Slug flow in pipeline-riser systems• Modelling of pipeline-riser systems for control

applications• Controllability analysis

• Effect of input rate limitations• Controller design• Extended slug control• Conclusions

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• Explicit lower bounds on required input rate derived– Stabilization

– Disturbance rejection

• Controller design with limited input rates

Effect of input rate limitations• Limitation on input rate can limit performance for

control systems

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Controller design

• Controllers design based on simplified 3-state model– Stabilizes both two-fluid model and OLGA model

• Measurement selection from controllability analysis confirmed– Controllers based on upstream pressure measurement

robust and effective– Controllers based on only a flow measurement tends to

drift off– A flow measurement combined with another measurement

can be used for stabilizing control

25 PID controller with measured inlet pressure H∞controller with measured inlet pressurePID controller with measured riser base pressure

Single-loop controllers

PID controller with measured outlet flow

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Cascade and MISO controllers

Cascade controller, y1=PI, y2=Q Cascade controller, y1=DP, y2=Q H∞-controller, y=[DP Q]

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Extended slug control

• Anti-slug control combined with functionality to mitigate surge waves and startup slugs

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Summary

• Simplified model of pipeline-riser systems at riser slugging condisons for controllability analysis and controller design

• Controllability analysis gives clear recommendations for measurement selection for stabilizing control

• Input rate limitations may be important• Controller design• Extended control application

• Effect of water, different geometries• New measurements

Further work