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Stabilizing control and controllability:. Control solutions to avoid slug flow in pipeline-riser systems Espen Storkaas Trondheim 7.6.2005. Thesis summary. Introduction Controllability analysis of a two-phase pipeline-riser systems at riser slugging conditions - PowerPoint PPT Presentation
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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