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 A low-dimensional dynamic model of severe slugging for control design and analysis Implications of input rate limitations on controllability and controller design Stabilization of multiphase flow in pipelines with single-loop and cascade controllers Model-based anti-slug controllers Extended slug control – An industrial application Conclusions and further work

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

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

Slug flow in pipeline-riser systems Riser slugging Hydrodynamic slugging ....... Terrain slugging Transient slugging *Pictures from SINTEF Multiphase flow laboratory

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)

Outline Modelling of pipeline-riser systems for control applications 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

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

Bifuracation diagram for riser slugging

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

Modelling (2)- Design specs for simplified model 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 (3) – Simplified 3-state model Three dynamical states From entrainment model Given by valve equation:

Modelling (4) – Entrainment model

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

Modelling (6) – Model comparison

Outline Controllability analysis 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

Controllability analysis Investigation into a plants achievable control performance Independent of controller Inverse response Step in valve opening:

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

Measurement evaluation Achievable performace can be represented by lower bounds on closed-loop transfer function For example: Small numerical value for lower bounds on closed loop transfer functions indicate a good measurement candidate Chen (2000) Glower (1986)

Measurement evaluation Unstable system at 30% valve opening pi=0.0007±0.0073 y RHPZ MS=MT PI - 1 0.11 DP 0.016 1.9 0.25 Q 0.09 Low steady-state gain Similar results from two-fluid model

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

Outline Effect of input rate limitations Controller design 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

Effect of input rate limitations Limitation on input rate can limit performance for control systems Explicit lower bounds on required input rate derived Stabilization Disturbance rejection Controller design with limited input rates

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

Single-loop controllers PID controller with measured outlet flow PID controller with measured riser base pressure H∞controller with measured inlet pressure PID controller with measured inlet pressure

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

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

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 Further work Effect of water, different geometries New measurements