Instability and Feedback
Stabilisation of
Desired Pipeline Flow Regimes
Truls Larsson
Trondheim 25.08.2000
Trial lecture for the
Doktor ingeniør degree
NTNU
Truls Larsson Slide 1
Trial lecture
Outline
• Stability
• Classical stability analysis
– Exemplified by stability of laminar flow
• Classification of two phase flow regimes
– Stability of slug flow
• Unstable flow:
– Severe slug flow
• Feedback stabilisation of severe slugging
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Truls Larsson Slide 2
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Flow Regime in Pipeline
Flow regime:
velocity profile
distribution of phases
• One phase flow:
– Laminar and turbulent
• Two phase flow:
– Spatial distribution of the phases
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Truls Larsson Slide 3
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Classification of Flow Instabilities
Bergles (1981):
• Steady flow:
The variables are a function of time only, except for rapid
variations (turbulence, slug flow)
• Static instability:
if small changes leads to a new and different steady state
• Dynamic instability:
The system behaves in a “dynamic manner”
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Truls Larsson Slide 4
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Classical Stability Analysis:
Is laminar flow stable? (White 1974)
1 Solution to Navier-Stokes
2 Add a small disturbance to the solution
3 Insert into Navier-Stokes, and remove the
solution and ignore square terms
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Truls Larsson Slide 5
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Classical Stability Analysis:
Is laminar flow stable? (White 1974)
4 The result: A set of linear partial equations with
variable coefficient
– The Orr-Sommerfield equations
– Difficult to solve numerically
– Still they show that laminar flow is unstable at high
Reynolds numbers
• Turbulent flow is stable in “our” timeframe
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Truls Larsson Slide 6
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Multiphase Flow - Two Phase Flow
• Gas, liquid and/or solid flows in the same pipe
– Gas/solid flow
– Gas/liquid flow
• Offshore pipelines
– Liquid/solid flow
– Liquid/liquid flow
– Gas/liquid/liquid flow
• How are the phases distributed in the pipe?
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Truls Larsson Slide 7
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Flow Regimes - Gas/liquid Flow
Distributed flow
Separated flow
– Annular
– Bubbly
Gas
Liquid
– Slug flow
– Stratified Gas
Gas
Liquid
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Truls Larsson Slide 8
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Liquid
Flow Maps
Bubbly
Liquid velocity
Flow maps: the stability
of flow regimes for a
particular fluid and
geometry
Usually obtained by
experiments
Too simple!
Slug
Annular
Stratified
Gas velocity
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Truls Larsson Slide 9
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Kelvin-Helmholtz Stability Criteria
• Pressure drop due to smaller gas area
• Gravity force on the perturbation
Gas
Liquid
• Conditions for stability of stratified flow:
• Taitel and Duckler (1976)
• Lin and Hanratty (1986)
• And others
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Truls Larsson Slide 10
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Stability of Slugs
• Is slug flow stable?
– The front velocity of the slug has to be larger than
the tail velocity (Bendiksen and Espedal 1992)
• Equivalent to the minimum slip criteria
– Ruder et.al (1989) gives two conditions for the
stability of the slug
• Low pressure: both stratified and slug flow is stable
stable slug flow may be generated with a disturbed inlet
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Truls Larsson Slide 11
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Stability of Slug Flow
Modelling and Control
• Can it be stabilised? How?
• Proper modelling of “dynamic” slug flow?
• Is “normal” slug flow an instability?
• A limit cycle?
• Are new models needed?
• Present models: mainly concerned with either slug or
stratified flow. Slug initiation quasi stationary
• Model need: A complete model which describes the
whole cycle
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Truls Larsson Slide 12
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Severe Slugging
• Normal slug: High gas and liquid flow rates
• Severe slug: Longer period
– Generated at the base of a riser (Schmidt 1980)
– Generated at low elbows (Zheng et.al 1994) and (de
Henau and Raithby 1995)
– Other: start up, transients
• Not a rigid classification:
– Growth of normal slugs in long pipelines
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Truls Larsson Slide 13
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Simplified explanation of
Severe Slugging in Pipeline Riser
Step 1: Initiation
Gas velocity is not large
enough to sustain the
liquid film in the riser,
which falls down and
blocks the gas flow
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Truls Larsson Slide 14
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Dynamic Instability
Severe Slugging in Pipeline Riser
Step 2: slug generation
Liquid accumulates
Gas pressure increases in
the pipe
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Truls Larsson Slide 15
Trial lecture
Dynamic Instability
Severe Slugging in Pipeline Riser
Step 3: slug production
When the gas pressure
equals the liquid head,
the gas penetrates into
the riser. As gas enters
the liquid plug is
accelerated
Large peaks in the liquid
flow rate
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Truls Larsson Slide 16
Trial lecture
Dynamic Instability
Severe Slugging in Pipeline Riser
Step 4: gas blow-down
The pressure drops as the
expanding gas bubbles
leaves the pipe
The gas bubbles becomes
continuos, leaving a
liquid film at the wall
The gas velocity becomes
too small to …..
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Truls Larsson Slide 17
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Severe Slugging in Hilly Terrain
Liquid blocks a low
elbow, and gas
pressure builds up.
(Zheng et.Al. 1994) and
(de Henau and Raithby 1995)
Liquid is blocking
the low elbow.
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Truls Larsson Slide 18
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When to Expect Severe slugging
• Assumed: the flow regime in the pipe before the riser
should be stratified (Schmidt 1985)
– Not consistent with experiments by Hedne and Linga 1990
• Condition for severe slugging in risers given in:
Bøe 1981, Schmidt et.al. 1985, Taitel et.al. 1985, Pots et.al. 1987,
Taitel et.al. 1990 and others
• However:
• Based on steady state analysis
• Variables which are not readily available are needed
• Not able to predict if the system will be stable
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Truls Larsson Slide 19
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When to Expect Severe slugging
• Dynamic model/simulation of the pipe is needed!
• Slug initiation and growth
• Still: (taken from the OLGA training course)
– Pipeline with many dips and humps
• High flow rates: steady flow
• Low flow rates: dynamic flow
– Low gas-oil ratio: dynamic flow
– Gas-condensate lines: dynamic flow
• Low liquid velocities, long transients in liquid
– Decreasing pressure: dynamic flow
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Truls Larsson Slide 20
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Operational Problems Caused by Slugging
• Operational problems on the platform
– Separators
– Compressors
– Mechanical stresses
• Reservoir
– Pressure fluctuations are bad for the reservoir
• Pipeline
– The average flow is reduced?
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Truls Larsson Slide 21
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Reducing the Effect of Severe Slugging
• Design changes
• Pipeline and separator
• Extra/New equipment: slug catchers, venturi and gas lift
• Operation
• Increasing the separator pressure, may reduce production
• Choking: changes the flow-pressure drop profile of the riser
Schmidt et.al. (1985). Choking and terrain slugging?
– Hedne and Linga 1990: “success of manual choking depends
also on the upstream pipeline topology”
• Tighter separator control (Xu et.al. 1997)
• Feedback control of pipeline
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Truls Larsson Slide 22
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Feedback Stabilisation
of Severe Slugging
• From control theory
– Only feedback control moves the poles!
• Requirement for feedback stabilisation:
– Measurement:
– Actuator:
see the instability
react faster than the instability
• Not as sometimes suggested:
– To deduce that the pipeline is slugging form
measurements. (See Mcnulty et.Al 1999 for an example)
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Truls Larsson Slide 23
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Feedback Stabilisation
of Severe Slugging
Riser induced slug stabilisation:
• Hedne and Linga 1990:
– Experimental work on the SINTEF two phase flow loop
• Downward sloping pipeline ca. 950 m. Long and 60 m high riser
– Experiments with different pressures and velocities
– Manual choking: 80% valve closure to suppress all the terrain slugging
– Automatic choking: completely removed terrain slugging
• PI control of the pressure drop in the riser using the choke valve
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Truls Larsson Slide 24
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Feedback Stabilisation
of Severe Slugging
Riser induced slug stabilisation:
• Courbot (1996):
– The Dunbar pipeline was expected to show severe slugging in the riser.
– Tests showed that:
“It would be difficult, if not impossible, to operate the pipeline …
without any slug control device”
– Pressure in the bottom of the riser is controlled with the choke valve
• Plus switches and overrides
– Severe slugging was removed
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Truls Larsson Slide 25
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Feedback Stabilisation
of Severe Slugging
Terrain slugging stabilisation:
• Havre et.al. 2000:
– Severe slugging at the Hod-Valhall pipeline caused large operational
problems, and caused platform shutdown
– A simulation in OLGA reproduced the severe slugging behaviour
• It was due to the hilly terrain
– The slug controller uses:
• The pressure and temperature on both Valhall and Hod
• The choke valve
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Truls Larsson Slide 26
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Summary of feedback control
• All of them used the choke valve
• Both used a pressure upstream of the instability
– The pressure build-up is upstream the liquid
blocking.
• A linear analysis/controllability analysis
– to see where the instability is and where it could be
observed
• What is the flow regime after stabilisation?
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Truls Larsson Slide 27
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Summary
•
•
•
•
Stability of flow regimes in pipelines
Stability of stratified and slug flow
Dynamic instability: severe slugging
Industrial stabilisation severe slugging
• Pointed to some open issues.
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Truls Larsson Slide 28
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Acknowledgements and Literature
• Inputs and assistance from:
– K. Havre
– K. Falk
– J. Morud
– Hugo
– T. Ytrehus
– O.J. Nydal
– M.S. Govatsmark
– V. Olaissen
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Truls Larsson Slide 29
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Literature
Bendiksen, K. and M. Espedal (1991): onset of slugging in horizontal gas-liquid pipe flow. Int. J. Multiphase flow vol. 18 no 2, pp 237-242
Bergles, A.E. (1981): instabilities in two phase systems.
Courbot, A. (1996): prevention of sever slugging in the dunbar 16” multiphase pipeline. Offshore technology conference, pp 1- 8
De henau, V. and G.D. Raithby. (1995): A study of terrain induced slugging in two phase flow pipelines. Int. J. Multiphase flow, vol. 21, no 3, pp 365379.
Falk, K. (2000): personal communications
Havre, K., H. Stray and K.O. Stornes. (2000): stabilisation of terrain induced slug flow in multiphase pipelines. Submitted to ABB review.
Hedne, P. and H. Linga (1990): suppression of terrain slugging with an automatic and manual riser choking. Advances in gas-liquid flow, pp 453-460
Lin, P.Y. and T.J. Hanratty (1986): A model for prediction flow regime transition
Mcnulty, G., C. Wordsworth and I. Dis (1999): predicting, detecting, and controlling slugs in pipeline-riser systems. BHR group multiphase 1999, pp
105-118
Pots, B.F.M., I.G. Bromilow and M.J.W.F. Konijn. (1987): severe slugging
Schmidt, Z., Brill, J.P. And beggs, H.D. (1979): choking can eliminate severe pipeline slugging. Oil & gas journal -pp 230-238.
Schmidt, Z., Brill, J.P. And beggs, H.D. (1980): experimental study of severe slugging in a two phase flow pipeline riser system. Soc. Pet. Eng. J. Pp
407-414.
Schmidt, Z., D.R. Doty, K. Dutta-roy. (1985): severe slugging in offshore pipeline riser-pipe systems. SPE J, pp 27-38.
Taitel, T. and A.E. Dukler. (1975): A model for flow regime transition in horizontal and near horizontal gas-liquid flows. Aiche J. Vol. 19 no3, pp 4755.
Taitel, T. (1986): stability of severe slugging. Int. J. Multiphase flow, vol 12, no 2, pp 203-217
White F.M. (1974): viscous fluid flow. Mcgraw-hill.
Xu, Z.G., P. Gayton, A. Hall and J. Rambeak (1997): simulation study and field measurement for mitigation of slugging problem in The hudson
transportation lines. BHR group multiphase 1997, pp 497-507
Yocum, B.T. (1973): offshore riser slug avoidance: models for design and optimisation. SPE European meeting.
Zheng, G., J.P. Brill and Y. Taitel (1994): slug flow behaviour in a hilly terrain pipeline. Int. J. Multiphase flow, vol. 20, no 1, pp 63-79
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Truls Larsson Slide 30
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