Transient multiphase flow modelling

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Transient multiphase flow modelling

• On the problem
• Multiphase flow regimes
• Transients in oil-gas pipelines
• On types of models
• Multi fluid models
• Mixed models
• Simplified slug flow models
• Example
• Terrain slugging simulationProst meeting 4 May
  2000, Ole Jørgen Nydal

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Multiphase flow regimes

• Separated gas-liquid flows Annular, stratified
• Mixed gas-liquid flows Slug, bubble
• Oil-water regimes Continuous change from
  separated to mixed
• Complex flows!

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Transients in oil-gas pipelines

• Operator induced
• Shut-in, Start-up, change in production
• Pigging
• Unstable flow
• Liquid slugging

Liquid fraction
Time
Terrain slugging, severe slugging
Hydrodynamic slugging
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Model needs and requirements

• Application
• Design capacity, stability
• Operational support forecasting, control,
  supervision
• Computation of phase fractions, pressure and
  temperature along pipeline and in time
• Holdup H H(x,t)
• Pressure P P(x,t)
• Temperature T T(x,t)
• 1 D models
• 1D balance equations
• Simplifications require empirical closure laws
• Phase distribution
• Friction laws
• Heat transfer

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Methodology

• General purpose models
• Formulate general balance equations
• Establish closure relations for each regime
• Apply suitable numerical method
• Discretize each case sufficiently small to
  resolve transients
• Improvements with front tracking techniques
• Specialized slug flow model
• Data structure slugs and bubbles
• Formulate balance equations for slugs and for
  bubbles
• Slugs liquid flow
• Bubbles separated flow
• Closure bubble propagation characteristics
• Apply slug initiation models and let them go...

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General purpose models

• Mass equations for each field
• Continuous phases
• Dispersed phases
• 2 fluid models
• Momentum equation for each field
• Drag relations
• Drift flux models
• Momentum equation for mixture
• Relation for relative velocity

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General purpose models Status

• OLGA2000
• World wide industry standard
• IFESINTEF Scandpower
• Features 3 phase, network, process control,
  front tracking
• Equations
• 5 mass equations (continuous phases droplets)
• 2 momentum equations Gasdroplets, oilwater
• 1 energy equation
• Subgrid front tracking
• Petra
• Modern OLGA, in progress
• Statoil (IFE)
• Equations
• 3 momentum equations Gasdroplets, oil, water
• Grid moving with fronts

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Simplified slug tracking model

• Bubbles and slugs as computational objects
• P in bubble from compressibility
• Uls from dynamic force balance
• Bubbles and slugs are open control volumes
• Bubble propagation velocity imposed Ub Ub(Uls)
  
• Bubbles covering bends are split into sections
• H, Ulb computed in each section
• Slugs are initiated after liquid accumulation in
  bend

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Simplified slug tracking model
Outlet pressure
BUBBLE
SLUG
New inlet slug
Inlet mass flows
New terrain slug after accumulation in bend
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Example terrain slugging
Outlet pressure
3m
Inlet mass flows
20m

• Laboratory set-up (Tulsa) I.D. 2.5cm, air-water
• Modifications
• Small tube connecting inlet and outlet
• PI controller on inlet pressure

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Gas tube I.D. 0.25cm
Position of slug tail (m)
Length of slug (m)
Holdup in riser
Inlet pressure (Pa)
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Gas tube I.D. 0.3cm
Position of slug tail (m)
Length of slug (m)
Holdup in riser
Inlet pressure (Pa)
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Gas tube I.D. 0.4cm
Position of slug tail (m)
Length of slug (m)
Holdup in riser
Inlet pressure (Pa)
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Control
Holdup in riser
Inlet pressure (Pa)