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 simulation Prost meeting 4 May 2000, Ole Jørgen Nydal. Stratified. Bubble.

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Transient multiphase flow modelling

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Transient multiphase flow modelling

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


Multiphase flow regimes

Stratified

Bubble

Slug

Annular

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!


Transients in oil gas pipelines

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


Model needs and requirements

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


Methodology

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...


General purpose models

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


General purpose models status

General purpose models: Status

  • OLGA2000

    • World wide industry standard

    • IFE+SINTEF Scandpower

    • Features: 3 phase, network, process & control, front tracking

    • Equations

      • 5 mass equations (continuous phases + droplets)

      • 2 momentum equations: Gas+droplets, oil+water

      • 1 energy equation

    • Subgrid front tracking

  • Petra

    • Modern OLGA, in progress

    • Statoil (+IFE)

    • Equations

      • 3 momentum equations: Gas+droplets, oil, water

    • Grid moving with fronts


Simplified slug tracking model

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


Simplified slug tracking model1

Simplified slug tracking model

Outlet pressure

BUBBLE

SLUG

New inlet slug

Inlet mass flows

New terrain slug

after accumulation in bend


Example terrain slugging

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


Gas tube i d 0 25cm

Gas tube I.D. 0.25cm

Position of slug tail (m)

Length of slug (m)

Holdup in riser

Inlet pressure (Pa)


Gas tube i d 0 3cm

Gas tube I.D. 0.3cm

Position of slug tail (m)

Length of slug (m)

Holdup in riser

Inlet pressure (Pa)


Gas tube i d 0 4cm

Gas tube I.D. 0.4cm

Position of slug tail (m)

Length of slug (m)

Holdup in riser

Inlet pressure (Pa)


Control

Control

Holdup in riser

Inlet pressure (Pa)


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