1. 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

2. 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!
Stratified
Bubble
Slug
Annular

3. 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

4. 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

5. Methodology General purpose models Specialized slug flow model
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...

6. 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

7. 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)
3 momentum equations: Gas+droplets, oil, water
Grid moving with fronts

8. 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

9. Simplified slug tracking model
Outlet pressure
BUBBLE
SLUG
New inlet slug
Inlet mass flows
New terrain slug
after accumulation in bend

10. 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

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

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

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

14. Control
Holdup in riser
Inlet pressure (Pa)