Transient multiphase flow modelling On the problem Multiphase

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

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! Bubble Slug Stratified Annular

Transients in oil-gas pipelines • Operator induced • Unstable flow – Liquid slugging Liquid fraction – Shut-in, Start-up, change in production – Pigging Time Terrain slugging, severe slugging Hydrodynamic slugging

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 – 1 D balance equations – Simplifications require empirical closure laws • Phase distribution • Friction laws • Heat transfer

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 • 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 • OLGA 2000 – – 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 • • • 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 model Outlet pressure BUBBLE SLUG New inlet slug Inlet mass flows New terrain slug after accumulation in bend

Example: terrain slugging Outlet pressure 3 m Inlet mass flows 20 m • Laboratory set-up (Tulsa) I. D. 2. 5 cm, air-water • Modifications – Small tube connecting inlet and outlet – PI controller on inlet pressure

Gas tube I. D. 0. 25 cm Position of slug tail (m) Holdup in riser Length of slug (m) Inlet pressure (Pa)

Gas tube I. D. 0. 3 cm Position of slug tail (m) Holdup in riser Length of slug (m) Inlet pressure (Pa)

Gas tube I. D. 0. 4 cm Position of slug tail (m) Holdup in riser Length of slug (m) Inlet pressure (Pa)

Control Holdup in riser Inlet pressure (Pa)