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