Flerfasestrøm Ved NTNU, Insitutt for Energi- Og Prosessteknikk 1998-2003

Multiphase flow. Dept. Energy and Process Technology, .NTNU

PhD

Objective

Study the flow of oil-water-air in pipes at very low liquid flow rates.

Background

The activity is part of the project Pressure Loss Reduction in Gas Pipelines. The question addressed in this task is whether pressure loss reduction methods can be applied to gas flows with small amounts of liquid, as occurring in gas-condensate pipelines.

Activities 2003

Experiments

Pressure drop and phase fraction measurements for three-phase air-oil-water flow were completed with a series performed at slightly upward inclination and higher liquid loading than before. This allowed to study gravity dominated three-phase flow: local air-oil separation, stratified-slug transition.

Modelling

Measurements performed in 2001-2002 were compared with one-dimensional prediction models for two-phase and three-phase flow. The simulator PETRA was also applied in the study. Three-layer models were extended to capture the preferred wetting of the wall surface by one of the liquid phases and the resulting curving of the fluid-fluid interfaces. This phenomenon is expected to play a role for three-phase gas-liquid-liquid flow at low liquid loading. The model was tested against experimental data (see pictures below).

The work is completed and thorougly documented in the thesis, which was defended on December 12th 2003.

Post Doc.

Objective

Improve and apply the slug tracking scheme for example : the pigging simulations.

Background

The slug tracking scheme is based on a formulation where bubbles are compressible units and slugs are incompressible units. As the implementation is object oriented it is well suited for extension to pigging simulations.

Activities

Simulations

A pig is implemented as a section in a slug and contributes with friction and gravity terms in the dynamic formulation of the incompressible slug unit. Slugs can be created in front of the pig when liquid is scooped up, or liquid in front of the pig can disappear in downwards flow when a bubble penetrates the slug and reaches the pig. The pig is initiated at a user given time and location, and propagates according to the mass and momentum equations of the series of bubbles and slugs in the pipeline.Computations have been compared with some pigging experiment made in the laboratory.

Experiments

Two types of experiments have been made. Start-up experiments, in which a pig blows out initially stagnant liquid residing in low parts of a pipeline system. Pigging in flowing systems, both stratified and slug flow.

Objective

Identify flow mechanisms which can cause unstable flow in wells.

Background

Oil companies report the occurrence of unstable flow in gas lifted oil wells when the liquid rate is low and the watercut reaches sufficiently high levels. The gas rate in these wells is constant, but the liquid inflow varies with the inlet pressure.

Activities

Density wave instability theory

One cause of flow instability can be density waves. The density can change locally because of local changes in the gas/liquid ratio, or in the oil/water ratio, or a combination of these two. The local density change can be transported through the system as a density wave. If the system is gravity dominated, the inlet pressure will vary as the density waves travel through parts of the pipeline with different inclinations. At certain flow and geometry conditions, resonance can occur with oscillating flows.

Experiments

An experimental set-up has been made in order to investigate the density wave mechanisms. A flexible test section is configured similar to a well, consisting of three parts: A horizontal section, an inclined section and a vertical section. Gas lift is provided in the inclined section and pressure dependent water and oil inlet flows are obtained with an overflow system.

Experiments with pure oil-water flow demonstrates that unstable flows can occur, and the flow oscillation shave been reproduced in a simplified flow model (implemented in Matlab).

The activity is a part of the Petronics project.

Pictures: Unstable oil-water flow in laboratory

PhD

Objective

Arrive at robust and efficient methods for transient multiphase simulations based on two fluid and/or drift flux formulations.

Background

A two fluid model is suitable for separated flow modeling, whereas a drift flux model, together with an algebraic slip relation, may be preferred for mixed flows. Flow in pipelines include both regimes, meaning that both a separated and a mixed formulation may be considered solved for all regimes.

Model based control and optimization need robust and simplified models and an objective of the work is to arrive it a suitable scheme for this purpose. The activity is a part of the Petronics project.

Activities

Activities have included include:

• Development of a direct numerical scheme with non-staggered grid
• Extending methods derived for single phase to two phase flows
• Testing of the scheme on benchmark problems with gas-liquid fronts
• Reporting in the form of 4 publications and a thesis

The defense of the PhD thesis took place on March 5th 2003.

Project

Objective

The objective of the work is to explore the potentials, and the limitations of computational methods for slug/pig tracking in pipelines.

Background

Instead of solving numerically the general two phase flow equations on a user specified grid, a mathematical formulation, and an object oriented implementation, is based on liquid slugs and gas bubbles as discrete, computational objects.

The exploratory work during the last years has showed that this type of modelling has potentials, and the further work is focusing on robustness and efficiency. Pascal completed his EU financed PostDoc period in mid 2003 and Gael completed his PhD period in mid 2003, and both continue as Post Docs within this slug tracking project.

Activities

Activities are:

• Increase efficiency, the methods is computationally too slow
• Increase robustness, in particular related to the management of the moving slug/bubble grid, and to the slug initiation methods.
• Incorporating methods developed by other PhD’s (Fabien, Trygve, Jørn)
• Comparisons with available data

Participants

Norsk Hydro, Statoil, Total

Picture: Post processing of simulation results

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