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Consortium on Transient and Complex

Multiphase Flows and Flow Assurance

TMF

PROSPECTUS – 2017

Managed by

Department of Chemical Engineering

Imperial College London

London SW7 2BY

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Consortium on Transient and Complex

Multiphase Flows and Flow Assurance (TMF)

PROSPECTUS - 2017

TABLE OF CONTENTS

Section Description Page

1 Executive Summary 3

2. Background 4

3 Modus Operandi 6

3.1 Participation 6

3.2 Project Selection 7

4. Research Capabilities And Expertise 8

5. Potential Projects 21

5.1 Gas-liquid flows 21

5.2 Liquid-liquid flows 22

5.3 Three-phase flows 23

5.4 One-dimensional models 25

5.5 Effect of additives 26

Appendix Previous TMF Research 27

A.1 TMF Programmes and the sub-projects undertaken within each

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A.2 Highlights from previous TMF programmes 25

A.2.1 Improvement of slug flow modelling and slug control 28

A.2.2 Annular flow 31

A.2.3 Phase behaviour in flowing systems 32

A.2.4 Thermal aspects of bundles 32

Solids transport and erosion 33

Understanding scale-up to industrial systems 33

A.2.7 Application of advanced measuring systems to multiphase flow

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A.2.8 Non-Newtonian and highly viscous fluids 38

A.2.9 Data library and technology transfer 39

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1. Introduction

A major issue in the oil-and-gas industry is the ability to predict sudden regime changes in multiphase flows following a change of parameters. Predictions of this kind, however, pose significant challenges since the flows involved have deformable interfaces interacting with turbulent fields that possess highly complex topology, and encompass a very wide range of length and time-scales. This major research challenge is addressed by the Consortium on Transient and Complex Multiphase Flows and Flow Assurance (TMF) in direct collaboration with the oil-and-gas industry.

The main aim of the Consortium is to enhance the fundamental understanding of industrially-relevant multiphase flows and, by so doing, improve safe and efficient handling of fluids in well bores, flowlines, pipelines, risers and piping (both on and offshore). Within the Consortium, this aim is addressed through a two-pronged approach: the creation and use of unique experimental facilities that enable data measurements to be performed under realistic, industrial conditions; and the development of computer codes which predict flow regime transitions, and individual flow regime properties.

Bringing together users, design organisation, computer code developers and suppliers, and renowned academics from multiple centres of excellence, the Consortium allows in-depth interaction between key experts with the aim of best improving the effectiveness and accuracy of the major commercial software packages. Research projects are chosen, funded and coordinated by individual sponsors with the general progress, results and deliverables shared with all participants. The knowledge within the TMF Consortium continues to lead to improved design and operation of flowlines, pipelines, risers and facilities in the oil-and-gas industry, with associated increase in process efficiency and reduction in risk and carbon footprint.

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2. Background

This Prospectus for the Consortium on Transient and Complex Multiphase Flows and Flow Assurance (TMF) gives the background to the TMF projects and describes the TMF modus operandi. TMF was founded by Professor Geoff Hewitt of Imperial College (winner of the Global Energy International Prize in 2007) in 1996 and comprises sponsors from oil companies, design-houses and code-developing organisations working directly with three university partners: Cranfield, Imperial College and Nottingham (http://www.tmf-consortium.org).

The systems being researched range from multi-component gas-liquid to gas-liquid-liquid-solid, including the presence of additives, with much of the experimental research using hydrocarbons. Past and current research themes include:

The development of world-leading experimental techniques to image multiphase flow regimes and measure their flow characteristics.

The development of cutting-edge numerical techniques for faithful simulation of complex, transient multiphase flows.

The development of effective, one- and two-dimensional models for efficient and reliable computation of large-scale features of multiphase flows and flow regime transitions.

The development of reduced-order models from limited information available on complex flows.

Elucidating the intricate coupling arising from complexities related to flow geometry and/or heat transfer, for instance, on the nature of multiphase flows in industrially-relevant situations.

Understanding the effect of additives, e.g. surfactants, or drag-reducing agents on the dynamics of multiphase flows.

The nature of the work ranges from the fundamental to the applied, and involves a blend of long-term projects (3 years involving Ph.D. students) and targeted, shorter term projects (1-2 years or less undertaken by postdoctoral research associates).

The TMF Consortium is under the direction of Professor Omar Matar1 (http://www3.imperial.ac.uk/o.matar) of the Department of Chemical Engineering at Imperial College London. Professor Geoff Hewitt remains on the TMF Executive Committee whose members are:

Professor Omar Matar (Chairman and Consortium Director)

Professor Barry Azzopardi (University of Nottingham)

Professor Gioia Falcone (Cranfield University)

Dr Buddhi Hewakandhamby (University of Nottingham)

Professor Geoff Hewitt (Founder and former Programme Director)

Dr Raad Issa (Imperial College London)

Dr Tim Lockett (BP, Chairman of the Participants’ Steering Committee)

Dr Lee Rhyne (Chevron, Deputy-Chairman of the Participants’ Steering Committee)

Mr. Colin Weil (Consortium manager2)

This Executive Committee meets quarterly. The partner universities each have a local TMF Committee with membership consisting of key academic and research staff.

There have been five successive previous TMF Projects as follows:

1Department of Chemical Engineering, Imperial College London, Email: o.matar@imperial.ac.uk, Tel: +44 (0) 207

594 9618, Mobile: +44 (0) 7979 243 026, Fax: +44 (0) 594 5638. 2 Email: colin.weil@sky.com

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Managed Programme on Transient Multiphase Flows (TMF1): 1996-1999.

Coordinated Project on Transient Multiphase Flow (TMF2): 1999-2002.

Joint Project on Transient Multiphase Flows (TMF3): 2002-2006.

Joint Project on Transient Multiphase Flow and Flow Assurance (TMF4); 2006-2009.

Joint Project on Complex and Transient Multiphase Flows and Flow Assurance (TMF5): 2009-2015

The continuation of work in the generic area of multiphase flows from one project to the next is an indication of the ongoing importance of the area to the oil and natural gas industry. The research priorities have been established throughout by intensive consultation with the industrial partners.

Two key features of the TMF projects have been the following:

a) Though all the TMF projects have been managed by Imperial College, an integral part of the research structure has been to involve major universities in the UK who have complementary experimental, theoretical and numerical expertise in the multiphase flow and related areas (e.g. Cranfield and Nottingham). More recently, the University of Birmingham, and UCL, have become affiliated with the TMF programme (through the MEMPHIS programme – see Section 4).

b) Funding from the UK Government has played a vital role in enabling the projects to address the industrial problems by providing support for study of the underlying engineering science. The majority of this funding has been provided through the Engineering and Physical Sciences Research Council (EPSRC) which includes several recently funded related multiphase projects detailed in Section 4.

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3. Modus operandi

3.1 Participation

There are three levels of sponsorship in the TMF Consortium:

A sponsor (e.g. operating companies) that is funding one or more projects is referred to as a “Full Sponsor”.

A sponsor (e.g. a company providing services to the Operating Companies, for instance, design-houses) that is paying a reduced fee is referred to as an “Associate Sponsor”.

A sponsor (e.g. an organisation developing and supplying design programs to the oil and natural gas industry) that is not paying fees but offering in-kind contributions is referred to as an “in-kind sponsor”. The level of contribution of “in-kind” sponsors should be commensurate with the fee paid by oil and natural gas companies. This value to be assessed and agreed by the members of the steering committee and, in particular, the Chairman of the Steering Committee or his nominated representative.

The structure for TMF projects is as follows:

Each project has a “Full Sponsor” TMF member company taking a leading role in the project (the “Lead Company” concept).

The funding of the project would come from a commitment by the Full Sponsor to participate in TMF. This commitment would typically be over the period of the Ph.D. study (3 years), but could alternatively be financing a Post-Doctoral researcher for a different (shorter or longer) duration.

The projects could start at any time.

The fees charged to the participant company would cover the employment cost of the student, cover costs for any associated experiments, plus a small additional charge to cover the management costs of having the project in the TMF Consortium. Note that for non-European Union students, the fees charged will be sufficient to cover the non-EU fees.

All the information generated in all the projects would be shared with the other member companies of TMF (including the “Associate”, and “In-kind sponsors”).

The benefits to the company would include not only the information on the project in which they are taking the lead, but also access to current and previous TMF projects, including experimental data.

In addition to receiving the added value of sharing the output from the whole of the TMF project, the Lead Company for a given project would have additional benefits which include:

Direct involvement in a project which they have identified as being of high priority.

Involvement in the selection of the student.

Participation at dedicated meetings with the researchers.

Potential involvement in publications.

Optional periods of attachment by the student to the company (subject to appropriate extension of Ph.D. studies, where appropriate).

Possible recruitment of the Ph.D. to the company in due course.

NOTE: The costs of participation are provided in an addendum to this prospectus. These fees are normally updated every 3 years following a 12-month notice period. Existing projects would not be affected by updated fees.

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3.2 Project selection

The steps involved in TMF participation by Full Sponsors are as follows:

1. The selection by the Lead Company of a research project for which the Company would take a leading role. To assist in this process, we have included a list of project themes in Section 3 of this prospectus. The selection could be directly from this list, based on a modification of a listed project or be a new topic nominated by the company (though, in the latter case, the project would still have to be in the general area covered by TMF).

2. There would be a negotiation between the company and TMF management to ensure that appropriate equipment and academic supervisors are available to support the project. The University partners in TMF have an extensive array of suitable experimental equipment available; large-scale rig facilities exist at Cranfield, Imperial, and Nottingham, and advanced instrumentation available includes electrical capacitance tomography (ECT) , wire mesh sensors, traversing and multi-beam gamma densitometers and X-ray tomography, particle image velocimetry as well as laser-induced fluorescence systems. Additionally, collaborations with other universities or centres of excellence would be sought if beneficial to the research. The optimal placement of the project will follow naturally from the project requirements and the staff and facilities available.

3. The signature by the company of the standard contract document.

4. The appointment of the student (or more senior researcher) to the project. It may be that the company would wish to nominate the researcher (many companies have schemes for appointing Ph.D. students, for instance) or the university partner within TMF may be able to suggest a suitable person. Advertising the post would also be possible in the case of a Ph.D. student; for postdoctoral research associates, advertising is mandatory.

5. When the researcher takes up the post, execution of the project would then commence. The lead company could participate in the project in a number of ways which could include:

a. Participation in discussing and agreeing the overall objectives and time schedule for the project.

b. Participation in review meetings on the project and/or receipt of notes on review meetings.

c. Hosting the researcher within the company for periods to pursue the project and related company objectives.

In each of the previous TMF programmes, in-kind participation has been included from organisations that can offer added value input in lieu of the fees. Such participation requires agreement from the existing sponsors (see also the information provided on “In-kind Sponsors” in 3.1 above).

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4. Research capabilities and expertise

In this section, we provide a brief overview of some of the TMF research capabilities and expertise at the various TMF university partners.

MEMPHIS is a £ 5 M EPSRC-funded programme into Multiscale Exploration of Multiphase Physics in Flows (http://memphis-multiphase.org/) which interacts directly with the TMF Consortium and offers all its research and deliverables to its sponsors. Led by Omar Matar at Imperial College; this is a 5-year joint project with Birmingham (Prof. Mark Simmons – TMF affiliate), Nottingham (Prof. Barry Azzopardi – TMF Exec. member), and University College London (Prof. Panagiota Angeli – TMF affiliate). The aim of this programme is to develop the next-generation of predictive tools for complex multiphase flows, working closely with the TMF Consortium and its researchers. MEMPHIS will achieve this goal by developing a single modelling framework that establishes, for the first time, a transparent linkage between input (models and/or data) and prediction; this will allow systematic error-source identification and, therefore, directed, optimal, model-driven experimentation, to maximise prediction accuracy. The framework will also feature optimal selection of massively-parallelisable numerical methods, capable of running efficiently on 105-106 core supercomputers, optimally-adaptive, three-dimensional resolution (see Figure 1), and sophisticated multi-scale physical models. This framework will offer unprecedented resolution of multi-scale, multiphase phenomena, minimising the reliance on correlations and empiricism. The modelling framework will be sufficiently general to address a number of TMF research challenges. The Programme will produce ready-for-use predictive tools that can be assimilated easily within existing industrial codes/software, minimising the time-to-impact.

The programme kicked off in January 2013 and results from the work undertaken as part of MEMPHIS have been presented at several TMF sponsors’ meeting. Outputs from MEMPHIS will continue to be made available to TMF sponsors.

Figure 1 - Adaptive meshing: density, adapted mesh and parallel domain

decomposition in a lock-exchange flow with Kelvin-Helmholtz billows

Centre for Doctoral Training at Imperial College is a £4.5M EPSRC and industry funded centre in Fluid Dynamics Across Scales (http://www.icfluids.org/) has Omar Matar as a Deputy-Director. It will train at least fifteen 4-year Ph.D. students per annum for 5 years in technical aspects of fluid dynamics including multiphase flows, and endow them with transferable and professional skills. The students will also have the opportunity to spend extended secondments with companies. The training provided will ensure that students will make a flying start in their industrial careers post-graduation from the CDT. The CDT started in October 2014 when the first CDT cohort were put in place. TMF companies can sponsor CDT students who can undertake research that falls under the TMF remit.

Gas-liquid and liquid-liquid flows

At Nottingham University experimental work on gas-liquid flows in large-diameter risers (67-127 mm diameter), led by Barry Azzopardi, has focused on slug, churn and annular flows (see Figure 2).

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Figure 2 - Gas-liquid studies in large-diameter pipes at Nottingham using wire-mesh

The studies at Nottingham, which were performed using a combination of wire mesh sensors, multi-point electrode film probes and electrical capacitance tomography (ECT) together with Phase Doppler Anemometry for drop size, velocity, concentration and entrained fraction, demonstrated the occasional absence of slug formation in pipes of sufficiently large diameter. However, wisps were identified in both of the pipe diameters studied (see Figure 3).

Figure 3 - The development of complex flow structures in gas-liquid vertical pipe flows

at Nottingham obtained using wire mesh sensors This research continues in order to determine time and space-resolved film thickness distributions and will contribute to our extensive database and allow analysis of the data to understand the processes underlying flow regime transitions and characteristics in vertical and inclined risers (see Figure 4). Currently, there is on-going work on rising bubbles in ‘heavy’ (high-viscosity) oils, horizontal films sheared by turbulent gas flows, and on understanding the mechanisms of slug flows in (relatively) small-diameter pipes. Recently added rigs allow investigations in gas-liquid two-phase flow transition around bends and liquid–liquid as

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well as three-phase flows. Development of Brightness Based Laser-Induced Fluorescence (BBLIF) at Nottingham enables the investigations into film dynamics and droplet formation and gas entrainment.

Figure 4 - Gas-liquid studies in large-diameter pipes at Nottingham using wire-mesh

sensors (left) and electrical capacitance tomography (right) At Cranfield University, the work led by Hoi Yeung, has involved the use of large-scale rigs to study gas-liquid flows in risers through a number of techniques including ECT. This work has been successful in constructing flow regime maps as a function of gas and liquid superficial velocities in large-diameter risers (see Figure 5).

Figure 5 - Flow regime map for gas-liquid flows in large-diameter risers at Cranfield

obtained through electric capacitance tomography

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The experimental research on slug flows and control theory work at Cranfield has led to the development of reliable controllers for flows that exhibit slugging. Feedback control stabilised open loop unstable system, and a controllability analysis provided guidance for measurement selection leading to patented inferential slug control particularly suitable for brown fields. This combines topside measurements to form a control parameter avoiding the use of riser base measurement. Following a successful field trial in North Sea, the production was shown to increase by 9 % over the trial period. At Cranfield, work has also been carried out on gas-liquid and liquid-liquid flows involving heavy oils. An example of this is provided in Figure 6.

Cranfield have also researched clamp-on ultrasonic techniques that are used to determine detailed velocity distribution in the slug body and film zone. The use of ultrasonics has the advantage over many other techniques of being non-intrusive. Results to date have compared favourably with PIV/PLIF as is shown in Figure 7.

Figure 7 - Slug body and film zone velocity distribution comparisons at Cranfield

The unique 44 m flow line with its 11 m riser at Cranfield not only enables the study of severe slugging as described above, but allows well research to be carried out simulating the effect of liquid loading both in vertical and horizontal wells (see Figures 8 and 9).

Key: ST Stratified CAFTnTWF Thin Top Water Film CAF Core Annular Flow OF Oil Film OLP Oil Lump

Figure 6 - High viscosity oil-water flow regime experimental and CFD comparisons at Cranfield

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Figure 8 - 4” and 2” Flowline/riser facility at Cranfield

Figure 9 - Configuration at the base of 2” riser

In contrast to pipe flows, multiphase flow in the annulus is also under research at Cranfield (see Figure 10). Preliminary work using a new air/water two inch rig (see Figure 11) has indicated that the correlations widely used for pipe flows fail to adequately predict hold-up, pressure gradient and flow regimes. The agreement depends on the choice of equivalent diameter. This is not surprising as the presence of the inner surface affects the flow development (see Figure 12).

Figure 10 - Slug flow in annulus at Cranfield

Figure 11 - Schematic of 2” annulus rig at Cranfield

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Figure 12 - Flow regime comparison at Cranfield of experimental

results versus Beggs and Brill correlation-based predictions

At Imperial College, the work led by Geoff Hewitt involved the use of a 32 m high-pressure water-air-sand flow facility (WASP, see Figure 13). The 3 inch test section is equipped with axial-view photography and an

Figure 13 - The WASP rig at Imperial College (top); axial-view visualisations of stratified/stratifying horizontal gas-liquid flows (bottom)

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X-ray system for an extensive study of slug flows in horizontal (and near-horizontal pipes). Of particular interest here is studying the development length of disturbances introduced at the inlet as a function of flow rates and inlet geometry. Other rigs at Imperial are also currently being used to study a variety of flow regimes in horizontal and near-horizontal gas-liquid flows: from wavy-stratified to annular flows.

Work on vertical flows in both co- and counter-current configurations is also being undertaken at Imperial College. In the former case, work on air-water upwards-annular flow using capacitance probes has demonstrated that disturbance waves become circumferentially coherent after a certain development length from the inlet (see Figure 14). In downwards-annular flow, a combination of laser-induced fluorescence (LIF), particle image velocimetry (PIV) and particle tracking velocimetry (PTV) developed by Dr Christos Markides yielded valuable information regarding the complex structure of the wall-bounded thin film and the velocity distribution within it (see Figure 15). The work on vertical gas-liquid counter-current flows led by Omar Matar commenced at the end of 2014 and will focus on determining the onset of flooding in the presence of surface-active additives.

Figure 14 – A demonstration of circumferential coherence in disturbance waves in air-water upwards-annular flows using capacitance probes by Imperial College

In addition to the experimental work at Imperial College, considerable progress has been made in modelling flow pattern transitions and flow-pattern specific (phenomenological) models for vertical flows. These models are embodied in the GRAMP computer code. A notable feature of these models is the recognition of the great importance of churn flow; in this regime, there is a continuous gas core but the liquid film (though on average flowing upwards) undergoes periodic flow reversals.

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Figure 15 - An investigation of downwards-annular flows using laser-induced fluorescence (left),

particle-image velocimetry (centre), and particle-tracking velocimetry (right) at Imperial College

Examples of TMF research in liquid-liquid flows include work carried out at Imperial College’s Two-phase Oil Water Experimental Rig (TOWER) facility (see Figure 16). This work, led by Dr Christos Markides, uses LIF, PIV and PTV to determine the flow structure and velocity distribution in each phase (see Figure 17).

Figure 16 - Two Phase Oil Water 7 m long, 27.5 mm ID Experimental Rig (TOWER) at Imperial College

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Figure 17 - A study of oil-water horizontal flows using laser-induced fluorescence at Imperial College

Computational modelling

At Cranfield University and Imperial College, TMF research has also involved the development of sophisticated multi-dimensional computational techniques, as well as one-dimensional (1D) methods for the prediction of how interfacial waves develop into slugs and how these slugs subsequently travel along the test section, entraining gas at their front. The higher-dimensional and 1D work is led by Chris Thompson at Cranfield University and Raad Issa at Imperial College Mechanical Engineering, respectively. In fact, one of the main successes of previous TMF programmes (see Section 5) has been the application of the 1D model to simulate the initiation and development of slug flow in horizontal and nearly horizontal pipes (so called “slug capturing” technique). The technique has been proven to predict slug flow characteristics (such as slug length and frequency) with remarkable accuracy. In Figure 18, examples are shown of the predictions provided by the 1D models developed at Imperial (the TRIOMPH code from the group of Raad Issa), and Cranfield (the EMAPS code from Chris Thompson's group). Slug capturing techniques are now being adopted within commercially available codes available to the oil-and-gas industry including Ledaflow, PROMPT and MAST. Furthermore, work from the TMF programme also contributes data which can be used to evolve the slug tracking capability of OLGA. The modelling work on gas-liquid flows has also involved the use of computational fluid dynamics (CFD) techniques to understand the behaviour of entrained droplets in the turbulent gas phase of stratified/stratifying flows.

Figure 18 - 1D models based on two-fluid theory: predictions of slug formation in vertical flows using TRIOMPH (Imperial, left), and horizontal flows using EMAPS (Cranfield, right)

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At Imperial College's Mechanical Engineering Department, the team led by Raad Issa is concentrating on delivering a methodology for reliable calculation tools for the multiphase flow in pipelines to be employed in order to optimise designs of pipelines and topside processing facilities. Current flow calculation methods are based mainly on empirical formulae and much simplified analyses and hence fail to provide the desired degree of certainty. In particular, they are unable to yield accurate time-variations of the flow associated with the most problematic regime of slug flow, yet it is these variations that are needed for designs which are cost effective.

Multi-dimensional multiphase CFD simulations for pipelines that are several kilometres long are prohibitively expensive computationally with current high-performance computing resources. Present calculation methods are therefore based on a one-dimensional approach and may be classified into two categories: steady-state and slow transients. The first generation methods cover the entire range of two-phase flow patterns (on an empirical basis), but is only suitable as a rough-guide to obtaining pressure drop and hold-up in the pipes. They are not able to cope with imposed changes in inlet flows, e.g. bringing an additional well on line or well shutdown as they assume a steady flow to prevail at all times. Second generation methods comprise more sophisticated mechanistic models that use different sub-models for each flow pattern and account for the unsteady flow behaviour that occurs over a long timescale. These are based on the two-fluid model which is solved numerically on a coarse grid with each pipe segment being of the order of several metres (or scores of metres). However, those approaches cannot deal with the rapid intermittency that occurs naturally as part of a nominally steady state flows, e.g. slug flows. Such intermittency can drastically change the flow properties, including changes in flow regime, as the flow encounters variation in pipe inclinations. Moreover, the methods, invoked to determine the prevalent flow regime in each section of the pipe for the calculations to proceed, are often empirical and/or based very loosely on physics. The effect of inclination is handled by interpolation between forces arising in horizontal and vertical pipes. This crude approximation compounds the uncertainty of multiphase analyses especially because the flow can change significantly with angle of inclination.

To develop a successful prediction methodology requires an approach that is time resolved to allow for the possibility that more than one type of structure can occur at any set of flow rates. Moreover, it is desirable to develop this capability with a view that it can ultimately be used readily as a tool for analysis in real engineering applications. To this end, a one dimensional model is currently the only practicable approach considering that the applications envisaged are in risers that can be several kilometres long which cannot be tackled efficiently using multidimensional approaches with current computer resources. For these reasons Imperial College are advocating the flowing modelling approach.

Figure 19 - One-dimensional two-fluid model using a fine mesh

numerical solution by Imperial College

The one-dimensional two-fluid model has been shown (using fine mesh numerical solution) to predict the transition from the stratified to slug regimes and from the stratified to annular flow in horizontal pipes in a seamless manner without arbitrarily assuming the nature of the flow regime a priori (see Figure 19). What is significant is that inherent flow unsteadiness and intermittency can be simulated naturally as an outcome of the numerical solution of the governing conservations equations (for momentum and mass for each

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phase). The flow characteristics in both the slug and annular regimes were predicted to a good degree of accuracy, which is especially remarkable in slug flow as the slug initiation and subsequent development and transport were all captured automatically. This capability was extended to three-phase flow as well where oil and water stratification and dispersion can also be predicted (see Figure 20).

Figure 20 - Thee phase flow where oil and water stratification and

dispersion can be predicted by Imperial College

The same methodology is being applied by Raad Issa's team to vertical flow where inherent unsteadiness can be a dominant feature (namely for churn and slug flow). Whereas the flow unsteadiness in horizontal pipes is primarily driven by the Kelvin-Helmholz instability, in vertical pipes, intermittency may be generated within the pipe by liquid fall-back possibly leading to flooding of the pipe (slug formation), (as shown in Figure 18), or to a rapid succession of large waves moving in an oscillatory manner (churn flow). The flow entering the pipe may already be intermittent and this intermittent behaviour can be sustained up the vertical pipe. Both of these phenomena can also, in principle, be captured automatically by the two-fluid model without a priori assuming the nature of the regime. This indeed has been shown by preliminary calculations performed at Imperial College.

Drag-reducing and surface-active agents

Imperial College's Chemical Engineering Department has recently built a dedicated rig for the study of drag-reducing agents (DRAs) of interest to the oil industry. The rig is a once-through design capable of achieving very high Reynolds numbers. The liquid is forced through the test pipe from a gas-pressurised vessel. This avoids the use of a pump which may degrade the DRA (see Figure 21).

Figure 21 - Results from a study of polymeric drag-reducing agents (DRAs) at Imperial College

showing the DRA's effect as a function of their concentration (top), and associated near-wall flow field information (bottom)

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More recently, the effect of surfactant additives on the dynamics of multiphase flows has been investigated. The work carried out at Imperial College, led by Omar Matar, focused on the effect of certain defoamers on the dynamics of stratified/stratifying gas-liquid flow in horizontal tubes. This flow regime is characterised by the thin liquid films that drain under gravity along the pipe interior, forming a pool at the bottom of the tube, and the formation of large-amplitude waves at the gas-liquid interface. This regime is also accompanied by the detachment of droplets from the interface and their entrainment into the gas phase. The study involved carrying out an experiment involving axial- and radial-view photography of the flow and demonstrated that increasing effect of defoamer concentration leads to the promotion of wavy flow at lower gas flow rate for fixed liquid flow rate (see Figure 22). This study also demonstrated that the average size of the entrained droplets decreases with increasing defoamer concentration (not shown). Further studies of the effect of surfactant on multiphase flows (e.g. vertical co- and counter-current flows, as mentioned above) are currently on-going.

Figure 22 - The effect of defoamer concentration on flow regime transitions in horizontal gas-liquid flows by Imperial College

Sand transport and erosion

Prediction methods for sand transport and accumulation are often based on correlations derived from hydro-transport studies. Solid concentrations in these studies are normally much higher than those found (when sand screens are working), in oil production. Limited work has been carried out in a previous TMF project on single solid particles. Recent studies carried out using the Cranfield University rigs under the lead of Hoi Yeung (see Figure 23) have demonstrated that fluid properties (predominantly viscosity) have a major influence in the transportation or accumulation of sand particles. As the viscosity of the carrier fluid increases, the lack of turbulence invalidates correlations derived from water based experiments. The presence of water also complicates matters further, as water-wetted sand could behave very differently from that in the absence of water. There is also very little information in the open literature on the behaviour of this water/sand mix in flowing oil; this will be the subject of future TMF research.

A further topic of interest to the oil industry and the TMF Consortium is that of sand-driven erosion. In gas pipelines with a small amount of liquid present, the flow regime is annular with a liquid film on the surface of the pipeline. A lot of work has been reported in the literature on how to improve sand erosion prediction models. These models rely on properties of sand i.e. concentration, particle distribution and velocity. Thus, accurate estimation of the distribution of sand in the gas core (travelling with the liquid drops) and liquid film is required. It is likely that the erosion mechanism for sand moving with the liquid film is different from that moving in the gas core. The objective of future TMF work in the area of sand-driven erosion is to carry out detailed investigation of sand behaviour in both the film and the gas core. This work will likely involve bends of varying geometry since they are the most vulnerable to erosion in industrial application because the change in flow direction moves the denser liquid/solid particles towards the outer wall increasing the collision frequency. The rate of erosion (or bend wear) should depend on the surface material's properties (base material and/or lining) the liquid/solid particles in the suspension, the flow velocity and particulate density (liquid droplet/solid to air ratio), particle shape and size distribution and the impact angle. Liquid

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droplet impingement cause erosion but it could also form a film on the outside of the bend. This can absorb the impact of the particles dissipating the energy. There is also the possibility of abrasion from solid particles being dragged over the wall. Furthermore, each of these mechanisms for mechanical damage may interact with corrosion processes in a system where the fluid interacts chemically with the pipe wall.

Figure 23 - Sand transport rigs at Cranfield University

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5. Potential projects

Projects for research within TMF could be chosen directly from this list or, can be based on a modification of a listed project; or it could be a new project nominated by the company (though, in the latter case, the project would still have to be in the general area covered by TMF).

5.1 Gas-liquid flows

CFD modelling of slug flows: With the increasing power of computers and with the growing sophistication of codes capable of carrying detailed interface tracking in multiphase flow systems, it is clear that CFD is now capable of calculating situations which are much more complex than those addressed in earlier work. Specifically, it is now possible to make full calculations of developing slug flows, showing how the interfacial waves in stratified flow develop into slugs and how these slugs subsequently travel along the test section, entraining gas at their front. The deliverables proposed here are aimed at exploiting and developing this technology. The first step in this project will be to carry out sets of calculations relating to existing TMF data sets on developing slug flow. The aim will be to examine the methods not only from the point of view of the (already established) qualitative capability, but also on the predictions of quantitative data such as the evolution of slug frequency and velocity with length along the tube. One of the main objectives is to assess the effect of choice of turbulence model on the numerical predictions; this will be explored by carrying out a limited range of calculations with advanced models and the results will be compared with those obtained using simpler RANS methods.

Large diameter systems: The experiments carried out on the larger diameter riser at Nottingham have highlighted some unexpected features of churn and annular flows. They have provided a very useful data base. They have also identified what additional measurements are necessary. This project extends the previous work by applying new instrumentation to determine time and space resolved film thickness. The deliverables will contribute to TMF's and Nottingham's extensive database and allow analysis of the data to understand the processes occurring. Experiments will be carried out on pipes vertical and inclined at 10° to the vertical. Bubbly/annular/churn flow will be examined. 5 and 20 mPa·s silicone oils will be used as the liquid and nitrogen and sulphur hexafluoride as the gases. Wire Mesh Sensor, multi-point electrode film probes and Electrical Capacitance Tomography will be applied together with Laser diffraction for drop concentration and entrained fraction. Further to these, Nottingham have developed experimental facilities to investigate the influence of fittings such as bends on two-phase flows. The system uses compressed air and 5 cP silicone oil as the working fluids. Electrical Capacitance Tomography and Capacitance based Wire Mesh Sensors are used together with a number of axially distributed pressure measurements.

Stratifying annular flow: In the work proposed, the Imperial College WASP facility (a high pressure multi-phase flow facility) will be used to study stratifying annular flow in a horizontal 79 mm diameter pipe. The facility is already being used for air-water stratifying annular flows; in the proposed project, the measurements would be extended to include air-oil two-phase flows and air-oil-water three phase flows. In addition to the use of axial view photography for observation of the flow structure, the deliverables for this project would also include measurements of droplet mass flux, liquid film flow rate distribution and pressure gradient. In the air-oil experiments, the effect of adding an additive would be investigated; what would be the effect of such an additive on the entrainment and deposition processes?

Droplet/turbulence interaction: The motion of droplets in stratifying annular flow in horizontal and inclined pipes is highly complex. However, this motion is of considerable practical importance since liquid transport (in the form of droplets) to the upper part of the pipe plays a crucial role in transporting corrosion inhibitor (in the liquid) to that part of the tube. The droplets may predominantly develop from the thick liquid layer at the bottom of the pipe and the trajectories of the droplets depend on the initial release velocity and release direction and also on the turbulent field in the gas core. An important aspect of the problem is the distribution of effective roughness around the core/wall liquid layer interface and this provides an additional challenge to the turbulence modelling. In this proposed Ph.D. project, the problem would be modelled using a variety of turbulence models (RANS, LES etc.) and the deliverables would include conclusions towards the optimum choice of model. Other applications of the modelling to a number of practical situations would be explored.

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Entrainment rate measurements: Rate of entrainment and deposition are essential closure relationships for churn and annular flow models. However, they are very difficult to measure, particularly in larger diameter pipes. The available data base is dominated by air/water information. A very powerful technique, developed some years ago, used tracers in the liquid (dyes or salts) to obtain these rates. Measurements were made of concentrations in the film. It is proposed to extend the technique to silicone oils which have a surface tension in line with hydrocarbon oils and can be obtained in a range of viscosities. The proposal is to use liquids with viscosities of 5 and 20 mPa·s in an existing large riser facility to get deliverables that are directly relevant to hydrocarbon services. A challenge to be sorted initially is the selection of a tracer (a dye) which is detectable and removable from the (expensive) silicone oils.

Experimental determination of the effect of turbulence on small drops: Questions have been raised as to the correctness of the effect of the drop/turbulence interaction used in CFD codes. It is difficult to identify the motion of drops under turbulence from those travelling under ballistics. One technique could be to set up a stratified/atomising flow in a horizontal pipe with a cylindrical insert (an annular geometry). The central cylinder would intercept drops, created at the bottom, which travel on ballistic paths. Only those drops moving by interaction with turbulence would reach the annular gap above the central cylinder where measurements would be made using Phase Doppler anemometry. The results would be compared with predictions of a CFD code.

Liquid accumulation in pipelines and wells: In upwards pipeline flow of gas-liquid mixture (both in vertical systems and upwards inclined systems), a certain gas velocity is required to carry the liquid upwards, at gas velocities less than this, liquid accumulation takes place and the liquid phase may block or partially block the pipe. An extreme case of this is “well death” where the liquid head building up in the well tubing blocks the passage of gas into the well and hydrocarbon production ceases. In the proposed project, liquid build-up would be studied for a range of inclination angles and the proposed deliverable would be to develop a predictive model to predict the occurrence of accumulation and to give guidelines on operational strategies to minimise it. The outcomes from this project will lead to an improvement of the methodology employed currently in the oil-and-gas industry.

5.2 Liquid-liquid flows

Liquid-liquid and liquid-liquid-solid flows: The project proposed here addresses both experimental measurements on, and modelling of, liquid-liquid and liquid-liquid-solid systems with the aim being to promote a synergy between the experiments and models so as to achieve an optimum modelling strategy. The proposed experimental work will be based on the use of the Imperial College's TOWER (Two-phase Oil Water Experimental Rig) facility. The key measurement technique used will be Laser Induced Fluorescence (LIF). In parallel with the experimental work, modelling studies will be performed; for liquid-liquid flows these will focus on the development of one-dimensional two-fluid models, the flows being treated as consisting of two layers with an aqueous-continuous layer at the bottom of the pipe and an oil-continuous layer at the top. Oil drops are dispersed in the aqueous layer and water droplets are dispersed in the oil continuous layer; interfacial entrainment and capture of the drops is included in the model and methods based on the experimental data will be used to describe these processes. In the modelling of liquid-liquid-solid systems, the main focus will be on improving the accuracy of multi-dimensional CFD predictions of both Eulerian and Lagrangian models.

CFD modelling of dispersed flows: Liquid-liquid and liquid-liquid-gas flows involving emulsions are commonly encountered in the oil and natural gas industry. Such flows are highly complex and their properties are, so far, beyond the reach of detailed models. Nevertheless, the engineering challenge of dealing with such flows remains and the objective of this proposed project is the development of practical CFD models which deal with the flows at a sufficient level of detail to yield useful results and which can be used as predictive tools for both simple and complex geometries. This work will develop new RANS-type models which will predict the effective mean physical properties (e.g. viscosity) by considering local emulsion properties. They will also develop models of the interactions between the fluid turbulence and the emulsion. These will be used to calculate the development of the emulsion – in terms of drop size distributions et al and the damping of turbulence by eddy-drop interactions.

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Prediction of flows involving highly viscous liquids: As the fields with less viscous oils are depleted, hydrocarbon liquids of higher and higher viscosity are being encountered. Where the viscous oil is co-existing with an aqueous phase, then the behaviour of the aqueous phase may be crucial in governing the flow behaviour. A good example here is that of core-annular flow of oil and water where a surrounding water layer (in contact with the wall) encloses a viscous oil phase (flowing at the centre of the pipe). The objective of the project proposed here is to develop CFD methods for predicting such flows. A crucial issue is the prediction of the occurrence and consequences of interfacial instabilities which occur at the phase interfaces and exert a strong influence on flow behaviour. The study will also consider the effects of variations in flow rates, volume fraction and inclination. A critical issue for selecting the core-annular flow transport mechanism for operations is the ability to restart a pipeline which was operating in core-annular flow but which is now not flowing. Important questions that need to be addressed include: how does the core annular pattern get re-established from a stratified starting point? Can one only ever generate core-annular patterns by introducing the phases in a core-annular way at the inlet?

5.3 Three-phase flows

Modelling of solid deposition and removal processes: Large temperature variations often destabilise crude-oils triggering phase separation phenomena leading to deposition of certain components of the crude and subsequent fouling of solid surfaces. The mechanisms of deposition, removal and ageing of the deposit are not yet fully understood. Currently, models for crude-oil fouling are mostly based on empirical or semi-empirical correlations with relatively poor understanding of the physics involved. It is therefore essential to accurately predict and understand the underlying mechanisms. The work in this project is aimed at developing capabilities to simulate accurately, reliably and efficiently the spatio-temporal evolution of the fouling process as a function of the chemical, physical and thermal characteristics of the system. They will adopt an approach combining the mass momentum and energy conservation with a chemical equilibrium model based on the Gibbs free energy to account for the formation of the deposit and with a rheological model for the deposit to consider “ageing-type” phenomena. Two distinct models will be developed: simple models based on the use of lubrication theory and an integral method to derive a single, non-linear evolution equation for the interface between the deposit and the overlying laminar or turbulent core in the presence of mass exchange between the two fluids (resulting in deposition); another, more complex model, which employs the Diffuse Interface Method (DIM) coupling the conservation equations with a convective Cahn-Hilliard equation for the volume fraction to analyse the flow dynamics. The DIM allows us to account for changes in interface topology which will model portions of the deposit being sheared off via interactions with the laminar or turbulent core. Again, phase change and ageing phenomena will be incorporated in the latter model.

Sand transport and accumulation: Prediction methods for sand transport and accumulation is often based on correlations derived from hydro-transport studies. Solid concentrations in these studies are normally much higher than that found (when sand screens are working), in oil production. Limited work has been carried out in a previous TMF project on single solid particles. Recent studies carried out at Cranfield have demonstrated that fluid properties (predominantly viscosity) have a major influence in the transportation or accumulation of sand particles. As the viscosity of the carrier fluid increases, the lack of turbulence invalidate correlations derived from water based experiments. The presence of water complicates matter further as water wetted sand could behave very differently. There is very little information in open literature on the behaviour of this water/sand mix in flowing oil. The prime aim of the project is investigate the effect of viscosity and water on sand transport and accumulation. The work will initially be based on visual observation and image analysis. Attempts will be made to develop new prediction models which could account for the effect of viscosity and water.

Three phase flows in risers: Much of the work on risers has concentrated on two phase (gas and liquid) flows. Very little has been reported on three phase (gas, oil, water) flows. Examining the data collected during the slug control project executed in previous TMF programmes have revealed the following: a) during the slug build up stage, oil and water could separate in the riser resulting in an oil slug followed by a water slug hitting the topside facility, b) the total pressure gradient (gravitational + frictional) is related to water cut and there is an optimal water cut when it reaches a maximum pressure gradient. Both effects could have serious implications on design and operation especially to deep risers. The aim of the project is

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to carry out more detailed investigations to establish both phase distribution/evolution and pressure drop information over a range of water, oil and air combinations. Wire mesh, ECT and fast gamma densitometers are the tools used to examine local values, in addition to differential pressure and high speed videos. The results will also be made available to validate the latest version of multiphase simulators or CFD simulations.

Horizontal annular flow with sand: There are two main concerns with the presence of sand a) sand deposition leading to blockage and corrosion and b) erosion. In gas pipelines with a small amount of liquid present, the flow regime is annular with a liquid film on the surface of the pipeline. A lot of work has been carried out to improve sand erosion prediction models. These models rely on properties of sand i.e. concentration, particle distribution and velocity. Thus, accurate estimation of the distribution of sand in the gas core (travelling with the liquid drops) and liquid film is required. It is likely that the erosion mechanism for sand moving with the liquid film is different from that moving in the gas core. The objective of this project is to carry out detailed investigation of sand behaviour in both the film and the gas core. A special extraction device will be developed to extract the liquid film from around the annulus to determine both the film flow rate and sand loading. Film velocity will also be measured using a tracer technique. After the film is extracted, liquid droplet and sand particle behaviour will be determined using a laser based technique (as the pipe wall is now clear of liquid) and sampling. It is expected that work will be carried out on the existing 4 inch flow loop and cover a range of total sand loading, distribution, air and liquid velocities.

Downward inclined pipelines: effect of undulating pipelines: Two-phase flow research to date has been concentrated on horizontal and vertical up flows. Realising the fact that real pipe lines are not horizontal, limited work on effects of inclinations has been carried out. However most of the test arrangements are designed to investigate ‘developed’ flow behaviour. The effects of change in inclination have been mostly ignored. Gas phase could collect at the top of a ∧ blocking the passage of the liquid and increasing the pressure loss of the system. In the opposite manner, a heavier phase could collect at the bottom of a ∨leading to local corrosion. It is proposed to carry out detailed investigations of the flow behaviour of the ∧ and ∨ sections. Conditions for purging the accumulated phase will be established. Tests will be carried out using the 20 m long, 4 in inclinable facility (up to 20 degrees) at Cranfield for low inclination. Vertical and near vertical flows, i.e. ∩ and ∪ , could be studied by modifying the existing 4 in Serpent rig. It is envisaged that some CFD simulations will be carried out for selected situations. One of the outcomes of the study would be developing relationships between flow, geometric parameters and development length. An additional deliverable would be a better understanding of the local flow behaviour which would give insight into phase distribution, thus assisting in improved removal of the heavier phase.

Internal multiphase flows and structure interaction: Cranfield has recently carried out some preliminary work on the prediction of slug forces using the OLGA/CFD coupling tool created by CD Adapco. The results to date have been promising as the forces predicted on a horizontal 90 degree bend compared well with the test data collected as part of the TMF2 programme. This work points to the possibility of using 1D simulation tools to predict the behaviour of a long flowline and using CFD to simulate local fluid behaviour. Two point coupling (pipe (1D) – component (CFD) - pipe (1D)) will be developed. One typical example to study will be a subsea flowline going over a pipe support. There had been reported incidents that the forces (inertia, pressure and buoyancy) induced by slug flows cause excessive bouncing of the flowline which could lead to fatigue failure. Force characteristics will be extracted from simulation as input to fatigue analysis. Another example is to use to the approach to examine the effects of restrictions, e.g. an orifice in a pipe. This will enable companies to design these systems with confidence.

Solid impingement/erosion on bends: Bends in flow lines are the most vulnerable for erosion because the change in flow direction moves the denser liquid/solid particles towards the outer wall increasing the collision frequency. The rate of erosion (or bend wear) then should depend on surface materials properties the liquid/solid particles in the suspension, the flow velocity and particle density (liquid droplet/solid to air ratio), particle shape and size distribution and the impact angle. Liquid droplet impingement cause erosion but it could also form a film on the outside of the bend. This can absorb the impact of the particles dissipating the energy. There is also the possibility of abrasion from solid particles being dragged over the wall. Investigations will be carried out using an atmospheric pressure rig fed by an air blower with drops or solid particles being injected towards the bend. In house equipment will capture particle trajectories to

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infer all three velocity components in drop/particle suspensions and give their momentum and impact angle. Erosion will be determined by a number of techniques including making bends in plaster.

5.4 One-dimensional models

Phenomenological modelling of pipeline flows: In the TMF and related projects, considerable progress has been made in modelling flow pattern transitions and flow-pattern specific (phenomenological) models for vertical flows. These models will be incorporated into the GRAMP computer code as a demonstration of their methods and impacts so that commercial codes may benefit from these developments. A notable feature of these models is the recognition of the great importance of churn flow; in this regime, there is a continuous gas core but the liquid film (though on average flowing upwards) undergoes periodic flow reversals. The work in this project will focus on the effect of inclination angle on churn flow and the associated transition models.

Modelling of vertical slug flow using the one-dimensional multi-fluid model: One of the main successes of previous TMF programmes has been the application of the 1D model to simulate the initiation and development of slug flow in horizontal and nearly horizontal pipes (so called “slug capturing” technique). The technique has been proven to predict slug flow characteristics (such as slug length and frequency) with remarkable accuracy. The aim of the project is to develop a similar methodology to predict slug flow in vertical risers where the mechanism responsible for generating slugs is quite different in nature from that responsible for initiating hydrodynamic slugs in horizontal pipes. In vertical flow a falling liquid film leading to flooding of the pipe is thought to be responsible for slug formation rather than the Kelvin-Helmholtz type instability prevalent in horizontal pipes. The project is intended to deliver a unified model for predicting intermittent and slug flows in both horizontal and vertical pipes in a seamless manner and the work will explore how far the 1D methodology can be taken to predict slug flows in general. The research will make use of experimental data generated by Nottingham University in previous as well as current TMF projects especially in large diameter risers.

One-dimensional modelling of intermittent flow: The project objectives are to investigate the applicability of the 1D multi-fluid model to the simulation of intermittent flow in vertical risers and its ability to predict the onset of such regimes, particularly that of churn flow in transient flow situations. Recent computations in vertical flow using the 1D model, have shown that with suitable numerical techniques, it may be possible to capture, in a natural way, liquid flow reversal leading to intermittent flow. This is thought to be a precursor to churn flow, which in reality exhibits a very similar behaviour. The project will explore the development of this “intermittency capturing” capability of the model and develop a general methodology that can be integrated with the slug capturing technique to yield a unified method for the prediction of slug and intermittent flow in a seamless manner. The work will examine suitable closure relations for the model that are appropriate to this type of flow and will engage in an extensive validation exercise against available experimental data, including those generated at Nottingham University.

Generic 1D model: One-dimensional models continue to be the most practical means of predicting transient multiphase flow in long pipelines. However, there are several limitations, some of which are severe, affecting the reliability of the models employed. In particular, existing methods for the determination of which flow regime prevails at any location in the pipeline are somewhat ad-hoc and are not entirely based on sound foundations. Based on those criteria, different closure relations are used to represent the flow regime that is determined to exist. The proposed project aims at developing a general methodology for the prediction of flow regimes and transition between those based on mechanistic principles. The concept is based on the solution of additional transport equations for dispersed elements of the liquid phase in the continuous gas phase and for the dispersed elements of the gas phase in a continuous liquid phase. In this way, all flow regimes can in principle be accounted for: e.g. bubble, annular mist and stratified flow. For each of these equations, closure models are required, in particular for the entrainment and deposition rates of phases from one stream to the other. These are to be formulated from existing phenomenological models some of which have been developed and are being developed at Imperial College (Chemical Engineering). Improvements to the closure relations embodied in the basic model such as accounting for velocity profiles (as introduced for example in the Biberg model) will also be undertaken.

5.5 Effect of additives

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Experimental evaluation and modelling of Drag Reducing Agents (DRAs): The Chemical Engineering Department at Imperial College has built a special rig for the study of DRAs of interest to the oil industry. The rig is a once-through design capable of achieving very high Reynolds numbers. The liquid is forced through the test pipe from a gas-pressurised vessel. This avoids the use of a pump which may degrade the DRA. The objective of the work proposed in this project would be to modify this facility so that it can be used with two-phase gas-liquid flow. Though some work has been done on drag reduction in multiphase flow, this work has been very limited and the use of this unique facility should allow a much more thorough investigation. The results would be analysed using appropriate models such as the multi-fluid model.

Experimental evaluation of the effect of surfactant on multiphase flows: We consider the dynamics of a stratified/stratifying gas-liquid flow in horizontal tubes. This flow regime is characterised by the thin liquid films that drain under gravity along the pipe interior, forming a pool at the bottom of the tube, and the formation of large-amplitude waves at the gas-liquid interface. This regime is also accompanied by the detachment of droplets from the interface and their entrainment into the gas phase. This regime involves carrying out an experimental study involving axial- and radial-view photography of the flow, in the presence and absence of surfactant. The aim is to show that the effect of surfactant on the dynamics in terms of the average diameter of the entrained droplets, the characteristics of the interfacial waves, and the pressure gradient that drives the flow. This study is extensible to a range of multiphase flows.

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Appendix

Previous TMF research

The following is a list of the previous TMF programmes and their projects, followed by highlights of their deliverables. Participants in the TMF Consortium have access to the theses, reports and presentations from all the previous TMF Programmes.

A.1 TMF Programmes and the sub-projects undertaken within each

TMF1 - Managed Programme on Transient Multiphase Flows (1996-1999)

TMF1 was jointly funded by the oil industry, its contractors and by the UK Engineering and Physical Sciences Research Council (EPSRC). The research was organised in six projects carried out at two universities: Imperial College London and Cranfield:

Transient Three-Phase Flows (Geoff Hewitt, Imperial College).

Phase Interactions in Transient Stratified Flow (Geoff Hewitt, Imperial College).

Slug Growth and Collapse in the Flow of Gas-Liquid Mixtures in Pipes (Raad Issa and Geoff Hewitt).

Flexible Risers Severe Slugging (Hoi Yeung, Cranfield University).

Numerical Techniques for Simulating Multiphase Flow (Chris Thompson), Cranfield University.

Transient Mass Transfer in Co-existing Hydrocarbon Liquid and Gas Flows (Stephen Richardson, Imperial College).

TMF2 - The Co-ordinated Project on Transient Multiphase Flows (1999-2002)

Funded jointly by the oil industry, its contractors and by the EPSRC. The research was organised in seven projects carried out at four universities: Imperial College London, Cranfield, Nottingham and Cambridge:

Modelling Bases (Chris Thompson, Cranfield University; Barry Azzopardi, Nottingham University and Geoff Hewitt, Imperial College);

Three Phase Flows (Geoff Hewitt and Raad Issa, Imperial College and Chris Thompson, Cranfield University);

Thermal Management of Tube Bundles (Joe Quarini, Bristol University and Geoff Hewitt, Imperial College);

Transport Behaviour of Particulate Solid Constituents (Rex Thorpe, Cambridge University);

Transient Hydrodynamic Loading (Rex Thorpe, Cambridge University and Hoi Yeung, Cranfield University);

Flexible Risers (Hoi Yeung, Cranfield University);

Coupled heat and mass transfer effects (Stephen Richardson, Imperial College).

TMF3 - Joint Project on Transient Multiphase Flows (TMF3, 2002-2006)

The research was supported by a single overall grant from EPSRC and by funds contributed by oil industry sponsors plus from the Department of Trade and Industry (now the Technology Strategy Board). The research was organised in ten projects, five of which were multi-centred, carried out at four universities: Imperial College London, Cranfield, Nottingham and Bristol.

Multi-fluid Modelling (Raad Issa, Imperial College and Chris Thompson, Cranfield University).

Slug Tracking, and active control of slug flows (Chris Lawrence, Imperial College).

Large Diameter Risers (Barry Azzopardi, Nottingham University).

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Thermal Management of tube bundles (Joe Quarini, Bristol University and Geoff Hewitt, Imperial College).

Three-Phase Flows (Geoff Hewitt and Raad Issa, Imperial College and Chris Thompson, Cranfield University).

Junction Flows (Barry Azzopardi, Nottingham University).

Flexible Risers (Hoi Yeung, Cranfield University).

Non-Newtonian Flows (Chris Thompson, Cranfield University and Geoff Hewitt, Imperial College).

Active Control of Slug Flows (Hoi Yeung, Cranfield University and Geoff Hewitt, Imperial College).

Technology Transfer (Chris Thompson, Cranfield University)

TMF4 - Joint Project on Transient Multiphase Flow and Flow Assurance (2006-2009)

Funded jointly by the oil industry, its contractors, with associated projects supported by the EPSRC. The research was organised in six projects, two of which were multi-centred, carried out at four universities: Imperial College London, Cranfield, Nottingham and Bristol:

Multiphase flow in vertical and deviated pipes (Barry Azzopardi, Nottingham University and Omar Matar, Imperial College))

Slug flow (Raad Issa, Imperial College)

Modelling of complex flows (Chris Thompson, Cranfield University and Geoff Hewitt, Imperial College)

Interfacial development and behaviour in stratified flow (Peter Spelt, Imperial College)

Thermal management (Joe Quarini, Bristol University)

Slug control (Hoi Yeung, Cranfield University)

TMF5 - Joint Project on Complex and Transient Multiphase Flows and Flow Assurance (2009 2012)

Funded jointly by the oil industry, its contractors, with associated projects supported by the EPSRC. The research was organised in six projects, two of which were multi-centred, carried out at three universities: Imperial College London, Cranfield and Nottingham.

Multiphase vertical flows with fluids of high viscosity (Barry Azzopardi, Nottingham University)

Experimental studies of slug flows (Geoff Hewitt, Imperial College)

Mechanistic Models to Simulate Slug Flow in Horizontal and Vertical Pipes (Raad Issa, Imperial College)

Multiphase horizontal flows with fluids of high viscosity (Hoi Yeung, Cranfield University)

Modelling of interfacial behaviour in separated flows (Peter Spelt, Imperial College)

Compilation of Experimental Data Bases (Imperial College and Nottingham University)

A.2 Highlights from previous TMF programmes

The overall aim of the first of the TMF programmes (TMF1) was to advance the knowledge of transient multiphase phenomena, through the use of demonstration computer codes, and through making both data and models available to developers of commercial codes. It did this by a blend of experimental work, modelling and computational developments. This aim has continued throughout all the TMF programmes through to the TMF Consortium.

A.2.1 Improvement of slug flow modelling and slug control: On the experimental side at Imperial College, the team led by Geoff Hewitt, using the WASP rig, were able to simultaneously measure the key parameters in stratified flows in both steady state and transient conditions. These included local velocities in the gas and the liquid (using hot wire and hot film probes), wall shear stress in the gas and liquid regions (using hot

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film probes), interfacial structure (using multiple twin-wire probes), pressure drop using rapid response transducers, holdup using multiple gamma densitometers, X-ray tomography and visualisation using special high pressure visualisation methods, including axial view video. The experiments were compared with closure models used in predictive codes and, in the case of transients, with the code predictions. Perhaps the most important finding was that transients cannot be adequately predicted using closure laws based on steady state data. Using the same rig, large sets of new experimental data were produced for both steady state and transient flows in 1.5 ° downhill inclined and downhill-uphill (V-section) configurations Imperial Colleges Mechanical Engineering applied its 1D two-fluid model to the slug flow case using an in-house code (TRIOMPH) as a test bed. It was shown that, given a small enough mesh size, the 1D two-fluid model could automatically simulate the initiation and growth of hydrodynamic slugs (slug capturing) and yield flow details that had hitherto not been possible to achieve.

Figure A1 - Liquid hold-up showing slug initiation, and a comparison of the predicted slug frequency against measurements by Imperial College

An example of the capability of TRIOMPH is shown in Figure A1 which respectively depicts successive snapshots of the computed liquid hold-up showing slug initiation, and a comparison of the predicted slug frequency against measurements. The TRIOMPH code was further developed to account for the distribution of entrained gas within a liquid slug yielding better agreement with experimental data. It was later modified to simulate three-phase slug flows reliably and efficiently. This is achieved by combining the two sets of equations for the two liquid phases into one set for both liquid components. In order to account for the relative motion of these liquid phases, a new scalar transport equation for the phase fraction of one liquid component in the liquid phase was formulated. The new model showed very good agreement with experimental data obtained at Imperial using dual-energy gamma-ray densitometry; these data helped

The latest development in modelling at Imperial College's Mechanical Engineering department was aimed at improving the modelling of horizontal slug flow, specifically in respect of the well-conditioning of the two-fluid model, this being the basis for the slug-capturing methodology that was developed in earlier TMF projects. Additional model closures were introduced and tested, namely for (i) surface tension, (ii) momentum flux correction factor, (iii) virtual mass force, and (iv) momentum and mass diffusion. All of those effects were found to ensure the well-posed nature of the model but the best results (compared with laboratory experiments on slug flow) were obtained with the introduction of mass and momentum diffusion. The results were published in conference papers and a thesis was written.

At Cranfield University a wide-ranging series of experiments were carried out into severe slugging on a 10 m high S-shaped flexible riser over a range of pressures (2, 4 and 7 bara) and a range of buffer vessel volumes. Comparisons were made between six selected data sets and the predictions from commercial codes. The agreement between code predictions and data was not good; perhaps equally significant, the agreement between the selected codes was also not good, indicating the great uncertainties involved in such prediction

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methods. Further research at Cranfield demonstrated the importance of modelling the entire riser systems in order to capture faithfully the salient features of severe slugging. The work also extended the studies done into the case of two-phase (oil-water) and three-phase (oil-water-gas) flows. Problems addressed included the mechanism of gas penetration at the riser base. Additional experiments generated new data on two- and three-phase flows in flexible risers and modelling of such flows, with particular reference to severe slugging. A new phenomenon observed was that of liquid-liquid separation in the riser.

The work on slug control of risers builds on years of severe slugging research at Cranfield and the realisation that slugging could not be avoided during the life of a field. Analytical models of flowline riser systems were developed. This model was then used to carry out controllability analysis and controller design. The controller was tested using the Cranfield riser facility. One of challenges of the project was to develop controllers that are robust and can cope with changing field conditions. Since the completion of the TMF project, extra funding was secured to develop the technology further. The patented controller required only information normally available on the platform and was successfully demonstrated on a platform in the North Sea. During the field trial, the controller continued to maintain optimal production even when one of the gas lift compressors was shut down. Data also revealed that production was increased by about 6% with the controller on. This demonstrated clearly the benefits of active control.

Numerical simulations of multiphase flow were investigated at Cranfield University where the main objective was to develop new numerical methods for application in the solution of the drift flux and multi-fluid (two-fluid and three-fluid) models for multiphase flow. A new (three-layer) model was also developed for three-phase stratified flow. It was demonstrated that enormous savings in computing resources was possible using grid adaptivity or, more likely, an ability to calculate at more detail with the same resources. A new code framework, EMAPS, was developed which supported automatic adaptivity of the problem dimensionality. Further development of the three-fluid model enabled it to take account of mixing between the water, oil and gas. Experimental data obtained at Imperial using dual-energy gamma-ray densitometry on the WASP rig helped to calibrate multi-layer models for three-phase stratified flow; for instance, a four-layer model (water, oil-water mixture, oil and gas, in ascending order from the bottom to the top of the pipe). A collaboration involving novel experimental work in Nottingham and numerical simulations using a two-fluid model in EMAPS (at Cranfield) yielded exciting results that addressed the problem of wave propagation in T-junctions whilst the experiments showed that upstream propagation of waves from the T junction was possible in some circumstances. Under a grant from the UK Department of Industry, work was carried out at Cranfield University on making EMAPS available to the TMF industrial partners in source code form.

If slug flows occur in pipelines, then transient hydrodynamic loadings may be generated in fittings such as bends and junctions. Cambridge and Cranfield Universities addressed the problem of predicting the magnitude and frequency of such transient (and potentially destructive) forces. Measurements were made at Cambridge (see Figure A3), and in larger scale equipment at Cranfield of the bend forces in slug flow and these measurements were used to validate models for the prediction of the forces which were embodied into a spreadsheet model.

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Figure A3 - Experimental rig for transient hydrodynamics

loading on pipe bends at Cambridge University

A.2.2 Annular flow: At Imperial College, a project investigated the initiation, growth and development of circumferential coherence of disturbance waves in annular flow. Most data for two-phase gas-liquid flows has been obtained with tubes of small diameter. In annular flow, the crucial phenomenon is the formation of disturbance waves. These waves are “squally” regions of the film which are typically of the order of 5 times the film thickness in height and extend typically for 10 – 30 mm in the flow direction. The existence of these waves is a necessary condition for liquid droplet entrainment and the waves have a crucial role in governing wall shear stress (and hence pressure gradient) and heat transfer. In small diameter tubes, the disturbance waves are coherent around the circumference of the tube but there is evidence that such coherency disappears when the tube diameter is large. The objective of this project was to conduct a more thorough investigation of the processes of creation and growth of disturbance waves in both small diameter and large diameter pipes. Experiments were carried out that featured simultaneous high-frequency film thickness measurements from multiple conductance probes positioned circumferentially and axially along a vertical pipe; these measurements were aimed at studying the three-dimensional development of these interfacial structures as a function of distance from the tube inlet. The results indicate that the disturbance waves begin to achieve their circumferential coherence from lengths as short as 5-10 pipe diameters downstream of the liquid injection location; this coherence gradually strengthens with increasing distance from the inlet. The frequency of occurrence of the disturbance waves first increases away from the inlet as these waves form, reaches a maximum at a length between 7.5 and 15 pipe diameters that depends on the flow conditions, and then decreases again. This trend becomes increasingly evident at higher gas and/or liquid flow-rates.

Mechanistic Models to Simulate Slug Flow in Horizontal and Vertical Pipes were investigated at Imperial College. One of the most important outcomes of the TMF programme has been the development of 1D two-fluid models which are applied in a sufficiently detailed way that they are capable of capturing the features of the initiation and evolution of slugs in horizontal and near-horizontal flows (slug capturing). A prime objective of the work in this project was to extend the generic 1D models to flows in tubes inclined at angles covering the full range from horizontal to vertical. Though this seems a simple extension, difficulties occur because of the fundamental ill-posedness of the 1D two fluid models and various solutions to this problem are being investigated. What are needed are practical methods for resolving these issues and the aim will be to explore a number of such approaches taking into account of experimental evidence and also evidence from three-dimensional CFD calculations. Account had to be taken of the fact that the precursor to slug flow in horizontal pipes is stratified flow whereas in vertical flows, the precursor are bubbly flow. At some angle of inclination, the dominant precursor regime changes.

Modelling of interfacial behaviour in separated flows at Imperial College researched Stratified and Stratifying Annular Flows. The crucial issue in stratified flows is that of the formation and development of

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interfacial waves. A key issue here is the interaction between turbulence and interfacial waves; even for small-amplitude waves, turbulence in the gas is distorted as it is advected over the waves. Previous work in the TMF projects had concentrated on addressing these problems in the context of parallel channel flows. This project aimed to extend and develop this work to the (more practical) case of round tubes. The initial work was conducted using the Immersed Boundary Method (IBM) as this method has the advantages of allowing a simple grid generation which can deal with complex shapes of channel. Tests confirming the validity of the IBM method for simple single and two phase round tube flows had previously been completed. More recently, the IBM-based code had been used to core-annular flows in the laminar flow regime, as well as the rise of Taylor bubbles.

A.2.3 Phase behaviour in flowing systems: Transient mass transfer in co-existing hydrocarbon liquid and gas phases in flowing systems was investigated at Imperial College, Chemical Engineering where the main objective was to gain better information on mass transfer phenomena in hydrocarbon pipeline transport systems. In computer codes for predicting both steady state and transient flows in pipelines, it is the common practice to assume local equilibrium between the respective phases. A new test facility was constructed in which stratified propane/methane mixture flows were set up in a 25.4 mm (1 inch) diameter, 2.4 m long horizontal tube. A series of transient tests were carried out with various changes of pressure level in the gas phase, with a constant liquid flow rate and with a range of gas flow rates; the time for equilibration was determined. It was concluded that departures from equilibrium may not be too significant in steady flows in very long (tens of kilometres) pipelines but that departures from equilibrium may affect local phenomena in crucial regions. This work was later extended at Imperial College to investigate the effects of Coupled Heat and Mass Transfer. The experimental facility was modified to allow transients in composition rather than pressure, as had been used previously. Again, it was concluded that the equilibrium assumption was reasonable for calculations on long pipelines.

A.2.4 Thermal aspects of bundles: Experiments simulating a pipe carrying hydrocarbon product passing through a sleeve pipe which carries a heating fluid were undertaken using a water-filled simulated four-tube bundle at Imperial College and the data was used to validate CFD calculations at Bristol on the natural convective heat transfer between the tubes. A spreadsheet model for interpolating the CFD calculations and predicting the performance of the whole bundle system was also developed at Bristol. The work on thermal management of tube bundles continued at Bristol where a their spreadsheet estimation tool was developed to predict thermal performance based on in-depth CFD computations which fit heat transfer coefficients using neural networks which calculate temperature profiles along the bundle using a simple ‘1D’ code was extended to cover vertical and inclined bundles. The trained neural network outputs detail of heat fluxes and heat transfer coefficients (see Figure A2). A further step in this work at Bristol on the prediction of gas-filled multi-tube bundles was validation of the estimation tool by large scale experiments at Imperial College. The final step in extended the methods developed was to develop the estimation tool for the case of inclined or vertical bundles. For the horizontal case, the convective flows are essentially two-dimensional; for an inclined or vertical bundle, the flows become three-dimensional and prediction of bundle behaviour is more difficult (though still feasible using the CFD/neural network methods developed in the earlier TMF programmes.

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A.2.5 Solids transport and erosion: Cambridge University investigated the transport behaviours of particulates with the aim of estimating the fluid velocities necessary for the transport along a pipeline of sand particles present in the (multiphase) streams from wells. Measurements were made of the critical velocity for initiation of transport of sand particles in single-phase water flow, two-phase stratified flow and two-phase slug flow over a range of flow rates. Stand-alone spreadsheet programs were developed which allow the conditions in which particulate material is transported to be estimated.

A.2.6 Understanding scale-up to industrial systems: Vertical and inclined large diameter (over 100 mm diameter) risers were studied at Nottingham highlighting the complexity of the regimes that flow in these systems and the absence of certain regimes (e.g. slug flow) in large diameter pipes. In addition, tests were carried out on the large diameter (192 mm) risers at SINTEF and Rossendorf operating with steam/water at 46 bar using a Wire Mesh Sensor probe. At Nottingham, new measurements were also made on air-water flows in combining junctions and on air-kerosene flows in dividing horizontal T-junctions. In combining slug flows, a new phenomenon was found in which slugs could be broken up into a series of shorter slug, termed “caravan slugs”. In further research at Nottingham new data were obtained on very large scale facilities which confirmed the severe challenges to the methodologies for predicting flow regime, phase fractions (holdups) and pressure gradient because the flow regimes (and hence the other parameters) may be very different in large diameter pipes (see Table A1 and Figures A4 and A5). In addition to these various experiments, SINTEF (a TMF participant) made available data from earlier experiments where, instead of the fluids being mixed at the bottom of the riser, the riser was preceded by a long pipeline and a 90° bend. Clear differences in flow pattern are seen between the two cases, Figure A6, though there is little difference in mean void fraction, Figure A7.

Table A1 - Large diameter vertical and steeply inclined riser systems experimental rigs at Nottingham

Experiments Riser diameter

(mm)

Height (m)

Fluids Pressure (bar)

Number of runs

Instrumentation

Nottingham 127 10 Air/water 3 50 conductance ring probes for time series of void fraction

SINTEF 189 50 Nitrogen/naphtha 20, 90 150 pressure and gamma probes

HZDR1 192 10 Steam/water 46 16 Wire Mesh Sensors*

Note 1 HZDR - Forschungszentrum Rossendorf, now Helmholtz Zentrum Dresden Rossendorf

Figure A2 - Modelling the CFD results using neural networks by Bristol University

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NOTE: The pressure employed on the HZDR experiment was chosen to give the same gas density as the 20 bar experiments at SINTEF

Figure A4 - View corresponding to what would be seen if the pipe were transparent (a) and phase distribution at a diameter (b) for liquid superficial velocity of 0.01 m/s and different gas superficial velocities, by

Nottingham University using Wire Mesh Sensor

Figure A5 - Gas velocity (gas superficial velocity divided by void fraction) versus mixture velocity for SINTEF (Omebere-Iyari et al., 2007), HZDR (Present work) and data from literature (Shen et al., (2004)

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Figure A6 - Flow pattern map for experiments on riser mounted on SINTEF large facility: (a) 20 bar; (b) 90 bar working with Nottingham

Figure A7 - Mean void fraction in riser mounted on SINTEF large facility: configuration 1 - mixer at bottom of riser; configuration 3 - pipeline/bend/riser geometry by Nottingham

Industrial systems include networks of pipes which typically join multiphase flows together and in some instances split multiphase flows into two pipes. The effects on flows at junctions were investigated at Nottingham where measurements were made on air water flows in combining junctions and on air-kerosene flows in dividing horizontal T-junctions. In combining slug flows, a new phenomenon was found in which slugs could be broken up into a series of shorter slug (“caravan slugs”). The research was undertaken on the Intermediate facility at SINTEF with 67 mm diameter pipes using diesel and sulphur hexafluoride with gas densities up to 30 kg/m3. The experiments including rig modification and operational support funded by EU Project on Access to Large Facilities. An example of the result in the form of time traces of liquid hold-up from probes mounted around the combining junctions are shown in Figure A8 and illustrate clearly the upstream slugs being divided into a train of smaller ones.

b a

AnnularBubble

Intermittent

Naphtha-nitogen at 90bar in a 0.189m diameter vertical conduit 1987 data

0.01 0.1 1 100.001

0.01

0.1

1

10

Gas superficial velocity, m/s

Liq

uid

sup

erfi

cial

vel

ocit

y, m

/s

Transition

Churn

Bubble

Intermittent

(Semi)

Annular

SemiAnnular

AnnularBubble

Intermittent

SemiAnnular

Naphtha-nitogen at 20bar in a 0.189m diameter vertical conduit 1987 data

0.01 0.1 1 100.001

0.01

0.1

1

6

Gas superficial velocity, m/s

Liq

uid

su

per

ficia

l vel

ocit

y,

m/s

Bubble

Churn

Intermittent

(Semi)

Annular

Slug

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Figure A8 - Time traces of liquid hold-up at combining junctions from an experiment at SINTEF undertaken by Nottingham

A.2.7 Application of advanced measuring systems to multiphase flow: Nottingham University and Imperial College, Chemical Engineering, worked together with Nottingham focussed on experimental measurements using their large diameter vertical flow rig whilst Imperial worked on modelling, in particular, the transition region between bubble flow and churn flow. The experiments focussed on measurements using the large facility (127 mm, 10 m), and inclinable facility (67 mm, 6 m). Measurements on the large facility were made using air/water and employing Wire Mesh Sensors (WMS), conductance probes, hot film probes for wall shear stress measurement, pressure gradient measurements, Phase Doppler Anemometry for drop size and velocity determination. Data was obtained covering churn and annular flow patterns. On the inclinable facility experiments were carried out with air/water and air/silicone oil and employed Wire Mesh Sensors and differential pressure cells. For the silicone oil work Electrical Capacitance Tomography (ECT) was also employed and here the flow patterns studied included bubbly, slug, churn and stratified. The full range of inclinations between vertical upwards, though horizontal to 20° downwards were addressed.

The state of the art instrumentation employed at Nottingham included the first full rig application of a capacitance version of the Wire Mesh Sensor systematic testing of the instruments was required e.g. the WMS and ECT were run simultaneously and shown to give good agreement [see Figure A7 (a)]. In addition, the WMS was tested in simultaneous runs with gamma densitometry, again with good agreement [see Figure A9 (b)].

(a) (b) Figure A9 - Results from air/silicone oil on 67 mm pipe on inclinable rig mounted vertically at Nottingham:

(a) Comparison of mean void fraction measured using ECT to the from WMS; (b) Time traces of cross-sectionally averaged void fraction from WMS and two ECT planes with the latter transposed to compensate

for different axial positions showing excellent agreement

0

0.2

0.4

0.6

0.8

1

0 1 2 3 4 5

Time (s)

Void

fra

ctio

n

ECT (1) ECT (2) WMS

0 0.2 0.4 0.6 0.8 10

0.2

0.4

0.6

0.8

1

Void fraction (WMS)

Voi

d fr

action

(E

CT

)

0 0.1 0.2 0.25 0.7Liquid superficial velocity(m/s)

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Axial view photography was used to show for the first time that the gas core in churn flow is continuous though with extensive droplet entrainment. Examination of the existing data for wave coherence in smaller (32 mm) diameter tubes showed that (after a length of around 1 metre) the waves were ring-like and their frequency had approached an asymptotic value with length.

An advanced interface-tracking scheme by Imperial College has been developed for the simulation of large bubbles rising in vertical pipes with codes developed for single large bubbles that are tracked in a frame moving with the bubble, and for trains of large bubbles, validated against steady state results in the literature. The results have revealed how bubbles break up due, mainly due to gas entrainment at the tail of a bubble after amplification of oscillations in the “skirt” (or tail) of the bubble, or direct breakup before a skirt is formed, although a stable bubble was possible in some cases wherein breakup was followed by coalescence. It was established also that there exists a maximum pipe diameter even for laminar flow conditions, before a Rayleigh-Taylor instability would be expected.

An investigation at Imperial College of the bubble flow/slug flow transition in small and large diameter pipes using the void wave growth model of Biesheuvel and Gorissen (1990) was shown to give excellent predictions when compared with previously unpublished data on bubble/slug transition in a 32 mm diameter pipe showing that the transition to slug flow was dependent only on the phase flow rates. Transient analysis of the large diameter data obtained in the Nottingham experiments showed that void wave growth leading to slug flow did not occur. An investigation of the viability of an existing Imperial College code (GRAMP) to predict churn and annular flows over a wide range of conditions (including larger diameter tubes) was also carried out. In this work, the code was compared with a number of data sets obtained at Imperial College for flow in a 32 mm diameter pipe and with data from the larger diameter pipes obtained at Nottingham. A new correlation was developed for liquid entrainment in churn flow (based on data for 32 mm tubes) and this was found to give reasonable predictions of the entrained fraction in the larger diameter tubes when the variability of interfacial stress was accounted for.

A simplified model was developed for bubbles in large diameter tubes which considered the gas phase to consist of only two types of bubble, namely small bubbles of a fixed size (typically with a diameter around 3 mm) and spherical cap bubbles. The rates of small bubble entrainment from and coalescence with the spherical cap bubble come into equality at a certain spherical cap bubble diameter. The spherical cap bubble will grow until this diameter is reached. Approximate calculations revealed that this model was consistent with the observations.

As part of the modelling of complex flows activity at Imperial College Chemical Engineering, focus was on flows with two liquid phases, carrying out Laser Induced Fluorescence (LIF) studies on liquid-liquid flow (see Figure A9) and on larger scale measurements on three-phase flows using X-ray tomography etc. Experiments were carried out on interface behaviour and on droplet transport using advanced optical methods, and in particular axial view photography on stratified and stratifying annular flow. The project included modelling work in which droplet motion was being studied using CFD. An investigation was pursued of the influence of the type of turbulence model (RANS, LES) on the predictions.

Figure A9 - Non-Newtonian Flow: Annular flow with water dispersion in the oil

core obtained using the laser induced fluorescence technique at Imperial College

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A.2.8 Non-Newtonian and highly viscous fluids: An assessment survey was carried out by Imperial College and Cranfield of the occurrence of non-Newtonian fluids in the oil-and-gas industry, which led to the following work. At Cranfield University advanced numerical methods, which were successful in predicting the development of slugs from stratified flow conditions, were deployed to study non-Newtonian flow systems. The computational work focused on non-Newtonian liquid drag reduction by gas injection. Two regimes were considered: fully stratified gas/shear-thinning liquid flow and gas/shear-thinning liquid slug flow regimes. Predictions of drag reduction ratio and holdup were presented for the stratified flow of gas and non-Newtonian Ostwald-de Waele liquid. Fully stratified flow was modelled and the approach developed inTaitel and Dukler (1976) used. For these regimes, CMC (CarboxyMethyl Cellulose) solution is used to investigate the behaviour of the gas and non-Newtonian liquids in horizontal pipes. Results have been reformulated and an extension to interfacial Andreussi and Persen (1987) correlation has been carried out for stratified regimes. For slug flow regimes, the mechanistic slug unit model is adopted in order to estimate the pressure gradients along the slug unit. The slug unit model is rearranged and reinterpreted as inviscid Burgers's equation for incompressible phases.

For both stratified and slug flow regimes, three dimensional Coarse-Mesh Finite Difference (CMFD) simulations were performed in order to compare the drag reduction ratio and pressure gradients. In stratified flows, CMFD is also used in an attempt to evaluate the liquid wall friction factor and to compare the obtained values with those given by empirical standard correlations.

Figure A10 - Electrical Capacitance Tomography is used to determine detailed flow behaviour in a 250 mm riser at Nottingham

Extensive research into flows of viscous fluids has been undertaken. Nottingham University investigated the effect of liquid viscosity on two-phase gas-liquid flows in vertical pipes for the case where the liquid phase has a very high viscosity using pipe diameters up to 125 mm with the liquid phases including sugar solutions and dead crude oil. Since the liquids used can be opaque, the flow structure was investigated with the aid of Electrical Capacitance Tomography (ECT) (see Figure 10). The results from this technique were processed to yield mean holdup values in addition to giving more detailed information on phase distribution (see Figure A11). The prime aim of the experiments was to evaluate the effect of high viscosity on the flow patterns (determined by phase distribution measurements), pressure gradient and liquid holdup.

Additional experiments on the 127 mm diameter vertical riser on the Intermediate Facility at SINTEF employed a viscous oil (35 times the viscosity of water) and sulphur hexafluoride as the working fluids at pressures which gas densities in the range 28-45 g/m3. Structures similar to those shown in Figure 3 were

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observed. In addition, experiments with downflow of liquid to maintain the Taylor bubbles of slug flow stationary were carried out with water and two oils (viscosity 42 and 152 times that of water). The results showed that there was a strong trend for the creation of small bubbles from the tail of the Taylor bubbles which diminish with increasing liquid viscosity.

Figure A11 - Probability Density Function of void fraction at the wake section for air-water, low and high viscosity liquids at superficial liquid velocity of 0.33m/s and gas

superficial velocity of 0.057 m.s. by Nottingham

Cranfield investigated the effect of liquid viscosity on two-phase gas-liquid flows in horizontal pipes for the case where the liquid phase has a very high viscosity. The research used an existing facility which can operate with high viscosity liquids (liquids of 70 cP and 250 cP viscosity). The experimental work in this project included comprehensive studies of the flows of air/high viscosity liquid flows in horizontal tubes and

in inclined tubes with inclinations (upwards and downwards) up to 20. Data was collected on flow pattern, pressure gradient and void fraction and on detailed parameters such as slug frequency and slug body holdup. The experiments were initially on gas-liquid two-phase flows and then extended to three-phase (liquid-liquid-gas) flows by introducing a second liquid phase. In addition to the data collected an improved slug model was developed which could be used to determine pressure gradient, slug frequency and slug length. Crucial to the model is the ‘apparent interfacial friction factor’ for high viscosity oil gas flows. The model was validated with experimental data from various sources covering viscosities ranges up to 4000 cP.

A.2.9 Data library and technology transfer: The ongoing TMF programmes have amassed a very large catalogue of experimental data at Nottingham and Imperial College. Compilation of these data bases are of great importance and value to both the academics and the participants in TMF. At Imperial College extensive experimental research on slug flow with particular reference to initiation phenomena (including the influence of inlet conditions), to the behaviour of entrained bubbles in slugs (using optical probe systems) and on the prediction of slug flows using CFD models, ID two-fluid models etc. A systematisation of the vast amount of data obtained for slug and other flows on the Imperial College WASP facility was carried out; the purpose is to provide a readily accessible data base for access by Participants in TMF. Similar efforts at Nottingham resulted in a large database, which is available to all TMF sponsors.