Experiments in stratified gas-liquid pipe flow

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Experiments in stratified gas-liquid pipe flow

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. ir. K. C. A. M. Luyben, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 26 mei 2015 om 10:00 uur

door

Miloš BIRVALSKI

diplomirani inženjer mašinstva Universiteit van Belgrado, Servië

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Dit proefschrift is goedgekeurd door de promotor: Prof. dr. ir. R. A. W. M. Henkes

Copromotor: dr. ir. M. J. Tummers Samenstelling promotiecommissie:

Rector Magnificus, voorzitter

Prof. dr. ir. R. A. W. M. Henkes, Technische Universiteit Delft, promotor dr. ir. M. J. Tummers, Technische Universiteit Delft, copromotor Prof. dr. A. Jensen, Universiteit van Oslo, Noorwegen

Prof. dr. W. van de Water, Technische Universiteit Eindhoven Prof. dr. ir. W. S. J. Uijttewaal, Technische Universiteit Delft Prof. dr. R. F. Mudde, Technische Universiteit Delft Prof. dr. ir. J. Westerweel, Technische Universiteit Delft

Printed by: CPI-Wöhrmann Print Service - Zutphen

Front & Back: Two versions of Blind men crossing a log bridge by Ekaku Hakuin (1685–1768), Japanese Zen master, painter, calligrapher. The in-scription on the front page reads: ‘Both the health of our bodies and the fleeting world outside us are like the blind men’s round log bridge – a mind/heart that can cross over is the best guide’.

Copyright c 2015 by M. Birvalski ISBN 978-94-6203-833-2

An electronic version of this dissertation is available at http://repository.tudelft.nl/.

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Summary

The growing demand for energy in the future will necessitate the production of natural gas from fields which are located farther offshore, in deep water and in very cold environments. This will confront us with difficulties in ensuring continuous production of the fluids (natural gas, condensate and water) emerging from a natural gas well. Often, all the three phases are transported together through a multiphase flow pipeline to processing facilities onshore.

The natural gas production pipelines that carry the wellstream fluids from the subsea wells to the processing facilities are designed using engineering multiphase flow models. It is known that the currently available flow models cannot predict the most important flow parameters such as pressure drop and liquid holdup with sufficient accuracy when the gas production is low. At these conditions, the liquid accumulates in V-sections (lower elbows, low spots) of a pipeline, which are present because the pipeline profile follows the undulations in the seafloor topography. When the flow is low, the existing engineering models show large uncertainty in predicting the shear stresses, particularly at the gas-liquid interface. This uncertainty leads to the inaccurate prediction of, for example, the liquid holdup, which can cause production problems connected with liquid slugs, corrosion and the formation of gas hydrates.

In this Thesis, the problem of liquid accumulation in an undulating pipeline is studied both at a more general, macroscopic level in a flowloop that contains a V-shaped section, as well as on a detailed, fundamental level in a straight horizontal pipe. Both configurations, however, have the same flow pattern: stratified gas-liquid flow. The main issues addressed in the V-section setup are the conditions of liquid accumulation and removal in the case of zero net liquid flow with gas flowing over a stagnant liquid pool, as well as the appearance of multiple steady-state solutions in the two-phase models at these conditions. The detailed measurements in the horizontal setup aimed at simultaneously capturing the velocities in both phases in the entire streamwise cross-section of the pipe and the position of the gas-liquid interface. This provided detailed information on waves and turbulence, which are the main phenomena in stratified flow.

The occurrence of multiple solutions in stratified flow models was studied by applying a steady state and a transient model to various conditions for which lab experiments exist and by verifying the structural stability of the obtained solutions. It was found that the applied transient model (supplied with the criterion for struc-tural stability) can qualitatively predict the measurements in zero net liquid flow, at conditions where hysteresis occurs and in experiments with a holdup discontinuity. Based on the comparison with available experimental data, it was concluded that hysteresis can only occur in fully laminar flow of both phases, and it is not expected to occur at typical field conditions. However, to achieve a better quantitative

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viii _Summary

ment with the measurements, the closure relations used in the model need to be improved.

Measurements of the gas velocity, liquid holdup and pressure drop in zero net liquid flow were performed in a setup containing a V-shaped section. It was shown that the critical gas velocity (i.e. the minimum gas velocity at which the liquid is removed from the low spot) and pressure gradient increase with increasing inclina-tion angle and with increasing liquid density and viscosity, while the liquid holdup stays approximately the same. The results were compared to the predictions of a mechanistic flow model, which was modified to account for the recirculation in the stagnant liquid layer at critical conditions by employing a theoretical solution for the wall shear stress at laminar flow conditions of the liquid. Good agreement was found between the measurements and the simulations.

Stratified two-phase flow of air and water was measured with Particle Image Velocimetry (PIV) in a horizontal transparent pipe, with a laminar, transitional or turbulent liquid, and a smooth or a wavy interface. An advanced experiment was designed and built, which uses two lasers and three cameras to simultaneously measure the liquid velocity, interface shape and gas velocity. The data were time-and phase-averaged to obtain detailed time-and accurate insight into the turbulent time-and wavy structures.

The cases with a smooth interface were shown to approximately follow the ve-locity laws valid in single-phase flows. The wavy region of the flowmap (constructed with superficial gas and liquid velocities at the axes) had waves which are asym-metric, with gravitational and capillary forces of similar magnitude. The linear wave theory provided a good approximation of the wave-induced velocity profiles, although the wave non-linearity caused a deviation close to the interface. The sep-aration of wavy and turbulent motion was, however, only partly successful due to the wide range of wavelengths and wave heights in all the wavy cases.

The laminar-to-turbulent transition of the liquid phase in stratified gas-liquid flow was also studied. The boundaries of transition were determined in both the smooth and the wavy region of the flowmap. In both regions, the Reynolds number at the start and at the end of transition decreased with increasing gas flowrate. The two occurring wave patterns (labelled ‘2D small amplitude’ and ‘3D small amplitude’ waves) corresponded to the capillary-gravity and the gravity-capillary solutions of linear wave theory. This led us to recast the flowmap of the wavy region into Weber number – Froude number coordinates, which in turn provided a physical interpretation of the interaction between the developing turbulence and the changing wave patterns.

Finally, the interfacial characteristics and the velocities were investigated in both phases of stratified flow in two wave patterns: ‘3D small amplitude’ and ‘2D large amplitude’ waves. The 2D LA waves (corresponding to gravity waves) had higher and longer waves, that changed the liquid velocities in almost the entire liquid layer. The 3D SA wave pattern (corresponding to gravity-capillary waves) had smaller and shorter waves whose influence was limited to only a part of the liquid height. The effect of the two wave regimes on gas phase velocities, however, was rather similar. In all cases, waves produced an increase in the Reynolds stresses in the air close to

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Summary ix

the interface, which was linked to the occurrence of boundary layer separation at the interface. The occurrence of separation correlated well with the wave properties. The results of the current study provide a step forward in understanding strat-ified gas-liquid flow through straight pipes. The physical explanations and insights of the measured phenomena can be used to develop new correlations for the engi-neering flow models, which is the type of models that are currently widely applied in the industry. Furthermore, a high-quality experimental database for an elementary two-phase pipe flow configuration has now been established. This database can be used for the improvement and validation of more advanced models for such two-phase flows. In particular, these are Computational Fluid Dynamics (CFD) models of increasing complexity, such as Reynolds-Averaged Navier Stokes (RANS), Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS).

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Samenvatting

De groeiende vraag naar energie in de komende jaren maakt het noodzakelijk om ook aardgas te produceren uit velden die zich verder van de kust, in dieper water en in zeer koude omgevingen bevinden. Dit stelt ons voor uitdagingen bij de realisatie van een continue productie van de het gas en de vloeistoffen (namelijk aardgas, con-densaat en water) uit de ondergrondse aardgasbronnen. Vaak worden de genoemde drie fasen gezamenlijk door een meerfasen pijpleiding naar een verwerkingsinstallatie op het vaste land getransporteerd.

De aardgasproductiepijpleidingen die het gas en de vloeistoffen vervoeren van de onderzeese putten naar de verwerkingsinstallaties zijn ontworpen met behulp van ingenieursmodellen voor meerfasenstromingen. Het is bekend dat de beschikbare stromingsmodellen de belangrijkste stromingsparameters, zoals drukval en vloei-stofophoping, niet voldoende nauwkeurig kunnen voorspellen als de gasproductie door de pijpleiding laag is. Onder deze omstandigheden hoopt de vloeistof zich op in V-secties (lagere ellebogen, lage punten) van een pijpleiding die aanwezig zijn vanwege de hoogtevariaties langs de zeebodem. Wanneer de gasdoorzet laag is vertonen de bestaande ingenieursmodellen een grote onzekerheid in het voorspel-len van de schuifspanningen, vooral bij het grensvlak tussen de vloeistof en het gas. Deze onzekerheid leidt tot een onnauwkeurige voorspelling van bijvoorbeeld de vloeistofophoping wat kan leiden tot productieproblemen die verband houden met vloeistofslokken (‘slugs’), corrosie en de vorming van gashydraten.

In dit proefschrift wordt het probleem van de vloeistofophoping in een pijplijn met hoogtevariaties bestudeerd zowel op een meer algemeen, macroscopisch niveau in een experimentele stromingsopstelling met een pijp met V-sectie, alsmede op een gedetailleerd fundamenteel niveau in een rechte horizontale pijp. Beide configuraties hebben echter hetzelfde stromingspatroon: gestratificeerde gas-vloeistof stroming. Het belangrijkste probleem dat in de V-sectie opstelling wordt onderzocht is de vloei-stofophoping en de vloeistofverwijdering in het geval dat de netto vloeistofdoorzet nul is en het gas over een stilstaande hoeveelheid vloeistof in de V-sectie beweegt, en het optreden van meerdere stationaire oplossingen in de tweefasen modellen on-der deze omstandigheden. De gedetailleerde metingen in de horizontale opstelling hebben tot doel tegelijkertijd de snelheden in beide fasen in de gehele dwarsdoor-snede van de buis en de positie van het vloeistof-gas grensvlak vast te leggen. Dit geeft gedetailleerde informatie over de golven en de turbulentie in de gestratificeerde stroming.

Het optreden van meerdere oplossingen in modellen voor gestratificeerde stro-mingen is bestudeerd door het toepassen van een stationair en een tijdsafhankelijk model voor verschillende omstandigheden waarvoor laboratorium experimenten be-staan, en door de structurele stabiliteit van de verkregen oplossingen te verifiëren. Het blijkt dat het toegepaste tijdsafhankelijke model (met het criterium voor

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xii _Samenvatting

turele stabiliteit) de experimenten waarbij de netto vloeistofdoorzet nul is kan voor-spellen onder omstandigheden waarin hysteresis optreedt, alsmede de experimenten met een discontinuïteit in de vloetstofophoping. Uit de vergelijking met beschikbare experimentele gegevens kan geconcludeerd worden dat hysteresis alleen kan plaats-vinden bij een volledig laminaire stroming in beide fasen, en het is niet te verwachten dat dit optreedt bij typische veldomstandigheden. Echter, om een betere kwanti-tatieve overeenkomst met de metingen te verkrijgen, moeten de sluitingsrelaties in het model verbeterd worden.

Metingen van de gassnelheid, vloeistofophoping en drukval, voor het geval dat de netto vloeistofdoorzet nul is, werden verricht in een opstelling met een V-vormige sectie. Er werd aangetoond dat de kritische gassnelheid (de minimale gassnelheid waarbij de vloeistof uit het lage punt verwijderd wordt) en de drukgradiënt toene-men met toenetoene-mende hellingshoek en met toenetoene-mende vloeistofdichtheid en visco-siteit, terwijl de vloeistofophoping ongeveer hetzelfde blijft. De resultaten werden vergeleken met de voorspellingen van een mechanistisch stromingsmodel, dat werd aangepast voor het effect van de recirculatie in de stilstaande vloeistoflaag bij kri-tieke omstandigheden door gebruik te maken van een theoretische oplossing voor de wandschuifspanning onder de laminaire stromingsconditie van de vloeistof. Er is een goede overeenkomst tussen de metingen en de simulaties.

De gestratificeerde tweefasenstroming van lucht en water werd gemeten met Par-ticle Image Velocimetry (PIV) in een horizontale doorzichtige buis, met een lami-naire, transitionele of turbulente vloeistof, en een glad of een golvend vloeistof-gas grensvlak. Er is een geavanceerd experiment ontworpen en gebouwd waarbij door het gebruik van twee lasers en drie camera’s tegelijkertijd metingen konden worden verricht voor de vloeistofsnelheid, de vorm van het vloeisof-gas grensvlak en de gas-snelheid. De gegevens werden gemiddeld in de tijd en in fase om gedetailleerd en nauwkeurig inzicht te krijgen in de structuur van de turbulentie en van de golven.

Er werd aangetoond dat de gevallen met een glad vloeistof-gas grensvlak onge-veer de snelheidswetten volgen die ook geldig zijn in éénfase stromingen. In het stro-mingsoverzicht (met de superficieële gas- en vloeistofsnelheden op de assen van de overzichtsfiguur) is een gebied met asymmetrische golven waarbij de zwaartekracht en de capillaire krachten van dezelfde orde van grootte zijn. De lineaire golftheorie geeft een goede benadering van de golf-geïnduceerde snelheidsprofielen, alhoewel de niet-lineariteit van de golven een afwijking nabij het vloeistof-gas grensvlak ver-oorzaakt. De scheiding van de golf- en turbulente bewegingen was echter slechts gedeeltelijk succesvol ten gevolge van het brede scala van golflengten en golfhoogten voor alle gevallen met golven.

De omslag van de laminaire naar de turbulente toestand van de vloeistof fase in gestratificeerde gas-vloeistofstroming werd ook bestudeerd. De omslaggrenzen wer-den bepaald in zowel het gladde als het golvende gebied van het stromingsoverzicht. In beide gebieden daalde het Reynoldsgetal aan het begin en aan het einde van de omslag met toenemende gassnelheid. De twee gevonden golfpatronen (aangeduid als ‘2D small amplitude’ en als ‘3D small amplitude’ golven) komen overeen met de capillaire-zwaartekracht en de zwaartekracht-capillaire oplossingen van lineaire golftheorie. Dit was de aanleiding om het golvende gebied in de figuur met het

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Samenvatting xiii

stromingsoverzicht weer te geven met het Webergetal en het Froudegetal als coör-dinaten, die zorgen voor een fysische interpretatie van de wisselwerking tussen de ontwikkeling van de turbulentie en de veranderende golfpatronen.

Ten slotte zijn de karakteristieken van het vloeistof-gas grensvlak en van de snelheden onderzocht in beide fasen van de gestratificeerd stroming voor twee golf-patronen: ‘3D small amplitude’ en ‘2D large amplitude’ golven. De 2D LA golven (die overeenkomenen met zwaartekrachtgolven) hadden hogere en langere golven die de vloeistofsnelheden veranderden in vrijwel de gehele vloeistoflaag. Het 3D SA golfpatroon (die overeenkomenen met zwaartekracht-capillaire golven) had kleinere en kortere golven waarvan de invloed beperkt was tot slechts een deel van de vloei-stofhoogte. De invloed van de twee golfbewegingen op de snelheden in de gasfase was echter min of meer hetzelfde. In alle gevallen veroorzaakten de golven een toe-name van de Reynoldsspanningen in de lucht nabij de water-lucht grensvlak, hetgeen verband houdt met het loslaten van de grenslaagstroming bij het grensvlak. Het op-treden van grenslaagloslating heeft een goede correlatie met de golf-eigenschappen. De resultaten van de huidige studie vormen een stap voorwaarts in het begrijpen van gestratificeerde gas-vloeistofstroming in rechte pijpen. De fysische verklaringen en inzichten van de gemeten verschijnselen kunnen worden gebruikt om nieuwe cor-relaties te ontwikkelen voor de ingenieursmodellen, die momenteel op grote schaal worden toegepast in de industrie. Bovendien is een hoogwaardige experimentele database voor een elementaire tweefasen pijpstroming tot stand gekomen. Deze database kan worden gebruikt voor de verbetering en de validatie van meer gea-vanceerde modellen voor dergelijke tweefasen stromingen. In het bijzonder zijn dit Computational Fluid Dynamics (CFD) modellen met toenemende mate van com-plexiteit, zoals Reynolds-Averaged Navier Stokes (RANS), Large Eddy Simulation (LES) en Direct Numerical Simulation (DNS).

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1

Introduction

The World energy consumption is projected to grow by 40% in the next 25 years. Fossil fuels are expected to continue to provide some 80% of this energy. Although the reserves of fossil fuels are large, new fields that are being discovered are far offshore, in deep water and in environments with a subzero temperature. This creates large challenges in a number of technical disciplines, amongst which is flow assurance in oil and gas production. One recently developed gas field that highlighted some of the flow assurance issues that will be encountered more frequently in the future is the Ormen Lange gas field. A combination of a very long pipeline transporting the wellstream fluids (gas, condensate and water) over a rough seafloor topography resulted in large uncertainties in the predictions of state-of-the-art flow models. The main difficulty that the models have is to accurately capture the shear stress at the gas-liquid interface. This is particularly true at low production rates where - due to the large number of upward and downward inclined pipe segments making up V-shaped sections where liquid can easily accumulate - the error in predicting the shear stress resulted in large uncertainties on the liquid inventory and, consequently, on the pressure drop along the pipeline. In this Thesis, the issue of liquid accumulation at low flow is approached in two ways. Firstly, the conditions under which large liquid accumulation occurs - and the related existence of multiple solutions in two-phase flow models - are studied experimentally in a setup containing a V-shaped section, and they are simulated using a transient two-phase flow model. Secondly, the physical processes that influence stratified flow in general, and the shear stress at the gas-liquid interface in particular, are studied in detail by applying Particle Image Velocimetry in an experimental setup with a straight horizontal pipe. The experiments were especially focused on two key phenomena of the current problem, which are waves and turbulence, and their mutual interaction.

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2 _1._Introduction

1.1. Research motivation

1.1.1. Energy demand and supply: past, present and future

The World energy consumption currently stands at about 600 EJ/year [1], with most of the energy (83%) coming from fossil fuels (i.e. coal, oil and natural gas); see figure1.1a. Despite the large efforts being invested into increasing the energy efficiency, especially in the developed countries, the rising world population and the increasing percentage of people moving into the middle classes (particularly in China and India) is expected to cause the total energy demand to increase at a rate of 1.5% per year (being only 0.5% in OECD1 _{countries, and 2.2% in non-OECD countries)}

in the next 25 years, with the total consumption being approximately 40% higher in 2040 compared to the level today [1]. The production of nuclear energy and energy from renewable sources is going to rise, bringing the percentage of the total energy supplied by fossil fuels from 83% to an expected 79% over the same period. Despite this, the share of energy coming from natural gas - which is the cleanest burning fossil fuel - is expected to increase slightly; this is the only fossil fuel for which the demand rises according to all three scenarios in the report published by the International Energy Agency in 2012 [2], and it is expected to provide almost a quarter of the world’s energy in 2040 [1].

2010 2020 2030 2040 0 200 400 600 800 Time [year] W or ld en er gy con su m p ti on [E J/ ye ar ] 1960 1970 1980 1990 2000 20100 1 2 3 4 5 6 7 8 9 Time [year] Na tu ra l ga s re se rv es [1 0 3 E J] 40 50 60 70 80 R es er ve s/ P ro d u ct io n [y ea r] a) b) Natural gas Total Fossil fuels

Figure 1.1: a) World energy consumption in the next 25 years according to the U.S. Energy Information Administration [1] and b) historical trend of the reserves and the reserves/production (R/P) ratio for natural gas, according to OPEC [3]. EJ (exajoule) = 1018_J.

The rising demand for natural gas will have to be matched by its production. This means exploiting the natural gas reserves, which are typically defined as ‘the estimated quantities which analysis of geological and engineering data demonstrate with reasonable certainty to be recoverable in future years from known reservoirs under existing economic and operating conditions’ [4]. Due to large efforts in ex-ploration, but also due to the improving technology of exploitation, the amount of 1_{Organisation for Economic Co-operation and Development}

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1.1. Research motivation

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Figure 1.2: Different types of natural gas resources found in the Earth’s crust. The conventional resources can be either non-associated or associated gas, depending on whether there is also oil in the gas reservoir. The non-conventional resources are: coalbed methane, tight sand gas, shale gas and gas hydrates, also known as ‘methane clathrates’ (not shown). Image source: U.S. Energy Information Administration.

natural gas resources that are being converted to reserves, i.e. which have become

recoverable under existing economic and operating conditions has been steadily rising

during the last 50 years (see figure1.1b). An indication of how long the reserves will last is given by the reserves/production (R/P) ratio, which has had a value between 50 and 65 for the last 30 years [3]. This means that the current proven reserves of natural gas are abundant; they are expected to last for at least another half a century. Currently, the amount of yet undiscovered natural gas reserves worldwide is estimated to be ∼6400 EJ (a mean estimate by the U.S. Geological Survey [5]), which is about 70% of the current proven reserves; the true amount is, however, expected to be even larger, since the USGS study assessed only a limited number of geological provinces, that were all outside of the United States.

All the estimates for the reserves mentioned so far are conventional natural gas reserves. These are reserves that can be exploited using well-established technologies that have been employed (with improvements over time) already for several decades. Natural gas from conventional reserves exists in two forms: as non-associated gas and as associated gas (see figure 1.2). As the name suggests, non-associated gas is present in the reservoir with no accompanying oil, while the associated gas shares the reservoir with some amount of oil. To produce from these reserves, a well needs to be drilled through the subsurface formation into the reservoir, after which the gas flows under its own pressure to the surface. In older wells, for which the reservoir pressure has decreased, special stimulation techniques can be employed to produce the remaining gas, which will be done as long as the production is still economically viable.

The main non-conventional natural gas, on the other hand, appears in four types of deposits (figure 1.2): gas shale, tight sand gas, coalbed methane and gas hydrates (methane clathrates). Although there is a high uncertainty in the

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4 _1._Introduction

mates of non-conventional reserves, it is widely recognised that they are vast; for example, the U.S. Energy Information Administration estimated the world shale gas reserves at ∼8200 EJ [6], and the total resources of gas hydrates are estimated at 40000-200000 EJ [7] (although only some of these are expected to become reserves). Currently, natural gas is commercially produced from two non-conventional sources: gas shale and coalbed deposits. The production from gas-rich shale in particular has increased dramatically over the last decade; it now accounts for 40% of the U.S. natural gas production, and it is expected to rise both there, but also in China, which holds the world’s largest reserves [6].

Despite the large non-conventional resources and the increasing production of some of them in selected countries, it appears that they will remain of secondary importance compared to the conventional ones also in the foreseeable future. This is mainly because it is still cheaper and cleaner to produce natural gas using conven-tional techniques; for example, the production from gas shale has been linked to an increased risk of groundwater contamination with chemicals used in hydraulic frac-turing (which is a technique used in production) and with methane seeping through the geological strata into the aquifers, while the gas extraction from coalbeds typi-cally produces large amounts of brine (salty water), which has to be properly treated to minimise the impact on the local water supply [8].

The conventional reserves, however, also come with a number of production challenges, which are expected to increase in quantity and complexity in the coming years. The reason for this is that ‘easy oil’ reserves (i.e. easily accessible hydrocar-bon deposits) are being depleted and production is therefore moving to increasingly difficult reservoirs: they are located farther offshore, in deep sea and deep under-ground, at geographical locations with long periods of subzero air and (sea)water temperatures, etc. Currently, the ratio of offshore to onshore gas production stands at ∼0.6, and is expected to grow in the next decade to ∼0.8 [9]. Furthermore, it is expected that a substantial amount of new development is going to occur in the Arctic, as it holds large natural gas resources; according to a recent assessment by USGS [10, 11], approximately 30% (1900 EJ) of the world’s undiscovered conven-tional gas reserves is located above the Arctic circle, of which 84% is offshore.

1.1.2. Multiphase flow in natural gas production

Once the development of a particular natural gas reservoir has been assessed to be economically and technologically feasible, several wells are typically drilled, which bring the reservoir fluids to the surface. In most cases, three fluid phases arrive at the well head: natural gas, gas condensate and water. Natural gas is composed of over 80% methane, with ethane, nitrogen, carbon-dioxide and water vapour consti-tuting the rest (not counting small amounts of other hydrocarbons and inorganic gases). In order to be sold to the customers, the gas needs to be separated from the gas condensate - the latter composed of heavier hydrocarbons which can be sold separately as natural gas liquids (NGL) - and water. When developing a gas field, a decision needs to be made on where this phase separation is going to take place (see figure1.3): sub-sea at the well-head, at a nearby floating platform, at a platform located further away from the well-heads in shallower water, or at a

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pro-1.1. Research motivation

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5

Ormen Lange - Scenario A Ormen Lange - Scenario B

Ormen Lange - Scenario C _{Ormen Lange - Scenario D}

A spar with gas piped to a tie-in point.

A spar with gas piped to the shore. Subsea comple-tion to the shore. Subsea comple-tion to platform, then to shore. shore shore tie-in point Kollsnes well-head well-head

well fluids well fluids sales gas sales gas

Figure 1.3: Various options to develop an offshore natural gas field. The wellstream fluids can either be processed on location (figures A and B) or transported to processing facilities in shallower water (figure D) or on the shore (figure C). In the example of the Ormen Lange gas field shown here, option C was chosen due to economic and strategic reasons. Image source: www.offshore-technology.com

cessing facility onshore. Sometimes, due to economic and environmental reasons, it is decided to apply full wellstream transfer to onshore processing facilities, although this is also the technically most demanding solution because it means operating a multiphase flow pipeline instead of several single-phase pipelines. This can create many problems in maintaining continuous flow (i.e. stable production) which are addressed within the discipline called flow assurance.

In multiphase flow, several phases with different density and viscosity flow through a pipe, while the combined effect of gravity and shear forces at the walls and at the interfaces determines the flow characteristics. At low to moderate gas velocities in near horizontal pipes, gravity will cause stratification of the phases. In natural gas pipelines, which carry small amounts of liquids along with a large amount of gas, the liquids will flow as a layer along the bottom of the pipe, and the gas will flow as a layer on top. This flow arrangement is called the ‘stratified flow pattern’.

Gravity and shear forces acting together determine the flow area (also called ‘holdup’) that each phase takes up in the pipe cross-section and the velocity of the phases. Importantly, in upward inclined pipe sections, the gravity works against the pressure gradient; this causes the holdup of the liquid phase in upward inclined sec-tions to be higher than in downward inclined secsec-tions (while keeping the same gas and liquid throughput). Also, it means that the pressure drop curve (see figure1.4) has a minimum; decreasing the flowrate through a pipe decreases the pressure drop (the pipe is in the so-called ‘friction-dominated’ regime). There is a certain pro-duction rate (marking the minimum in the pressure drop curve), below which a

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6 _1._Introduction 50 100 150 200 250 300 0 2.5 5 7.5 10 Production rate [kg/s] L iqu id an d w at er co n te nt [1 0 3m 3] 0 25 50 75 100 P re ssu re d ro p [b ar ] pressure drop liquid content water cont. natural gas gas condensate water

Figure 1.4: A pipe cross-section showing stratified three-phase flow of natural gas, condensate and water (schematic, left). The dependence of the liquid content and the pressure drop of a pipeline on the production rate (right). The characteristic minimum in the pressure drop curve, and the associated sharp increase in the liquid content for low flow rates, is the result of gravity working

against the pressure forces in the upward inclined sections of the pipeline. Data source: Holm [12].

further decrease in the flow rate leads to an increase of the pressure drop (the pipe is in the ‘gravity-dominated’ regime). The increasing pressure drop is caused by the increasing liquid holdup. For a low elbow (low spot) in a pipeline, this means that the holdup tends to be relatively high in the upward inclined leg. In figure1.4, this is seen as a sharp increase in the liquid content - also called the ‘liquid inventory’ or ‘total liquid holdup’ - as the pipeline moves into the gravity-dominated region.

The combination of the mentioned friction and gravity effects creates complexity and makes it difficult - given the flowrates of the phases and the pipeline profile - to determine the holdup and the pressure drop, especially at low flowrates i.e. close to and below the transition between the friction- and the gravity-dominated regions. Despite the difficulty, a large effort is being invested into developing models that can better predict the holdup and the pressure drop, because these are the two key parameters for pipeline operation. It is especially important to know the liquid inventory accurately in the following cases [13, 14]:

1. When there is a risk of generating large liquid slugs in some parts of the pipeline (e.g. severe slugs in risers) or in piping connecting the various pro-cessing equipment onshore (reactors, separators, heat exchangers, etc.). When the flow conditions are changed, the accumulated liquid can be displaced from the low spots, producing slugs which could pose a hazard to pipe and equip-ment integrity.

2. When changing the production rate of an operating pipeline, since the dif-ference in steady-state liquid inventory (content) will be produced out of the pipeline (at ramp-up) or accumulated in the pipeline (at turn-down). The amount of liquid produced from the pipeline at the downstream end can be too large for the downstream equipment (e.g. slug catchers) to handle, which disrupts the production. In the other case, when liquid is accumulating, it is

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1

7

important to know its amount, because this determines the total amount of hydrate and corrosion inhibitors needed for efficient operation.

3. When depressurising the pipeline, since this determines the maximum gas ve-locity at which depressurisation can be performed. During this operation, the goal is to keep the liquids inside the pipeline, in order not to flood the liq-uid drum of the flare system. The depressurisation itself is performed after production shutdown to prevent the formation of gas hydrates at low ambient temperature. Gas hydrates are solid ice-like structures made of light hydro-carbon molecules (like methane) trapped in the crystal structure of water and they occur at low temperature and high pressure; the formation of hydrates is a serious threat to continuous production, since they can obstruct or even block the flow, and this should thus always be prevented.

On the other hand, it is sometimes required to remove all the accumulated liquid by using gas sweeping; this can be a part of the hydrate or corrosion prevention strategy. In this situation, it is of interest to know at which mini-mum gas velocity the liquid is removed from all the low spots of the pipeline. Also, it is of interest to know how much, and at what flowrate, liquid will be produced at the downstream end, since the downstream liquid handling facilities have a limited throughput and storage capacity.

Currently, the industry determines the pressure drop and the liquid holdup for a operational design by using the industry-standard OLGA or LedaFlow simulation packages, or the in-house codes of various oil and gas companies such as the Shell Flow Correlations of Shell. These packages solve conservation equations for mass, momentum and energy though after simplification to one spacial coordinate only, and with inclusion of correlations for various terms, like the wall friction and the interfacial stress. None of these packages, however, gives reliable predictions for the gravity-dominated conditions (i.e. at low flow in upward inclined pipes), at least not without substantial tuning of the shear stress correlations used by these models (see later).

At the same time, the need for accurate flow modeling will increase in the near future, with the development of more complex natural gas fields that are farther away from the shore. For these conditions, any error in determining e.g. the liquid holdup will amplify due to the pipeline length, which can potentially create many problems in the production. In figure1.5, data on the pipeline length, liquid loading (the volumetric ratio of gas to liquid flowrate) and water depth for several recently developed gas and oil fields are presented. It is seen that the gas fields, which are characterised by low liquid loading, have a pipeline production system that has already reached a water depth of 2 km and a length of 150 km of full wellstream transfer to the shore, and these numbers are likely to increase in the future.

To better illustrate the difficulties currently encountered by multiphase flow mod-elers working in flow assurance, several flow assurance design problems encountered during the development of the Ormen Lange field are presented (see figure 1.5) in the next section.

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1

8 _1._Introduction 0 50 100 150 200 101 102 103 104 Pipeline length [km] L iq u id lo ad in g [m 3 /1 0 6 m 3 st. ] 0 50 100 150 200 2.5 2 1.5 1 0.5 0 Pipeline length [km] W at er d ep th [k m ] 1998, Mensa 2002, Canyon Express 2005, Simian-Sienna 2007, Snøhvit 2007, Tweedsmuir 2001, Mica 2009, Tyrihans 2004, Coulomb 2004, Coulomb 1998, Mensa 2001, Mica 2002, Canyon Express 2005, Simian-Sienna 2007, Ormen Lange 2007, Snøhvit 2009, Tyrihans 2007, Ormen Lange 2003, Penguins gas fields oil fields b) a) 2003, Penguins 2007, Tweedsmuir

Figure 1.5: Pipeline length, liquid loading and water depth of several recently developed oil and gas fields. Data source: Henkes [15]

.

1.1.3. Flow assurance issue in the Ormen Lange field

Ormen Lange is a large gas field (∼16 EJ) discovered in 1997 approximately 100 km offshore from mid-Norway. Production started in 2007, through a pipeline system consisting of two parallel 30” (0.762 m) trunklines that carry the wellstream fluids to the gas processing facility onshore, 120 km away. The transported fluids are natural gas (20-35 106_m3

st./day/pipe2), condensate (100 m3/106mst.3 gas) and water (4 m3_/106_m3

st.gas). The fluids enter the pipeline at the well-heads, positioned at the sea bottom ∼850 m below the surface. The pipeline route follows the very rough seafloor topography, surrounded by water of -1 ◦C, making a total (cumulative) upward elevation change of ∼1800 m.

The main difficulties in developing this field lie in the very long and uneven profile of the pipeline needed to connect the remote subsea manifolds to the shore and in the subzero temperatures of the surrounding water. The profile of the Ormen Lange pipeline is shown in figure1.6a. It starts with a steeply upward inclined part over a horizontal distance of about 25 km, after which it continues at a depth of less than 250 m for the next 95 km. A closer look at the various parts of the profile (figure1.6c-d) reveals that it is composed of many upward and downward inclined sections which make the profile quite rough, especially when compared to some of the other pipelines, shown in the same figure. Statistically, slightly inclined upward and downward sections constitute the greatest part of the profile: sections with inclinations less than ±0.5◦ make up 50% of the profile, those of ±1◦60% and those of ±2◦ 80% of the profile (figure 1.6b). Besides this, almost 60% of the length is upward inclined at 0◦-3◦.

As mentioned earlier, the many inclined parts of the pipeline form a large number 2_{The subscript ‘st.’ refers to ‘standard’ conditions, which are 15.6}◦_{C and 101.325 kPa, commonly}

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1

9 0 25 50 75 100 125 −1000 −750 −500 −250 0 Pipeline length [km] S ea d ep th [m ] −30 −8 −3 −0.5 0 0.5 30 8 30 2 4 6 8 10 Pipe angle [◦_] P ip e le n gt h [% of to ta l] 0 20 40 60 80 100 C u m u l. le n gt h [% of to ta l] 0 2500 5000 7500 10000 −40 −20 0 20 40 Distance [m] E le va ti on ch an ge [m ] 40 45 50 55 60 −200 −180 −160 −140 −120 Distance [km] S ea d ep th [m ] detailed profile simplified profile Ormen Lange a) b) d) c) other pipelines

Figure 1.6: Characteristics of the Ormen Lange pipeline: a) profile of the entire pipeline, b) pipe angle distribution, c) profile of the first 8 km, compared with profiles of other pipelines and d) simplified and detailed profile between the 40th_{and the 60}th_{km. Data source: Holm [}₁₂_{], Henkes}

and Rudrum [16].

of low spots (V-sections, lower elbows) where liquid can accumulate under certain flow conditions, which are typically not reached during operation at the nominal flowrate, but are reached during e.g. decreasing or stopping production or depres-surisation of the pipeline. As mentioned earlier, it is important to know the liquid content and the pressure drop of the pipeline both under nominal and special oper-ating conditions. The prediction of these quantities for the Ormen Lange field has been obtained using OLGA and the in-house Shell Flow Correlations (SFC). These are transient one-dimensional three-fluid models, which means that the governing equations for mass, momentum and energy - dependent only on the streamwise coordinate and on time - are discretised and solved for each phase.

The governing equations for the different phases are connected to each other and to the flow boundaries through (semi-)empirical closure models. In the momentum equations, the closure models are for the shear stresses at the fluid-fluid and at the fluid-solid boundaries. Over the years, as the understanding of the phenomena influ-encing the shear stresses grew, the implemented closures were becoming increasingly

mechanistic (i.e. based on physical principles), but nevertheless they still include

an empirical part, which is tuned against a set of lab and field data. For example, the Shell Flow Correlations determine the interfacial stress by using a well-known wall shear stress correlation (that of Churchill [17]), with the effect of the waves rep-resented through an increased surface roughness; the relative roughness is in turn derived using a formula that was fitted to experimental data [18]. A recent model

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1

10 _1._Introduction

for stratified flow - that of Biberg [19], a version of which is presently included in OLGA - models the ‘interfacial turbulence level’ (which is the scaled eddy viscosity) on each side of the gas-liquid or liquid-liquid interface using algebraic expressions containing non-dimensional numbers (Reynolds number, Froude number and the density ratio) along with constants that were obtained through, again, fitting to experimental data.

During the design phase of the Ormen Lange field - in an effort to ultimately achieve better flow predictions - two experimental campaigns were conducted (by IFE in 1999 and by Norsk Hydro in 2001), which gathered data under conditions similar to the ones expected in the field. Using these data, and other experimental and field data collected previously, the latest version of the OLGA model available at the time (OLGA2000 v.4) was modified and re-tuned to better predict the newly-acquired database. Some of the models that have been modified and/or re-tuned were the gas-liquid interfacial friction factor at low velocities in near horizontal pipes and the gas-liquid and oil-water interfacial friction factors for gravity-dominated flows [20]. Besides this, the criterion for transition between laminar and turbulent liquid flow was also modified, in order to get a better prediction of the liquid-wall shear stress at low Reynolds numbers [20].

The changes implemented in the OLGA model resulted in better predictions of the experimental data; however, once Ormen Lange started production and enough field data were gathered, studies showed that there were still some inaccuracies in the OLGA predictions under real operating conditions [16,21].

An example of this is the prediction of the time needed for the return of glycol injected into the pipeline before the production started in September 2007. Namely, in order to prevent the formation of gas hydrates (once the production starts) by any water that might be present in the pipeline before its startup, 5200 m3 _of mono-ethylene glycol (a chemical which mixes with water, creating a solution with a significantly reduced hydrate formation point) was added to the pipeline. The glycol was added both onshore to pipeline A and subsea to pipeline B. After this, natural gas from the Sleipner field was circulated from the shore through pipeline A and back through pipeline B in order to distribute the glycol throughout both pipelines. At the same time, a certain amount of glycol was being continuously added both onshore and offshore. Once the first quantities of glycol returned onshore, the glycol injection could be stopped, because it meant that glycol is distributed along both pipelines, i.e. the potential formation of gas hydrates is prevented [16,21]. As shown in figure1.7a, at the moment of the first glycol return a total of 8100 m3 _of glycol was stored in the two pipelines.

Figure1.7 shows this gas and glycol recirculation operation as it was measured in the field and simulated by the improved OLGA model. The total amount of glycol in the pipelines is shown on the left, and the amount returning to shore is seen on the right. The OLGA simulations were performed for two pipeline profiles: simplified and detailed (see figure 1.6d). The results with the original model for both profiles underestimate the time needed for the glycol to return; the majority of it arrives onshore after ∼50 hours for the simplified, and after ∼67 hours for the detailed profile. This is an error of 44% and 25%, respectively. Since the glycol was

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1.2. Research objectives

1

11 0 25 50 75 100 4 5 6 7 8 Time [h] G ly co l h ol d u p [1 0 3 m 3 ] 0 25 50 75 100 0 100 200 300 400 Time [h] C u m u la ti ve gl yc ol re tu rn [m 3 ]

field org. simp. org. det. tuned simp. tuned det.

b) a)

Figure 1.7: Comparison of OLGA simulations with field data of the glycol distribution operation in the Ormen Lange pipeline: a) total glycol content in the pipeline, b) cumulative glycol returning onshore. Transient simulations were performed for the simplified and the detailed pipeline profile (see figure1.6d). To achieve better agreement with the field data, the interfacial friction factor in the OLGA simulations needed to be reduced (see original vs. tuned results). Data source: Henkes

et al. [21].

predicted to return too early, it means that the interfacial shear stress between the gas and the glycol was overestimated. When the model was tuned by reducing the interfacial friction (to 0.03 and 0.4 times the original value for the simplified and detailed pipeline profile, respectively), a better match between the field data and the models was achieved.

The case of glycol return gave the flow assurance engineers of Ormen Lange two insights. Firstly, since the predictions of both the original and the tuned model were better for the detailed than for the simplified profile, it underlined the importance of representing the pipeline as accurately as possible, including all the short upward and downward sections where liquid can accumulate. Although it is computationally beneficial to simplify the profile, it can produce large errors in predicting the liquid inventory, particularly at low flow. Secondly, it again stressed the necessity of improving the closure models for the shear stresses, both at the wall and at the interface(s), especially under gravity-dominated conditions and with higher viscosity liquids. By developing closures that include more physical understanding of the processes involved, the need to modify the fitting constants each time a new system is simulated would be reduced, if not completely eliminated.

1.2. Research objectives

M

ultiphase flow through pipelines under gravity-dominated conditions is the central topic of the current Thesis. In the previous sections, it is shown that this type of flow is often encountered in natural gas pipelines that carry the

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well-1

12 _1._Introduction

stream fluids (gas, condensate and water) from the wells to the shore. An example was also given (§1.1.3), which shows that current multiphase flow models cannot predict the flow under gravity-dominated conditions with sufficient accuracy.

This complex fluid flow problem is investigated on several levels. On a macro-scopic level, the objective of the study is to find out more about the flow behaviour in a pipeline with undulating topography when only the gas is transported through the pipe, while the liquid remains inside. This relates to the hydrate or corrosion pre-vention strategy (see §1.1.2), and to the possible formation of terrain slugs. Another objective is to determine how well the available multiphase flow models - originally developed for situations where all phases are flowing - predict this specific situation in which only the gas is flowing over accumulated liquid. Related to this, it is of interest to study the problem of the occurrence of multiple steady-state solutions in multiphase models under these conditions. This will enable us to see in what way the steady-state solutions correspond to what is observed in the pipeline.

In the previous sections, it was also shown that much of the inaccuracies in the model predictions stem from correlations representing the shear stress at the gas-liquid interface. Also, it is clear that tuning the correlation coefficients cannot solve the problem, since it does not employ any knowledge about flow physics. The objective of the study on a more microscopic level is, therefore, to understand the details of the phenomena occurring within the fluid layers of stratified flow, since it is these small-scale mechanisms that are finally responsible for producing the shear stresses at the fluid-fluid and fluid-solid interfaces. In particular, it would be interesting to study the evolution of waves at the interface, and their interaction with the gas and liquid layers. Also, referring to the improvements made in the OLGA model for the Ormen Lange pipeline (see §1.1.3) and the inaccuracy when predicting the flow of a relatively viscous liquid such as glycol, the objective is to measure the flow under both laminar and turbulent conditions and to see how these affect the waves and the resulting shear stresses.

1.3. Approach and scope

I

n this Thesis, two-phase flow of gas and liquid through circular pipes is consid-ered. Both an experimental and a modelling approach are adopted to address the problems stated in the previous sections.

The flow under gravity-dominated conditions in pipelines is studied in an exper-imental setup containing a V-shaped section, that can be considered a model for the pipeline as a whole. Experiments are performed on liquid accumulation and sweep-out under the so-called ‘zero net liquid flow’ conditions, i.e. when only gas is transported through the pipe, while the liquid remains in the lower elbow. The focus is on fluid states close to the accumulation/sweep-out critical point, which means that the gas-liquid flow is always stratified. The initiation of slugs in this configuration (which are known as ‘terrain slugs’) is not considered, nor is the situa-tion when both phases are transported through the pipe. The angle of the V-secsitua-tion is kept below 3◦, since this is also the inclination of the majority of the real pipeline profiles. Both the angle of the V-section and the viscosity and density of the liquid are varied, and the experimental findings are compared to predictions of a

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two-1.4. Thesis outline

1

13

phase flow model, using various sets of shear stress correlations. Furthermore, a transient version of the same model is implemented to investigate whether it can predict the behaviour observed in both the present study and in the experiments of others, regarding the existence of multiple steady states of the liquid layer under these conditions.

The investigation of the small-scale phenomena in the gas and liquid layers is performed in another experimental setup consisting of a straight pipe. It is done using Particle Image Velocimetry (PIV), which is an optical technique to measure velocity in a planar cross-section of the flow. The study is restricted to investigating the flow in a horizontal pipe, but detailed measurements are performed in both phases of the flow simultaneously. A horizontal pipe is chosen because it can be seen as a base case for studying the detailed phenomena of stratified flow in near-horizontal pipes. Besides this, it is expected that the mechanisms responsible for the shear stresses at the wall and at the interface in horizontal pipes are the same as those in slightly inclined pipes. Both here and in the V-section setup air and water are used as working fluids, since these can be easily and safely handled under laboratory conditions. In the straight pipe setup, the flowrates can be selected to be low to moderate, so that both laminar and turbulent fluid layers can be achieved, along with a smooth and a wavy interface. Besides the velocities in the two phases, the interfacial characteristics are measured as well, using an optical technique that works simultaneously with PIV. The combination of the two techniques provides us with plenty of information about the small-scale phenomena in stratified two-phase pipe flows.

1.4. Thesis outline

C

hapter 2 describes the two experimental setups used in this study. Special attention is given to the dual PIV system, that is configured to measure the velocities in both phases of the horizontal stratified pipe flow, as well as to determine the position of the gas-liquid interface.

Chapters3 and4 present the results of the measurements and simulations per-formed in the V-section setup, focusing on liquid accumulation and sweep-out. Chapter 3 considers the simulations performed using the transient two-phase flow model, along with the comparisons to experiments performed under conditions were multiple steady-state solutions are predicted to exist. Chapter4 presents predom-inantly experimental results obtained under conditions where the liquid layer is in the so-called ‘critical’ position, i.e. at the point of being swept out of the pipe. The results are compared to the predictions of the steady-state two-phase flow model using various closures described in the literature.

Chapters5, 6 and7are devoted to the results of the PIV measurements in the horizontal two-phase flow setup. The experimental technique used to determine the instantaneous position of the gas-liquid interface is described in Chapter 5, along with the results of the PIV measurements in the liquid phase, performed under laminar and turbulent conditions of the liquid layer, as well as with a smooth and a wavy interface. Chapter 6 reports on the investigation of the transition to turbulence in the liquid layer of stratified flow and how this affects the various

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wave-1

14 _References

types observed on the gas-liquid interface. Chapter 7 presents the results of the simultaneous measurements in both phases of stratified flow, covering - along with the measurements discussed in the other Chapters - all the regular wave patterns observed in gas-liquid pipe flow.

Finally, the conclusions of the entire study and recommendations for further research are given in Chapter8.

References

[1] International Energy Outlook 2013, U.S. Energy Information Administration (2012).

[2] World Energy Outlook 2012, International Energy Agency (2012). [3] Annual Statistical Bulletin 2013, OPEC (2013).

[4] Definitions, Sources and Explanatory Notes at www.eia.gov, U.S. Energy Infor-mation Administration.

[5] An Estimate of Undiscovered Conventional Oil and Gas Resources of the World,

2012, U.S. Geological Survey (2012).

[6] Technically Recoverable Shale Oil and Shale Gas Resources: An Assessment of

137 Shale Formations in 41 Countries Outside the United States, U.S. Energy

Information Administration (2013).

[7] A. Milkov, Global estimates of hydrate-bound gas in marine sediments: how

much is really out there? Earth-Science Reviews 66(3-4), 183 (2004).

[8] World Energy Outlook Special Report on Unconventional Gas: Golden Rules

for a Golden Age of Gas, International Energy Agency (2012).

[9] Prediction at www.energyfiles.com, Energy Files.

[10] Circum-Arctic Resource Appraisal: Estimates of Undiscovered Oil and Gas

North of the Arctic Circle, U.S. Geological Survey (2008).

[11] D. L. Gautier, K. J. Bird, R. R. Charpentier, A. Grantz, D. W. Houseknecht, T. R. Klett, T. E. Moore, J. K. Pitman, C. J. Schenk, J. H. Schuenemeyer, K. Sørensen, M. E. Tennyson, Z. C. Valin, and C. J. Wandrey, Assessment of

undiscovered oil and gas in the Arctic, Science 324(5931), 1175 (2009).

[12] H. Holm, Long gas/condensate tie-backs, Norsk Hydro (2005).

[13] R. A. W. M. Henkes, Liquid accumulation in nearly horizontal pipelines with

multiphase flow at low gas production rates; Proposal submitted to the Shell Science Cluster on Novel Engineering Solutions (2009).

[14] B. Guo, S. Song, J. Chacko, and A. Ghalambor, Offshore Pipelines (Gulf Professional Publishing - Elsevier Inc., 2005).

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References

1

15

[15] R. A. W. M. Henkes, Het zit in de pijplijn; Rede, uitgesproken bij de

aanvaard-ing van het ambt van deeltijdhoogleraar in het vakgebied ’Multiphase Pipeline Flows’ aan de faculteit der Technische Natuurwetenschappen, TU Delft (2010).

[16] R. A. W. M. Henkes and G. A. Rudrum, Benchmarking of the wells and pipeline

models in the Ormen Lange Flow Assurance System (FAS), Shell Global

Solu-tions (2009).

[17] S. W. Churchill, Friction-factor equations spans all fluid-flow regimes, Chem. Eng. 7, 91 (1977).

[18] R. A. W. M. Henkes, A. J. N. Vreenegoor, and G. Haandrikman, Threephase

model for the stratified flow of water, oil and gas in pipelines, Shell Global

Solutions (1998).

[19] D. Biberg, A mathematical model for two-phase stratified turbulent duct flow, Multiphase Sc. and Technol. 19(1), 1 (2007).

[20] H. Kvandal and A. Valle, OLGA2000 development for inclined flow, Norsk Hydro (2003).

[21] R. A. W. M. Henkes, K. Beran, and P. J. J. Moeleker, Comparison of the

OLGA and COMPAS dynamic models for the Ormen Lange pipeline, Shell

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2

Experimental setups

This chapter describes the two experimental setups used in the present study. The first setup is an air/(glycerol-)water flowloop that includes a V-shaped section used to study the flow at Zero Net Liquid Flow (ZNLF) conditions. The second setup is an air-water flowloop that consists of a straight horizontal pipe used to study stratified gas-liquid flow under non-wavy and wavy conditions.

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2

18 _2. _{Experimental setups}

2.1. V-section setup

T

he flowloop containing a V-section (low elbow) was constructed by modifying an existing setup located in the Kramers Laboratory for Physical Technology of TU Delft. It is a 137 m long pipe made out of transparent PMMA (Perspex) segments of ∼2 m length and 50.8 mm (2”) nominal internal diameter through which air and water at atmospheric conditions can be circulated. The pipeline consists of two straight sections of 68 m length, connected through a 180◦bend. At the middle of the second (return) section, the supports holding the pipes were extended to allow for making a ∼18 m long V-shaped elbow (see figure2.1). The angle of the inclined legs (with respect to the horizontal) can be set, using a self-made auxiliary tool and a spirit level, to inclinations between 0◦and 2.1◦. In the experiments, the inclination of the upward leg (which is 9.8 m long) was set to 1.3◦, 1.7◦and 2.1◦.

p

∆p

downward inclined leg upward inclined leg

V-section, L≈ 18 m air supply rotameter, QG l open vessel Pipe diameter: 50.8 mm

Gas: air (atmospheric conditions) Liquid: water, 60% glycerol/water (w/w) liquid inlet, VL

Figure 2.1: Schematic of the experimental setup containing the V-section.

Since zero net liquid flow is studied, the pumps and rotameters, that normally circulate and measure the liquid, were not used. The original inlet section was closed off, and the air inlet was brought closer to the test section. Pressurised air provided by the laboratory utilities flowed from the inlet through ∼60 m of straight pipe before reaching the V-section. The air flowrate QG was regulated

with a valve and measured using a calibrated rotameter (Kytola Instruments HK-4DC, accuracy ±5 % full scale). A gauge pressure meter (Observator Instruments) was used to measure the overpressure p in the device. This quantity was used to arrive at a correct value of the mass flowrate through the rotameter (which is a volumetric measurement device), since the air density can change with flowrate. The volumetric flowrate (and therefore the velocity) in the test section can then be obtained by using the air density calculated with the temperature and pressure at the V-section. After the lower elbow, the gas flowed through a ∼35 m long horizontal section until it reached an open vessel where it discharged freely into the atmosphere.

The desired amount of liquid VL was measured using a measuring beaker and

then brought into the V-section through a tapping point in the downward inclined leg before the start of an experiment. The tapping point was closed and the experiment was started by increasing the airflow. The pressure drop in the upward inclined

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2.1. V-section setup

2

19 pressure tap 1 liquid inlet

a)

pressure tap 2

c)

pressure tap 2 rotameters

b)

Figure 2.2: Photographs of the V-section setup: a) the bottom of the V-section showing the first pressure tap and the liquid inlet, b,c) views downward along the inclined leg, showing the second pressure tap, the rotameters, and a horizontal section of the flowloop.

leg was measured over a 1.7 m long section. The first pressure tap (a 1.5 mm diameter opening at the top of the pipe) was positioned 15 cm from the lowest point of the elbow, while the second was 1.7 m away, i.e. at 1.85 m from the lowest point. The pressure taps were connected to a calibrated differential pressure meter (Furness Controls Ltd. FCO12, accuracy ±0.5 % full scale), which measured the static pressure drop ∆p in the gas.

In critical conditions, the length of the liquid layer l was recoded. In subcritical conditions, both the length in the downward ld and in the upward lu inclined leg

were measured using a common measuring tape.

Fluids used in the setup were air, tap water, and a 60% (w/w) glycerol/water mixture. The temperature of the fluids was monitored and was typically between 16 and 24◦C. The initial amount of liquid poured into the test section was less than 1 liter in all experiments. The airflow could be increased such that all the liquid is expelled from the V-section. When working with the viscous glycerol/water mixture, some liquid could remain attached to the pipe wall, especially in the downward inclined leg. This liquid was removed by pouring a large amount of water into the pipe and flushing it out with air.

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2

20 _2. _{Experimental setups}

PIV test section ∆p1

water tank air fan

turbine flow meter

Y inlet section with

Coriolis flow meter water pump T

∆p2

water PIV laser

flow straightener 50 D

200 D

horizontal pipe, D = 50 mm

air PIV laser

p,T air flow seeder

Figure 2.3: Schematic view of the straight horizontal pipe setup.

obtained using this flowloop are used in Chapter3, while the measurement procedure and the final results are presented in Chapter4.

2.2. Straight pipe setup

A

new two-phase gas-liquid flowloop was constructed in the Applied Physics building of TU Delft (see figure 2.3). The design goal was to achieve strati-fied flow of air and water in a relatively long horizontal or slightly inclined (±5◦) pipe at atmospheric conditions.

2.2.1. General configuration

The length of the pipe was 10.3 m, which is the maximum allowed by the size of the laboratory. The inner diameter was selected to be 50 mm to maximise the length/diameter L/D ratio (which should be high in order to achieve fully developed flow), while still remaining large enough to achieve similar physical phenomena that occur in natural gas pipelines. The pipe was constructed by joining shorter segments made of transparent PMMA (Perspex) and glass (see table2.1). High-quality glass pipes (Schott) with 2.6 mm wall thickness were used for the test section because of their high optical quality and their precise inner diameter. The rest of the pipe was made of PMMA, which is cheaper and less brittle. The disadvantage of using PMMA is that the production tolerances for the pipe geometry are quite high. In table2.1, it is seen that both cast and extruded PMMA pipes usually have different diameters at the two ends. For cast pipes this difference is typically larger than for extruded pipes; on the other hand, cast pipes have a better roundness. For the present setup, a number of pipe segments were selected from a large collection such that each pipe segment: 1) has a diameter that is as close as possible to the 50 mm of the glass pipes, and 2) has the smallest difference in diameters between the subsequent pipe sections, which includes the Y-shaped inlet section which has an outlet diameter of 51.6 mm.

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2.2. Straight pipe setup

2

21

Table 2.1: Pipe segment characteristics in the straight pipe flowloop. Segment No. 1 is the Y-shaped inlet, which has a 126 cm long horizontal section, see figure2.4.

Pipe length Inlet diameter Outlet diameter

Pipe material [cm] [mm] [mm] 1. 126.0 - 51.6 PMMA, cast 2. 203.5 52.0 49.6 PMMA, cast 3. 203.5 49.7-50.6 49.5-50.4 PMMA, extruded 4. 100.0 49.7 49.9 PMMA, cast 5. 100.0 50.0 50.0 glass

6. 100.0 50.0 50.0 glass (test section)

7. 204.0 49.8 49.2 PMMA, cast

flow straightener

air inlet

water inlet open vessel

atomiser pipe outlet

water

Figure 2.4: Photograph of the Y-shaped inlet section (left) and the outlet part of the setup (right). The water enters through a flexible hose, 17-18 mm in diameter. The two fluids meet 40 cm from the left edge of the Y-inlet.

by using a digital protractor (Pro 3600, accuracy ±0.05◦) and it was aligned with the previous pipe segment. In addition to this, the overall alignment was checked by filling the pipe with water almost to the top and checking visually if the lengthy air bubble had the same size along the length of the pipe. Air and water entered through a Y-shaped inlet section (see figure2.4), which consisted of a 126 cm long horizontal part and a 40 cm long branch that has a 30◦ angle with the horizontal. The air entered from the top, through the branch, and the water flowed horizontally. The length of the Y-inlet section following the location where the two streams meet was 86 cm. After this point, the flow developed over a length of 7.5 m (150D), before it reached the test section that was positioned halfway along the second glass pipe segment. After this, there is another 2.5 m (50D) before reaching the pipe exit. Water flowing through the pipe discharges freely into an open container, see figure2.4.

The water was circulated using a radial pump. It transported water from an open container at the end of the pipe to the inlet section through a flexible hose (17-18 mm in diameter). A Coriolis mass flow meter (Rheonik RHM08, accuracy ±0.2 %) measured the liquid flowrate, which was controlled by adjusting the frequency of the pump and a set of valves. The air was driven through flexible hoses of large

Experiments in stratified gas-liquid pipe flow