Journal of Applied Fluid Mechanics, Vol. 9, Special Issue 1, pp. 1-9, 2016. Selected papers from the 7thInternational Exergy, Energy and Environment Symposium, IEEE7-2015 Available online at www.jafmonline.net, ISSN 1735-3645, EISSN 1735-3645.
Slug Catcher Multiphase CFD Modeling: Optimization and Comparison with Industrial Standards G. Montenegro1†, G. D’Errico1, A. Della Torre1, L. Cadei2, S. Masi2 1.
Politecnico di Milano, Department of Energy, InternalCombustionEngines Group, via Lambruschini, Milano, I-20156, Italy 2 Via Emilia, 1 Piazza Ezio Vanoni 1, 20097 San Donato Milanese (MI) – Italy †Corresponding Author Email:
[email protected] (Received October 20, 2015; accepted December 10, 2015)
ABSTRACT In the oil & gas industry, the traditional procedure for slug catcher design is based on the Stokes' law. Design equations are obtained from a 1-D analysis and validated with experimental data. Therefore, this method basically relies on simplified models and empirical correlations. For this reason, an over margin factor from 20 to 40% is usually applied. In this paper, a simplified CFD procedure for the modelling of the gas-liquid separation is presented. Steady state and transient models have been considered for single phase and multiphase fluids, using OpenFOAM. The influence of flow model and mesh grid on results have been evaluated as a trade-off between solution accuracy and computational efforts, in order to assess the applicability of these models to industry. A comparison of the industrial validation procedure with the CFD analysis has been realized, focusing on the pros and cons of the two different approaches. A new application solver has been constructed and programmed in order to get the most accurate results with the minimum computational efforts. This solver is based on a completely new and innovative approach to the Navier-Stokes equations for multiphase flow. New model proposed has been used for the evaluation of design for the two slug catchers studied, in order to get a better separation and fluids management. Keywords: Slug catcher; Multiphase; OpenFOAM; CFD.
NOMENCLATURE A
amplitude of oscillation
Fy
a Cp Cx Cy c dt Fx
cylinder diameter pressure coefficient force coefficient in the x direction force coefficient in the y direction chord time step X component of the resultant pressure force
f, g h i j α γ
1.
INTRODUCTION
The global energy demand has grown exponentially since the last century, and according to the main energy agencies an increase of 41% is expected in 2035. In this context, the fossil fuels, and in particular the oil and the natural gas, will maintain their primary role, achieving the 75% of the market share (BP, 2014). The inexorably depletion of the traditional and easily exploitable reservoir has led the major Oil
component of the resultant pressure force acting on the lower side generic functions height time index during navigation space index angle of attack dummy variable
Company to focus their attention on the improvement of production efficiencies. One of the main challenges in the optimizations process of the production systems is the development of innovative and precise methodologies able to handle the multiphase flow, which is a very common phenomenon in the actual Oil and Gas applications and can cause the onset of critical working conditions in upstream and downstream apparatus. This last characteristic leads inexorably to the loss of production capacity of the entire production system, composed by the reservoir, the well and the
G. Monttenegro et al. /JA AFM, Vol. 9, Sppecial Issue 1, ppp. 1-9, 2016. surface facilities, and fiinally to the deccrease of the companyy profit.
of slug catcher c is the rreduced buffer storage s volume, generally less thhan 100 m3.
In the cuurrent work, a CFD C analysis is used u to study the first stage of the sep paration process between gas and oil occurring at the slug catch her facility, considerring also the cap pacity of the sluug catcher to handle and a distribute thhe multiphase flow f coming from th he wellhead. Th he Oil & Gass world has usually preferred to exxploit experimenntal data or simplifieed approach to design d and valid date the parts of the slug catcher facility fa responsiible for the separatioon of the phasess and for the management of the incooming flow. Thhe project startts from the analysis of the main currrent standard meethodologies adopted in the petroleum m industry for the design and t slug catcher facility. validatioon procedure of the The num merical calculation are carried out o using the OpenFO OAM® code (O OpenFOAM docu umentation), an advaanced and freee CFD Toolbo ox, able to customizze and extend software soluttions to the simulatio ons process acro oss a wide rangee of physical phenomeena and at different level of in-depth analysis. OpenFOAM has an extensivve range of features to solve anny continuum mechanics problem m, ranging froom complex fluid f flows involvinng chemical reactions, turbulennce and heat transfer, to solid dynam mics and electrom magnetics. It is writteen in a highly effficient C++ objject oriented program mming, and, since it is open source, s it is easily customizable. c Foor this reason it has been adopted for this industtrial project as it allows to develop solvers ad hoc for f a physical pro oblem.
2.
7.
Multi-pippe type (also ccalled finger-typ pe): in this case the pipeline froom the well is directly d connecteed to a manifoold that collectts and distributees the flow too several tubess. This configuraation offers a technical-econnomical advantagge both to managge the level of prressure and to realize a bufffer storage volume, v ensuring a continuous flow to the downnstream facility. Moreover, type the finger configuraation gives to the operator larger layout fllexibility along with the capaccity to handle laarge slug flow froom the well. Hoowever, a large number n of pipes is required to provide p sufficientt volume and tthis results in a wide slug catcher footprint.
8.
Parking loop type: thiss hybrid configuuration joins the features of the vvessel and thosee of the finger tyype slug catcheers. This slug catcher c type is ad dopted to managge liquid carry over o in counter current gas/liqquid flow. It is i also suitable for the offshoree application thaanks to its particuular geometry. T The main disadvvantage of this device is the dependence on n strict operationnal conditions thhat reduces flexibbility.
The configurration analyzed in this work is the finger type one, o which is rrepresented in Fig. F 1. However the CFD methodoloogies developedd in the project are also a applicable in case of diifferent geometry mainntaining the righht level of accuraacy.
THE SLUG CA ATCHER
The slugg catcher is statio onary equipmennt used in the upstream m oil production system. The maain functions of a slugg catcher can be summarized as follows: f 1.
It provides p a bufferr and storage voolume for the fluiids coming from m the well;
2.
It maintains m constaant and close to the t optimum the operating conddition of the uppstream and dow wnstream facilityy, allowing to maximize m the oil production. Th his function perm mits also to f rate to the gas line and enssure a constant flow to the t oil treating seection;
3.
Fig. 1. Fllow regimes in horizontal pipees.
3.
It manages m the intermittent slug floow generated in the upstream section of the production faciility, distributting homogeneeously the incooming flux;
4.
it provides p a first stage s of separattion between the gas and the liquuid phases.
5.
Acccording to their geometrical characteristics, theyy can be classiffied in three maiin categories (Enngineering data book, b 2004):
6.
Vesssel type: simpple two phasee separation vesssel. The vessel needs to be larg ge enough to hanndle large liquid d slugs and a high design pressure. The conffiguration of th his device is ore plot size suittable for limitedd offshore/onsho in the t field. The maain disadvantagee of this type
THE E MULTI-PHAS SE FLOW PATTE ERN
The multiphaase flow cominng from the well, w is composed mainly by gas and liquid. The flow w inside the horizontaal pipes of the slug catcher can c be classified co onsidering the distribution of o the different phases inside the geoometry that definnes the ( 2). The distribution assum med by flow regime (Fig the different phases p inside thee pipes and the devices d is an importtant parameter in the study of the multiphase flo ow behavior. Thhe specific distribbutions are usually diivided into flow w regimes modeel, with particular feattures and charactteristics, for verttical or horizontal pip pes. Here, the attention a is focu used on the last ones (Ergun, 1952). 1.
2
The bubble flow: the ssmall gas bubblles are d into the continuuous liquid phasse. It is dispersed
G Montenegro et G. e al. /JAFM, Vol. 9, Special Isssue 1, pp. 1-9, 20016. dynam mics and on thhe experience in i engineering these devices. The technical siziing procedure currenntly adopted forr a finger typee slug catcher consissts of the followiing main steps:
typical for high h volumetric flow rate. 2 2.
3 3.
The stratified flow: generaated by the very low velocity off both phases, which leads tto a complete seeparation of the mixture. The liquid is separatedd by an undistu urbed interface ffrom he gas.
1. 2. 3. 4. 5. 6. 7.
The stratifiied-wavy flow: as the gas veloocity increases, small s waves in th he flux directionn are generated. The dimensio on of the w waves n the relative veelocity betweenn the depends on phases. Thee waves do not reach r the upper part of the pipe.
4 4.
The interm mittent flow: the continuoously increase off the gas velocityy conducts the small waves to reeach the top of th he pipe. This reggime is characterrized by the preesence of big waves w alternated to t small waves. This regime cann be divided intoo two sub-categoory:
5 5.
The plug fllow: this regimee is characterizedd by big elongatted bubbles of gas, which do not reach the dimension d of thee full section off the pipe. Thus, a liquid volume is always pressents in the bottom part of the pippe.
6 6.
The slug floow: when the gaas velocity increeases another tim me, the bubbles achieve a a dimennsion comparablee with the pipe diameter. d Hence, the flow insidee the pipe is characterized c byy an alternation between the gas g and the liquid phase.
7 7.
The annuular flow: increasing the gas volumetric flow rate, the liquid is pushedd on the pipe waall, forming an annular a film, thiicker on the botttom of the ducct. The interfacce is characterizeed by small waves and small droplets com ming from the liiquid film.
8 8.
The mist fllow: for very hiigh gas velocity, the liquid is transported t in the flux in small droplets disspersed in the gaas continuous phase.
Sttorage volume caalculation; Seelection of numbber and diameterr of fingers; Sizing of fingers; Sizing of inlet heaader; Sizing of downcom mer; Sizing of riser; Sizing of outlet heeader and equalizzation lines.
This procedure p involvves an iterative process that is compoosed of series of steps based on n the selection of the number and diaameter of the fin ngers, followed by thee validation of thhis selection. Iff the selections made are shown to bee inadequate, thhe procedure is repeatted starting froom the second point of the previoous list, varyingg the number of o the fingers, and/orr the diameter. O Once the design is successfully complleted, the overalll dimensions arre evaluated in order to t check the com mpatibility of theese parameters with thhe plant layout and a pipe-rack. It is important to focus the atteention on the relatioonship implemeented in this procedure to compuute the length of separation. In fact, some experiimental equationns depending onn the geometry of the slug catcher devvice and on the composition c of oil andd gas are deriveed and interpolatted from graph that shhows the separattion length as a function f of the superfficial velocity annd the density of the gas (as depictted in Fig. 3). In thiss project an alteernative industrial approach to the deesign of the slug catcher separatiion mechanism is brieefly introduced. This proceduree will be used for quuick comparisonss with the CFD solutions. This proceddure is basedd on the onne-dimensional application of the Stokes law. The particle segreggation is modelled with a connstant terminal velociity that producess a flux of liquiid towards the bottom m of the geomeetry. The terminnal velocity is compuuted rearrangingg the balance beetween gravity, buoyaancy and drag fforces acting on n the particles. The particles segregattion starts from the top of the pipe su uddenly at the teerminal velocity.
The flow is conssidered completely dispersed inn this T w work, taking intto account the worst conditionn for t separation process the p allowedd inside the faciility, a and it is compposed by two phases: p natural gas ( (continuous phaase) and raw oiil (dispersed phhase, s small droplets).
4gd drop l g Vt 3Cd l
(1)
The drag d model can be chosen from m a variety of differeent experimentaal or semi-empirrical relations. In thiss work the Schhiller Neumann formulation is considdered [5]:
24 Re 1 0.15 Re p Cd p 0.44
w regimes in horrizontal pipes Fig. 2. Flow The first step needed to prooceed with a CFD T C a analysis of the multiphase m flow inside i a slug cattcher i the study of th is he actual design procedure for finger t type slug catch her already useed in the indusstrial p practice. It is based b on simplee concepts of ffluid
Ree p 1000
0.687
Re p 1000
(2)
Wheree the Reynolds nnumber depends on the relative velociity between the ggas and the liquid phases:
Re p
3
l d droplet U r l
(3)
G. Monttenegro et al. /JA AFM, Vol. 9, Sppecial Issue 1, ppp. 1-9, 2016. stress Reynoldds number, M is the averagedd interphase momenntum transfer terrm. For clarity sake, s it is reported thee volume phase fraction , defined d as:
Fig. 3. 1D Stokes S approach h
a
The induustrial standardss introduced above, implies the utilization of severaal simplifying assumptions. a Thus, many m theoretical problems arisee from these procedurres, generating some sourcess of errors, which can affect the fin nal results. Theese problems are brieffly listed below
3.
(6)
t two Combining thee two continuityy equations for the phases a and b it is possible to forrmulate the volumetricc continuity equaation as:
U 0
THE TWO PH HASE APPROAC CH
(7)
Where, U = aUa bUb .
Usually the dynamicss of two-phasse flow is modelled d using the Naavier-Stokes equuations for a Newtoniian fluid. The main approacches to the simulatio on of this kind of phenomena are (Weller, 2002; Hill, H 1998; Gosssman et al. 1992; Rusche, 2002):
quations is rearrranged into a prressure The set of eq equation and then t solved. In particular, for f what concernn the calculation n of the interphase moomentum, its vaalue id determin ned by different contrributions. In ourr specific case, due to the limited siize of the dropplets considered lift is neglected andd only drag annd turbulent drag are modelled.
1. Direct Numerical Simulation (DNS): ( the Naviier-Stokes equattions are emplo oyed without furthher manipulatio on. The topolo ogy of the interrface between thhe phases is deteermined as a part of the solution.
M V
2. Disppersed Phase Eleement (DPE): thhis model is also referred to as the Eulerr-Lagrangian apprroach. The equ uation of motiion for the dispeersed phase is expressed e in thee Lagrangian form mulation, while the conservatioon equations for the t continuous phase are expreessed in the Euleerian frame.
Fd Ftd
(8)
The turbulence (Gossman, 19992) is introduceed as a standard k-ε model, whichh is suitable for f the particular flow w conditions uunder study, inccluding source terms to t incorporate thhe dispersed phhase on turbulence. This approach h can provide thhe best approxim mation of a real two-p phase physical phenomenon. p Thhe main drawback is the t computationnal burden required to perform the simulation, mainnly due to the in ntrinsic unsteady naturre of the model.
3. Twoo-Fluid Model: in i this case both h phases are characterized using the Eulerian conservation c equaations. Hence thee model is also referred r to as the Euler-Euler E fram me.
3. SIMPL LIFIED SINGLE E PHASE APPRO OACH
The Tw wo-Fluid model has been adop pted in this work in order to simulaate the two-phaase dynamic. This methodology m reqquires less coomputational efforts than t the DNS and a it is more suitable for every floow regime than the DPE, thankss to the twoway cou upling granted byy the Euler-Eulerr approach.
To overcome the high dem mand of compu utation resources wheen the simulationn of a real geom metry is addressed, an innovative moddel, based on a singlephase approacch has been deeveloped, is prooposed. The Navier-Sttokes equation is solved only for f the continuous phhase, the gaseouus phase consideered as incompressible, and the segreegation process of the liquid droplet is modelled wiith the transport of the scalar variable α, which reprresents the voluumetric fluid flow. fraction of liquuid in the total fl
The govverning equations are written in the t averaged form forr both the fluidss, considering eaach phase as continuoous, allowingg in this way the interpenetration of one phase p into the othher one. The momenttum transfer between b the tw wo different phases is taken into account through thee term M
The idea is too represent the eeffect of the disspersed phase with thhe help of a siimple scalar traansport equation, reduucing in this waay the equationss to be solved to sim mulate the mulltiphase phenom menon. Instead of sollving the continuuity equation an nd two momentum traansfer equationss, one for each phases, p thus three equations. e The simplified approach presented heree allows solving only one conntinuity equation and one momentum m transfer equatiion for quation the continuouus phase. Thee continuity eq introduced, sim mulates the disppersed phase floow and the separationn process betweeen the liquid and a the gas. Applying g the steady staate condition and a the
t interface that connsiders the forcces acting on the betweenn the fluids: drrag, lift, virtuaal mass and turbulennt effect. For eaach phase it is possible to express the mass andd momentum conservation c respectivvely as:
U 0 t
Va V a V b
(4)
M U U U Reff p g (5) t
Where Reff is the combined turbulent and viscous
4
G Montenegro et G. e al. /JAFM, Vol. 9, Special Isssue 1, pp. 1-9, 20016. hhypothesis of incompressiblee flow, the m mass c conservation equuation is rearrangged in the follow wing f form, which is implemented in the solver ccode ( (Malalasekera, 2007; 2 Ferziger): U 0 (9) t T general forrm of the mom The mentum conservaation e equation, obtainned with the Eulerian approoach f fixing a cell vo olume in the space, can be wrritten w the followin with ng notation: U (10) U T S f UU t T is the sttress tensor of a Newtonian fluidd. w where A Applying the steeady state and incompressible i f flow c condition and neglecting the diiffusive transporrt of t liquid particcles, which are transported onlyy by the c convection, the final equation for the transporrt of t droplet volu the ume fraction caan be formulated as f follows:
U S p t
the seettling Vt . Thee value of Vt is, therefore, determ mined by means oof an iterative prrocedure. An adv vantage of this ssimplified approoach is that the system m of equations iss now decoupledd. There in no more interaction beetween the moomentum and contin nuity equations oof the two phasess, but there is a single phase system of o equation pluus a decoupled transport equation w which exploits the t convective term. This means thatt the system cann be modelled also ass a steady state process, p resultingg in a dramatic reducttion of the compuutation demand.
5 5.
To vaalidate the simpplified approachh, simulations were carried c out on a pipe with sqquared section. This allowed a having a calculation mesh m free from typical issues that generate g errors,, in order to quicklly optimize the t CFD appplication. The geomeetry is representeed by a 16 meteers pipe, with a squareed cross section cross section off 0.36 m2 (Fig. 4). In n this way it is possible to simplify the calculaations, reducingg the time necessary to solve the prooblem, and provviding the possib bility to easily analysse the results. The mesh in this case is compo osed by a simplee block.
(11)
S
The source term T m p , is mathem matically introduuced a a scalar quaantity inside th as his formulation and s should mimic thhe separation of the t liquid phasee due t the gravity efffect. Considerinng the physics off the to s sedimentation prrocess, it is posssible to undersstand t the gravity produces that p a flow of liquid particlees in t direction off the gravity. Thhis flow producces a the f flux that can be conveniently represented byy the d divergence operaator. For this reaason, it is possible to r rearrange the souurce term as a flux fl term, given by a v vertical velocityy multiplied by the orthogonal area o the cells: of
U Vt . t
(12) Fig. 4:Square duct ggeometry and mesh m detail.
The volumetric flux of this neew divergence tterm T c comes from the terminal t velocityy calculated in E Eq.1. T face center value of the terrminal velocity used The u i the discretizzation of the divergence in d term m is o obtained by lineear interpolationn of the cell ceenter v values of the terrminal velocity field. Equationn 12 c be also writtten in a more compact can c formulaation c considering the volumetric v flux given by the sum m of t two convecctive contributioons: gas convecction the a deposition: and
U - Vt 0 t
PRELIMIRA ARY SIMULATIIONS
The simulations are m managed focusing the attention on thee influence on tthe final solutioon of different param meters, such as thhe mesh size, thee discretization schem mes and the soluution algorithm ms. The results obtain ned have allowed the comparisoon of both the CFD approaches: thee eulerian-euleriian two phase approaach and the sim mplified single-pphase one. The simulaated cases are illlustrated in the Figs 5 and 6, where a threshold ffilter highlights a separation t the length efficieency of 99%. It can be noticed that of the complete segreegation of the liquid l phase is wo approaches, approxximately the saame for the tw approxximately 14 m. In this particulaar case there is no expperimental correelation that cann be exploited, therefo ore, the CFD reesults are compaared to the 1D Stokess’ approach. Thee particle diametter used for the calculaations has been cchosen equal to 150 μm.
((13)
IIn this frameworrk, it is really im mportant to definee the c correct boundarry on the terminal velocity. A As a M Matter of fact, im mposing at the boundary a Neum mann c condition wouldd imply to assiggn an incoming and o outgoing flux att those patches where there shhould n be any kind of flow. To acco not ommodate this iissue a slip type bouundary conditio on on the term minal v velocity field hass been imposed. The drag coefficcient is computeed with the SchiillerT N Neumann modell previously presented. In this ccase, s since there is onnly one velocityy (the gas veloccity), a assumptions neeed to be made in i order to estim mate t the relative vellocities. In partticular, it has bbeen a assumed that thee two phases arre moving one with r respect to the oth her only by the term responsiblee for
Fig. 5:Singlle-phase approaach.
5
G. Monttenegro et al. /JA AFM, Vol. 9, Sppecial Issue 1, ppp. 1-9, 2016. experimental database. d An analysis of o the velocity vvector in the dirrection orthogonal to the main flux reevealed the preseence of a lifting effectt at the end of tthe downcomer, where the rotational flow caused byy the variation of the geometry distuurbs the depositiion process. Thee liquid droplets, moddelled as sphericcal particles, aree lifted by this flow causing the ooverestimation of the separation length. This effect iis shown in Fig. 9.
Fig. 6. Two-phases approach h. The 1D D Stokes’ meethod underesttimates the segregattion length, provviding a value off 9.68 m. The maximum m error obtaineed is about 40% %. The main reason of o this discrepanccy is due to the fact that the 1D theoory approximatees the flow as a 1D flow, whose component c of the velocity in i the axis direction n is uniformly diistributed over thhe flow area. This is obviously o not reaalistic and thereffore does not account that in the midddle on the pipe thhe velocity is higher, causing c an increease of the lengtth necessary to allow the separaation. Imposin ng uniform distribution of the velociity and setting sllip boundary condition ns, the case beecomes equivaleent to a 1D case, ressulting in a sep paration length close c to one given byy the 1D theory.
Fig. 8. a) Sin ngle-phase thresshold filter; b) TwoT phase threshold d filter.
After hav ving validated thhe two approachhes, a simple part of a real slug catcheer, composed off a finger and a 45° incclined downcom mer has been considered (Fig. 7). This geometry perm mits to test the behaviour b of CFD sollvers, comparinng the results obbtained with the exxperimental inndustrial standdards. The downcom mer is 4 m lonng, with a diam meter of 30 inches. The T finger is incclined by 1.15°, and is 40 m long, wiith a diameter of o 48 inches. Thhe mesh size has beenn chosen after a sensitivity anaalysis on the volumetrric fraction disstribution. The calculation grid, connsidering these two parameterrs, has been generateed using the messh cartesian mesh generator of Open nFOAM, namelly snappy HexxMesh. This mesh gennerator creates a high quality, heex-dominant mesh sttarting from a background mesh m and a surface representation r o the geometry that will be of studied. Furthermore, surface refinnement and boundary y layers have been included in the final mesh. Also A in this case several test sim mulations are performeed, in order to further optimiize both the CFD sollvers. The attenntion has been posed p on the influencee of the mesh and a the boundarry condition characteristics.
Fig. 9. Rootational flow in nside the fingerr. The solver does not take into account that thee liquid deposited on the bottom off the pipe cann not be dragged in thee same way of siingle droplets. For F this reason, a coallescence model has been implem mented to simulate th he real behaviouur of the liquid already a collected at the t bottom of tthe geometry, liimiting this lifting efffect. The idea inn this case is to find a threshold valuue of the volum me fraction of liquid, above whichh the droplets start to agggregate, generating parrticles with largeer diameter. The model implemented individuates the αlim value whenn a cell w is completelyy full of spherrical particles without merging phenomena (Fig. 10)).
Fig. 7. Downcomer D + finger f geometryy and final mesh h detail. The finaal results are shown s in Fig. 8. As it is possible to see, in this case c the results obtained, in terms off liquid volumetrric fraction, usinng the singlephase ap pproximation is not in agreemeent with the Euleriann-Eulerian approoach. In particullar, it seems to overestimate the to otal length off separation, whereas the two-phasse simulation computes c a ment with thee industrial separatioon in agreem
Fig. F 10. α limit ccondition.
6
G Montenegro et G. e al. /JAFM, Vol. 9, Special Isssue 1, pp. 1-9, 20016. incom ming flow. For this purpose a single phase solverr, without conssidering the sccalar transport equatiion of the volum metric fraction, haave been used.
The model maintains constantt the value off the T d droplet diameter until the alphha limit is reacched. A After this pointt, the particle diameter d is obtaained w an exponenntial fit, which innterpolates the innitial with d diameter of thee liquid sphere and the maxim mum d diameter achiev vable, or in oth her words the cell e equivalent lengtth. The result obtained using the m modified singlee-phase solver similar to the one a achieved with thhe two-phase so olver (Fig. 11). This c confirms the impprovement in thee prediction thatt can b achieved inntroducing the coalescence moodel. be H However, the result is coonditioned by the a assumption madde to define the limit level of α and t interpolate thee values of the maximum to m diametter.
Fig. 12. a) Slug catch her finger-type 3D 3 model.
Fig. 11. a) Single-phase S coaalescence modell threshold filteer; b) Two-phasse threshold filter. The final compaarison with the industrial standdards T i illustrates a suubstantial underestimation of the e experimental prrocedure and of o the 1D Stookes’ a approach compaared to the CFD D analysis (Tab. 1). T This comparisoon reveals the lack of accuuracy a achieved with the main typical process deesign a adopted in the industry which does not take into a account 3D effeccts.
Fig. 13. Sim mplified 3D model. The fllow is assumed tto be completely y composed by gas. The T solver usedd for this simulaation is already implem mented in OpeenFOAM, for incompressible i flow and a exploits the SIMPLE algoritthm for steady state problems. The simulation is performed considdering a turbulent incompressibble flow and a steadyy state conditionns. The result is shown in Fig. 14.
Table 1 Compaarison of the diffferent approacches Expeeriment D 1D CFD al ap pproach Stookes approaach 10.06 11.29 13.000 Lsep[m] Moreover, it iss important to highlight that the M r reduction of tim me achieved usiing the new sinnglep phase approach to the study of the multiphase flow f d dynamics, is around the 25
5.
CONC CLUSION
Every simulationn presented in the E t previous secction i performed with is w simplified. In this section,, the f facility considerred is a real fingger type slug cattcher ( (Fig. 12).
Fig. 14. Gaas flow distributtion.
The 3D model can be simpliified exploiting the T s symmetry of thee device. The fin nal 3D model useed is r reported in Fiig. 13. The mesh m is generrated c considering the results of the previous p preliminary s simulations, and d in particular thhe downcomer plus f finger configurattion.
F shows an accceptable level of o symmetry in The Fig the flu uid distribution, which can be observed o in the velociity field obtainedd inside the pipes. In order to have a clearer unnderstanding on o the flow distribbution, the volum metric flow havve been probed acrosss general planes defined by the user allowing to evaaluate how the flow is distribuuted along the fingers. This is impportant informattion since the usual procedure is to assume a uniforrm distribution
A single phase simulation of thhe slug catcherr has b been performed to assess the cap pacity of the faccility t receive and to distribute homogeneously to h the
7
G. Monttenegro et al. /JA AFM, Vol. 9, Sppecial Issue 1, ppp. 1-9, 2016. of the gaas flow over all the t fingers.
design and vaalidation proceddure augmented with a fixed oversizee margin of 20% %. Moreover thee result achieved by the experimenttal process dessign is affected by a too high unncertainty, due to the experimental correlation that does not considder the dispersed phasse particle diameeter.
Tablle 2Percentage of the total flow ws in the differrent pipes Q [%] [ 51.116% Splitter 1 48.884% Splitter 2 14.776% Downcomer 1 15.110% Downcomer 2 20.112% Downcomer 3 20.114% Downcomer 4 15.111% Downcomer 5 14.777% Downcomer 6 As it is possible p to see (T Tab. 2), the current layout of the slugg catcher seem ms to gather annd distribute homogennously the incooming flow from the well. Having evaluated the fluuid dynamics off the device, it is now w possible to prroceed with the multi-phase simulatio on using the single s phase appproximated olver (F Fig. 15) and thee multiphase app proach (Fig. 16). Thee average particcle diameter is set to 1000 μm, in agreement withh the specificattions of the extractioon site.
Fig. 17.. Design evaluation procedure.. The final syntthesis of all the m models, consideerations and solutionss reported in tthe previous parts is combined in a general designn evaluation validation procedure abble to combinne the computtational lightness achieeved with the sinngle-phase steaddy state solver and thee high level of aaccuracy typicall of the two-phase appplication. In this way the perfectt tradeoff, between accuracy and reduction of reequired simulation tim me, is reached, making econom mically and technicallly feasible the aapplication of th he CFD approach in thhe Oil and Gas seector.
Fig. 15. Single phase simulatiion.
The proceduree (Fig. 17) is ann iterative proceess that starts from the current exxperimental ind dustrial standard and consequently aapplies the CFD D tools presented prevviously in the reeport. The singlee-phase steady state appproach is usedd to obtain a quiick and accurate distrribution of the phases p inside thhe slug catcher. The results r obtainedd in this way arre then used as initiall condition for thhe two-phase unnsteady application, reducing r the tiime required for f the simulation.
p simulatioon. Fig. 16. Two phases assumption a of o incompresssible flow The approxim mates the real flow characteeristics, but considerring that the pressure drop allong a slug catcher is limited, the expansion n ratio is approxim mately equal, to one. This allow ws to neglect the coompressibility without losinng solution accuracyy. Moreover, thee flow velocitiees inside the pipes coorrespond to loow Mach numbbers (
