aTC 6414

A New Approach for Finger Storage Slug Catcher Designc. Sarica, O. Shoham, and J.P. Brill, U. of Tulsa

Copyright 1990, Offshore Technology Conference

This paper was presented at the 22nd Annual OTC in Houston, Texas, May 7-10, 1990.

This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submiUed by the author(s}. Contents of t~e paper,as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The matenal, as presented, does not necessardy reflectany position of the Offshore Technology Conference or its officers. Permission to copy isrestricted to an abstract of not more than 300 words. illustrations may not be copied. Theabstract should contain conspicuous acknowledgment of where and by whom the paper IS presented.

ABSTRACT

Two options are available for separating thegas liquid mixture at the exit of a two phase flowpipeline operating under slug flow conditions.These are a traditional vessel type separator and afinger storage type slug catcher. Use of a vesselseparator is usually due to space limitations thatexist, for example, on offshore platforms. Fingerstorage slug catchers are the obvious choice forlong, large diameter pipes, especially those whichundergo pigging. They are more cost effective andmore simple to construct and operate.

In the past, sizing of finger storage slugcatchers were based primarily on experience andrules of thumb. Not surprisingly, most of theexisting slug catchers have been oversized. Withthe recent trend of using a subsea compact fingerstorage slug catcher upstream of the platformriser, the need for more accurate design methodsis even more crucial.

This paper presents a new, innovativeapproach for the prediction of the reqUireddimensions of slug catcher fingers. The approachis based on the effect of the finger pipe diameterand inclination angle on the transition boundarybetween slug flow and stratified flow. Predictionof the slug characteristics at the slug catcherinlet, under normal flow or pigging conditions,are incorporated. Based on the new approach,the reqUired length and optimal downwardinclination angle of the fingers can bedetermined.

The new approach has been used to designa finger storage slug catcher for actual field

References and illustrations at end ofpaper.

639

conditions. The effect of operational conditions,e.g. pipeline diameter and finger inclination angleon the reqUired slug catcher dimensions isdemonstrated.

INTRODUCTION

Pipelines can be operated under two phaseflow conditions for several reasons. In hostileenvironments such as arctic and offshore fields,oil and gas are often transported in a singlepipeline to reduce the construction cost. Innatural gas transportation, due to pressure andtemperature drop during flow in the pipeline,condensation causing two phase flow may occur.Depending upon the operating conditions, normalor terrain induced slug flow, may develop. Also,artificial slugs, possibly the largest ones, can becreated dUring the removal of accumulated liqUidby a pigging (sphering) operation in gas pipelines.It is a common practice to install a slug catcher toaccommodate liqUid slugs at the exit of a pipeline.A slug catcher can serve as both a separator andas temporary storage.

There are several unconventional slugcatcher types, such as a prorock slug catcher1 , aself supporting fluid separator2 , and a flexiblesubsea slug catcher3 . However, the vessel andfinger storage types of slug catchers are the mostwidely used in the petroleum industry.

Use of traditional vessel type separators asslug catchers are mainly dictated by spacelimitation and relatively small slug sizes. Anumber of studies have been conducted to designsuch catchers4 ,5,6.7.8.9.1O. In references 4, 5, and6 the acceleration of the sluga during theirproduction into the catcher and resultant loadson bends, fittings and slug catcher internals havebeen investigated for a specific slug catcher

A New Approach for Finger Storage Slug Catcher Design

where, vd is the drift velocity given by,

OTC 6414

for normal slug flOW) ..(2)for pigging

V d =(0.35 yg~

[LiqUid Input] _[LiqUid Discharge] =[UqUid Accumulation]

Mass Rate Mass Rate Mass Rate

Lsavg= eX~-2.099 + 4.859 [In d]O.s}",.......(4)

v t = 1.2 Vm + Vd· • • (l)

Estimation of maximum liqUid accumulation

The accumulation rate of liqUid in thecatcher can be given by a liqUid mass balancebetween the inlet and outlet of the catcher, asfollows

(Lp

Lsrnax=~s)O EpdL (8)

Vs= Yl.. (7)Es

For large diameter pipelines, the Prudhoe Baycorrelation 16 can be used to obtain the averageslug length and the maximum anticipated sluglength.

VL = A'JOr.. EpdL (6)

Lsrnax=exp{ 1.54 + In LsavgL (5)

For pigging applications, a conservative estimateof the volume of the liqUid in front of the pig canbe obtained by integration of the liqUid holdupalong the pipeline, assuming no liqUid sheddingat the back.

E - 1s-( 1 + (--!nL)1.39) (3)28.4

The slug volume and the maximum slug lengthcan be expressed as

Slug liqUid holdup can be predicted by theGregory et a1 correlation15.

include slug length, slug holdup, slug velOCity, andthe translational velocity. The slug velocity isequal to the mixture velocity under steady stateflow conditions. The translational velocity isexpressed as,

640

Slu~ characteristics

A proper design of any slug catcher requiresapriori information about the characteristics ofthe slugs at the pipeline exit conditions, eithernormal or artificially created by pigging. These

design. Giozza7 has modified a model given bySchmidt et alB to simulate vessel type separatorbehavior under slug flow conditions to handlevariable slug characteristics. Later, Miyoshi et al9

modified Giozza's model to incorporate severeslugging effects, and proposed a procedure topredict separator foam layer thickness. Gencell etal lO have developed a model utilizing linearcontrol theory to optimize the gains and resettimes of separator controllers and ensure stablesystem operation.

A finger storage type slug catcher ispreferable to the vessel separator type for tworeasons. First, it is cost effective when largevolumes of liqUids must be handled. Secondly,although more space is required compared to thevessel separator type, it causes less operationalproblems. Very few studies have been conductedfor designing finger type slug catchers. Bos anddu Chatinier11 have investigated two phase flowbehavior (I.e. slug distribution and liqUid carryovermechanism) in a high-pressure multiple-pipe slugcatcher, simulated by a two-liquid (kerosene/zincchloride) laboratory facility at atmosphericconditions. They claimed that the observedmaldistribution of the liqUid slug among thefingers can be corrected by properly instalIin.s:rdowncomer constrictions in the facility. Oranje1~presented an application of both platform andonshore finger type slug catchers. Later, Oranje13reported that the application of different slopesfor the separation and storage sections presentsconsiderable improvements in the storagecapacity of the catcher, while achievingsignificant reduction in installation cost. Herecommended that countercurrent flow in a slugcatcher, which may result in severe carryover,should be eliminated by providing gas escapeheaders along the fingers to ensure cocurrentflow conditions. Contrary to Oranje's13recommendations, in most of the existing fingertype slug catchers, counter current flow occurs.One of the crucial problems in countercurrentflow type slug catchers is liqUid carryover(flooding). A flooding diagram has been proposedby Senni et al14. However, there is not enoughinformation about the diagram development, andhow one should use the diagram to size a slugcatcher properly.

In this paper, a new approach based on twophase flow hydrodynamics is presented for thedesign of a finger type slug catcher. The effect ofthe finger inclination angle on the slug catcherdimensions is emphaSized.

ANALYSIS

2

ocr 6414 Cem Sanca. Ovadia Shoham. James P. Brill 3

Assuming a constant liquid density in the catcher.and no acceleration dUring the slug production.the liquid accumulation rate can be written as,

qaccum = vmEsAp - qdis (9)

The accumulated liquid volume can be stated as.

the available volume to handle the liqUidaccumulation in the catcher. Thus. for a particularcatcher diameter. the catcher length can beobtained by.

L - Vaccum ( 1 3)catc - A rE E ] .catcl tran- oper

641

or

Vaccum = tsp qaccum ( 10)

RESULTS AND DISCUSSION

The proposed model is applied fordesigning a slug catcher for a typical field case. Allrelevant information is given in Table 1. Theresults in this section are for the example case.

Fig. 1 shows a flow pattern map for a 20-in.diameter horizontal slug catcher (same diameteras the pipeline). Shown on the map is theoperation point for the exit conditions of thepipeline. The flow pattern in both the pipeline.and the slug catcher is intermittent or slug flow.Transition from slug flow to the stratified flowpattern can be achieved by increasing the slugcatcher diameter. The minimum slug catcherdiameter which will cause stratification under thesame flow conditions is 26 in.. as shown by theflow pattern map in Fig. 2. This is based on theaverage flow rates of the gas and the liquid phasesat the slug catcher. However. due to theintermittent nature of slug flow. the liquid andgas flow rates into the slug catcher are notconstant. but vary signIficantly when the slug bodyor the gas pocket are produced. Thus. one mustuse a larger slug catcher diameter to ensurestratification dUring the production of the liquidslug body. Once the stratification is ensured. theavailable space and the cost will be the dominantfactors in sizing the slug catcher. The flowpattern map for a 52 in. diameter slug catcher isshown in Fig. 3. The map shows that theoperation point for the same inlet flow conditionis located in the stratified flow regime. Thedifference between the operational and transitionsuperficial liquid velocities and correspondingliquid holdups determines the volume of the

The model involves two significant conservativeassumptions. First. dUring the production of theliquid slug body, liquid continues to be shed intothe trailing liquid film below the gas pocket. Thevolume of the liquid accumulation is thus lessthan calculated by Eq. (11). Second, it is assumedthat the liquid in the slug catcher prior to theslug production is Eoper. In reality, the liqUid levelin the catcher will fall during the production ofthe gas pocket and film. Both of theseassumptions result in predicting slightly largerslug catcher dimensions than needed andintroduce a safety factor.

Although the model is given only for onefinger. it is capable of handling more than onefinger, prOVided that the liquid distributionamong the fingers is known.

(PL-Po) g cos ex AodAL .... (12)

Po dhL

_ ( hL )VOtran - l-d

Vaccum =L~~ax [vmEsAp - qdiJ ............ ( 11)

When VG. the actual gas velocity. is less thanVGtran. stratification is expected to occur in thecatcher. Once the minimum slug catcherdiameter is obtained. the next step in the designis to accommodate the incoming liquid flow. Dueto accumulation of the liquid. stratified flow cannot be maintained in the catcher at the abovementioned minimum diameter. Therefore. thecatcher diameter should be increased toaccommodate the liquid in the catcher. and toavoid liquid carryover. For an increased catcherdiameter and the same production conditions.the operational liquid holdup based on theaverage flow rates of the gas and the liquid phasesat the catcher. Eoper. can be obtained by solvingthe combined momentum equation understratified flow conditions. Also. for a given gassuperficial velocity. VSG, the maximum possiblesuperficial liquid velocity for stratified flow toexist can be determined, using the transitioncriterion given by Eq. (12). The correspondingliquid holdup is the transition holdup. Etran. Thedifference between the transition liquid holdupand the operational liquid holdup will determine

Fin~er type slu~ catcher sizin~

The present approach is based on thecriterion for the transition boundary between slugand stratified flow. presented by Taitel andDukler17. This approach was originally proposedby Machado18• and modified recently. Following isa detailed description of the new approach.

Transition from slug flow to the desiredstratified flow can be achieved by increasing theslug catcher diameter and/or introducing adownward inclination of the catcher. Theminimum slug catcher diameter which will causestratification can be determined using thetransition criterion given by Taitel and Dukler17

If the discharge rate is expected to fluctuate. it isrecommended to use the minimum rate.

4 A New Approach for Finger Storage Slug Catcher Design OTC 6414

liquid that the slug catcher can handle. For theexample case, using Eqs. 11 and 13, andassuming q~i~ = O, a 1458 ft. long 52 in. internaldiameter horizontal slug catcher length iscalculated.

Diameter effects

Fig. 4 presents the relationship between theslug catcher diameter and both the operationaland the transition liquid holdups for a horizontalslug catcher. For increasing slug catcherdiameter, the transition liquid holdup increaseswhile the operational holdup stays almostconstant. On the other hand, as shown in Fig. 5for -5° downward inclined slug catcher, theoperational liquid holdup decreases significantlywhen the catcher diameter increases, althoughthe transition holdup has the same trend as forthe horizontal case.

DoWnwar d inclinationn ande effec~ IFig. 6 shows the behavior of the transition

liquid holdup as a function of the slug catcherdiameter for four different inclination angles. Asshown, the effect of the inclination angle on thetransition holdup is insignificant because theaCtual t.ranSit,iOngaS velocity, vctra~, given by Eq.12 stays almost constant for the range ofinclination angles of interest.

The relationship between the operationalliquid holdup and the slug catcher diameter forfive different inclination angles is given in Fig. 7.As can be seen, the operational liquid holdupdecreases significantly, even for small downwardinclination angles.

A typical plot of the inclination angle versusthe slug catcher length is given in Fig. 8. Thedecrease in the slug catcher length is drasticbetween 0° and -0.5° inclination angles, while it is

insignificant between -0.5° and -5° inclinationangles for all catcher diameters. Therefore, after acertain increase in the downward inclinationangle, further increases do not yield the sameorder of reduction in the slug catcher length. Thedecrease in the operational liquid holdup, due todownward inclination of the slug catcher, willpromote stratification and reduce the requiredslug catcher length for a given sIug catcherdiameter, as shown in Fig. 9.

CONCLUSIONS I● A new innovative approach for the

prediction of the required dimensions of a slugcatcher is presented. The approach utilizes thetransition boundary criterion between the slugand stratified flow regimes.

“ Proper design of a slug catcher requiresapriori information about the characteristics of

. ..

the design slugs, either normal or artificiallycreated during a pigging operation, at thepipeline exit conditions.

● The analysis is given only for one finger.The model is capable of handling multiple fingersprovided that the liquid distribution amongfingers is known.

. The larger the catcher diameter, thesmaller the catcher length that is required.

● Small downward inclination anglesreduce the slug catcher dimensions significantly.After a certain increase in downward inclinationangIe, further increase does not yield the sameorder of reduction tn slug catcher length.

● Once stratification is ensured in thecatcher, space limitation and cost are the mainfactors in sizing slug catchers,

REFERENCES

1. Hubertz, T., et al.: “A Cost EffectiveSubsurface Concept for OnshoreTermination of Multiphase ExportPipelines, ” Seventh International Conf. onOffshore Mechanics and Arctic Engineering,Houston, TX, (Feb. 7-12, 1988), pp 291-295.

2. Wheeler et al.: “Hydrocarbon FluidSeparation at an Offshore Site and Method,”United States Patent, No 4793418.

3. Huntley, A.I?.: “Flexible Subsea SIug Catcheris Designed for Use in North Sea’s TrollField,” Oil & Gas J., Vol. 84, No 30, (July28, 1986), pp 84-86.

4. Huntley, A.R. and Silvester, R. S.:“Hydrodynamic Analysis Aids Slug CatcherDesign,” Oil & Gas J., Vol. 81, No. 38, (Sept.19, 1983), pp 95-100.

5. Silvester, R.S.: “Hydrodynamic Analysis AtdsSlug Catcher Design,” 2nd InternationalBHRA Multiphase Flow Conf., London,England, (June 19-21, 1985), pp 443-453.

6. Silvester, R.S. and Gordon, I.G.: “Design ofSlug Catcher Systems for Dynamic Loading,”Offshore Separation Processes SymposiumProc., Middlesbrough, England, (May 15,1986), pp 33-45.

7. Giozza, W. F.: “Simulation of Gas-OilSeparator Behavior under Slug FlowConditions,” M.S. Thesis, The University ofTulsa

8. Brill Engineering Co.: “Evaluation ofPrudhoe Bay Field Slug Attenuation Test

I

Data,” Report to Prudhoe Bay Unit, (Sep.1980), pp 61-79.

L Length (ft)P Pressure (psia)

9. Miyoshi, M., et al.: q“Slug Catcher Design for ~pVolumetric rate (ft3/s)

Dynamic Slugging in an Offshore production tProducing gas-oil ratio (SCF/STB)

Facility,” SPE 1986 International Meeting ~Time (s)

on Petroleum Engineering, Beijing, China, Temperature (oF)(March 17-20, 1986) SPE 14124, pp 119- Velocity (ft/s)135. ; Volume (ft3)

10. Genceli, H., et al.: “Dynamic Simulation of Subscrit)tsSlug Catcher Behavior,” 63rd SPE AnnualTechnical Conference, Houston, TX, (Oct. 2- accum Accumulation5, 1988), SPE 18235, pp 549-562. catc Catcher

d Drift11. Bos, A. and du Chatinier, J. G.: “Simulation of dis Discharge

Gas/Liquid Flow in Slug Catchers,” SPE G~7;~;~ Engineer~g. (August, 1987), pp Gtran Gas transition

GSin Superficial gas inputL Liquid

12. Oranje, L.: “Handling Two-Phase Gas LSin Superficial liquid inputCondensate Flow in Offshore Pipeline m MixtureSystems,” Oil & Gas J., Vol. 81, No. 16, 0 Oil(Apr. 18, 1983), pp128-130. oper Operation

P Pipe13. Oranje, L.: “Terminal Slugcatchers for Two- s slug

Phase Flow and Dense-Phase Flow Gas savg Slug averagePipelines,” J. Energy Resources Technology, Smax Slug maximumVol. 110, No.4, (Dec. 1988), pp 224-229. sp Slug passage

t Translational14. Senni, S., et al.: “The process Design ‘of a tran Transition

Subsea Booster System (S.B. S): Problemsand Solutions Related to the Hydraulics of Greek LettersMultiphase Flow, ” Seventh InternationalConf. on Offshore Mechanics and Arctic aEngineering, Houston, TX, (Feb.7. 12,

Inclination angle (degree)

1988), pp 297-305. v viscosity (Cp)

P Density (ibm/ft3)15. Gregory, G.A., et al.: “Correlation of the a Surface tension (dyne/cm)

Liquid Volume Fraction in the Slug forHorizontal Gas-Liquid Slug Flow,” Int, J,Multiphase Flow, 4., (1978), pp 33-39.

16. Norris, L.: “Correlation of Prudhoe BayLiquid SIug Lengths and Holdups Including1981 Large Diameter Flow Line Tests, ”Exxon International Report (October 1982).

17. Taitel, Y. and Dukler, A. E.: “A Model forPredicting Flow Regime Transition inHorizontal and Near Horizontal Gas LiquidHOW,” AIChE J., 22, (1976), pp 47-55.

18. Brill, J.P. and Beggs, H.D.: ‘Two Phase FIOW

in piDes”, The University of Tulsa (1983).

NOMENCLATURE

~L DESCRIPTION

A Area (ft2)d Diameter (in.)E Liquid holdup

g Gravity acceleration (ft/sZ)Height (ft)

OCT 6414 Cem Sarica, Ovadia Shoham, James P. Brill 5

643

— —— —————— ——

TABLE 1

TYPICAL FIELD DATA FGR FINGER TYPE SLUG CATCHER DESIGNEXAMPLE

~o= 70.000 STWDAY

Rp = 1,000 SCF/STB

p = 450 paig

T=800F

PO= 45 lbm/tuft

pG = 2 lbm/tuft

%= lcp

vG = 0.01 Cp

a = 23 dynefcm

dp = 20 in,

VSI,M= 2.5 ftjs

Vwln = 5.5 ft/r+

_ = 550.6 ft.

krsx s 2568.4 ft.

102 Dis~med Bublem> -

Or.cration Fuint$5& 101 ?

ahrtenn[ttent

&0

% ●

: 100?Amlldar

~

a

%

~o-l

Stratltled Smcoth

i& 1o”’Z

~o-3

~:-’ ~o-l o 110= 103

Superfici&”Gas Vef%ty (ft/s)

Fig. I: Ffow pattern Map for a 20 in. Horizontal Slug Catcher.

lo~ Dlsperaed Bublez

~-

Operation Point

~ 101I“tennittent

~ L

+ 100~

~l-l lo-l=

a Strattlkd Smcoth StratUlcd Wavy

4g 10-’!

$,

lo-’~~o-’ ~o-l o 1

10’Superffci~l”Gas Ve&ity (tl/s)

10s

Fig,2 : Flow Pattern Map for a 26 in. Horizontal Slug Catcher.

Intermittent

llansitlon Point

$

foperation PQtnt

Stratloed wavyStratlIlcd Smmtb

Fig.3 : Ffow Pattern Map for a 52 fn. Slug Catcher.

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0,8

I

--------------------

----- ----------

--------

#-. .

0.6

1

I— Operatton‘-----S Transition

0,4 !25 35

Diameter, in.45 55

Fig.4: Liquid Holdup vs Diameter for a Horizontal Slug Catcher.

I

1 — Operation

0,2— Transition

o.o~25 35 45 . .

00

Diameter, in.

Fig. 5; Liquid Holdup vs Diameter for a -5° Inclined Slug Catcher.

1.C

0.8~

zz 0.6~

~3 fJ.4

0,2

0.025

— a. ac)

— a .-0,01. . . . . . . . a = -!3.03— C.-m. . . . . . . . . . a= -5.0

~--------------------

----------- -------------------------- .. .. .. .. ..

35 45 55. .Diameter, in.

Ffg.7: Operational Holdup vs Diameter.

FJE 0.8-m

~

3

0.7-— a= 0,0— a-o,rx-. . . . ..-

a-os— E-s,o

0.6-2s 35 45

Diameter, in.

Fig.& Transition Holdup va Diameter

&

1— d=36ti.

“5mo — d=40h.

‘------ d= 44 in.

?m— d=52in.

I

I

“:.I-4 -3 -2 -1 0 1Inclination Angle, degree

Fig.8: Slug Catcher Length w Inclination Angle.

10000-

— a= 0.0

‘. 8000- — a= -0.01

~

. . . . . . . . --0,1— a= -5.0

J 6000-gg% 4000-0 ‘.,

\...

2000-

0-25 35 45 55

Catcher Dimneter, III.

Ffg. 9: Effect of Inclination Angle

845

I

I

I