Slug Catcher Conceptual Design
Slug Catcher Conceptual Design
Slug Catcher Conceptual Design
+
=
M
LLS
v
H
(1)
For viscosity greater than 500 cP the holdup is determined using a correlation obtained at PDVSA Intevep as,
( )
eL
R
LLS
e H
=
0022 . 0
0046 . 1 (2)
SPE 122829 3
The holdup in Taylor bubble is determined using a correlation obtained at PDVSA Intevep as,
( )
TB
LLS L TB
LTB
v
H v v
H
=
(3)
The gas void fraction is expressed using the Beggs (1991) correlation
LLS S
H = 1 (4)
The film length of the slug unit is predicted using a correlation developed with high viscosity liquid experimental data at
PDVSA Intevep.
8606 . 0
Re Re
Re
0365 . 0
+
=
SG SL
SL
F
L
(5)
The slug length correlation is obtained by inserting L
F
into the correlation developed by Shoham (2000) to predict the film
length. This correlation only considers hydrodynamic slug flow.
1
Re Re
Re
0365 . 0
8606 . 0
SL
LLS L
SG SL
SL
S
v
H v
L
(6)
The correlations already presented require the estimation of velocities, such as: mixture velocity, liquid superficial velocity,
and gas superficial velocity that are easy of obtaining. Also, the prediction of the translational velocity and the drift velocity
is necessary. The translational velocity is given by the Nicklin (1962) correlation and depends on the velocity profile, mixture
velocity and drift velocity:
D M TB
v Cv v + = (7)
Where C is given according to the flow type:
If the flow is laminar: 2 = C , while if it is turbulent 2 . 1 = C and when it is between laminar and turbulent the Taitel (2000)
correlation is used.
2 2
Re
Re
1
20 . 1
Re
Re
1
0 . 2
+
+
+
=
L
CL
CL
L
C
(8)
The drift velocity is calculated depending on the pipe inclination, such as:
For pipes slightly inclined the drift velocity is estimated by Bendiksen (1984) correlation as,
( ) ( ) ( ) ( ) sin cos + =
vertical D horizontal D D
v v v
(9)
The following correlation is used for horizontal pipes
( ) gD v
horizontal D
54 . 0 = (10)
And for vertical pipes the following correlation is used.
( ) gD v
vertical D
35 . 0 = (11)
The slug frequency is estimate as,
4 SPE 122829
U
TB
S
L
v
f =
(12)
Other parameters of interest for slug flow characterization are the liquid and gas instantaneous flow at the inlet of the catcher.
They are calculated using the Miyoshi et al. (1988) model.
For the liquid,
LLS p Mins insL
H A v Q = (13)
And for the gas,
( )
LLS p Mins insG
H A v Q = 1 (14)
Prediction of Liquid accumulation
The liquid accumulation into the slug catcher can be estimated applying a liquid mass balance between the inlet and outlet of
the equipment (Sarica et al. 1990) as,
rate mass
on accumulati Liquid
rate mass
discharger Liquid
rate mass
input Liquid
(15)
Where the liquid input mass rate is determined through the Miyoshi et al. (1988) model to calculate the liquid instant flow.
While the liquid discharge mass rate is related at the liquid flow to the outlet of the catcher. And this liquid flow depends on
the flow control valve size.
Based on the liquid mass balance presented by Sarica et al. (1990), the accumulated liquid volume is given as,
[ ]
dis p LLS M
TB
S
acum sp accum
Q A H v
v
L
Q t V = =
max
* (16)
Finger type slug catcher dimensioning
The most important parameter in the slug catcher design is the diameter of the slug catcher fingers, which is calculated to
obtain stratified flow into the finger. In this sense, models to predict the transition from slug flow to stratified flow are
necessary, such as: the inviscid Kelvin-Helmholtz instability criterion (IKH), the viscous Kelvin-Helmholtz instability
criterion (VKH), Taitel and Dukler (1976) model, etc.
To predict the transition Sarica et al. (1990) used the inviscid Kelvin-Helmholtz instability criterion (IKH) presented by
Taitel and Dukler (1976). However, in this work is proposed to use the viscous Kelvin-Helmholtz instability (VKH) criterion
presented by Barnea and Taitel (1993) to determine the transition from slug flow to stratified flow because it predicts better
the transition for a wider range of viscosities (100-5000 cP): The criterion is expressed as,
( )
2 / 1
+
L
L
P
G L
G L
L G G L V
h
A
A
g R R K v
Gtran
(17)
In this expression, K
V
is a correction factor given as,
SPE 122829 5
( )
L
L
P G L
IV V
V
dh
dA
A
g
C C
K
cos
1
2
(18)
The VKH criterion provides the minimum diameter from which can be obtained stratification, i.e. it works on the transition
curve, such as is shown the green point at the Fig. 1.When the actual gas velocity is less than the transition gas velocity is
expected the stratified flow. Thus, the catcher diameter should be bigger than the minimum diameter to receive the incoming
liquid. The catcher diameter is determined increasing the minimum diameter to insure stratified flow into the equipment, such
as is shown in the Fig 1 as operation point. Also, it must consider the incoming liquid flow, available space for installation
and the costs.
0.01
0.1
1
10
100
0.1 1 10 100
VsG (m/s)
V
s
L
(
m
/
s
)
VKH HL/D = 0.5 Operation point Transition point
Intermittent
Annular
Stratified
Fig. 1. Flow partner map for the designed slug catcher.
For a given gas superficial velocity there is a transition liquid holdup and an operation liquid holdup. The first is given by the
maximum liquid superficial velocity for stratified flow and is calculate using the VKH criterion. The second is given by the
average operation flow rates of liquid and gas at the slug catcher. The difference between these two holdups will provide the
available volume to handle the accumulation of liquid in the slug catcher. Thus, the catcher length for the designed diameter
is given as,
[ ]
oper L trans L finger
accum
finger
H H A
V
L
=
(19)
6 SPE 122829
RESULTS AND DISCUSSION
The proposed methodology is used to design a finger type slug catcher for a heavy oil field in Orinoco Belt. The designed
equipment was economically compared with a design of a conventional horizontal separator that is commonly used in the
field. The data for the design are given in Table 1 and the location of the operation point in the flow pattern map for this
information is shown in the Fig 2.
Table 1. Field data
API 16
Temperature ( F) 72 95
Pressure (psig) 105.80
Q
L
(BPD) 14.343,55
Q
G
(MMSFD) 8,556
BS&W (%) 42,65
G
0.55
0.01
0.1
1
10
100
0.1 1 10 100
VsG (m/s)
V
s
L
,
(
m
/
s
)
VKH HL/D = 0.5 Operation point
Intermittent
Annular
Stratified
Fig. 2. Flow pattern map for the inlet conditions of the catcher.
In the catcher design four fingers were considered and the distribution of flow rate through of finger is uniform. Under these
considerations the catcher diameter and length are determined. Thus, the diameter is 0.508 m and the length of every finger is
8 m. The weight of the catcher considering every pipe section is 3573 kg. The operation point of the finger is showed in the
Fig. 1 as the blue point.
On the other hand, a horizontal conventional separator for the same conditions will have approximately a diameter of 1.83 m
(6 ft) and a length of 6.10 m. (20ft).The separator weight is 4627 kg.
Based on to the weights of the equipments, the type finger slug catcher is 23% lighter than the horizontal conventional
separator. For this reason, it is expected a similar reduction in the fabrication cost. Whenever is considered that the other cost
relating to the fabrication are almost the same.
CONCLUSIONS.
The slug flow characteristics must be known to develop a proper design of finger type slug catcher. Thus, various correlation
and models are selected to predict the slug flow characteristics for heavy oil in a rigorous way.
SPE 122829 7
It is proposed a improvement of the Sarica et al. (1990) methodology based on the use of the viscous Kelvin-Helmholtz
instability (VKH) criterion to predict the stratified-no stratified transition in a more rigorous way to determine the dimensions
of a finger type slug catcher to handle viscous liquids. The improvement allows performing a better design of the slug catcher
to guarantee the segregation and separation of the phases while the slug mitigation is achieved.
The economical comparison based on the weights demonstrated that the fabrication cost of the slug catcher can be 23% less
expensive than the conventional separator.
REFERENCES
1. Barnea, D., Taitel, Y., 1994. Interfacial and structural stability of separated flow. Int. J. Multiphase Flow 20: 387-414.
2. Barnea, D., y Taitel, Y., 1993. Kelvin Helmholtz Stability Criteria for Stratified Flow: Viscous Versus Non-Viscous
(Inviscid) Approaches. Int. J. Multiphase Flow. 19(4): 639-649.
3. Barnea. Dvora., 1990.On the effect of viscosity on stability of stratified gas liquid Flow application to flow pattern
transition at various pipe inclinations. Chemical Engineering Science, 46(8): 2123-2131.
4. Beggs, H.D., 1991. Production Optimization Using NODALTM Analysis. OGCI Publications Oil & Gas Consultants
International Inc. Tulsa.
5. Bendiksen, K. 1984. An Experimental investigation of the motion of long Bubbles in Inclined Tubes, Int. J. Multiphase
Flow 13(1): 1-12.
6. Lin P. and Hanratty, T. 1986. Prediction of the initiation of slugs with linear stability theory. Int. J. Multiphase Flow 12:
79-98.
7. Miyoshi, M., Doty, D. and Schmidt., 1988. Slug Catcher Design for Dynamic Slugging in Offshore Production
Facility. JGC Corp. SPE 14124.
8. Nicklin, D. 1962. Two phase bubble flow, Chem. Eng. Sci, 17: 693-702.
9. Sarica, C., Shoham, Ovadia., and Brill, J.P., 1990. A New Approach for Finger Storage Slug Catcher Design. OTC 6414.
10. Shoham, O., 2000. Two-Phase Flow Modeling. Department of Petroleum Engineering. University of Tulsa. TOMO 1.
11. Taitel, Y. and Dukler, A., 1976. A Model for Predicting Flow Regime Transitions in Horizontal and Near Horizontal Gas
Liquid Flow. AICHE J.,22,:47-55
NOMENCLATURE
API API gravity
A Cross-sectional area [m
2
]
C Wave velocity
fs Slug frequency [slugs/s]
g Gravity acceleration [m/s
2
]
H Holdup
Kv Coefficient of stability
L Length [m]
Q Flow rate [m
3
/s]
Re Reynolds number
t Time [s]
Velocity [m/s]
V Volume [m
3
]
BS&W Bottom sediment and water
Subscripts
accum Accumulation
D Drift
dis. discharge
F Liquid film (Taylor bubble)
G gas
ins instantaneous
IV inviscid
8 SPE 122829
L liquid
LS Liquid slug
M Mixture gas-liquid
max Maximum
oper Operational
p Pipe
S Slug
sG Superficial gas
sL Superficial liquid
sp Slug passage
TB Translational
trans Transition
U Slug unit
V Viscous
Greek Letter
specific gravity
Inclination angle
Density [kg/m
3
]