1. Guidelines for OLGA 2000 Slugtracking

2. Slug tracking module General
Tracking front and tail of each individual slug

3. Slug tracking module General
4 types of slug initiation mechanisms
Hydrodynamic
Terrain
Level (start-up)
Pigging
Follows individual slugs through pipeline
Calculates the slugs growth or decay
Eliminates numerical diffusion of liquid fronts
Two-phase or three-phase flow
The normal OLGA Scheme is numerically diffusive (all finite difference schemes are that).

4. Numerical diffusion Real liquid front Standard numerical solution
The standard scheme in OLGA is implicit and so in one single time step any liquid with a velocity will be smeared out in the entire pipe (if there is a driving pressure difference of course).
This can be illustrated in this way: liquid will be distributed in the entire (numerical) pipe section (averaged).
Since the liquid has a velocity it will cross the next boundary and thereby an amount of liquid will appear in the next section and this is also
distributed in the entire section - and son on.

5. Slug tracking module
When slug flow is indicated, the module sets up a slug and a slug bubble
Slug growth
Slug front an tail tracked
Slug material balance determines whether slugs grow or decay

6. Slug tracking module Terrain slugs

7. Terrain slugging Standard OLGA predicts terrain slugging very well
Liquid fronts are sharper with slug tracking than in standard OLGA
Slug tracking may give terrain slugging in cases where standard OLGA does not

8. Slug tracking module Level slugs
Setup of level slugs at local liquid concentration jumps over more than one section
Level detection in j-1: 1  voidj-1 > BUBBLEVOID (min void in the bubble next to level slug at initiation) 0  voidj < SLUGVOID (max void in level slug at initiation)
Level detection in j +1:
0  voidj < SLUGVOID
1  voidj+1 > BUBBLEVOID
Startup slug 1
Startup slug 2
Startup slug 3

9. Start-up slugs (level slugs)
Initial gas fraction in a level slug is taken from the sections covering the slug - including the sections where the levels are detected.
In order to start level tracking in cases with long sections the use of LEVEL = OFF and HYDRODYNAMIC = MANUAL is an alternative.

10. Start-up slugs (level slugs)
Experiments in 8” loop.

11. Slug tracking module Correlations
Classification of type of tail and front (bubble nose or level/breaking front)
Bubble nose velocity
Gas entrainment into slug according to standard OLGA correlation for gas fraction in slugs.
A NOSE is characterised by a slug bubble which propagates into the liquid slug. A LEVEL is characterised as a breaking front.
Note! FRONT is always the RIGHT-hand side boundary of the liquid slug.
TAIL is always the LEFT-hand side boundary of the liquid slug.
Example: A riser slug in growth phase may have a level/breaking front in both end.

12. Standard OLGA Hydrodynamic slugs
Standard OLGA gives average pressure drop, holdup and flow rates for slug flow
Standard OLGA does not show individual slugs or impact of slugging on downstream facilities

13. Slug tracking module Hydrodynamic slugs
Flow regime must be slug flow according to the standard OLGA slug model
Minimum initial slug length: 1 DPipe (default)
(Initial) gas fraction in the slugs are calculated from the same correlation for gas fraction in slugs that is used in the standard OLGA slug model.

14. Slug tracking input Hydrodynamic slugs
Available input parameters with slug tracking
Initial slug length
Initial frequency
Delay constant
Illegal sections

15. Slug tracking input Hydrodynamic slugs cont.
Be aware of intrinsic ILLEGAL SECTIONS: (a section where the slug front “stops” and the slugs that reach this point will eventually vanish )
first and last section in any Branch
consequently at MERGE and SPLIT nodes
process equipment
but not valves
Turn on more ILLEGALSECTIONS only if you have problems
Use the example of a slug into a separator. It can obviously not maintain its front and when the tail also reaches the
separator inlet the slug is ”vanished”

16. Tuning Hydrodynamic slugs cont.
Available field data are limited (Prudhoe Bay, Alaska)
One could calculate slug frequency by a method like the Shea correlation
One could tune slug tracking to match frequency from measurements or estimated by other methods by adjusting the DELAYCONSTANT
Use DELAYCONSTANT to tune model rather than initial frequency

17. Shea correlation
FsL = slug frequency (1/s) (= no of slugs/observation time period)
D = pipeline diameter (m)
L = pipeline length (m)
UsL = superficial liquid velocity (m/s)
You may tune DELAYCONST so that resulting slug frequncy is of same order as FsL for hydrodynamic slugging with moderate terrain effects.

18. Slug lengths Hydrodynamic slugs cont.
Hydrodynamic slugging appears to be a statistical phenomenon
correlations for slug length distribution predictions are uncertain
Rule of thumb: Max. length of hydrodynamic slugs can be in the order of 6 times the average slug length

19. Performing tracking of hydrodynamic slugs (1)
Start with running OLGA without slug tracking until steady state is established
If slug flow is predicted, prepare for slug tracking.
Pipe sectioning: keep in mind that OLGA Slugtracking requires at least 10 time steps to transport a slug through any pipe section
Estimate run-through-time (residence time): T = (L·A)/QT
L = Pipeline length
A = Pipe cross-sectional area
QT = Average total volumetric volume flow

20. Performing tracking of hydrodynamic slugs (2)
Turn on slug tracking in a first (of two) restart simulation
(slug tracking can not be turned off in a RESTART)
HYDRODYNAMIC = ON
Use default values for the other parameters1)
Initial slug length - default = 1Dpipe
Delay constant default = 150 Dpipe
Illegal sections default = intrinsic
This is the safe way. It is of course fully possible to start with slugtracking immediately.
1) Initial frequency is defined as a minimum number of pipe diameters between slugs. One could use a high number (10000) to avoid any influence from this parameter

21. Performing tracking of hydrodynamic slugs (3)
Model the first slug tracking case with a moderate plotting frequency
Plot time trends of QLT1) and ACCLIQ2) at pipe outlet and LIQC3) and NSLUG4) of each branch.
Specific slug tracking variables apart from NSLUG are seldom needed
1) total liquid volume flow,
2) accumulated total liquid volume flow,
3) total liquid inventory in a branch,
4) total # of slugs in a branch

22. Performing tracking of hydrodynamic slugs (4)
Run the case
At least one run-through-time
Until NSLUG is quasi stable
Until LIQC (total liquid content in a branch) is quasi stable
Quasi stable means quasi stable (i.e a certain pattern in e.g. QLT is repated – watch out for long periods).

23. Performing tracking of hydrodynamic slugs (5)
Make a second restart - from the first slugtracking
Reduce the plotting interval sufficiently (s)
Run the case several run-through-times
Use the 2nd case to analyze liquid surges out of the pipe
use ACCLIQ
We emphasize that it is not necessary to plot all sorts of slug variables unless one is concerned with mechanical impact
on pipe from slugging.

24. If OLGA crashes with slug tracking
Try one or more of the following:
Review the sectioning of geometries. Avoid large differences in lengths of neighbour sections
Limit maximum time step (MAXDT in INTEGRATION)
Switch off temperature calculation (TEMPERATURE = OFF in OPTIONS)

25. Papers about slugging
Prevention of Severe Slugging in the Dunbar 16” Multiphase Pipeline
Paper presented at OTC, Houston 1996
Total Oil Marine, Aberdeen, U.K.
Simulation Study and Field Measurement for Mitigation of Slugging Problem in the Hudson Transportation Lines
Paper presented in Cannes -97
Amerada Hess & NEL (UK)
Scandpower & IFE (Norway)