Guidelines for OLGA 2000 Slugtracking

Slug tracking moduleGeneral

• Tracking front and tail of each individual slug

Slug tracking moduleGeneral

• 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

Numerical diffusion

Real liquid front Standard numerical solution

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

Slug tracking moduleTerrain slugs

Terrain slug

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

Slug tracking moduleLevel 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

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.

Start-up slugs(level slugs)

Slug tracking moduleCorrelations

• 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.

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

Slug tracking moduleHydrodynamic 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.

Slug tracking input Hydrodynamic slugs

• Available input parameters with slug tracking

• Initial slug length• Initial frequency• Delay constant• Illegal sections

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

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

Shea correlation

6.02.1

SLsL

LD

U68.0F

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.

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

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

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

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

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 NSLUGare 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

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

Performing tracking of hydrodynamic slugs (5)

• Make a second restart - from the first slugtracking

– Reduce the plotting interval sufficiently 1 - 5 (s)

– Run the case several run-through-times

– Use the 2nd case to analyze liquid surges out of the pipe• use ACCLIQ

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)

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)