GUIDELINES FOR OLGA SLUGTRACKING

Slug tracking moduleGeneral

• Tracking front and tail of each individual slug

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

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 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 OLGAHydrodynamic 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 samecorrelation for gas fraction in slugs that is used in the standard OLGA slug model.

Slug tracking inputHydrodynamic slugs

• Available input parameters with slug tracking• Initial slug length• Initial frequency• Delay constant • Illegal sections

Slug tracking inputHydrodynamic 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 except valves

• Turn on more ILLEGALSECTIONS only if you have problems

TuningHydrodynamic slugs cont.

• Available field data are limited (Prudhoe Bay, Alaska)

• One could calculate slug frequency in pipelines by a method likethe 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

6.02.1SL

sLLDU68.0F⋅⋅

=

FsL = slug frequency at pipe outlet (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 (which affects the initial slug frequency) so that the resulting outlet slug frequncy is of same order as FsL for hydrodynamic slugging with moderate terrain effects.

Tracking hydrodynamic slugs (1)

• Start with running OLGA without slug tracking until steady stateis established

• If ID = 3 you should run slugtracking

Tracking of hydrodynamic slugs (2)

• Turn on slug tracking in a RESTART simulation• (slug tracking can not be turned off in a RESTART)

– HYDRODYNAMIC = ON– Use the default value of 150 for the Delay Constant

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

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

Tracking of hydrodynamic slugs (5)

• Make a second restart - from the first slugtracking case– 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