Guidelines for OLGA 2000 Slugtracking

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

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).

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.

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 module Terrain slugs

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 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

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) Experiments in 8” loop.

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.

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 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.

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 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”

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 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 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

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

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).

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 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.

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)