The Transient Multiphase Flow Simulator

Contents • Introduction

• Physical models and numerical solutions

• Network topology

• How to make fluids flow

• Fluid properties

• Heat transfer

• Process equipment and modules

• File structure and execution

Fundamental features

• OLGA is– transient ( df/dt # 0 )– one-dimensional (along pipe axis)– “complete”– a modified “two-fluid” model– realised with a semi-implicit numerical solution

• staggered grid– made for (relatively) slow mass transients

The dynamic three phase flow simulator

OLGAOLGA

8 Conserv. Equations

mass (5)momentum (2)

energy (1)

Fluid Properties

Closure Laws

mass transfermomentum transfer

energy transfer

BoundaryConditions

Initial Conditions

The OLGA three phase flow model• Mass conservation

– Gas– Hydrocarbon bulk– Hydrocarbon droplets– Water bulk– Water droplets

• Momentum conservation– Gas + droplets– Liquid bulk

• Energy conservation– Mixture

• Constitutive equations

Variables

• Primary variables– 5 mass fractions (specific mass)– 2 velocities– 1 pressure– 1 temperature

• Secondary variables– Volume fractions– Velocities– Flow rates– Fluid properties

Conservation of mass

Conservation of energy

energy = mass (thermal energy + kinetic energy + potential energy)spec

energy flow + work = mass flow (enthalpy + kinetic energy + potential energy)

Force balance equation(Conservation of momentum)

j j+1

Pj Pj+1

dZj

liquid

gas

M - MomentumV - Velocitym - Mass M = m ·V

S = Shear = wall shear + interfacial shearG = Gravity = m · gravity accelerationF = Force = pressure · flow areaMT = Momentum Transfer =

mass transfer - entrainment + deposition

dM /dt = ((M·V )j - (M·V )j+1) /dzj - S j + G j + F j + F j+1+ MT

The OLGA volume equation

Qi

i i+1

Qi+1Pi

source / sink

i

for all sections the local fluid volume time variations

The OLGA volume equationCombine all mass conservation equations

T and RS constant 0= CdP/dt = Qin - Qout [m3/s]

+ volume change by mass transfer+ volume change by mass sources+ Volume Correction

C = “compressibility” i.e. dVfluid/dP [m3/Pa]

Net change of total fluid volume in a section = 0

0)t/T)(T/V( temperature change is assumed to be zero in this step :

Calculation sequence

for each time step:

b) solve momentum - and volume equation simultaneously U and P

d) solve mass equations

c) perform flash

first for gas

and then for water film and water droplets

then for oil film and oil droplets

a) solve model relations to give friction factors etc.

e) solve energy equation Tf) perform flash

Sources of numerical errors in general• Linearisation of strongly non-linear

models– Iteration is not performed

• Thermal expansion or contraction– Temperature decoupled from pressure may give volume errors

• Local changes of total composition neglected in standard OLGA – taken into account in CompTrack

Volume errorAt each time step when all equations have been solved the net fluid volume change in each section usually is 0and the volume error can be expressed as

VOLi = 1- Vi f / Vsectioni 0 f

Vi f = mif /i f

Vi f = fluid volume in section no imif = mass in pipe section no i

i f = density of fluid in section no i

(f indicates liquid , gas and droplets)

.

Volume error cont.

The main source of high volume error peaks is change of phase velocity direction from one time step to the next (i.e. from time step tn to tn+1).The volume flow at tn+1 is based on massflows from tn and thus the volume flow and velocities may have opposite directions.

The volume error is corrected by “adding or subtracting fluid volume” over several time steps (not by iteration).

This correction may also cause unphysical pressure and/or temperature peaks.

(VOL is an output variable which should be plotted together with phase velocities during fast transients)

Modeling the pipeline profile in OLGAThe assumption

OLGA topology

• GEOMETRY is a sequence of PIPES– a PIPE is defined by its

• LENGTH• INCLINATION• INNER DIAMETER• ROUGHNESS and• WALL

OLGA topology cont.

a BRANCH consists of one GEOMETRY and two NODES

a BRANCH has flow direction

NODE-1NODE-2

OLGA topology cont.

An OLGA network consists of a number of BRANCHES

a NODE is either TERMINAL or MERGE or SPLIT 1)

1) v 4.00

OLGA topology cont.

PIPE_1

1 2 3 4

PIPE SECTIONS

1 2 3 4 5

PIPE SECTION BOUNDARIES

1

2

3

OLGA topology cont.

OLGA calculates different types of output variables:

VOLUME VARIABLES e.g. pressure (PT), temperature (TM) and volume fractions (e.g. HOL) are calculated in the midpoints of the pipe sections.

BOUNDARY VARIABLES e.g. velocities, flowrates and flow patterns which are calculated on section boundaries.

e.g. a VALVE is always positioned on a section boundary.

OLGA topology cont.

BOUNDARY of type ”CLOSED” –i.e. no flow across boundary

BOUNDARY of type ”PRESSURE” –i.e. flow across boundary.

OLGA topology cont.

- Pressure, - Temperature, - Gas Mass Fraction - Water Mass Fraction

Pressure boundary

Gas fraction = 1

Gas fraction = 0

Pressure boundary; gas mass fractionAnalogy

Gas fraction = 0.5P,TP,T

P,T

How to make fluids flow

• a mass SOURCE• pressure boundaries• the standard WELL

a mass SOURCE

BOUNDARY TYPE = CLOSED

a mass source into the pipe

BOUNDARY TYPE = PRESSURE

Total mass rateTGas mass fractionWater fraction

SOURCE-1

OLGA calc. this P and T

mass SOURCE cont.

• a SOURCE feeds its mass regardless of the pressure in the pipe

• a SOURCE can be positioned anywhere• one pipe section can have several SOURCES• a SOURCE can be negative

a negative SOURCE

BOUNDARY TYPE = PRESSURE

a mass source out of the pipe

BOUNDARY TYPE = CLOSED

SOURCE-out

OLGA calc. this P

two PRESSURE BOUNDARIES

BOUNDARY TYPE = PRESSURE

BOUNDARY TYPE = PRESSURE

Pin Pout

Pin > Pout

Pin Pout

Pin < Pout

a WELL

BOUNDARY

TYPE = CLOSED

WELL-1

BOUNDARY

TYPE = PRESSURE

Reservoir P & T PI (productivity index) Injection indexGas mass fractionWater fraction

Pres

a WELL cont.

• a WELL is essentially a pressure boundary

• fluid flows into the well when the bottom hole pressure is less than the reservoir pressure

• a WELL can be positioned anywhere along a pipe • a pipe can have several WELLs

• the Advanced Well Module provides numerous additional options.

Starting the dynamic calculation sequence

calculated by the steady state pre-processor

calculated from user given Initial Conditions: i.e. profiles of T, P, mass flow, gas volume fraction, water cut

or be

conditions at t = 0 can either be

Steady state pre-processor

• Activated when setting STEADYSTATE = ON in mainkey OPTIONS

• Gives a full steady state solution at time 0 (when STARTTIME = ENDTIME = 0 in INTEGRATION)

• The subsequent dynamic simulation will tell you if the system is stable or not

time0

Wall heat transfer

• Standard heat transfer correlations• Averaged fluid properties• Radial heat conduction in pipe walls

Pipe wall discretisation How OLGA represents walls

liquid

gas

Need to input:

Conductivity

Heat capacity

Density

Calculated by OLGA: Heat transfer coefficient

Need to input: Heat transfer coefficient

Fluid propertiesGeneral

• The fluid properties are pre-calculated tables as a function of P and T and for one fluid composition– It follows that the total composition is constant

throughout a fluid table1)

• The exact value of a property for a given P and T is found by interpolating in the relevant property table

1) Compositional tracking available with OLGA 2000 v3.00

Properties in the fluid tables

More on Rs: the gas mass fraction

• the mass transfer between gas and liquid as a function of variations in P and T is determined by a rate of mass transfer (it takes time)

• the change of Rs w.r. to P and T determines this rate

• thus: Rs (P,T) = constant gives no mass transfer

mass of gas at P and T

mass of gas + HC-liquid at P and T Rs =

Restrictions - limitations• Table based fluid properties

– Total composition is constant for one fluid table.

• the solution is thus approximate due to slip, phase separation, network systems and varying sources of different compositions

Well B has

Fluid Table 2

Well A has

Fluid Table 1

Flowline has

fluid properties ?

Process equipment with OLGA 2000 basic

• Separators• Compressors • Heat exchangers• Chokes and Valves (CV)

- critical, sub-critical• Check valves• Controllers

PID,PSV,ESD etc.• Controlled sources and leaks• Pig/plug• Heated walls

OLGA 2000 Modules

• Slugtracking – with pigging

• Water

– three-phase flow• FEM -Therm

– conductive 2-D (“radial”) heat transfer– finite elements– integrated with OLGA bundle– grid generator

OLGA 2000 Modules • Comptrack

– compositional tracking• MEG-track• UBitTS

– Under Balanced interactive transient Training Simulator

• Advanced Well• Multiphase Pumps

– positive displacement– rotodynamic

• Corrosion• Wax

OLGA 2000 file structure

OLGA 2000

reflex of the Input File +results from OUTPUT

Trend Plot Fileresults from TREND

Profile Plot Fileresults from PROFILE

Restart File(binary)

Input File .inp

Fluid Properties File .tab

.out

.tpl.ppl

.rsw

ASCII file

Principles of OLGA 2000 execution

OLGA 2000 GUI

Input File .inp

OLGA 2000

PVTsim

Fluid Properties File .tab

.tpl

.ppl

.out .rsw

OLGA 2000 Support

• olga@scandpower.com– OLGA 2000 Helpdesk mail address

• http://www.olga2000.com– The OLGA 2000 web site

• ONN@scandpower.no– OLGA 2000 News Network (ONN): Mailbox at

Scandpower for sending news and important messages to the OLGA users that have signed on.