INTRODUCTION TO OLGA

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 transf.momentum transf.

energy transf.

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 (only one temperature)

• 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– etc.

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

Sources of numerical errors in general

• Linearization 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*) – may give volume errors

*)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- Σ Vi f / Vsectioni ≠ 0f

Vi f = mif /ρi fVi f = fluid volume in section no imif = mass in pipe section no iρi f = density of fluid in section no i(f indicates liquid , gas and droplets)

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

Modeling the pipeline profile in OLGA

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 INTERNAL *)

*) MERGING or SPLITTING

OLGA topology cont.

PIPE_1

1 2 3 4

PIPE SECTIONS

1 2 3 4

PIPE SECTION BOUNDARIES

1

2

1

Volume variables

2 PIPE_3

PIPE_2

32

1PIPE_4

Boundary variables

OLGA topology cont.

1 2 3 41 2 3 4

1

1

2

Volume variables e.g.Pressure (PT)Temperature (TM)Volume fractions (HOL)

2

2

1

PIPE_3

PIPE_2

3

PIPE_4

PIPE_1

Volume variables calculated in section mid-points

OLGA topology cont.

1 2 3 41 2 3 4

1

2

1

2

2

PIPE_3

PIPE_2

3

1PIPE_4

Boundary variables e.g.VelocitiesFlow-ratesFlow-pattern

PIPE_1

Boundary variables are calculated on section boundaries

Valves are always located on section boundaries

OLGA topology cont.

a TERMINAL NODE is either type ”CLOSED” –i.e. no flow across node

or of type ”PRESSURE” –i.e. flow across the node.

OLGA topology cont.

You must specify:

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

Pressure node

Generally:

flow in both directions

How to make fluids flow

• a mass SOURCE• pressure boundaries• the standard WELL

a mass SOURCE

NODE TYPE = CLOSED

A mass source into the pipeNODE TYPE = PRESSUREYou must specify it’s

Total mass rateTemperatureGas mass fractionWater fraction

OLGA calculates this P and T

mass SOURCE cont.

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

• a SOURCE can be positioned in any pipe section

• one pipe section can have several SOURCES

• a SOURCE can be negative (a sink)

a negative SOURCE

NODE TYPE = PRESSURE

a mass source out of the pipe

NODE TYPE = CLOSED

SOURCE-out

OLGA calc. this P

two PRESSURE NODES

NODE TYPE = PRESSURE

NODE TYPE = PRESSURE

Pin Pout

Pin > Pout

Pin Pout

Pin < Pout

a WELLNODE

TYPE = CLOSED

WELL-1

NODE

TYPE = PRESSURE

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

Pres

a WELL cont.

• a WELL is essentially a pressure NODE• 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 theOLGA Steady State pre-processor

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

OR BE

Conditions at t = 0 must be available.They can either be

Steady State pre-processor

• Activated when setting STEADYSTATE = ON in mainkey OPTIONS

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

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

time0

Basic wall heat transfer in OLGA

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

symmetrical around pipe axis• OLGA calculates heat accumulation in the pipe walls

as well as heat conduction through walls

Tfluid

Tambient

Tambient

Tambient Tambient

How to represent pipe walls in OLGA

For improved accuracy you should specify several layers for each material layer.

For each WALL you specify sequences of MATERIAL and the thickness of each layer -starting with the innermost layer

Tfluid

Tambient

Tws

For each wall MATERIAL you specify> Density> Cp > Thermal conductivity.

Heat transfer cont.• Conduction through pipe walls

– Assumptions• One dimensional radial heat conduction

(axial conduction not accounted for)

an example

PIPE_1

1 2 3 4

Numerical PIPE SECTIONS

PIPE_3

PIPE_2

PIPE_4

PIPE-1 PIPE-2 PIPE-3

WALL-a global

Axial specification of pipe walls in OLGA

PIPE-1

WALL-1 WALL-2 WALL-3detailed

PIPE-2 PIPE-3

PIPE-n

PIPE-1 PIPE-2 PIPE-3

WALL-aglobal with exception(s)

PIPE-n

WALL-B WALL-a

Axial specification of pipe ambient conditions inOLGA

Pipe ambient heat transfer parameters may be specified on 4 levels:

• Global i.e. entire network

• Branch-wise

• Pipe-wise

• Section-wise

Axial specification of pipe ambient conditions in OLGA

PIPE-1 PIPE-2 PIPE-3

e.g.: exception for PIPE-2 of BRANCH B-2

PIPE-n

Tamb-B-22Vair-B-22

Axial specification of pipe ambient conditions in OLGA

Section#1Vwater-311

PIPE-1

Section#2Vwater-312

Section#3

e.g. exceptions for Sections 1 and 2 of PIPE-1 of BRANCH B-3

Temperatures when walls are specified:

Tfluid

Tws

You need to specify: Tambient and the outer wall heat transfer coefficient, directly or indirectly by a fluid velocity.

Tambient

Applicable for transients as well as for steady state.

The temperature in the fluid and in each wall layer is calculated by solving the general heat transfer equations:

TtTCp 2∇=∂∂⋅ λρ

Inner wall heat transfer coefficient. Calculated by standard correlations.

)( fluidwsii TThq −=

Assuming one temperature for the fluid mixture.

Inner wall surface temperature

Overall heat transfer coefficient; the U-value:

Tfluid

Tambient

You only need to specify: Tambient and U-value

OLGA calculates: Tfluid

Then the heat flux is:

q = U(Tambient -Tfluid) (W/m)

U-value assumed to be specified wrt. inner pipe diameter.

Only applicable for steady state.

Fluid properties with standard OLGA General

• 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 fluid property for a given P and T is found by interpolating in the relevant property table

1) The Compositional Tracking module allows for detailed fluid description as function of time and position.

Restrictions - limitations with fluid tables

Total composition is assumed constant for one fluid table.– the solution is accurate for steady state co-current flow.– It is more approximate in case of local phase separation, local

mixing and varying sources of different compositions

Well B has Fluid Table 2

Well A has Fluid Table 1

Flowline has fluid properties ?

During a shut-in, fluid re-distribution causes localcomposition changes.

Compositional Tracking is required in practical applications when…

0

50

100

150

200

250

300

350

-50 50 150 250 350 450 550 650

flowing total compositionoil phasegas phase

At steady state flow conditionsgas phase is at its dew pointoil phase is at its bubble point

After e.g. shutdown – oil and gas segregates and P and T changes locally

e.g.oil above its bubble pointgas in its retrograde area

Compositional Tracking is required in practical applications when…

Black-Oil Module

• Tracks Black-oil components (oil, gas and water) described by a minimum of information:– Specific Gravity of of the oil and gas components– Gas/Oil ratio or equivalent

• With water– Specific gravity of the water– Salinity – Watercut

• Water is assumed to be inert– no water vapor and no hydrocarbons in liquid water

Properties in the fluid tables

More on Rs: the gas mass fraction

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

mass of gas at P and T

mass of gas + HC-liquid at P and TRs =

)/( 3

sec

sec *

smkgt

RsV

mtionof

tionintotHC

ΔΔ

=ψ

e.g. local mass transfer from oil to gas:

*includes water vapor in gas

Process equipment with OLGA 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 Modules• Water

– three-phase flow • Slugtracking

– also with water• FEM -Therm

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

• CompTrack– compositional tracking

• MEG-track– allows for hydrate check as function of MEG

conc.

OLGA Modules cont. • Advanced Well

– including gas-lift valves and drilling functions

• UBitTS – under Balanced interactive transient

Training Simulator• Multiphase Pumps

– positive displacement– rotodynamic

• Corrosion• Wax

– with pigging

OLGA filesis reflex of the Input File +results from OUTPUT.out

.tpl Trend Plot Fileresults from TREND

.ppl Profile Plot Fileresults from PROFILE

.plt Animation Plot Fileresults from PLOT

Restart File.rsw

Input File

Fluid Properties File .tab

OLGA

OUTPUT

extract of the .out file

TREND

Liquid volume flow as function of time at a specific position

PROFILE Profiles of P and hold-up for a flow-line-riser at t = 0

PLOT

Liquid Hold-up as function of time along the flowline-riser-

animation by OLGA-viewer

OLGA execution

.out

.tpl

.ppl

.plt

.rsw

Input File

Fluid Properties File .tab

OLGAGUI

OLGAsimulator

PVTsim