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DESCRIPTION
OLGA Basic
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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
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