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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 8 Conserv. Equations mass (5) momentum (2) energy (1)
Closure Laws Fluid Properties
mass transf. momentum transf. energy transf.
Initial Conditions
Boundary Conditions
OLGA
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) Pj
Pj+1 gas liquid
j
dM /dt =
dZj
j+1
((M·V )j - (M·V )j+1) /dzj - S j + G j + F j + F j+1+ MT
M - Momentum V - Velocity m - Mass M = m ·V
S =
Shear
G= F = MT =
Gravity = m · gravity acceleration Force = pressure · flow area Momentum Transfer = mass transfer - entrainment + deposition
= wall shear + interfacial shear
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 error At each time step when all equations have been solved the net fluid volume change in each section usually is ≠ 0 and the volume error can be expressed as VOLi = 1- Σ Vi f / Vsectioni ≠ 0 f Vi f = Vi f = mi f =
mif /ρi f fluid volume in section no i mass in pipe section no i
ρi 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.
NODE-1
a BRANCH consists of one GEOMETRY and two NODES
a BRANCH has flow direction
NODE-2
OLGA topology cont. a NODE is either TERMINAL or INTERNAL *)
An OLGA network consists of a number of BRANCHES
*) MERGING or SPLITTING
OLGA topology cont.
PIPE_4 1
Boundary variables
2 2
PIPE_1
1
PIPE SECTIONS PIPE_2
1 1
2 2
4
3 3
PIPE_3
2 1
4
PIPE SECTION BOUNDARIES
Volume variables
3
OLGA topology cont.
PIPE_4 1 2
Volume variables e.g. Pressure (PT) Temperature (TM) Volume fractions (HOL)
1
2 2
4
3 3
PIPE_3
1 PIPE_2
PIPE_1 1
2
3
2 1
4 Volume variables calculated in section mid-points
OLGA topology cont.
PIPE_4 1
Boundary variables e.g. Velocities Flow-rates Flow-pattern
2 2
3
PIPE_3
1 PIPE_2
PIPE_1 1 1
2 2
4
3 3
4
Valves are always located on section boundaries
2 1 Boundary variables are calculated 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.
Pressure node
You must specify: - Pressure, - Temperature, - Gas Mass Fraction - Water Mass Fraction
Generally: flow in both directions
How to make fluids flow • a mass SOURCE • pressure boundaries • the standard WELL
a mass SOURCE NODE TYPE = CLOSED OLGA calculates this P and T
A mass source into the pipe You must specify it’s Total mass rate Temperature Gas mass fraction Water fraction
NODE TYPE = PRESSURE
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 = CLOSED NODE TYPE = PRESSURE
OLGA calc. this P a mass source out of the pipe
SOURCE-out
two PRESSURE NODES NODE TYPE = PRESSURE
NODE TYPE = PRESSURE Pin
Pout Pin > Pout
Pin
Pout Pin < Pout
a WELL NODE
NODE
TYPE = CLOSED
TYPE = PRESSURE
WELL-1 Pres
Reservoir P & T PI (productivity index) Injection index Gas mass fraction Water fraction
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
Conditions at t = 0 must be available. They can either be calculated from user given calculated by the OLGA Steady State OR BE Initial Conditions: i.e. profiles of T, P, pre-processor mass flow, gas volume fraction, water cut
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
0
time
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 Tambient
Tambient
Tfluid
Tambient
Tambient
How to represent pipe walls in OLGA For each wall MATERIAL you specify > Density > Cp > Thermal conductivity.
Tfluid Tws
Tambient
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
Heat transfer cont. •
Conduction through pipe walls – Assumptions • One dimensional radial heat conduction (axial conduction not accounted for)
an example
PIPE_4
PIPE_3 Numerical PIPE SECTIONS PIPE_2
1
2
3
PIPE_1
4
Axial specification of pipe walls in OLGA PIPE-1
PIPE-2
WALL-a
global
PIPE-3
PIPE-1
PIPE-2
PIPE-3
WALL-a
WALL-B
WALL-a
global with exception(s)
PIPE-n
PIPE-n
PIPE-1
PIPE-2
PIPE-3
WALL-1
WALL-2
WALL-3
detailed
Axial specification of pipe ambient conditions in OLGA
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 e.g.: exception for PIPE-2 of BRANCH B-2 PIPE-1
PIPE-2 Tamb-B-22 Vair-B-22
PIPE-3
PIPE-n
Axial specification of pipe ambient conditions in OLGA e.g. exceptions for Sections 1 and 2 of PIPE-1 of BRANCH B-3 PIPE-1 Section#1 Section#2 Section#3 Vwater-311 Vwater-312
Temperatures when walls are specified: You need to specify: Tambient and the outer wall heat transfer coefficient, directly or indirectly by a fluid velocity. Tfluid Tws
Tambient
The temperature in the fluid and in each wall layer is calculated by solving the general heat transfer equations:
∂T ρ ⋅ Cp = λ∇ 2T ∂t
Applicable for transients as well as for steady state.
qi Inner wall heat transfer coefficient. Calculated by standard correlations.
Inner wall surface temperature
=
h i ( T ws − T fluid )
Assuming one temperature for the fluid mixture.
Overall heat transfer coefficient; the U-value: You only need to specify: Tambient and U-value Tfluid
OLGA calculates: Tfluid Then the heat flux is:
Tambient
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 A has Fluid Table 1
Flowline has fluid properties ? Well B has Fluid Table 2
Compositional Tracking is required in practical applications when…
During a shut-in, fluid re-distribution causes local composition changes.
350
Compositional Tracking is required in practical applications when… At steady state flow conditions gas phase is at its dew point oil phase is at its bubble point
300
250
After e.g. shutdown – oil and gas segregates and P and T changes locally
200
150
e.g.oil above its bubble point gas in its retrograde area
flowing total composition oil phase gas phase
100
50
0 -50
50
150
250
350
450
550
650
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 Rs =
mass of gas at P and T mass of gas + HC-liquid at P and T
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 files .out Input File
OLGA
Fluid Properties File .tab
is reflex of the Input File + results from OUTPUT
.tpl
Trend Plot File results from TREND
.ppl
Profile Plot File results from PROFILE
.plt
Animation Plot File results from PLOT
.rsw
Restart File
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-riseranimation by OLGA-viewer
