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 8 Conserv. Equations mass (5) momentum (2) energy (1)
Closure Laws Fluid Properties
mass transfer momentum transfer energy transfer
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 • 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) 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
The OLGA volume equation Pi i
Qi
i
Qi+1
i+1
source / sink
The OLGA volume equation for all sections the local fluid volume time variations
Combine all mass conservation equations
Net change of total fluid volume in a section = 0 T and RS constant
0= CdP/dt = Qin - Qout + volume change by mass transfer + volume change by mass sources + Volume Correction temperature change is assumed to be zero in this step : [m3/s]
(V / T)(T / t ) 0 C = “compressibility” i.e. dVfluid/dP
[m3/Pa]
Calculation sequence for each time step: a) solve model relations to give friction factors etc.
b) solve momentum - and volume equation simultaneously U and P c) perform flash d) solve mass equations first for gas then for oil film and oil droplets and then for water film and water droplets e) solve energy equation T f) 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 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- Vi f / Vsectioni 0 f Vi f = Vi f = m if =
mif / i f fluid volume in section no i 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 OLGA The 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.
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 MERGE or SPLIT 1)
1)
v 4.00
An OLGA network consists of a number of BRANCHES
OLGA topology cont. PIPE_1 3
PIPE SECTIONS
2
1
1
2
2
3
3
4
PIPE SECTION BOUNDARIES
4
1 5
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 boundary
- Pressure, - Temperature, - Gas Mass Fraction - Water Mass Fraction
Analogy Pressure boundary; gas mass fraction Gas fraction = 1 P,T
Gas fraction = 0.5 P,T
Gas fraction = 0 P,T
How to make fluids flow • a mass SOURCE • pressure boundaries • the standard WELL
a mass SOURCE BOUNDARY TYPE = CLOSED OLGA calc. this P and T BOUNDARY TYPE = PRESSURE SOURCE-1 a mass source into the pipe Total mass rate T Gas mass fraction Water fraction
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
BOUNDARY TYPE = CLOSED
OLGA calc. this P
a mass source out of the pipe
SOURCE-out
two PRESSURE BOUNDARIES BOUNDARY TYPE = PRESSURE
BOUNDARY TYPE = PRESSURE Pin
Pout Pin > Pout
Pin
Pout Pin < Pout
a WELL BOUNDARY
BOUNDARY
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 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 conditions at t = 0 can either be calculated from user given Initial Conditions: calculated by the or be i.e. profiles of T, P, steady state mass flow, pre-processor 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 (when STARTTIME = ENDTIME = 0 in INTEGRATION) • The subsequent dynamic simulation will tell you if the system is stable or not
0
time
Wall heat transfer • Standard heat transfer correlations • Averaged fluid properties • Radial heat conduction in pipe walls
Pipe wall discretisation How OLGA represents walls
Calculated by OLGA: Heat transfer coefficient gas
Need to input: Heat transfer coefficient
Need to input: liquid
Conductivity
Heat capacity Density
Fluid properties 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 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 Rs =
mass of gas at P and T mass of gas + HC-liquid at P and T
• 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
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 Flowline has Well A has
fluid properties ?
Fluid Table 1 Well B has Fluid Table 2
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 Input File .inp Fluid Properties File .tab
ASCII file
OLGA 2000
.out .rsw reflex of the Input File + results from OUTPUT
.tpl
Trend Plot File results from TREND
.ppl Restart File (binary) Profile Plot File results from PROFILE
Principles of OLGA 2000 execution Input File .inp
OLGA 2000 GUI
OLGA 2000
Fluid Properties File .tab
.tpl PVTsim
.ppl .out
.rsw
OLGA 2000 Support •
[email protected] – OLGA 2000 Helpdesk mail address
• http://www.olga2000.com – The OLGA 2000 web site
•
[email protected] – OLGA 2000 News Network (ONN): Mailbox at Scandpower for sending news and important messages to the OLGA users that have signed on.
