2 OLGA Basic Upd

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

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

mtotHC* in sec tion ΔRs ψ= (kg / m3s) Vof sec tion Δt •

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

*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 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

OLGA execution .out OLGA GUI

OLGA simulator

.tpl Input File

PVTsim

Fluid Properties File .tab

.ppl

.plt

.rsw