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Verification and validation of consequence and risk models

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Verification and validation of consequence models

UKELG, 19-20 September 2007Cardiff, Wales

Henk W.M. Witlox, DNV Software

Det Norske Veritas AS. All rights reserved Slide 226 October 2007

Verification and validation of consequence models

Introduction

Discharge

Dispersion

Flammable effects


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Continuous release with rainout

jet fire

pool fire

explosion or flash fire

droplet trajectory

flashing two-phasedischarge from vessel vapour-plume

centre-line

poolpoint ofrainout

SUBSTRATE


Quality procedure for model development Define model

- Literature review- Formulate physical and mathematical model- Solution method and algorithm

Design (preliminary, detailed) and coding Model testing

Documentation

Review

Model integration into overall product


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Model testing

Verification - code solves correctly mathematical model- Against analytical solution- Against parallel verification Excel spreadsheet- Against other model

Validation against experimental data justify model assumptions- Small-scale experiments (isolating phenomenon)- Large-scale experiments

Sensitivity analysis overall robustness and effect parameters- Base case- Parameter variations (single or multiple parameters)


Key references for consequence and risk modelling

Dutch Yellow Book (1997) Loss Prevention Process Industries

- Lees (1996)- Updated Mannan (2005)

Perry Chemicals Engineering Handbook (1999) CCPS guidelines

- Dispersion (1996)- Flammable effects (1994)- QRA (2000)


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Discharge model

pipe

flow

expansion zone

leak orifice

atmosphere

vessel (stagnation)


Discharge model

Range of scenarios- Leak from vessel, short or long pipes, instantaneous, relief systems, - Sub-cooled liquid, flashing liquid, or gas release

Data - Flow rate, velocity and liquid fraction (both orifice and post-expansion data)- Droplet size

Literature survey- Numerous discharge models- No up-to-date overview of experiments (benchmark tests)- No published established systematic model evaluation

Literature review to establish experimental dataset

Application to Phast discharge models


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Discharge model verification and validation

Verification- Analytical flow-rate equations

- incompressible liquid (Bernoulli)- ideal gas (choked and un-choked)

- Process simulators- Worked-out examples in literature (e.g. CCPS publications)

Validation- Subcooled and saturated water jets

- Sozzi and Sutherland (varying pipe length)- Uchida and Narai (varying pipe length and stagnation pressure)- Many other experiments

- Hydrocarbon releases- Full-bore and orifice releases of liquid propane (Shell)- Orifice releases of butane (Shell)- Long pipe - validation against Isle of Grain (full-bore and partial leaks LPG)


Discharge model validation for sub-cooled water release

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.5 1 1.5 2 2.5Pipe length (m)

Rat

io o

f pre

dict

ed to

o

bser

ved

flo

wra

te (-)

PHAST6.53-Sozzi&Sutherland

PHAST6.53-Uchida&Narai


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Validation against P66: mass expelled against time (45.3% breach)

P66 - Inventory

0

20

40

60

80

100

120

0 10 20 30 40 50 60t(s)

Ma

ss (kg

)


Dispersion (example PHAST Unified Dispersion Model UDM - continuous dispersion)

z

x

TOUCHINGDOWN

ELEVATEDPLUMEy

GROUND-LEVELPLUME

circularcross-section

truncated cross-sectionsemi-elliptic cross-section

Elevation phases: elevated, touching down, ground-level, lift-off, elevated, mixing layer

Dispersion phases: jet, heavy, passive

WIND

jet release


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Dispersion module verification and validation

Near-field elevated/jet dispersion- Verification: analytical solution (horizontal jet), vertical jet correlations

Heavy dispersion- Verification: analytical solution (2D), HEGADAS (3D)- Validation: McQuaid (2D isothermal), HTAG (3D isothermal)

Passive dispersion- Verification: analytical solution, TNO Gaussian concentration profile

Finite duration- Verification: SLAB/HGSYSTEM [finite-duration correction model FDC]- Validation: Kit Fox [quasi-instaneneous model (QI) and FDC]


Dispersion module verification and validation (continued)

Thermodynamics- Verification: analytical, HEGADAS (pure component, mixture, HF)- Validation: Schotte experiment (HF)

Pool spreading/evaporation- Verification: GASP- Validation: spills on water/land, wide range of chemicals (LNG, propane, )

- spreading (non-volatile chemicals)- evaporation (confined pools)- simultaneous spreading and evaporation


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Dispersion Validation against large scale experiments

Continuous passive dispersion- Prairie Grass

Continuous elevated two-phase jet- Ammonia (Desert Tortoise and FLADIS)- Propane (EEC)- HF (Goldfish)

Continuous dispersion from pool - LNG (Maplin Sands, Burro, Coyoto)- LPG (Maplin Sands)

Un-pressurised instantaneous- Freon-12 (Thorney Island)

Continuous and finite-duration dispersion from area source- CO2 (Kit Fox)


Dispersion Validation for Kit Fox experiment(20 second release of CO2, experiment KF0706)

0.01

0.1

1

10

100

1 10 100 1000Downwind distance (m)

Cen

tre

line

co

nc

entr

atio

n (m

ol%

)

Phast 6.53 (FDC)Experimental


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Fireballs, jet fires and pool fires Mathematical model

- Empirical correlations for fire geometry and surface emissive power- Fireball (sphere)- Pool fire (tilted cylinder)- Jet fire (cone)

- Radiation at given location by means of integration along fire surface

Verification- Simple hand calculations and/or spreadsheet- Comparison against other models

Validation- Pool fire:

- LNG (Montoir, Johnson)- Hexane (Lois and Swithenbank)

- Jet fire: - Vertical natural gas (Chamberlain)- Horizontal natural gas (Johnson, Bennett et al.)- Two-phase LPG (Bennett et al.)- Horizontal liquid crude oil (Selby and Burgan)


Pool fire model (Phast model POLF)

H

D

WIND


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Validation against Johnson LNG pool fire experiments

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5 6 7 8Measured heat flux (kW/m2)

Pred

icte

d he

at flu

x (kW

/m2)

0% deviation

-15% deviation

15% deviation

-40% deviation

40% deviation

Johnson (1992)PHAST 6.53


Jet fire - Chamberlain pure-vapour model (no crosswind)

Z

X

W2

j

W1

b

RL

Lb

cone

tilt by wind

lift-off distance

widths, length

WIND


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Validation against Johnson LNG jet fire experiments

0

5

10

15

20

25

30

35

0 5 10 15 20 25Measured heat flux (kW/m2)

Pred

icte

d he

at flu

x (kW

/m2)

0% deviation-15% deviation15% deviation-40% deviation40% deviationJohnson (1994)-Simulated DataPHAST 6.53


Explosion modelling

Comparative study by Fitzgerald - Key models:

- TNO Multi energy (MULT)- Baker Strehlow (BSEX)- Shell Congestion Assessment Model (CAM)

- Validation: - LNG, LPG (EMERGE - TNO)- LNG (BFETS -SCI)


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Explosion - Validation against EMERGE 6 experiment(LPG, medium scale)

0.00E+00

1.00E+04

2.00E+04

3.00E+04

4.00E+04

5.00E+04

6.00E+04

0.00E+00 1.00E+01 2.00E+01 3.00E+01 4.00E+01 5.00E+01Distance from edge (m)

Ove

rpre

ssu

re (P

a)

Test AverageTest MaximumBSEX ResultsMULT Results

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