Post on 10-Mar-2020
CFD Based Approach for VCE Risk Assessment
2009 MKOPSC International Symposium
Anna Qiao, Steven Zhang, Asmund HuserDNV Energy
© Det Norske Veritas AS. All rights reserved Slide 203 December 2009
Objective
Determine the maximum design load at a specified frequency
Provide a detailed risk picture for large offshore platform
Provide recommendations on design issues based on CFD analysis results
© Det Norske Veritas AS. All rights reserved Slide 303 December 2009
CFD Based Approach Adopted
CFD ventilation anddispersion
CFD explosion
Start
Ventilation and cloud sizedistribution
Explosion loadResponse surfaces
Probabilisticdistribution of
leak sizes,wind rose, etc.
Acceptec criteria,
QRA, design
Geometry modelling FLACS
ProbabilisticassessmentEXPRESS
CFD references database
© Det Norske Veritas AS. All rights reserved Slide 403 December 2009
Major Steps
Identification of critical leak scenarios
CFD model (FLACS)- Geometry model – grid calculation- Ventilation and dispersion analysis- Explosion analysis
Probabilistic analysis (EXPRESS)
Sensitivity studies
Design recommendations
© Det Norske Veritas AS. All rights reserved Slide 503 December 2009
Ventilation
Purpose - Select the leak location,
leak rate, and wind speed to find the largest flammable cloud
- Determine the time to start leak in dispersion model
© Det Norske Veritas AS. All rights reserved Slide 603 December 2009
Dispersion Analysis
Purpose – to find the maximum flammable cloud size
© Det Norske Veritas AS. All rights reserved Slide 703 December 2009
Dispersion Analysis
0
10
20
30
40
50
60
70
80
90
0 1 2 3 4 5 6 7 8 9 10 11
Leak rate (kg/s)
Filli
ng d
egre
e Q
9 (%
)
CFD Results Buoyant GasBuoyant Gas TrendCFD Results Dense GasDense Gas Trend
Set up a series of scenarios - Wind speed, wind direction, leak rate, leak direction and leak location are
varied systematically- Select condition giving largest cloud size
© Det Norske Veritas AS. All rights reserved Slide 803 December 2009
Approach Limiting CFD Runs
Problem: CFD analyses are major calculations and time consuming
Solution: DNV adopted a “Frozen Cloud” approach
Assumption: a linear relation between gas concentration, c, and leak rate and the wind speed is assumed for each leak scenario
The effect on the flammable gas cloud size from running slightly smaller and larger leak rates can be assessed without CFD simulation of additional cases
umzyxc
),,(
© Det Norske Veritas AS. All rights reserved Slide 903 December 2009
Frozen Cloud Results
Case study shows that for this area, the highest filling, 60% of the process area, is expected to occur when R=0.15 (Ratio of volume flow of combustible gas to air)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.1 0.2 0.3 0.4 0.5 0.6
R=Qg/Qa
Vf/V
© Det Norske Veritas AS. All rights reserved Slide 1003 December 2009
Complete Cloud Response Surface
Huser et al, OTC 12951, 2001
© Det Norske Veritas AS. All rights reserved Slide 1103 December 2009
Explosion Analysis
Once cloud dispersion is established, the explosion calculation can also be executed by CFD modeling
Consider the effect of - Geometry: size, confinement, obstruction, minimum distance between
decks/plates, corner constant- Gas mixture: composition, location, quantity
Critical locations for explosion load determination- Control rooms, vessels, columns, decks
Critical loads is a combination of:- The maximum pressure pulse measured on monitors is used as
reference pressure- Maximum drag
© Det Norske Veritas AS. All rights reserved Slide 1203 December 2009
Explosion Analysis from FLACS
© Det Norske Veritas AS. All rights reserved Slide 1303 December 2009
Explosion Database of CFD Cases and Experiments
19 explosion models- One process platform (plant) for each model- 15 to 50 explosion simulations performed in FLACS for each model- Maximum explosion overpressure used to develop the correlations
15 experiments- Various module sizes- Different congestion levels- Different explosion venting areas
© Det Norske Veritas AS. All rights reserved Slide 1403 December 2009
Simulation Overpressure vs. Cloud Size Trend
Huser et al, Journal of Loss Prevention in the Process Industries, 22 (2009), 324
P
3/5~ fVP
© Det Norske Veritas AS. All rights reserved Slide 1503 December 2009
Smart Process
Conditions New ExplosionNew Ventilation New Dispersion
Database
Frequency
Exceedence Curve
New Installation
© Det Norske Veritas AS. All rights reserved Slide 1603 December 2009
Explosion Risk Analysis, EXPRESS
Purpose – Frequency of exceeding explosion load vs the load
Selected critical loads are used in probabilistic analysis
DNV Probabilistic tool EXPRESS is applied to find pressure exceedence curves
- EXPRESS establishes a response surface using a limited number of CFD runs
- The response surface rapidly generates the exceedance results- Pressure on other targets are found as a linear relationship between
the reference pressure and the other target
© Det Norske Veritas AS. All rights reserved Slide 1703 December 2009
CFD Based Approach Adopted
CFD ventilation anddispersion
CFD explosion
Start
Ventilation and cloud sizedistribution
Explosion loadResponse surfaces
Probabilisticdistribution of
leak sizes,wind rose, etc.
Acceptec criteria,
QRA, design
Geometry modelling FLACS
ProbabilisticassessmentEXPRESS
CFD references database
© Det Norske Veritas AS. All rights reserved Slide 1803 December 2009
Frequency/Probability
Leak frequency (for each leak category)
Probability of ignition – Time Dependent Internal Ignition Model (TDIIM)- Pump- Compressor- Generator- Electrical equipment- Personnel- Hot work- Other
Explosion frequency
© Det Norske Veritas AS. All rights reserved Slide 1903 December 2009
Time Dependent Internal Ignition Model (TDIIM)
Intermittent ignition sources- The total cloud size
Continuous ignition sources- The increase in cloud size
© Det Norske Veritas AS. All rights reserved Slide 2003 December 2009
Monte Carlo Simulation
Select specific outcomes of all stochastic variables from their statistical distributions
Calculate ignitable time dependent cloud size and ignition probability
Select ignition time from the ignition time probability distribution
Calculate explosive cloud size at time of ignition
Calculate explosion loading from explosion load response surface
© Det Norske Veritas AS. All rights reserved Slide 2103 December 2009
Pressure Exceedence Curves
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
0 0.2 0.4 0.6 0.8 1 1.2 1.4Explosion pressure load (barg)
Acc
umu l
ated
fre q
uenc
y(1
/yea
r ). ESD close after 60 s
ESD close after 30 s
Reduction in explosion load due to rapid initiation of ESD
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
0 0.2 0.4 0.6 0.8 1 1.2 1.4Explosion pressure load (barg)
Acc
umu l
ated
fre q
uenc
y(1
/yea
r ). ESD close after 60 s
ESD close after 30 s
Reduction in explosion load due to rapid initiation of ESD
© Det Norske Veritas AS. All rights reserved Slide 2203 December 2009
Prevention/Mitigation
Leak frequency reduction
Localized enclosure
Ventilation
Deluge
Gas detection, shutdown and blowdown
Reduced number of ignition sources
Blast wall
© Det Norske Veritas AS. All rights reserved Slide 2303 December 2009
Experimental explosion results used for the development of the correlation
CFD results directly applied to find cloud dispersion/size and resulting maximum explosion pressure
Special techniques allow fewer CFD runs- Frozen cloud approach- Response surface approach
Combined CFD and probabilistic analysis gives the detailed risk picture
Why Method is Appropriate
© Det Norske Veritas AS. All rights reserved Slide 2403 December 2009
Typical Risk Based Design Activities for Offshore
Explosion – FLACS & EXPRESS
Fire – KFX, PFPro & EXPRESS
WCI – Wind Chill Index
Ventilation - ACH
Gas detector location - GDOZ