ASSESSING EXPLOSION HAZARDS
OF LARGE HYDROCARBON CLOUDS
FORMED IN CALM CONDITIONS:
ARE WE DOING IT WRONG?
Jerry Havens
Department of Chemical Engineering
University of Arkansas, USA
55th UKELG Meeting on “Dispersion and Consequences of LNG Releases”
April 26, 2016 - HSE Laboratory, Buxton Derbyshire
LNG IMPORT / EXPORT HISTORY (UNITED STATES)
 First LNG Cargo (27,400 m3 ), 1964 – Lake Charles, Louisiana to Canvey Island.
 Federal regulation 49 CFR 193 (Department of Transportation) Liquefied Natural Gas
Facilities: Federal Safety Standards, promulgated in 1980, applicable to land portion of
terminals. Approving authority for operations beyond the ship-to-shore boundary is
Coast Guard.
 49 CFR 193 is primarily consequence based, requiring the applicant to ensure public
safety by enforcement of exclusion zones that do not extend beyond property lines for two specific hazard categories:
 Unignited vapor cloud travel - distance to the ½ LFL concentration level for
prescribed “Design” spills.
 Prescribed fire heat flux limits – distance from ignited “Design” spills.
 To now, UVCE hazard has been neglected based on assumption of handling pure LNG
(treated as methane). U. S. NOW APPROVING LNG EXPORT TERMINALS, WITH
HAZARDS OF ACCIDENTAL RELEASE OF HEAVIER-THAN-METHANE HC’s. THIS
PAPER IDENTIFIES POTENTIAL DEFICIENCIES IN DISPERSION MODELING AND
EXPLOSION PRESSURE-LOADING (OVERPRESSURE) METHODS IN CURRENT USE.
IMPROVED REGULATORY PROCEDURES ARE
REQUIRED TO ADDRESS EXPORT TERMINAL HAZARDS
 Large design spills presently being considered for both LNG and heavy hydrocarbons make it
difficult for applicants to meet the required vapor exclusion distances (distances to the ½ lfl
concentration level cannot extend beyond the property line). The “new” way-around this
problem is confinement of the gas cloud within property boundaries with gas-impermeable
vapor fences --- introducing uninvestigated potential for increases in damaging explosion
loading (overpressure).
 Confinement of vapor clouds by fences, in addition to increasing explosion potential, can
severely limit the utility of newly approved CFD dispersion computer methods for estimating
cloud concentrations and explosion overpressures.
 CFD dispersion models recently approved have not been evaluated adequately for
predicting gas-cloud concentration fields confined by fences – introducing additional
uncertainty in the accurate prediction of concentration profiles and prediction of flamerunup overpressures – most importantly with vanishingly low wind, stable, conditions.
 Recently approved predictions of overpressures for flammable HC clouds formed from
Design Spills in low or no- wind conditions for several export terminal applications in the
U.S. are using methodology for estimating the overpressures (explosive loading) that have
been shown by industry evaluations to be seriously non-conservative.
JORDAN COVE EXPORT TERMINAL (JCET)
Proposed for Coos Bay, Oregon
VERY LARGE, HIGH-COST, PLANT
FULL CONTAINMENT TANK DIAMETER ~ 80 M
FOUR LIQUEFACTION TRAINS
SHIP OVERALL LENGTH ~ 300 M
CONSTRUCTION COST ~ $6+ BILLION
This paper focuses on possible errors and unacceptable uncertainties
In the EIS for the JCET. The primary concern is the methodology used in
the EIS to evaluate the possible consequences of unconfined vapor cloud
explosions (UVCEs).
Artist’s Rendition
Predicting Explosion Overpressures
(from documents supporting the JCET EIS)
The analysis of overpressure hazards from a VCE requires two primary simulation steps.
• The dispersion of the flammable cloud from a given release;
• The flame front and overpressure propagation from ignition of the flammable cloud.
Each of the above steps is a complex function of several parameters, including the following:
• Release characteristics, location and orientation
• Wind speed and direction;
• Ignition timing and location.
The process to identify a worst-case vapor cloud explosion is a complex task typically requiring
numerous simulations. The following considerations were made (for JCET) to reduce the
simulation matrix:
• Releases from liquefaction trains 2 and 3 were considered;
• The same scenarios selected for vapor dispersion were used;
• Ignition locations selected to maximize flame acceleration to property lines.
In order to determine which scenarios will produce the largest overpressure hazards, a
mapping methodology is needed to convert the inhomogeneous vapor clouds into “equivalent
5
stoichiometric” clouds. The methodology used the Q9 criterion developed by Gexcon.
Focus on Selected Hydrocarbon Design Spills Specified for JCET
Storage
Volume, gallons
LNG Tanks (2)
89,662,000
Impoundment
Volume, gallons
112,338,200
Design Spill
Volume, gallons
0 (Full Containment*)
Propane (1)
15,670
43,935 (shared)
15,670
Isopentane (1)
31,030
43,935 (shared)
31,030
Amine (1)
17,205
17,245
17,205
Ethylene (1)
14,000
43,935 (shared)
14,000
LNG line 36 inch**
NA
833,400 (shared)
827,740
MR*** line 6 inch
NA
833,400 (shared)
61,060
*ASSUMES NEGLIGIBLE PROBABILITY OF ANY LIQUID RELEASE
**SHIP UNLOADING LINE (LARGEST LINE)
***MIXED REFRIGERANT
Vapor Cloud Exclusion Zone Issues
Vast Range of Design Spills for Tank and Largest Operated Line specified by Applicants
“Ship Unloading Line” design spills: range 28,900 gallons to 812,000 gallons; 2004-2009 (Left to Right)
Jordan Cove Ship-Unloading Line Spill appears to be the largest to date.
900000
spill size, gallons
800000
700000
600000
500000
400000
300000
200000
100000
0
The Large Spills at Jordan Cove
are confined to the Property by Vapor Fences
Jordan Cove Export
Ship Unloading Line
827,740 gallons
Into Concrete
Impoundment
833,400 gallons
width length depth (ft)
85
85 ~15
VAPOR
FENCES
900 m
For Scaling
LNG TANK
DIAMETER
~ 80 M
SHIP
LENGTH
~300 M
Overall Plant
Dimensions (fence location extremes)
800 m
SHIP UNLOADING LINE DESIGN SPILL
827,740 Gallons
Estimated Cloud Area (> ½ lfl)
720 m by 400 m = 288,000 m2
Estimated Cloud Volume
assuming 2 – 5 m depth
576,000 – 1,440,000 m3
Containment by Dikes and Fences?
JETTING AND FLASHING LNG
ASSUMED PARTLY CAPTURED IN
CONCRETE SPILL IMPOUNDMENT,
FROM WHICH EVAPORATION
IS PREDICTED
A SHIP AT THE DOCK IS ENGULFED.
THE CONCENTRATION DISTRIBUTION
OF THE CLOUD IS NOT REPORTEDNo concentration field specified;
preventing independent evaluation
of explosion overpressure
method used.
ETHYLENE DESIGN SPILL
14,000 Gallons
Estimated Cloud Area (> ½ lfl)
320 m by 400 m = 128,000 m2
Estimated Cloud Volume
assuming 2 – 4 m depth
256,000 – 512,000 m3
JETTING AND FLASHING ETHYLENE
ASSUMED PARTLY CAPTURED IN
CONCRETE SPILL IMPOUNDMENT,
FROM WHICH EVAPORATION
IS PREDICTED
PLANT IS LARGELY ENVELOPED
CONCENTRATION DISTRIBUTION
OF THE CLOUD IS NOT REPORTED
No concentration field specified;
preventing independent evaluation
of explosion overpressure
method used.
MIXED REFRIGERANT (MR) DESIGN SPILL
61,060 Gallons
Estimated Cloud Area (> ½ lfl)
720 m by 400 m = 288,000 m2
Estimated Cloud Volume
assuming 2 – 4 m depth
576,000 – 1,152,000 m3
JETTING AND FLASHING REFRIGERANT
ASSUMED PARTLY CAPTURED IN
CONCRETE SPILL IMPOUNDMENT,
FROM WHICH EVAPORATION
IS PREDICTED
THE PLANT AND SHIP ARE ALMOST ENGULFED.
THE CONCENTRATION DISTRIBUTION
OF THE CLOUD IS NOT REPORTED.
No concentration field specified;
preventing independent evaluation
of explosion overpressure method used.
THE MR IS PROPRIETARY AND IS NOT
IDENTIFIED TO THE PUBLIC – THIS CREATES
ADDITIONAL UNCERTAINTY
The Dispersion Phase
1. The first step in predicting overpressures for a UVCE is to estimate the
concentration field throughout the cloud as a function of time. In cases
considered here (U.S.) this is now done almost exclusively with the FLACS model.
2. Since the gas concentration field is not uniform, the overpressures experienced
will depend critically on the time and location of ignition; for each vapor cloud
predicted to result from a given “Design Spill”, numerous simulations must be
made to determine the importance of ignition time and location. Additional
variables that must be considered are wind direction and speed – for Jordan
Cove wind speeds of 1 m/s and 2 m/s were specified for (stationary) wind
directions. For each “Design Spill” numerous executions of FLACS were
necessary.
3. Only after these concentration fields are specified can the calculations for
overpressures proceed to attempt identification of the worst cases resulting
from the various times and locations of ignition.
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Focusing on
Modeling
Dispersion
in Calm
Conditions
Revisit
Buncefield
Parking
Lot
Assumed 2D Flow Area – Parking Lot
Concentration Decreasing Right to Left
110 m
If we can
understand and
predict this
approximately
2-D section’s
concentration
dependence
with time, can
proceed confidently with
estimation of
explosion
overpressures.
13
Considering Gas Gravity Current Dispersion Predictions
No-wind, sufficiently dense, gas gravity currents can be turbulence-free. This
has been demonstrated in laboratory (wind tunnel) experiments, justifying a
realistic prediction of the dispersion in the parking lot at Buncefield.
Laminarization Criteria (from published literature)
Authors
Criterion
Expt/Buncefied
McQuaid (1976)
Um /(g x go)1/3
<3
Stretch (1986)
(g x g ’)/U3
> 0.005
Britter (1989)
(g ’ x Qo)/(U3 x W) > 0.1
Hall & Waters (1989)
(g ’ x Qo1/2)/U2.5
> 1000
0.79/0.73
2.0/2.5
2.0/2.3
16329/87885
where
g = gravitational acceleration, m/s2
g ’ = reduced gravitational
acceleration, m/s2
Qo= volumetric flow rate, m3/s
U = fluid velocity, m/s
U* = friction velocity, m/s
W = bund perimeter, m
1. We verified a model of the concentration field in the laboratory experiment
simulating the gravity flow across the Parking Lot at Buncefield.
2. With this model we predicted concentrations in the Buncefield flow (2D section
of parking lot) assuming air mixing into the cloud limited to molecular diffusion.
Model Verification
STEADY FEED -
• Reynolds Number
• Froude Number
ν = kinematic viscosity, m2/s
g ’ = g((ρ – ρair)/ ρair), m/s2
Model Verification
Density 1 = 1.77 kg/m3 “CO2” (g’ = 4.87 m/s2)
- For illustration, checks for consistency, verify model
Density 2 = 1.35 kg/m3 “Buncefield” (g’ = 1.40 m/s2)
- Video records to measure timing
- Concentration measurements to define time-varying
cloud concentrations - Measure gas concentrations
with FID at specific heights and down-channel locations
16
Model Verification
Synchronized video capture: side-on views of flow (smoke visualized) at 5 stations
Use timed video to measure gravity current height and velocity as function of down-current time-varying position
Starting cloud depth 7 cm
Vertical concentration measured at downwind distances 1 m, 2 m, 3 m, 4 m at heights of 0.1, 1, 2, 3, 4, 5, 6, 7, 8 cm
3
2
Source box Cover
for filling
(removable)
1
5
4
Slaved
Video
Cameras
17
100
Concentration (mole%)
Model
Verification
80
8.0 cm
6.0 cm
4.0 cm
2.0 cm
0.1 cm
60
40
20
0
0
10
20
30
40
50
Time (sec)
60
70
80
90
20
30
40
50
Time (sec)
60
70
80
90
20
30
40
60
70
80
90
20
30
40
60
70
80
90
100
Concentration (mol %)
80
Experiment
Data
8.0 cm
6.0 cm
4.0 cm
2.0 cm
0.1 cm
60
40
20
0
0
10
Concentration (mole%)
100
80
8.0 cm
6.0 cm
4.0 cm
2.0 cm
0.1 cm
60
40
Model
Predictions
20
0
0
10
50
Time (sec)
Concentration (mole%)
100
80
8.0 cm
6.0 cm
4.0 cm
2.0 cm
0.1 cm
60
40
20
0
0
10
50
Time (sec)
18
Implications for or heavy gas clouds in no wind
Diffusion can act to increase the explosive volume!
110 m (300 s)
5m
These predictions
of Buncefield used
dike-overflow
concentrations
estimated by HSL as
starting concentrations for the cloud
that advanced
across the parking
lot. Pink color
indicates within
flammable limits.
19
Predicting Explosion Overpressures
The Combustion/Explosion Phase
 The Jordan Cove DEIS reported overpressure calculations for the mixed refrigeration
and ethylene spills which resulted in the flammable clouds illustrated previously
(dispersion results taken directly from the DEIS). The highest overpressures reported
were slightly greater than 2 psi in the congested equipment areas that corresponded
to the flammable cloud location, with maximum overpressures less than ½ psi at the
property boundaries. FERC stated that approximately 2.3 psi overpressures could
impact the surface of the LNG tanks, but that such overpressures were not expected
to pose a threat to the integrity of the tank.
 Havens and Venart requested FERC to supply information substantiating the
maximum overpressure predictions of approximately 2 psi. FERC replied to our
comments in the FEIS, but not helpfully - provide no additional information that we
could use to make an independent evaluation of the overpressure predictions that
led them to approve the terminal application. Professor Venart died about the time
the FEIS came out.
20
Predicting Explosion Overpressures
The Combustion/Explosion Phase

This following statements are based on the published paper identified immediately below. The paper,
published in 2008, describes an Industry/Academia-led evaluation of current methods for determining
flammable gas volumes to be considered in explosion modeling :
SIMPLIFIED FLAMMABLE GAS VOLUME METHODS FOR GAS EXPLOSION
MODELING FROM PRESSURIZED GAS RELEASES: A COMPARISON WITH
LARGE SCALE EXPERIMENTAL DATA
V.H.Y. Tam1, M. Wang1, C.N. Savvides1, E. Tunc2, S. Ferraris2, and J.X. Wen2
1EPTG, BP Exploration, Chertsey Road, Sunbury-on-Thames, TW16 7LN, UK
2Faculty of Engineering, Kingston University, Friars Avenue, London, SW15 3DW, UK
IChemE Symposium Series No. 154, 2008
Quoting from the Abstract
Gas explosion modelling is used widely to assess explosion loading due to overpressure or drag forces.
These loadings depend upon the conditions when ignition is assumed to occur. We consider one of these
conditions, namely the volume of a flammable gas cloud, specifically from a release of pressurized gas: and
the application to the most commonly used commercial explosion code, FLACS.
There is a number of ways this volume is defined. The three main methods are (i) volume enclosed by
the LFL contour surface, (II) volume bounded by LFL and UFL contour surfaces and (iii) burning velocity
weighted volume (Q9). These methods give vastly different flammable volumes …, this situation is not
satisfactory.
In this paper, we compared the prediction using different methods against data from large scale
explosion experiments (Phase 3b.) Our results show that the burning velocity weighted volume Q9
significantly underpredicted loads for explosion higher than 0.1 bar compared with the other two measures
21
which gave neutral to slightly over-conservative prediction of explosion loads.
Predicting Explosion Overpressures
The Combustion/Explosion Phase
Additional Quotes from the Paper
 Q9: Weighted Equivalent Stoichiometric Volume. Q9 is a volume measure which accounts for the effects
of gas concentration by weighting the volume with the effect of burning velocity and expansion ratio.
Experiments showed that burning velocity varies with concentration … For hydrocarbons, burning velocity
is maximum at or near stoichiometric concentration of 1 and dropping off rapidly as gas concentration is
rich … or lean …, reaching zero at UFL and LFL. Further, the expansion ratio follows a similar pattern …
 General Behavior of the (three) Methods. These measures give different flammable gas cloud volumes. …
>LFL gives the largest volume, followed by ∆FL, then Q9. Magnitude of explosion loading would follow
the same pattern. We found that Q9 measures are being used increasingly by consultants. We are
concerned that there has not been work to verify that this approach is indeed correct. Our observation is
that there appears to be little fundamental understanding of the Q9 measures by consultants we
encountered. Its application is based on a belief that since there is a varying gas concentration in a gas
cloud formed from pressurized gas release, assuming a uniform gas cloud concentration is thus ‘overconservative’, and using Q9 would remove this perceived ‘over-conservatism’. As we see … this is not
necessarily so.
22
Predicting Explosion Overpressures
The Combustion/Explosion Phase
And:
 Superficially, Q9 seems to be the most accurate measure out of the three as it accounts for the well known
effect of gas concentration on flame speed and expansion ratio. It may be a surprise that our results
showed that the Q9 measure performs poorly. However, one should not confuse complexity and accuracy.
 In reality, the three measures described in this paper are no more than a much simplified and idealized
representation of a complex situation… by focusing on a couple of obvious factors, many others are
overlooked – here are a couple of examples:
 Initial Turbulence – …. A pressurized gas leak can impart a large amount of and high intensity
turbulence which may be considered similar to turbulence produced by the obstacles. This is not
taken into account by (any of) the simple measures discussed here.
 Size of the gas cloud – Limiting the flammable gas cloud to a smaller effective volume reduces the
effect of flame acceleration over a larger distance and over longer period of time than that produced
by larger cloud volumes and could lead to lower and the wrong distribution of overpressure….
Another reason for possible underestimation of flammable volume is that volumes with rich gas
mixtures can be diluted with air or with lean gas mixtures during the course of a gas explosion,
rendering the rich mixture closer to the stoichiometric ratio of 1. Applying the Q9 method blindly, it is
possible to reach a conclusion that a very large leak of flammable gas would not pose a hazard.
23
Predicting Explosion Overpressures
The Combustion/Explosion Phase
And, finally:
 Following the completion of the Phase 3b exercise and publishing of the results
there was no published validation of the methodology for calculating effective
cloud volume measures for use with CFD codes. Any methods used should be
verified against experimental data as far as possible. It should be the duty of the
model developer or user of the model to verify any new methods against
available data…
 Based on the results of this work, we recommend that the non-conservative
flammable cloud volume measure ∆FL be adopted as the basis for FLACS at mid
to late stage of an engineering project definition. “>LFL is a conservative
measure which may be appropriate during the early stage of design of a process
facility where uncertainties in the design is high. THIS WORK DOES NOT
SUPPORT THE USE OF Q9 (emphasis added).
24
Conclusions – Dispersion (Cloud Formation) Modelling
• Low-wind, shallow clouds, and long time scales indicate potential
for important, if not controlling, molecular diffusion effects. In such
conditions, even in the presence of congestion, it should be
anticipated there will be little or no turbulence contribution to
dispersion – raising serious questions about the prediction of cloud
formation with the FLACS model.
• The requirements for accurate gas dispersion prediction (cloud
concentrations) under low-wind conditions, with plant equipment
and/or gas confinement with fences, have not been verified for the
CFD models presently used in the U.S. (FLACS). Further, the models
are proprietary, so the validity of the model predictions cannot be
checked independently by (or for) stakeholders.
• There is important potential for molecular diffusion effects to make
concentration more uniform and increase potential for explosion.
25
Conclusions - Prediction of Cloud Explosion Overpressures
Vapor Cloud Explosion overpressure predictions presented in the JCET EIS
are seriously deficient:
• The cloud concentrations are not specified. Without such specification, it
is impossible for any stakeholder to verify the predictions, which are
coupled to the subsequent estimation of overpressures.
• The composition of the mixed refrigerant liquid is not reported, further
denying stakeholders the possibility of assessing the overpressure
calculations.
• The explosion hazards (overpressures) are not considered for the pure
components of the MR, even though design spills for the components
were selected (ethylene, propane, isopentane, and amine).
• Serious questions have been raised by competent investigators regarding
the application in FLACS of the Q9 method.
• Approval of JCET, recently denied but requesting rehearing, should be
delayed until these questions and uncertainties have been satisfactorily
26
addressed.
Acknowledgments
Herman, Jeremy J., “An Experimental and Computational Evaluation of the
Importance of Molecular Diffusion in Gas Gravity Currents”, Doctoral Dissertation,
University of Arkansas, December 2013
27