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Copyright 2002, Offshore Technology Conference

This paper was prepared for presentation at the 2002 Offshore Technology Conference held inHouston, Texas U.S.A., 69 May 2002.

This paper was selected for presentation by the OTC Program Committee following review ofinformation contained in an abstract submitted by the author(s). Contents of the paper, aspresented, have not been reviewed by the Offshore Technology Conference and are subject to

correction by the author(s). The material, as presented, does not necessarily reflect anyposition of the Offshore Technology Conference or its officers. Electronic reproduction,distribution, or storage of any part of this paper for commercial purposes without the written

consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print

is restricted to an abstract of not more than 300 words; illustrations may not be copied. Theabstract must contain conspicuous acknowledgment of where and by whom the paper was

presented.

Abstract

A 3D CFD model with a reduced combustion capability canbe used with a dispersion model and either a 1D CFD code or

a blast curve method to provide an accurate explosion hazard

evaluation early in the life of an offshore project. The analysis

using this alternate approach is more accurate than using the

1D CFD code or blast curves alone because it accounts for

blast wave interactions with structures, actual vent surfaces,

venting into other areas of congestion and provides a more

realistic blast wave decay for the specific scenario.

IntroductionThe North Sea approach to predicting a vapor cloud

explosion (VCE) in offshore applications has been the use of

traditional three-dimensional computational fluid dynamics

(3D CFD) models. Traditional 3D CFD models can be costly

to obtain and are time consuming to operate. It would be

reasonable to expect several man-weeks per scenario to build

the traditional 3D CFD model and many days for the model to

run. Although this would provide the most accurate prediction

available, this evaluation is typically performed late in the life

of a new project because only then are sufficient details

known to develop the model. Unfortunately, this is also when

modifications are difficult and expensive to make. The toolsdiscussed in this alternate approach idealize the process

layout, hence a general knowledge of the process layout will

allows the using this alternate approach much sooner in

project life than typical for a traditional CFD analysis. This

has the potential for significant savings by discovering

problems early in project life. The proposed alternateapproach also results in a faster what-if analysis to evaluate

potential corrective actions.

An example application of this alternate approach is

presented for illustration. While this alternate approach is

primarily targeted for a new offshore construction project, it

also can also apply to existing offshore installations needing

an explosion hazards evaluation. This alternate approach does

not preclude the use of traditional CFD as a final validation of

the explosion hazards.

Analysis DescriptionThis alternate approach is broken into three tasks: 1) a vapor

cloud size evaluation; 2) a peak pressure analysis; and 3) an

actual venting and blast wave interaction analysis. The vapor

cloud size evaluation uses engineering judgement, a dispersion

model, ventilation calculations, or a combination thereof to

evaluate the vapor cloud size. The peak pressure analysis is

performed using either the 1D CFD model SCOPE [1] (Shel

Code for Overpressure Prediction in gas Explosions), for

confined explosions, or a simple blast curve method such as

the latest Multi-Energy Method [2] (MEM2) or Congestion

Assessment Method [3] (CAM2), for unconfined explosions.

The actual venting and blast wave interaction analysis isperformed using the 3D CFD model CEBAM [4(Computational Explosion and Blast Assessment Model) with

a simplified (reduced) combustion computation. This

alternate approach uses the strengths of each analysis method

to overcome inherent potential errors each method has in orde

to maintain simplicity. These steps are described in more

detail in the following sections.

Vapor Cloud Size Evaluation. This alternate approach uses a

dispersion model to evaluate the release rate of the source if it

is suspected that the release may not fill the entire area with a

flammable cloud. For small confined volumes, it can easily be

assumed that the entire volume will be filled with a flammable

cloud. For larger confined volumes, the release rate from the

dispersion model is used to determine a total release mass over

a given time period. Alternatively, ventilation calculations

can easily be performed where applicable to determine howmuch time is needed to obtain a flammable concentration. For

unconfined volumes, the dispersion model results can be used

to evaluate the cloud size and congested volume that is filled

by a flammable vapor.

OTC 14133

Offshore Explosion Hazard Evaluations using a 3D CFD Code with ReducedCombustion Combined with a 1D CFD Code or Blast CurvesG. A. Fitzgerald, ABS Consulting, Inc.

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2 GARY A. FITZGERALD OTC 14133

Explosion Peak Pressure Analysis. This task uses the

appropriate explosion prediction tool; SCOPE, MEM2, or

CAM2, as described below, to evaluate peak internal and

external (for external explosions) pressures. Confined offshore

process applications may have more vent surface available

than SCOPE can model since all the walls and the roof mightbe designed to fail under high pressures. Thus, the SCOPE

peak pressure results will be conservative.

Unconfined VCEs are presently best evaluated using either

the latest Multi-Energy Method (MEM2) or Congestion

Assessment Method (CAM2). Recent developments in these

blast curve based methods have greatly increased their

accuracy and confidence and comparisons to published

experiments [5] have shown them to be the best available

simple means to evaluate unconfined VCEs.SCOPE. SCOPE is a 1D CFD code. It is an alternative to

traditional 3D CFD for confined explosions that has been

recently made available to the general public. This code canbe set up in a matter of minutes and run in a few seconds.

SCOPE also simplifies the traditional CFD approach in that

the exact process environment does not need to be replicated.Instead, the user will idealize the environment in a number of

vertical congestion grids. The grids do not have to be evenly

spaced or have uniform blockage. This idealization approach

has resulted in a much faster means to evaluate internal VCEs

with accuracy comparable to traditional CFD models. SCOPE

is designed for confined explosions (roof and walls), but will

allow a significant amount of venting on three planes of the

scenario that can be open or covered and set to fail at a

specific pressure or time. SCOPE also allows for partial filland will predict when unburned gases are pushed out the vents

such that the turbulence induced by the vent causes an external

explosion. The major shortcomings in SCOPE are that

external pressure predictions are made based on blast wavedecay using blast curves from the main vent only and does not

account for interactions with buildings. Blast wave

interactions with buildings can either shield or focus a blast

wave, or result in a reduced blast load due to quick clearing

for small buildings.MEM2. The MEM2 method uses the original MEM blast

curves that are pressure and duration curves based onhemispherical VCEs and divided into 10 severity levels from 1

to 10. The original MEM was first published in 1984 [6] and

has been updated in 1998 [7] and 2001 [2]. With the MEM2, a

peak pressure is calculated and a severity level is solved for

that corresponds to the calculated pressure. The MEM2

calculations are made using the following inputs: Volume blockage ratio (VBR): Congested volume divided

by total volume.

Length of flame travel (Lp): A hemispherical radius is

calculated from the congested volume filled with the

flammable vapor.

Laminar burning velocity (LBV): Maximum burning

velocity of a slightly richer than stoichiometric fuel/air

mixture in a quiescent volume without congestion or other

turbulence inducing factors.

Average congestion diameter: Hydraulic diameter has

been found to provide best results for most process

layouts. Hydraulic diameter is defined as four times the

ratio between the cumulative volumes and the cumulative

surface areas of an object distribution. The actual average

diameter is used for repetitive obstacles with the

same diameter. Energy Efficiency: Less than 0.5 bar peak pressure results

in an energy efficiency of 20%. Less than 1 bar peak

pressure results in an energy efficiency of 50%.

CAM2. The CAM2 method is also based on hemispherical

VCE blast curves. It was published in 1995 [8], updated in

1999 [9] and an error corrected in 2001 [3]. Peak pressure is

calculated from one of two equations, one for 2D- and one for

3D-flame expansion. Additional calculations are used to decay

the blast with distance, determine duration and rise time to

peak pressure and blast wave shape. The CAM2 peak

pressure calculations are made using the following inputs:

Dimensions: Length, width and height, x, y, and z

respectively, of congested region.

Number of obstacles in each direction: Number o

obstacle layers the flame front passes in the x, y and z

directions.

Area blockage ratios (ABRs) in each direction: ABRs inthe direction of the flame front in the x, y and z directions

using the most congested regions in each direction.

Complexity factor: A variable ranging from 1 to 4 to

describe complexity of congestion. A complexity of 1

would be idealized, repetitive congestion. A complexity

of 4 would be a typical process layout.

Fuel factor and expansion ratio: Two variables provided

for several fuels to account for reactivity and amount of

expansion upon ignition.

Actual Venting and Blast Wave Interaction Analysis

Another new type of CFD model, referred to as a CFD codewith reduced combustion computation, is called CEBAM

CEBAM simplifies the traditional 3D CFD model by

foregoing vapor cloud dispersion or flame acceleration, thus

the set up time is only a few hours and run time is only a day

or two for most applications. However, this introduces

potential errors in that the user must specify a vapor cloud size

and flame speed to the VCE. While CEBAM is a fully

functional 3D CFD code that has been publicly validated [ 10

11], the accuracy of the results are directly dependent upon the

user assumptions for vapor cloud size and flame speed. Theseassumptions require an experienced user or additiona

modeling to produce accurate results. A major advantage ofusing CEBAM is the ability to model blast wave interactions

with structures and the ability to replicate true building failure

criteria that produces vent surfaces for the VCE.

The last step in the analysis involves building the CEBAM

model, identical to the SCOPE model if it was used in the

previous step, and adjusting flame speeds until the CEBAM

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model predicts the same peak pressures as was predicted bythe explosion peak pressure analysis model. In this process,

we are tuning the CEBAM model to duplicate the results

from the explosion prediction tool for an equivalent

configuration. With two major variables, flame speed and grid

size, many combinations of these two can exist to predict an

accurate peak overpressure. The pressure decay rate is alsodependent upon grid size. Communications with the CEBAM

developer have indicated that for most applications, a grid size

of 2 to 5 feet is sufficient to get accurate blast wave decay

rates at relatively low pressures.

If SCOPE was used as the source term model, once the

CEBAM flame speed and grid size is found to reproduce theSCOPE peak pressure results, the actual vent surfaces and

failure pressures (those beyond the capabilities of SCOPE) are

then included in the CEBAM model. Since the CEBAM

model can evaluate more vent surface area than SCOPE, the

resulting peak internal pressure in CEBAM may be less than

that produced by SCOPE, even though the flame speeds were

adjusted to produce the same initial peak pressures for the

same vent criteria. This is because the flame speed adjustment

to match pressure was performed using identical venting

criteria for both models. When that flame speed is later used

in CEBAM with increased venting, the effect of the increased

venting decreases the resulting pressure predictions. This

helps to remove some of the conservatism introduced in

SCOPE for cases where the modeled vent surface area is less

than actual and will result in a more realistic and accurate

answer while still being conservative.

Example Scenario

This example potential explosion scenario (PES) is fictitious

and is not meant to represent any specific facility but does

show the advantages of the proposed alternate approach. Afictitious Floating, Production, Storage and Offloading vessel

(FPSO) is shown in Figure 1. The scenario is identified to be

at the aft part of the process congestion, flanked by equipment

and/or vent panels that provide symmetrical confinement and

a grated deck roof with equipment above to provide vertical

confinement. The vessel is 800 feet long and 120 feet wide.

The scenario occupies a space with dimensions of 25 feet x100 feet x 25 feet tall. The scenario layout is shown in Figure

2. The outboard edges of the scenario are open. The scenario

assumes a process area with uniform congestion. This area

will be exposed to a release of LPG. An ignition source is

postulated at the center of the PES.

Vapor Cloud

Process Congestion

Turret

Helicopter Pad

Quarters

Figure 1 Fictitious FPSO Layout

Figure 2 Scenario Confinement Layout

Vapor Cloud Size Evaluation. The dispersion model used in

this evaluation was the Shell FRED [12] (Fire, Release

Explosion, Dispersion Hazard consequence modeling

package) model. A two-inch leak of saturated liquid LPG at96 psi resulted in a leak rate of 63 lb/s.

With the given compartment size and the calculated

release rate, it would be fair to assume the entire compartmen

could be quickly filled with a stoichiometric mixture. It would

also be fair to assume the confinement would prevent the

accumulation of a large amount of vapors outside this area.

Explosion Source Term Evaluation. Since the scenario

assumes center ignition but SCOPE requires ignition be

assumed at one edge, the SCOPE model would be considered

as a half-symmetrical simulation with a solid back wal

creating a plane of symmetry. This has the effect o

reproducing the actual scenario with center ignition. SinceSCOPE can only model vents in the planes behind, in frontand to one side of the ignition, the open vent in the roof was

moved to the side and the side vents that would relieve at 2 psi

were not modeled. The vents and confining planes modeled in

SCOPE are shown in Figure 3. The idealized SCOPE scenario

included five grids with round blockage ratios between 4 and

15%, square blockage ratios between 0 and 5% and average

obstacle diameters between 10 and 20 inches. No rear vent

was modeled due to the wall of symmetry that was used to

reproduce a center ignition scenario. The main vent was open

without any restrictions. Three side vents were used; one that

was open and two with panels that relieved at 1.25 psi and a

delay of 50 ms.

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View of PES from Aft

View of PES from Forward

Confinement Relieves at 1.25 psi

Fixed Confinement

No Confinement (open)

PortStarboard

StarboardPort

Ignition

Location

at Wall of

Symmetry

Figure 3 SCOPE PES Representation

The resulting peak internal pressure predicted by SCOPE

was 13.7 psi. An external explosion centered 8.2 feet outside

the vent with a radius of 39 feet was predicted to have a peak

pressure of 11.25 psi.

Actual Venting and Blast Wave Interaction Analysis. A

CEBAM model was first created with the same vents defined

in the SCOPE simulation to reproduce the results given bySCOPE. A flame speed of Mach 1.0 with a grid size of 2.5

feet was found to result in the same pressures and impulses

predicted by SCOPE at the point the SCOPE blast wave

begins to decay. Note that because of the constant pressure

prediction with external explosions, SCOPE does not beginthe pressure wave decay until the edge of the external

explosion is reached. These results are plotted against the

SCOPE results in Figure 4 and Figure 5. Since blast

predictions very close to the PES were not of concern for this

example, the 2.5 foot CEBAM grid was acceptable inside the

PES. If pressures near the PES were needed, a finer grid in

the PES would have been needed which would have had the

result of lowering pressures in the near field.

Communications with the CEBAM developer indicate that if

high pressures are of interest, then a small grid is needed but if

the pressures of interest are relatively low, then a coarse grid

is sufficient. Thus, if both near field and far field pressures

had been of interest in this example, the grid inside and near

the PES may have been set at 1 foot increments with the

remainder of the domain at 2.5 foot grid spacing.

1

10

100

1 10 100 1000Distance (feet)

Free-Fie

ldPressure(psi)

SCOPE Free-Field Pressure

CEBAM Free-Field Pressure (modeling SCOPE vent planes)

Figure 4 CEBAM Pressure Results with SCOPE Vent Planes VsSCOPE Pressure Results

10

100

1000

1 10 100 1000Distance (feet)

Free-FieldImpu

lse(psi-ms)

SCOPE Free-field Impulse

CEBAM Free-Field Impulse (modeling SCOPE vent planes)

Figure 5 CEBAM Impulse Results with SCOPE Vent Planes VsSCOPE Impulse Results

The CEBAM model, with important FPSO structures and

actual vent conditions with the previously determined flame

speed and grid spacing, was then run to determine loads on the

occupied areas of the FPSO. A model that represented onlythe aft portion of the ship with a wall of symmetry along the

keel was used to reduce computational time. The

computational domain is shown in Figure 6.

Figure 6 Symmetrical Domain Evaluated in CEBAM

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Blast loads were calculated at 7 probe locations shown inFigure 7. The SCOPE blast loads were free field loads and the

CEBAM blast loads were applied blast loads (included

reflections were applicable). Thus, reflection factors had to be

used to make the SCOPE loads applied loads. The resulting

applied blast loads are given in Table 1.

1

2

3

46

5

7

Forward

Forward

Aft

Aft

Figure 7 Plot of Probe Locations on Quarters Where Blast LoadsWere Calculated

Table 1 Pressures and Impulses Calculated bySCOPE and CEBAM

SCOPE CEBAMProbe Pressure (psi) /

Impulse (psi-ms)Pressure (psi) /Impulse (psi-ms)

1* 2.6 / 30 1.9 / 712 1.2 / 14 1.1 / 383 1.2 / 14 1.0 / 354 1.2 / 14 0.8 / 365 1.1 / 13 0.8 / 356 1.1 / 13 1.0 / 367 1.0 / 13 0.9 / 34

* This location has appropriate reflection factors applied

to the SCOPE free field blast loads

These results show lower predicted pressures for the

CEBAM results. The average pressure reduction was 20%

while probe 4 observed a 32% reduction. Since probe 1 was

less than twice the pressure of probe 2, probe 1 probably

benefited by clearing effects. Probes 2 and 3 likely benefited

from shock wave separation at the wall edges and probes 4-7

benefited due to shielding by the larger structure. Probe 6observed some reflection, but much less than a factor of two,

resulting in a pressure close to the SCOPE prediction without

any reflections. Had a reflection been applied to the SCOPE

result at probe 6, the pressure difference at that point would

have been much more than that shown.

In contrast, the predicted impulse was higher for theCEBAM analysis by a factor of up to three times the SCOPE

results. This is because impulse is primarily dependent upon

explosion energy, which in turn is directly dependent upon

PES volume. In SCOPE, the PES volume was defined as the

enclosure containing the vapor cloud and congestion externa

to the enclosure is not allowed. In CEBAM, the vapor cloudis allowed to vent into surrounding areas of congestion. In

this example, there was congestion above and forward of the

vapor cloud as previously shown in Figure 6. Thus, since

impulse is dependent upon explosion volume, the CEBAM

results are more realistic because it accounts for the venting of

the vapors into other areas of congestion that would beexpected to contribute to the explosion.

A contour plot of maximum pressures is provided in

Figure 8. This shows the pressure increases due to reflections

on the wall facing the PES (probe 1 location) and also some

reflections on the side wall where probe 6 was located. Note

that if the aft walls of the PES were not predicted to fail, then

the only failures would have been the PES forward walls. In

this case, the SCOPE results at the quarters may have

increased slightly (using 2 psi forward vents instead of 1.25

psi aft vents) but the CEBAM loads at the quarters would have

decreased due to increased directionality (the PES would have

been more like a shotgun). Cases have been observed where

CEBAM predicted pressures were half that of SCOPE due to

directional effects.

Figure 8 Contour Plot of Maximum Pressures at Deck HeightPredicted by CEBAM

There may be situations where CEBAM can predict higherpressures than SCOPE for locations near the explosion

primarily due to directional effects of the blast wave that

SCOPE cannot model. This is because SCOPE always decays

blast pressures from the external explosion centered at the

main vent. Thus, cases can exist where the area of concern is

near the side vent and SCOPE may under-predict the pressure

It would be conservative to apply the SCOPE external

explosion peak pressure to all vent surfaces, but depending on

the situation, it could be overly conservative.

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Unconfined Scenarios

Without a high degree of confinement, the current version of

SCOPE cannot accurately predict the peak overpressure.

Unconfined scenarios can be evaluated in an identical manner

to confined scenarios except a blast curve method is selected

to generate the peak pressure used to determine the CEBAMflame speed instead of SCOPE. Of the major public methods

available, comparisons to test data have shown that the latest

MEM2 and CAM2 methods will provide the most accurate

predictions [5].

Limitations of Alternate ApproachThe results show that situations like the example scenario

would benefit from this alternate approach due to the

significant explosion venting, congestion outside the PES

enclosure and some directional effects. However, where these

factors are not present, this method may not have much

advantage over a SCOPE analysis or MEM2/CAM2 blastcurve analysis alone. Thus, this alternate approach lends itself

to cases where the scenario has significant venting and/or

cases where directional effects may be significant and/or caseswhere the vapors could vent into other areas of congestion that

cannot be modeled by SCOPE.

SummaryThe proposed alternate approach of using a dispersion model

with an explosion overpressure prediction tool such as

SCOPE, MEM2 or CAM2 and the CFD code CEBAM has

shown to provide a method to predict offshore explosion

consequences without the use of traditional detailed 3D CFD.

The SCOPE code and the blast curve methods, MEM2 andCAM2, accurately predicts a peak explosion pressure, but uses

methods based on hemispherical VCEs for decaying the blast

wave without taking into account interactions with buildings

and only a limited amount of directionality. The CEBAMcode requires the user to input a flame speed to predict a

deflagration explosion overpressure, potentially resulting in

error introduction, but will accurately model a blast wave

directional affects and interactions with buildings with the

proper grid size. This alternate approach uses the strengths of

each analysis method to overcome inherent potential errors

each method has in order to maintain simplicity. The alternateapproach presented is best used in situations where the

scenario has significant venting and/or where directional

effects and blast wave interactions with buildings could be

significant and/or cases where the vapors could vent into other

areas of congestion that cannot be modeled by SCOPE. Thus,

this alternate approach allows the user to accurately predictboth the peak explosion pressure and the blast wave

interactions with buildings. This alternate approach results in

the most accurate prediction currently available without using

a detailed 3D CFD analysis and permits the evaluation of an

offshore construction project early in life when modifications

can be made more easily.

References

1 Shell Code for Overpressure Prediction in Gas Explosion

(SCOPE) Version 4.08.1, Shell Global Solutions, Chester

United Kingdom, June 2001.2 Mercx, W.P.M., van den Berg, A.C., and van Leeuwen, D.,

Application of correlations to quantify the source of strength of

vapour cloud explosions in realistic situations Final report forthe project: GAMES, TNO Report PML 1998-C53, TNO

Prins Maurtis Laboratory, October 1998 (work completed in1998 but subject to a publication embargo until 2001).

3 Puttock, J.S., Developments in the congestion assessmen

method for the prediction of vapour-cloud explosions,

Proceedings of the 10th International Symposium on Loss

Prevention and Safety Promotion in the Process IndustriesStockholm, June 2001.

4 Computational Explosion and Blast Assessment Mode

(CEBAM) Version 1.00.13, Analytical and Computationa

Engineering, Inc., San Antonio, USA, December 2001.

5 Fitzgerald, G.A., A Comparison of Simple Vapor CloudExplosion Prediction Methods, 2001 Proceedings of the Mary

Kay OConnor Process Safety Center Symposium, College

Station, USA, October 2001.6 Van den Berg, A.C., The Multi-Energy Method, a framework

for vapour cloud explosion blast prediction, Journal ofHazardous Materials, Vol.12, pp1-10.

7 Eggen, J.B.M.M., GAME: development of guidance for the

application of the multi-energy method, Prepared for Health

and Safety Executive, Prepared by TNO Prins MauritsLaboratory, 1995 (work completed in 1995 but subject to a

publication embargo until 1998).

8 Puttock, J.S., Fuel Gas Explosion Guidelines the Congestion

Assessment Method, Proceedings of the 2nd European

Conference on Major Hazards On - and Offshore, ManchesterEngland, October 1995.

9 Puttock, J.S., Improvements in Guidelines for Prediction of

Vapor-Cloud Explosions, Proceedings of The Internationa

Conference and Workshop on Modeling the Consequences of

Accidental Releases of Hazardous Materials, San FranciscoUSA, October 1, 1999.

10 Clutter, J.K., "A Reduced Combustion Model for Vapor Cloud

Explosions Validated Against Full-Scale Data,"Journal of Loss

Prevention in the Process Industries, Vol. 14, Feb 2001, pp.

181-192.11 Clutter, J.K., and Luckritz, R.T. "Comparison of a Reduced

Explosion Model to Blast Curve and Experimental Data,"

Journal of Hazardous Materials, Vol.79, Oct 2000, pp. 41-61.

12 Fire, Release, Explosion, Dispersion Hazard consequence

modelling package (FRED) Version 3.1.6.0, Shell GlobaSolutions, Chester, United Kingdom, March 2000.

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