Gavelli_edit.qxp 28/10/08 1:54 pm Page 81
Modelling Pool Fire Hazards from Large-scale Liquefied Natural Gas Spills
flux to a target (although several conservative assumptions are typically
Figure 9: Comparison of Hazard Distances to 5kW/m
2
for Four Models –
embedded in the analysis); conversely, a correlation predicting a ‘high’
Single-zone, Two-zone, Three-zone and LNGFIRE3
flame height could result in unnecessary restrictions being imposed on a
facility because of the large calculated hazard distances.
2,000
Single zone
Two zones
The amount of thermal radiation emitted by very large LNG pool fires is,
1,500
Three zones
like the flame height, a quantity that has not been measured and the
2
(m)
LNGFIRE3
balance between phenomena that determine burning rate and soot ~500m
1,000
formation remains poorly understood. Based on the data from smaller
LNG pool fires (diameters up to 35m), different authors have used SEP
Distance to 5kW/m
values spanning approximately a factor of two (i.e. between 175 and
500
325kW/m
2
). The effect of the SEP on the predicted hazard distances is
shown in Figure 8; the distances to a 5kW/m
2
hazard are plotted as a
0
function of pool diameter. The ratio between the hazard distance
0 100 200 300 400 500 600
Pool diameter (m)
predicted using the highest (325kW/m
2
) and the lowest (190kW/m
2
)
emissive power is approximately 1.25; that is, the hazard distance does Figure 10: Comparison of Hazard Distances to 5kW/m
2
for
not grow proportionally to the emissive power. However, in absolute
Constant and Variable Atmospheric Transmissivity (with 5,
50 and 95% Relative Humidity)
terms the predicted hazard distance can vary by hundreds of metres (e.g.
approximately 450m for a pool diameter of 500m). Therefore, the SEP
2,000
applied to the solid flame model can have a significant impact on the
Constant τ
consequence analysis for large-scale LNG spills. Variable τ, 5% RH
1,500
Variable τ, 50% RH
Another important parameter, and the subject of much debate, is the
2
(m) Variable τ, 95% RH
~250m
smoke shielding of the pool fire. Smaller LNG pool fires (up to
1,000
approximately 15m diameter) tend to burn cleanly with little or no soot
formation. However, when the diameter of the pool fire grows (as in
Distance to 5kW/m
the 35m diameter pool fires at Montoir, France), smoke has been
500
shown to form. Smoke shielding during the Montoir tests was observed
primarily on the upper half of the fire column. Based on the behaviour 0
0 100 200 300 400 500 600
of other hydrocarbon pool fires, the prevailing opinion among scientists
Pool diameter (m)
is that as the size of the LNG pool fire grows beyond 35m, smoke
shielding will increase. However, in the absence of data on larger pool is approximately 500m. Therefore, the effect of smoke shielding of the
fires the rate at which smoke shielding will change is unknown, and pool fire thermal radiation can also have a significant impact on the
models based on existing data need to be extrapolated well beyond consequence analysis for large-scale LNG spills.
their range of validation.
The last parameter to be evaluated in this study is atmospheric
Solid flame models with one, two and three zones are compared to transmissivity. As discussed earlier, τ is a factor (less than or equal to
determine the effect of the smoke shielding assumption on the hazard one) that accounts for the reduction in thermal radiation to a target
distances as a function of pool fire diameter (see Figure 9). For consistency, due to scattering and absorption by the atmosphere. Figure 10 shows
all three models use the Thomas flame height correlation. The SEP of the the effect of τ on predicted hazard distances to the reference heat flux
flame was set to 220kW/m
2
for the one-zone model, 265kW/m
2
for the level. The constant τ model (with τ=0.8) is compared with the variable
two-zone model and 325kW/m
2
for the three-zone model, as found in τ model, with the latter applied to different relative humidity values (5,
the published literature for the respective models. The three models are in 50 and 95%). The results show a small difference between all four
cases for pool diameters up to approximately 150m. The difference
increases as the distance travelled by the thermal radiation increases,
Another important parameter,
allowing for scattering and absorption to become more significant. The
and the subject of much debate,
constant τ model becomes more and more conservative as the hazard
distance grows, consistent with the trend shown in Figure 5: the
is the smoke shielding of the
constant τ model predicts hazard distances of approximately 5–15%
pool fire.
greater than the variable τ model for RH=5% and 95%, respectively.
Based on these results, the model for τ is not considered a critical factor
in hazard distance calculations.
agreement for small pool fires, as expected, since LNG pool fires are
essentially smoke-free for pool diameters up to approximately 15m. The Conclusions
effect of smoke shielding becomes progressively more significant as the The hazard distances from LNG pool fires are currently calculated using
pool size grows and the fires are predicted to become sootier. For solid-flame-based models. Several models have been developed and used
example, for a pool diameter of 500m the distance between the longest for LNG pool fires, particularly for the hazard analysis of large-scale LNG
(one-zone model) and the shortest (three-zone model) hazard difference spills on water. These models, although based on the same general
EXPLORATION & PRODUCTION – OIL & GAS REVIEW 2008 – VOLUME 6 ISSUE II
81
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