The Chad Risk Assessment
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Abo Homos-Nubaria Pipeline QRA
Weather Data
The Weather Data relevant to this study consists of a list of weather conditions in the form of different combinations of wind-speed, temperature, humidity and atmospheric stability. The weather conditions are an important input into the dispersion calculations and results for a single set of conditions could give a misleading picture of the hazard potential.
Mete oceanographic data gathered for Greater Cairo over a period of 5 years. This data included wind speed and direction; air temperature and pressure, as well as current speed, direction and wave height.
The general climatic conditions at North Cairo are summarised below:
Air Temperature oC: |
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o |
Minimum recorded |
- 1.1 |
o |
Maximum recorded |
52.2 |
o |
Yearly average |
28 |
Relative humidity %:
o Average daily minimum82
o |
Average daily minimum54 |
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o |
Annual average |
78 |
The recorded annual wind speeds at Cairo are shown in Table 10.1.
Month Jan.
Wind |
4.7 |
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speed |
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Table 10.1 Wind speeds at Cairo (Knots)
Feb. |
Mar. |
Apr. |
May |
June |
July |
Aug. |
Sep. |
Oct. |
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5.6 |
6.3 |
6.2 |
5.6 |
5.2 |
4.4 |
3.4 |
3.6 |
4.0 |
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Nov. |
Dec |
3.84.4
In wind Rose figures the radius = 10%
Average wind speed |
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2.44 m/sec. |
Wind Direction:
Three permanent high-pressure belts control the wind circulation over Egypt: the Azores, the Indian subtropical and the South Atlantic subtropical. In addition, there is a permanent low-pressure belt ‘the doldrums’ which crosses Africa near the equator. Seasonal high and low pressure systems also alternate over the continental mass, the red sea, the Mediterranean and the Arabian Peninsula.
Table 10.2 Wind Rose for North Cairo
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Abo Homos-Nubaria Pipeline QRA |
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345/ |
015/ |
045/ |
075/ |
105/ |
135/ |
165/ |
195/ |
225/ |
255/ |
285/ |
315/ |
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014 |
044 |
074 |
104 |
134 |
164 |
194 |
224 |
254 |
284 |
314 |
344 |
Jan |
5.4 |
8.5 |
7.4 |
3.0 |
1.3 |
2.7 |
13.5 |
14.8 |
12.6 |
8.5 |
6.3 |
5.8 |
Feb |
9.9 |
14.4 |
8.7 |
3.6 |
1.2 |
1.3 |
8.0 |
12.0 |
10.1 |
7.1 |
6.1 |
8.9 |
Mar |
15.8 |
16.3 |
9.3 |
3.6 |
1.4 |
1.6 |
5.0 |
7.5 |
6.4 |
7.9 |
8.2 |
10.1 |
Apr |
15.8 |
21.4 |
13.3 |
4.3 |
0.7 |
0.6 |
1.7 |
3.0 |
5.0 |
7.0 |
8.6 |
12.5 |
May |
21.3 |
24.9 |
13.3 |
3.0 |
0.6 |
0.3 |
0.7 |
1.3 |
2.0 |
4.3 |
7.7 |
14.4 |
June |
24.4 |
22.2 |
8.3 |
1.3 |
0.1 |
0.1 |
0.2 |
0.7 |
1.2 |
2.9 |
9.3 |
20.1 |
July |
26.3 |
16.4 |
4.8 |
0.8 |
0.2 |
0.0 |
0.1 |
0.2 |
1.2 |
3.1 |
7.7 |
23.4 |
Aug |
29.1 |
16.2 |
3.8 |
0.8 |
0.4 |
0.0 |
0.0 |
0.2 |
0.9 |
2.6 |
6.8 |
21.6 |
Sep |
22.8 |
23.9 |
8.1 |
1.4 |
0.2 |
0.2 |
0.7 |
1.2 |
0.8 |
1.3 |
6.3 |
15.4 |
Oct |
19.4 |
23.2 |
11.4 |
2.9 |
0.4 |
0.4 |
1.5 |
1.8 |
3.0 |
3.7 |
7.6 |
10.5 |
Nov |
16.6 |
17.2 |
6.6 |
1.9 |
0.5 |
0.8 |
3.3 |
5.7 |
6.6 |
6.3 |
7.2 |
9.7 |
Dec |
10.6 |
10.8 |
6.8 |
2.4 |
0.8 |
1.9 |
8.8 |
12.0 |
8.3 |
8.2 |
5.9 |
8.8 |
The prevailing winds are quite parallel to or heading towards the Northwest, mostly from west to north all year, except December and January, when they are from SE. When atmospheric low pressure is passing quite frequently and fast, the wind direction will change ‘anti-clockwise’, normally during a short period of one to two days. After a low pressure has passed, the wind returns to the prevailing direction (W-NW). The mean wind speed at Cairo is 2.44 m/sec.
Data on the direction of wind at North Cairo was obtained from the Egyptian Meteorological Office. Table 10.2 shows the analysis of the 12-months wind distribution data over a period of 10 years. FIGURE 10.1 gives the average wind directions at Cairo throughout the year.
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Abo Homos-Nubaria Pipeline QRA
Jan |
Feb |
March |
April |
May |
June |
July |
August |
Sept |
Oct |
Nov |
Dec |
FIGURE 10.1 Average wind directions at Cairo
The overall analysis of the wind data at Cairo is given in what is known as the wind rose. FIGURE 10.2 shows Cairo wind rose, based on data collected during 19922000. Note that winds blow towards the centre of the rose.
18% 20%
15%
7% |
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5% |
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6% |
2.4 |
4% |
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5% |
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3% |
N |
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5% 4% |
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8% |
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> 22.5 m/sec
20 - 22.5
17.5 - 20
15 - 17.5
12.5 - 15
10 - 12.5
7.5 -10
5- 7.5
2.5 - 5
0- 2.5
FIGURE 10.2 the Wind Rose at Cairo
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Stability Categories:
The two most significant variables, which would affect the dispersion calculations, are: Wind-speed and atmospheric stability. The stability class is a measure of the atmospheric turbulence caused by thermal gradients. Pasquill Stability identifies six main categories, which are shown in the Table 10.3.
Table 10.3 Pasquill Stability Categories |
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A |
B |
C |
D |
E |
F |
Very Unstable |
Unstable |
Moderately Unstable |
Neutral |
Moderately Stable |
Stable |
Neutral conditions correspond to a vertical temperature gradient of about 1(oC) per 100m.
Cairo weather data for the Geographical area is somewhat limited and do not show seasonal variations over a long time.
Therefore, the calculations included in this study have considered alternative stabilities for the average wind speed of 2.4 m/sec.
This was done with reasonable accuracy, since the stability is related to the wind speed, and the range of stabilities that is observed for a given wind speed is generally small, as shown in the Table 10.4.
As the range is large for a given wind speed, the calculations have initially considered four different combinations of wind speeds and stability classes to include the worst possible conditions.
The calculations have also considered atmospheric temperature (30oC), relative humidity 70% and surface roughness parameter of 0.1.
Table 10.4 The Relationship between Wind speed and Stability
Wind speed |
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Day-time |
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Night-time |
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Solar Radiation |
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Cloud Cover |
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(m/s) |
strong |
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medium |
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slight |
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thin |
medium |
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overcast |
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<3/8 |
>3/8 |
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>4/5 |
<2 |
A |
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A-B |
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B |
- |
- |
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D |
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2-3 |
A-B |
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B |
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C |
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E |
F |
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D |
3-5 |
B |
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B-C |
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C |
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D |
E |
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D |
5-6 |
C |
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C-D |
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D |
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D |
D |
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D |
>6 |
C |
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D |
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D |
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D |
D |
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D |
At night, the ground is often cooler than the air if the sky is clear, and this gives rise to the most stable conditions and potentially the greatest effect distances.
FIGURE 10.3 shows the criteria used for the selection of weather parameters used for the consequences modelling for this study.
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Abo Homos-Nubaria Pipeline QRA |
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Is the ground covered |
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Check category |
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m/s |
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0- 6 |
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F |
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in frost or snow? |
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against wind speed |
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> 7 |
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E |
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No |
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Is it night-time? |
Yes |
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Is sky overcast? |
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Yes |
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D |
m/s |
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F |
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No |
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No |
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< 2 |
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Sky more than half covered Y |
Check category |
2 |
E |
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Inland |
Coastal |
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> 3 |
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against wind speed |
D |
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sites |
sites |
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No |
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Sky clear? |
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Check category |
m/s |
F |
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against wind speed |
< 2 |
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Wind mainly from |
Yes |
D |
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2-4 |
E |
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from the sea? |
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> 5 |
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D |
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No |
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Time within 1 hr |
Yes |
D |
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before sunset? |
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No |
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Yes |
F |
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Time within 1 hr |
Yes |
Sky clear and |
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after sunrise? |
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wind calm/light? |
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No |
D |
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No |
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Is sky overcast? |
Yes |
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Check category |
m/s |
C |
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against wind speed |
0- 4 |
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No |
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> 5 |
D |
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Summer only |
Select weather type from |
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Hot |
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Warm |
Cool |
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Check category |
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Check category |
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Check category |
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against wind speed |
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against wind speed |
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against wind speed |
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m/s |
A |
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m/s |
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A |
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m/s |
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C |
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< 3 |
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< 1 |
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3- 4 |
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> 5 |
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5- 8 |
C |
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4- 7 |
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> 8 |
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> 7 |
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D |
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D |
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24
Abo Homos-Nubaria Pipeline QRA
FIGURE 10.3 Determinations of Modified Pasquill Stability Categories
Category D (neutral) is the most probable at inland sites, and appears to occur for up to 80% of the time at Cairo. To overcome the uncertainty of the accuracy of Cairo weather data results, the following cases were selected in this analysis to study the effects of normal and extreme weather conditions at Cairo.
Table 10.5 Sets of weather conditions initially selected for this study:
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Set 1 |
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Set 2 |
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Wind speed |
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Stability |
Wind speed |
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Stability |
2 m/s |
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F |
10 m/s |
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D |
The wind speed range between 1 to 5 m/s was considered to be reasonable representation of typical conditions at Cairo. This would overcome some of the uncertainty of the meteorological data provided by the meteorological office. Wind speeds in excess of 8 m/s are likely to disperse the cloud over long distances to well below LFL.
The weather set 2 was eventually selected to represent the most likely conditions; however the worst case conditions shall be defined by a sensitivity analysis study.
25
Abo Homos-Nubaria Pipeline QRA
Release Scenarios
Events associated with release, dispersion and ignition of flammable releases considered in this study can be summarized in the following figure.
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Release |
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Yes |
Ignites? |
No |
Dispersing cloud |
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Yes |
Ignites? |
No |
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More obstacles |
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Greater confinement |
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Flame acceleration |
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Jet fire |
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Pool fire |
Cloud fire |
Fast flame |
Internal |
Safe |
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explosion |
dispersion |
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Impinge? |
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Yes |
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Structural |
BLEVE |
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Failure |
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Figure 11.1 Hazardous events
These events can be more detailed as follows:
Jet fires |
A jet fire will result from an ignited pressurized hydrocarbon gas release. The |
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consequence of jet fires is directional depending on the on release orientation. |
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Jet fires typically have flame temperature of about 2,200 oF and can produce |
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high intensity thermal radiation. The high temperature poses a hazard from |
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direct effects of heat on humans and also from possibility of escalation. If a jet |
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flame impinges upon a target such as a vessel, pipe or structural member, it |
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can cause failure of the item to fail within several minutes. |
Jet (spray) fire will also result from ignited continuous releases of pressurized flammable liquid. The momentum of the release carries the material forwards in
26
Abo Homos-Nubaria Pipeline QRA
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a plume entraining air to give a flammable mixture as gas is released from the |
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plume. |
Flash fires |
If flammable gas accumulates in an unconfined area and is ignited, then the |
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result will be a flash fire within the flammable limits of the vapour cloud. |
Explosions |
Ignition of accumulated gas in semi-confined areas may also be accompanied |
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by an explosion; the overpressure generated will depend on the degree of |
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congestion and confinement of the process area, and the gas cloud size. |
Pool fires |
If a liquid release is ignited after it has time to form a pool, a pool fire results. |
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Because they are less well aerated, pool fires tend to have lower flame |
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temperatures and produce lower levels of thermal radiation than jet fires. They |
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also produce more smoke. Although a pool fire can still lead to structure failure |
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of items within the flame, this would take longer than in a jet fire. |
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An additional hazard of pool fires is their ability to flow. A burning liquid pool can |
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spread along horizontal surface or run down a vertical surface to give a running |
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fire. |
BLEVE |
BLEVE stands for Boiling Liquid Expanding Vapour Explosion. |
(Fire Ball) |
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A fire ball can occur if a vessel containing fuel ruptures in the presence of an |
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ignition source (usually a jet or pool fire). A fraction of the liquefied fuel |
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subsequently released will evaporate immediately and take part in a huge |
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fireball, which has the shape of a hemispherical burning cloud or ball of fire. |
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High degree of turbulent mixing and rapid air entrainment allows large quantities |
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of fuel to be consumed in a short period of time. |
Structural |
Loss of structure integrity due to overheating of structure members. The |
failure |
structure shall collapse under much lower load than the designed due to |
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increased temperature. |
Safe |
Dilution of the released gases beyond the lower flammability limits (LFL) leading |
dispersion |
to safe dispersion situation. |
27
Abo Homos-Nubaria Pipeline QRA
Impairment Criteria
This section defines the human injury and asset impairment criteria in caring out the consequence analysis of the identified hazardous events scenarios on the proposed facilities.
Table 12.1 represents standard human impact criteria as applied in consequence modelling.
Table 12.1: Criteria for Assessment of Fire Effects on Humans
Event Effect |
Distance to |
Effect |
Jet fire / Pool fire |
4.7kW/m2 |
Will cause pain in 15-20 seconds and injury after 30 |
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seconds exposure. |
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12.5 kW/m2 |
Significant chance of fatality for extended exposure |
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and high chance of injury. |
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37.5 kW/m2 |
Significant chance of fatality for people exposed |
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instantaneously. |
Flash fire |
LFL |
Fatal for people in the flammable cloud path |
Explosion |
0.05 Bar |
Will cause injuries from flying debris |
overpressure |
0.2 Bar |
20% chance of fatality to a person in a protected |
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enclosure |
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0.3 Bar |
Threshold for eardrum damage, |
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50% chance of fatality for a person within enclosure, |
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15% chance of fatality for a person in the open. |
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0.50 Bar |
Will cause 100 % fatality for a person within enclosure |
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or in the open. |
The criteria applied for assessment of the effects of fire on assets are summarised in Table 12.2.
Table 12.2: Criteria for Assessment of Fire Effects on Assets
Impairment Mechanism |
Level |
Effect |
Thermal Radiation |
4.7kW/m2 |
Impairment of evacuation/embarkation areas |
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6.3 kW/m2 |
Impairment of escape routes |
Thermal Radiation or Flame |
500 deg.C |
Structural Failure. |
Impingement on Load Bearing |
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Structural Steel |
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Both jet fires and explosions can lead to structure failure of items, though this will take several times longer for jet fires than for explosions. Table 13.3 presents indicative failure times under hydrocarbon fire impact conditions, where times to failure refer to burn through or loss of load bearing capacity.
Table 12.3: Structure Failure times in Fires (Indicative) |
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Component |
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Time to Failure (Min) |
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Jet Fire |
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Pool Fire |
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Unprotected structural steel beam |
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10 |
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10 |
Unprotected steel plate |
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5 |
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10 |
A-60 firewall |
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15 |
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60 |
H-120 firewall |
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60 |
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120 |
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28 |
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Abo Homos-Nubaria Pipeline QRA
Table 12.4 reports published information on the explosion overpressure effects.
Table 12.4: Explosion Overpressure Effects
Explosion
Damage
Overpressure - Bar(g)
0.0250% windows shattering
0.07Collapse of tank roof
0.07-0.14 |
Connection failure of corrugated panelling |
0.08-0.1 |
Minor damage to steel framework |
0.15-0.2 |
Wall of concrete blocks shattered |
0.2 |
Collapse of steel framework |
0.3 |
"Reparable damage" cladding blown off. Offshore bridjes and lifeboats |
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impaired |
0.34Steel walls blown off. Process plant within offshore module rupture, in neighbouring modules damaged. 50% chance for ESD valve closure failing
1.0 |
Columns and buoyant deck of semi-sub ruptured |
2.0 |
Riser wall rupture |
29