This Annex contains the details of the
Frequency Analysis for the QRA study of the terminal.
A detailed discussion on all the hazard
scenarios identified was given in Annex
13A5. It was concluded that internal process hazards, natural hazards and
external impacts are included in the generic failure frequencies. These failure
frequencies are summarised in this section.
Table
1.1 lists all the failure
frequencies adopted for the various release scenarios. Codes are assigned for
various source terms. Refer to Annex 13A7 for
code definitions.
Table 1.1 LNG
Release Event Frequencies
|
No. of Items |
Length of
Section (m) |
Hole Size (mm) |
Initiating
Event Frequency |
Unit |
Reference |
|
|
L01 |
1 |
450 |
10 |
3.00E-07 |
per meter per
year |
Hawksley [1] |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
L02 |
3 |
20 |
10 |
4.05E-03 |
per year |
COVO Study [2] |
|
|
|
|
25 |
4.05E-03 |
||
|
|
|
|
50 |
4.05E-03 |
||
|
|
|
|
100 |
4.05E-04 |
||
|
|
|
|
FB |
4.05E-05 |
||
|
L03 |
2 |
300 |
10 |
1.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
1.00E-07 |
|
|
|
|
|
|
50 |
7.00E-08 |
|
|
|
|
|
|
100 |
7.00E-08 |
|
|
|
|
|
|
FB |
3.00E-08 |
|
|
|
L04 |
1 |
30 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
L05 |
2 |
900 |
10 |
1.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
1.00E-07 |
|
|
|
|
|
|
50 |
7.00E-08 |
|
|
|
|
|
|
100 |
7.00E-08 |
|
|
|
|
|
|
FB |
3.00E-08 |
|
|
|
L06 |
1 |
N/A |
10 |
1.00E-05 |
per year |
Crossthwaite et al [3] |
|
|
|
|
25 |
5.00E-06 |
||
|
|
|
|
50 |
5.00E-06 |
||
|
|
|
|
100 |
1.00E-06 |
||
|
|
|
|
FB |
1.00E-06 |
||
|
G07 |
1 |
390 |
10 |
1.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
1.00E-07 |
|
|
|
|
|
|
50 |
7.00E-08 |
|
|
|
|
|
|
100 |
7.00E-08 |
|
|
|
|
|
|
FB |
3.00E-08 |
|
|
|
G08 |
1 |
108 |
10 |
1.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
1.00E-07 |
|
|
|
|
|
|
50 |
7.00E-08 |
|
|
|
|
|
|
100 |
7.00E-08 |
|
|
|
|
|
|
FB |
3.00E-08 |
|
|
|
G09 |
1 |
450 |
10 |
1.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
1.00E-07 |
|
|
|
|
|
|
50 |
7.00E-08 |
|
|
|
|
|
|
100 |
7.00E-08 |
|
|
|
|
|
|
FB |
3.00E-08 |
|
|
|
G10 |
1 |
24 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
G11 |
1 |
720 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
G12 |
1 |
300 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
G13 |
1 |
150 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
G14 |
1 |
20 |
10 |
4.05E-03 |
per year |
COVO Study |
|
|
|
|
25 |
4.05E-03 |
|
|
|
|
|
|
50 |
4.05E-03 |
|
|
|
|
|
|
100 |
4.05E-04 |
|
|
|
|
|
|
FB |
4.05E-05 |
|
|
|
P15 |
1 |
1 |
10 |
1.00E-04 |
per year |
COVO Study |
|
|
|
|
25 |
1.00E-04 |
|
|
|
|
|
|
50 |
1.00E-04 |
|
|
|
|
|
|
100 |
1.00E-04 |
|
|
|
|
|
|
FB |
1.00E-05 |
|
|
|
P16 |
2 |
1 |
10 |
1.00E-04 |
per year |
COVO Study |
|
|
|
|
25 |
1.00E-04 |
|
|
|
|
|
|
50 |
1.00E-04 |
|
|
|
|
|
|
100 |
1.00E-04 |
|
|
|
|
|
|
FB |
1.00E-05 |
|
|
|
P17 |
5 |
10 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
P18 |
5 |
10 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
P19 |
10 |
10 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
P20 |
1 |
150 |
10 |
3.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
3.00E-07 |
|
|
|
|
|
|
50 |
1.00E-07 |
|
|
|
|
|
|
100 |
1.00E-07 |
|
|
|
|
|
|
FB |
5.00E-08 |
|
|
|
P21 |
1 |
300 |
10 |
1.00E-07 |
per meter per
year |
Hawksley |
|
|
|
|
25 |
1.00E-07 |
|
|
|
|
|
|
50 |
7.00E-08 |
|
|
|
|
|
|
100 |
7.00E-08 |
|
|
|
|
|
|
FB |
3.00E-08 |
|
|
|
P22 |
1 |
1 |
10 |
1.00E-04 |
per year |
COVO Study |
|
|
|
|
25 |
1.00E-04 |
|
|
|
|
|
|
50 |
1.00E-04 |
|
|
|
|
|
|
100 |
1.00E-04 |
|
|
|
|
|
|
FB |
1.00E-05 |
|
|
|
T23 |
3 |
N/A |
Rupture |
1.00E-08 |
per tank per
year |
“Purple Book”
[4] |
The frequency of various outcomes
following a loss of containment event is estimated using an event tree model.
The various outcomes considered include pool fire, jet fire, flash fire and
vapour cloud explosions for liquid releases; jet fire and flash fire for
continuous gas releases and fireball and flash fire for instantaneous gas
releases. Event Tree Analysis is used to describe and analyse how an initiating
event may lead to a number of different outcomes, depending upon such factors
as the successful implementation of the various emergency response measures and
relevant protective safety systems in place.
A generic event tree used for this study
is shown in Figure 1.1.
The contributing factors taken into account in the event trees are discussed
below.
Figure 1.1 Generic
Event Tree
|
Detection &
Shutdown Fails |
Immediate Ignition |
Delayed Ignition
(1) |
Vapour Cloud
Explosion |
Delayed Ignition
(2) |
Event Outcome |
|||||
|
|
Yes |
|
Yes |
|
|
|
|
|
|
Pool fire/ Jet fire |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
No |
|
No |
|
Yes |
|
Yes |
|
|
Vapour cloud explosion |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
No |
|
No |
|
|
Flash fire over plant area |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Yes |
Flash fire full extent |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
No |
Unignited
release |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Yes |
|
|
|
|
|
|
Pool fire/ Jet fire |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
No |
|
Yes |
|
Yes |
|
|
Vapour cloud explosion |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
No |
|
No |
|
|
Flash fire over plant area |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Yes |
Flash fire full extent |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
No |
Unignited
release |
|
|
|
|
|
|
|
|
|
|
|
|
For loss of containment events from piping
and equipment, it has been assumed that detection and shutdown would occur 90%
of the time (based on safety integrity level 1 for emergency shutdown systems
which has an associated probability of failure on demand of 0.1).
As discussed in Annex 13A7 on the consequence analysis, if detection and shutdown
is successful, a 2 minute release is assumed based on the Emergency Shutdown
Device (ESD) provisions in the design (Annex
13D). For shutdown failure, a 10-minute release is assumed. The exception
to this is the unloading arms for which 2-minute and 30s releases are
considered for isolation failure and isolation successful. The release duration
does not have a major influence on the hazard distances determined from
dispersion modelling, but slightly different ignition probabilities are assumed
for these two cases.
Immediate ignition of an LNG release would
result in a pool fire, a jet fire or a fireball (for instantaneous gas
releases). For a liquid release under pressure, a jet fire is produced. For a
non-momentum liquid release, the liquid is assumed to spill onto the ground
producing a pool fire. Gas releases are all pressurised releases and ignition
would result in a jet fire. For instantaneous gas releases following a rupture
failure, a fire ball is assumed to occur.
In the event of non-ignition, a cloud of
natural gas would be formed by the gas release or evaporating liquid pool. A
flash fire would occur if this cloud were subsequently ignited.
If immediate ignition does not occur, the
dispersing cloud of natural gas may subsequently be ignited. Two delayed
ignition scenarios are considered. The first, “delayed ignition 1”, takes into
account the possibility that ignition could occur within the plant area due to
the presence of ignition sources on site. The second, “delayed ignition 2”,
assumes ignition occurs after the cloud has dispersed to its full (steady
state) extent.
Delayed ignition for an LNG storage tank
failure was treated a little differently given the much larger scale of the
release. Vaporisation from the liquid pool was observed to be highly transient
in nature. The liquid pool expands to its maximum extent after several minutes
and then begins to shrink again as the LNG pool “dries up”. The vapour cloud
was observed to expand rapidly with the initial pool expansion. Once
vaporisation diminishes, however, a sizable cloud of gas within the
flammability limits remains and is convected
downwind, gradually shrinking as it goes. Delayed ignition 1 was therefore
assigned to the cloud at its maximum footprint area, while delayed ignition 2
was applied to the remnants of the cloud at the maximum downwind extent.
Different ignition probabilities were also assigned to LNG tank release (Section 1.3).
If delayed ignition does not occur, the
vapour cloud disperses with no effect.
If a delayed ignition occurs within the
plant area (delayed ignition 1), the possibility of an explosion occurring
within the congested space of the process area is considered.