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Local Fuel – Shoreham Oil Terminal
Gasoline Ship Import – Layers of Protection Analysis
P & I Design Ltd
DOCUMENT NO: LF364002_RPT
2 Reed Street, Thornaby, UK, TS17 7AF
ISSUE: E DATE: 30.07.15
Tel: + 44 (0)1642 617444
PAGE 36 OF 38
Fax: + 44 (0)1642 616447
www.pidesign.co.uk5.4
Sensitivity (Ref. PSLG Guidelines, Appendix 2, Clause 150)
The calculations of frequency and probability for the initiating events are necessarily
subjective but the following sensitivity can be shown:
The worst case frequency of an overfill, without the SIL 2 layer of protection or any
conditional modifiers is calculated as 3.95 x 10
-4
/year.
For initiating events 1, if the HEART data was used to show a more conservative probability
of human error approaching 0.1, then the initiating event in Scenario 1 would be raised to 3
x 10
-2
and thus the frequency of an explosion would be raised to 9.35 x 10
-7
/year (Still
Tolerable if ALARP) and would predict that the overfill frequency increases to 3.12 x 10
-2
/
year which seems very high.
The probability of the ATG failure, during import operations, has been taken as 0.1 which is
less than the maximum that can be claimed for non SIL rated equipment. If this was
increased to 0.5, then the frequency of an explosion would be raised to 6.32 x 10
-8
/year. This
demonstrates that the effect is not significant.
If the number of import operations between the hours of 4:00pm and 8:00am were increased
to 10, then then the frequency of an explosion would be raised to 1.98 x 10
-7
/year. This
would still be in the Tolerable if ALARP region.
If the number of import operations between the hours of 8:00am and 4:00pm were increased
to 25, then then the frequency of a fire would be raised to 8.24 x 10
-7
/year. This would still
be in the Broadly Acceptable region.
Conditional Modifiers are not generic and should be subject to local site conditions,
properties of materials and location of facilities.
There is uncertainty about the figures used for the above conditional modifiers and it was
felt that a conservative approach has been taken. However, the sensitivity of the figures used
above have been considered.
A met office study suggest that throughout the whole year, between the hours of 4:00 PM to
8:00 AM, wind speeds less than 2m/s and stability within classes E&F occurs for 2% of the
time. If this probability is increased to 5% then the frequency would be increased to 9.89 x
10
-8
/year. This would still be in the Broadly Acceptable region.
For the environmental case following an explosion, the probability of bund failure has been
taken as 0.8, if this figure was raised to unity, the frequency of a release of gasoline and/or
fire fighting chemicals would increase to 1.98 x 10
-7
/year. This would still be in the Broadly
Acceptable region.