10 / 69
Simon Storage Ltd – Immingham East Terminal
Gasoline Import – Layers of Protection Analysis
P & I Design Ltd
DOCUMENT NO: SI057001_RPT
2 Reed Street, Thornaby, UK, TS17 7AF
ISSUE: G DATE: 29.06.12
Tel: + 44 (0)1642 617444
PAGE 10 OF 34
Fax: + 44 (0)1642 616447
www.pidesign.co.uk4
RISK TOLERANCE CRITERIA (Ref. PSLG Guidelines, Clauses 36-39)
The Layer of Protection Analysis (LOPA) SIL selection method is a quantitative method
that considers the initiating event frequency and probability of failures of the various layers
of protection. This method is based on BS EN61511-3 (CDV) Annex F. The unmitigated
event frequency is calculated using the initiating event frequency and probability of failures
of the various layers of protection. Based on the consequence of the hazard a tolerable
frequency is determined. From tolerable frequency and unmitigated event frequency the
required risk reduction and therefore required target SIL is determined. Risk Tolerance
criteria can be defined for process safety and environmental risks.
The PSLG final report into the Buncefield explosions provides some tolerable frequencies
and states that these figures, or a similar matrix, should form the basis of the assessment.
The tolerable risk frequency is the boundary between the ‘tolerable if ALARP‘ and the
‘Broadly acceptable’ regions.
Likelihood of ‘n’
fatalities from a
single scenario
Risk Tolerability
10
-4
/yr – 10
-5
/yr
Tolerable if
ALARP
Tolerable if
ALARP
Tolerable if
ALARP
10
-5
/yr – 10
-6
/yr
Broadly
acceptable
Tolerable if
ALARP
Tolerable if
ALARP
10
-6
/yr – 10
-7
/yr
Broadly
acceptable
Broadly
acceptable
Tolerable if
ALARP
10
-7
/yr – 10
-8
/yr
Broadly
acceptable
Broadly
acceptable
Broadly
acceptable
Fatalities (n)
1
2 - 10
11 - 50
Table 2 - Risk Matrix for Scenario Based Risk Assessments
Hence the following target risk criteria will be used:
No. Fatalities
Tolerable Risk Criteria
1
1 x 10
-5
.
2 to 10
1 x 10
-6
11 to 50
1 x 10
-7
Table 3 - Tolerable Risk Criteria