CDOIF Guidance

Safety and environmental standards for fuel storage sites

Final report

14 In algebraic terms, the LOPA is equivalent to calculating

in the equation below:

Where:

is the calculated frequency of consequence

summed over all relevant initiating failures

and with credit taken for all relevant protection layers and conditional modifiers.

is the frequency of initiating failure

leading to consequence

is the probability that enabling event or condition

will be present when initiating

failure

occurs.

is the probability of failure on demand of the

protection layer that protects against

consequence

for initiating event

is the probability that conditional modifier

will allow consequence

to occur for

initiating event

15 The calculated value of

is then compared with a target frequency. The target frequency

may be derived from detailed risk tolerance criteria, or may take the form of a risk matrix. This

comparison allows decisions to be made on whether further risk reduction is required and what

performance any further risk reduction needs to achieve, including the SIL, if the additional

protection layer is a SIS.

16 Some variants of the LOPA methodology determine the harm more precisely in terms of

harm caused to people and harm to the environment. This approach, which is required by the

tolerability of risk framework for human safety,

Reducing risks, protecting people

requires

consideration of additional factors such as the probability of ignition, the performance of

containment systems, and the probability of fatality. For a similar perspective of environmental

issues assessors should consult the relevant Environment Agency sector BAT guidance. All of

these factors may be subject to considerable uncertainty, and the way the LOPA is carried out

needs to reflect this uncertainty. Uncertainties are present in all calculations but sensitivity analysis

can be used to help understand the uncertainty.

17 The product of the LOPA should be a report which identifies the hazardous scenario(s)

being evaluated, the team members and their competencies, the assumptions made (including

any supporting evidence) and the conclusions of the assessment, including the SIL of any SIS

identified. The format and detail of the LOPA report should facilitate future internal review by the

operating company and should also reflect the likelihood that it may be scrutinised by an external

regulator and other third parties.

18 It is important to emphasise that the LOPA methodology is a team-based methodology

and its success relies on the composition and competence of the team. The team should have

access to sufficient knowledge and expertise to cover all relevant aspects of the operation. In

particular, for the risk assessment of an existing operation, the team should include people with

a realistic understanding of operational activities and tasks – recognising that this may not be

the same as what was originally intended by the designer or by site management. Any LOPA

study should be carried out from scenario definition to final result using the knowledge of what is

actually done.

PFD