13 / 40
11
Chemical Technology • July 2015
Layer of protection analysis (LOPA)
Layer of protection analysis is a simplified form of risk
assessment that evaluates the risk of an individual
hazard scenario. It only considers safeguards which
are independent layers of protection. The purpose of
this is to assist in avoiding the addition of excessive
protection incurring high costs, ie, over protection. It
uses an order of magnitude technique to evaluate the
adequacy of existing or proposed layers of protection
against known hazards.
In order to carry out a LOPA the following information
must be available:
• A hazardous event must have been identified, eg, fire,
explosion, toxic release, pollution, etc.
• The causes of the event.
• The consequences of an event, eg, fatalities, injuries,
damage, spoilt environment, etc.
• Existing safeguards.
The above information is often readily available from a
Hazard and Operability study (Hazop) and, therefore, a
LOPA can be done as part of such a study. Alternatively,
a separate LOPA can be done where the above informa-
tion is generated.
Setting tolerance criteria
Some criteria or target of tolerance or acceptability is
required to enable one to decide how much protection is
enough. In the following simple example a typical scale of
risk has been established. Risks can be classed as follows:
• Class I - Intolerable risk, not acceptable
• Class II - Undesirable risk, which is tolerable only if risk
reduction is impractical or the cost is disproportionate
to the improvement gained
• Class III - Tolerable risk if the costs of reduction exceeds
the improvement gained
• Class IV - Negligible risk.
It is normal to design hazardous processes or operations
to meet a Class III risk. A target risk table may be drawn
up for a particular industry or organisation. A hypothetical
example is shown below in Table 1. Note this matrix is not
an industry standard, see HSE (2001).
For example, if a particular hazardous event, eg, over-
filling of a tank, could lead to an irreversible health effect,
long term environment damage or a R50-m financial loss,
then this should not happen more than once in a 1 000
years, to meet the Class III risk criteria, that would make
the risk tolerable. Alternatively, this can be expressed as
PLANT MAINTENANCE, SAFETY,
HEALTH & QUALITY