The Case
2
Maintenance workers entering an oven to carry out breakdown
maintenance. The oven is part of a continuous production line,
so parts are hung from a conveyor and slowly progress through
the oven. When the conveyor gets jammed a quick fix saves
waiting three hours for the oven to cool down, so maintenance
people don some protective clothing and aim to get the job done
quickly in the heat. In the last five years they admit there have
been a ‘couple’ of times when they have felt overcome by the
heat and managed to get out quickly. On average this is done
about once per week.
The oven has been in use for 12 years.
Derek Viner
Version 1 Revision 0
Modelled Risk
3
STEP
no.
ESTIMATION
PROCESS
1 Define the Risk With accuracy, as explained
2 Consequence values
Define the LLC and LWC scenarios and the
Consequence Type components of each.
Decide on the values associated with each case.
3 Frequency
Seek a suitable estimate of the Frequency of the
LLC. In most cases this will be the starting point
for estimating the Risk line on the Risk Diagram.
4 Risk Line
a) Plot the point [LLC, Frequency] on the Risk
Diagram.
b) Draw a line from here at a slope of -45o up to
the LWC. Why? Many Risk lines approximate
this iso-Risk line and it is therefore a good
starting point in the absence of other
information.
c) Read off the Frequency associated with the
LWC and subject that to a reality check
Derek Viner
Version 1 Revision 0
Defining the Risk
4
Risk is properly defined when we are able to make a statement about the type of
adverse Consequences that could be experienced by an Asset of interest to us as
result of a defined process.
Energy sources and Event Definition:
Thermal energy. Control is lost once the worker enters the oven,
The mechanism of this is a voluntary and intentional act.
The Outcome of this is that they may or may not be overcome by the heat.
Derek Viner
Version 1 Revision 0
Modelled Risk
5
STEP
no.
ESTIMATION
PROCESS
1 Define the Risk With accuracy, as explained
2 Consequence values
Define the LLC and LWC scenarios and the
Consequence Type components of each.
Decide on the values associated with each case.
3 Frequency
Seek a suitable estimate of the Frequency of the
LLC. In most cases this will be the starting point
for estimating the Risk line on the Risk Diagram.
4 Risk Line
a) Plot the point [LLC, Frequency] on the Risk
Diagram.
b) Draw a line from here at a slope of -45o up to
the LWC. Why? Many Risk lines approximate
this iso-Risk line and it is therefore a good
starting point in the absence of other
information.
c) Read off the Frequency associated with the
LWC and subject that to a reality check
Derek Viner
Version 1 Revision 0
Consequence Values
6
Likely Least Consequence (LLC)
A worker manages to get out before collapsing and spends an hour or more
recovering. Say a cost of $100.
A single fatality is not unreasonable. Assume this would cost the company
$3,000,000. Of course there are moral implications here too, but QRE is not
equipped to take these into account directly. That is a job for management
decision-making.
Likely Worst Consequence (LWC)
Derek Viner
Version 1 Revision 0
Modelled Risk
7
STEP
no.
ESTIMATION
PROCESS
1 Define the Risk With accuracy, as explained
2 Consequence values
Define the LLC and LWC scenarios and the
Consequence Type components of each.
Decide on the values associated with each case.
3 Frequency
Seek a suitable estimate of the Frequency of the
LLC. In most cases this will be the starting point
for estimating the Risk line on the Risk Diagram.
4 Risk Line
a) Plot the point [LLC, Frequency] on the Risk
Diagram.
b) Draw a line from here at a slope of -45o up to
the LWC. Why? Many Risk lines approximate
this iso-Risk line and it is therefore a good
starting point in the absence of other
information.
c) Read off the Frequency associated with the
LWC and subject that to a reality check
Derek Viner
Version 1 Revision 0
Frequency estimation
8
for the Likely Least Consequence (LLC)
This is easy from the reported data – say ’a couple’ means 4 times in 5 years
or 10 times (the arithmetic say 9.6) in 12 years, or 0.83 scares per year on
average.
For the Likely Worst Consequence (LWC)
This has not happened yet. 1. We could estimate it based on a -45o line and
see if this makes sense. 2. Or we could conduct a formal logic analysis or
Outcome possibilities and synthesise an estimate. 3. Or we could deduce
something of the possibility using statistical theory (see p98 of the Text).
Derek Viner
Version 1 Revision 0
Frequency estimation
9
- Estimate it based on a -45o line and see if this makes sense.
Doing this (you’ll see this later on a Risk Diagram), we get a deduced
Frequency of the LWC of 8.3E-5, which is hardly believable in the
circumstances. This implies over 12,000 years between fatalities.
For the Likely Worst Consequence (LWC) - Synthesis from formal Outcome Analysis.
Not in the scope of this Unit - Estimate from statistical theory
p = 1/(2.5n), where n is the number of trials (48×12).
p = 0.00069 LWC per Event
See spreadsheet, LWC frequency is 0.0331 LWC pa, or once in 30 years.
Derek Viner
Version 1 Revision 0
Modelled Risk
10
STEP
no.
ESTIMATION
PROCESS
1 Define the Risk With accuracy, as explained
2 Consequence values
Define the LLC and LWC scenarios and the
Consequence Type components of each.
Decide on the values associated with each case.
3 Frequency
Seek a suitable estimate of the Frequency of the
LLC. In most cases this will be the starting point
for estimating the Risk line on the Risk Diagram.
4 Risk Line
a) Plot the point [LLC, Frequency] on the Risk
Diagram.
b) Draw a line from here at a slope of -45o up to
the LWC. Why? Many Risk lines approximate
this iso-Risk line and it is therefore a good
starting point in the absence of other
information.
c) Read off the Frequency associated with the
LWC and subject that to a reality check
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