Selecting a Calculation Mode
To select calculation mode:
From Outside a Bowtie:
- Navigate to Browse > Risk Studies > Risk Scenarios
- Double click a risk scenario from the grid view OR right click it and select properties
- Select the 2nd tab – “Analysis Options”
4. Select a “Risk Analysis Type” from the pull-down list.
From Within a Bowtie:
- Right click the top event in the bowtie view, select properties:
Follow Steps 3 and 4 as above.
Risk Analysis Types
Qualitative bowties provide the simplest overview with regards to understanding risk ratings and event likelihoods.
Qualitative mode hides ingoing and outgoing event likelihoods if they are configured to be shown in the current perspective.
The default cause likelihood ratings are as follows, these labels can be adjusted by the user in configuration:
No additional quantitative information will be displayed in qualitative mode.
Qualitative (Automatically Calculated)
Automatically calculated qualitative mode allows the user to understand event likelihoods in a more sophisticated way than simple qualitative analysis.
To understand calculated qualitative analysis, causal ingoing and outgoing likelihood should be toggled in the bowtie perspective.
For help with bowtie perspectives see the knowledge base article here.
The calculations occur in order from the causes, through controls to the Top Event. The top event can have its inbound likelihood set as either the sum of all connected cause likelihood, or the maximum of the connected cause likelihoods.
The outgoing likelihood of the top event can be set as the value of the top event or as its value divided by the number of outgoing consequences. These changes can be made in the site configuration.
This analysis option provides an overview that will display simple input and output likelihood values however the internal calculation variables such as RRF and PFD are not displayed. Likelihoods are rounded to a system defined level rather than an absolute value.
Cause Likelihood Values
The default values for cause likelihood are each different by 1 order of magnitude from 0.001 – 10:
There are several options for mapping quantitative values to qualitative bowties.
Floor function rounds the actual calculated value downwards to the nearest cause likelihood interval.
Ceiling function rounds the actual calculated value upwards to the nearest cause likelihood interval.
Value simply round to the nearest interval either upwards or downwards.
The qualitative rounding type can be changed in site configuration.
My Profile > Configuration > Register Policies > Analysis > “Analysis Floor Ceiling Value”
As explained in the configuration settings, the user must enter “floor”, “ceiling” or “value” in the field to set the option. These inputs ARE case sensitive.
RRF and PFD
Risk Reduction Factor (RRF) and Probability of Failure on Demand (PFD) are two alternative quantitative values calculating outgoing likelihood of any given node.
Outgoing likelihood and PFD can be calculated as follows:
And hence alternatively for outgoing likelihood:
These various values can be seen in the bowtie under the Semi-Quantitative analysis scheme.
The Semi-Quantitative analysis scheme provides the most in-depth overview of risk cause/consequence likelihoods. Semi-Quantitative analysis provides a view of calculations including the relevant PFD or RRF value; whichever is currently set to be used in the site configuration.
An example of the calculations for a control node can be seen below:
Values are expressed in scientific notation i.e. 1.000E-1 = 0.1, the above example’s output can be verified using the formulae mentioned prior.