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Calculations
0:06
Welcome to this application explainer video, part of our low per topic range.
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In this video, we’ll cover the subject of low PER calculations in SLM.
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SLM performs a series of calculations in the low per module to streamline the facilitation process, namely the risk gap for scenarios, barrier targets for Sifts, and barrier demand rates.
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The following information will be covered in this training.
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Chapter one will be scenarios, Chapter 2 will be barrier targets and Chapter 3 will be demand rates.
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In Chapter 1, we will cover the values that create the Lope A gap, adding the following to close the Lope A gap, barriers, enabling events, conditional modifiers and recommendations, and Lope A gap colour coding.
0:56
The main calculation with the LOPA scenario is the LOPA gap.
1:00
The LOPA scenario shows all values that contribute to the LOPA gap calculation, starting with the initiating event frequency.
1:11
The target mitigated event likelihood values for each of the consequence categories, probabilities for your enabling events and conditional modifiers, the PFDS for your Ipls, the mitigated event frequencies for the scenario, the low per gap value before the recommendation is applied, and the low per GAAP value after the recommendation is applied.
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The low per gap is the value of the gap between the mitigated event frequency or MEF and the target mitigated event likelihood, TMEL.
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It’s calculated by dividing the mitigated event frequency by the target mitigated event likelihood.
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When the value of the gap has been reduced to one or less than one, the gap is considered closed or the scenario passes.
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Any value greater than one means that the likelihood for the scenario has not yet been reduced down to an acceptable level to meet the target likelihood.
2:17
The gap is made-up of the TMEL and the initiating cause likelihood or initiating event frequency.
2:25
The gap is made-up of the TMEL and the initiating event frequency, which is equal to the mitigating event frequency before any conditional modifiers, enabling events and Ipls or barriers are applied to reduce the mitigated event frequency down to the target likelihood value, multiply your initiating event frequency by the probability of failure on demand for your barriers and the probabilities of your enabling events and conditional modifiers.
2:58
All these values multiplied together equal the mitigated event frequency.
3:03
If the looper gap still exists, the value of the PFD for the recommendation can be included in the mitigating event frequency calculation to show the value of the LOPE A gap.
3:17
With the recommendation included, the value of the gap showing in a particular scenario will be the value of the highest gap between any of the consequence categories.
3:29
The LOPE A gap is colour coded according to the sill needed to close the gap.
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If the loper gap is less than or equal to 1, the colour is green and the gap is considered closed.
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If the Loper gap is greater than one but less than 10, the colour is yellow and the barrier with a risk reduction factor of 10 will close the gap.
3:49
If the gap is greater than or equal to 10 but less than 100, the colour is orange and the seal One barrier will close the gap.
3:57
If the gap is greater than or equal to 100, the colour is red and a barrier of seal 2 or greater is needed to close the gap.
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A great summary view to see all of the gaps will be found in your LOPA study object on the LOPA Sheet List tab.
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This displays the PFD gaps for each of the LOPA scenarios.
4:24
In Chapter 3 we will cover selecting a target RRF calculation method and calculating the target RRF for a SIF applied to one or more scenarios.
4:35
Barrier targets can be calculated for Sifs applied to LOPA scenarios.
4:39
SLM will automatically calculate the target, PFD, RRF and sill of a SIF barrier based on the remaining Loper gap.
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For each scenario it is applied to.
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The calculation and result are found on the barrier object that has been classified as a SIF.
4:56
There are two ways that SLM makes this calculation, which is selected at the Site object.
5:01
In the Global module.
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On the Site Overview tab, look for the Loper Barrier Target Calculations field and select between cause summation or highest gap.
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The selection made applies to the entire site on the barrier objects in the risk reduction factor for the particular barrier.
5:22
ID section is a great way to see the maths behind the calculation or result shown here as the calculated target risk reduction factor Cause summation adds up all risk reduction factors for the consequence category for all scenarios the barrier is applied to, the sum of these RRFS is the target RRF.
5:44
The second option, highest gap, looks at all scenarios the barrier is applied to and determines which scenario has the highest gap.
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If the barrier was not applied, in this case the highest gap, the target risk reduction factor would be 50.
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Once the target risk reduction factor, PFD and seal are calculated, if the barrier is connected to a sieve in the instrumented systems module, the target information will be passed on to the instrumented systems to ensure the targets have been met in the SIFT design.
6:16
For more information on where to find these values in instrumented systems, see the video on barrier connections or module connections.
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In Chapter 3, we will cover the values required to calculate the demand rate of a barrier and how to calculate the demand rate.
6:37
The demand rates of each barrier can also be calculated in the local process.
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If these demand rates are known, they can be used to determine a barrier’s designed demand rate in the instrumented systems module and the measured demand rate calculated by demands recorded in Operate and maintain.
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The calculated demand rate is recorded by three pieces of information, all of which can be seen here on the barrier diagrams.
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For the barrier object, the initiate and cause likelihood of the scenario, the probability of failure on demand for any barriers that are activated before the activation of the barrier we’re looking at.
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And 3rd the order of activation seen on the Applicable Values tab of the local worksheet.
7:27
The order of activation shows the order in which each barrier will be activated on a given scenario.
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Barrier diagrams on the barrier objects are a great place to find these values needed.
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So to calculate the demand rate for a barrier, multiply the initiating cause likelihood with the probability of failure on demand for all barriers, which will be activated before the desired barrier for each scenario.
7:52
Then add the demand rate for each scenario altogether to get the total calculated demand rate for that barrier.
8:00
SLM automatically calculates this value and displays it on the barrier object along with barrier targets.
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See the Barrier Connections video and Module Connections video For more information on how to connect these values to functions in the Instrumented Systems module and where in that module these calculated values can be found.
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