Reliability HotWire

Reliability HotWire

Issue 120, February 2011

Tool Tips

*When I use the "Save Log of Simulations" feature in BlockSim 7 and calculate the average of all the individual results, why isn’t that value identical to the results from a single set of simulations using the basic approach?

To understand this question and the reason why the results are not identical, suppose you are interested in the reliability/maintainability performance for a system and you want to use simulation to estimate useful metrics, such as the mean availability and the total expected downtime after 1000 hours of operation.

If you model the system in BlockSim and then use the General tab of the Maintainability/Availability Simulation window to run 100 simulations for the specified mission time (1000 hours), the Simulation Results Explorer will display a variety of metrics obtained by calculating the average of the results from the 100 separate simulation runs. Furthermore, if you specify a "seed" that will be the starting point for the random number generation, the simulation routine will draw from a single stream of randomly generated numbers for the series of 100 simulations. Therefore, the results will be reproducible if you perform the exact same set of simulations again, using the same seed, for the exact same diagram.

As an example, Figure 1 shows the settings and some results from a basic simulation analysis.

Basic simulation settings and results
Figure 1: The settings for a single set of 100 simulations, along with the Mean Availability and Total Downtime metrics estimated for 1000 hours of operation.

Now suppose that you want to perform a more in-depth exploration that will help you to analyze the variability within the simulation results. For that purpose, you can use the Save Log of Simulations option, which is available in BlockSim 7.0.2 or higher. With this option, you still specify the number of simulations and seed on the General tab (let’s assume they are the same as in Figure 1) and then you also specify some additional inputs on the Display/Other Settings tab, as shown in Figure 2.

Additional settings for Save Log of Simulations feature
Figure 2: The additional settings for using the "Save Log of Simulations" feature include: a) a value by which to increment the seed for each individual simulation, b) the results that will be displayed in the log and c) the path/filename for the log file.

With the settings shown in Figures 1 and 2, BlockSim will perform one simulation using a seed of 1, then a second simulation using a seed of 11 (1 + 10 = 11), and so on until the utility has performed 100 separate simulations. Figure 3 shows a portion of the log file generated for this example. As you can see, the first data row shows the Mean Availability and Total Downtime obtained from the simulation performed with seed=1, the next row shows the results from the simulation with seed=11, and so on.

Log of simulation results
Figure 3: Portion of the log of simulation results, where each row represents the simulation performed using the specified seed.

Although both of these techniques will provide results based on 100 simulations of the diagram, the values will not be identical because the simulations will draw from different streams of randomly generated numbers. Specifically:

  • With the basic simulation approach described first, all 100 simulations will draw from a single stream of randomly generated numbers where the seed=1.
  • Whereas, with the "Save Log of Simulations" method, the first simulation will utilize a stream with seed=1, the second simulation will utilize a stream with seed=2, and so on.

Therefore, if you were to compute the average of all 100 of the individual availability (or downtime) results shown in the log, it would not be identical to the comparable result displayed in the Simulation Results Explorer. This is not cause for concern because there will always be variability in the results obtained via simulation. The range of values generated by the "Save Log of Simulations" feature will allow you to explore how significant that variability might be for any particular analysis.

*Can I configure Xfmea 5/RCM++ 5 to give a particular user access to some projects but not others?

Yes. In Xfmea 5 or RCM++ 5 when you are working with an Enterprise database or with a Standard database with login security enabled, the Access Group feature makes it possible to provide each user account with different types of access for different projects in the database. For example, you may want to grant a user read/write access to all projects in the "Department A" and "All Departments" groups, read-only access to all projects in the "Department B" and "Department C" groups and no access to projects in the "Confidential" group. There are three steps to use this functionality. (Note that only users with the Admin access level in the database can perform these steps. In the steps below these individuals are referred to as "authorized users.")

  1. An authorized user can choose File > Database Setup > Define Access Groups to open the Define Access Groups window and define the categories that are used within the database to differentiate the different types of access that can be granted to users.
  2. Define Access Groups window

  3. To restrict a user’s access based on group, an authorized user can choose File > Database Setup > Manage Users to open the Manage Users window. Open the user record, set the Access Level to By Access Group and then set the permission level (User, View or None) for each group.
  4. To restrict access to a project based on group, authorized users can select the project and then choose Project > Project Properties to open the Project Properties window. On the Access tab, click the Restrict access to project by selected user access groups option and select which access group(s) the project belongs to.