BlockSim Example: CM Triggered by Subsystem Down

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The purpose of this example is to illustrate how to model the case when CM is triggered by failure of a subsystem. We present two kinds of methods: a method without a subdiagram and a method with a subdiagram. The limitation of the method without a subdiagram is that it cannot model subsystem with more than two devices in parallel or more complex structure. The method with a subdiagram can break this limitation.

Method 1: Without subdiagram

Consider a system with three devices A, B and C are in parallel. A and B form a subsystem, AB. If both A and B fail (i.e., if subsystem AB fails), then start to repair A and B. Device C is in parallel with subsystem AB. The repair of C depends on its failure. When C fails, then start to repair C.

BlockSim Solution

The BlockSim modeling of this system is shown in the figure below.

• The reliability of all blocks follows the Weibull distribution with Beta = 1.5 and Eta = 500 hours.
• Block A and block B belong to maintenance group MG_AB.
• The corrective maintenance of block A and block B are set to "When found failed during an inspection". The CM duration is 120 hours. Upon repair, they are as good as new.
• There is an inspection task for both block A and block B. The inspection is performed based on events in maintenance group MG_AB. If a "Block fails" or a "Start of corrective maintenance" event occurs for any of the blocks in the maintenance group MG_AB, then the inspection is performed. The inspection duration is set to "Immediate Repair" and upon inspection; the device is restored to same as it was when it failed (ie.e, as bad as old).
• The corrective maintenance of Block C is performed upon failure of Block C and the duration is 20 hours. It is as good as new upon repair.

Block Up/Down plot

The system event log is shown in the figure below and is as follows:

1. At 271 hours, Block C fails and the repair is finished at 291 hours.
2. At 331 hours, Block B fails. Since the subsystem AB is not down (if both A and B fails, then subsystem AB fails), Block B is waiting for repair. (The failure of Block B cannot trigger an inspection of Block B, but this failure would trigger the inspection of Block A. However, Block A has not failed at this time, thus nothing happens).
3. At 381 hours, Block A fails and subsystem AB fails. The failure of the subsystem triggers the inspection and then inspection triggers corrective maintenance. (The failure of Block A triggers an inspection on Block B, which is currently failied. Thus corrective maintenance of Block B is performed. Performing corrective maintenance on Block B triggers the inspection of Block A. Since Block A is also currently failed, thus corrective maintenance of Block A is performed too.)
4. At 501 hours, both Block A and B are restored.
5. At 791 hours, Block B fails and is waiting for repair.
6. At 943 hours, Block C fails and get restored at 963 hours.
7. At 1358 hours, Block A fails and subsystem AB fails, triggering corrective maintenance for Block A and B.
8. At 1478 hours, both Block A and B are restored.
9. ...

Method 2: With subdiagram

Let’s a system consists of four devices A, B, C and D. Devices A, B and C are in parallel structure and form a subsystem ABC. Only all A, B and C fail (subsystem ABC fails), then start to repair devices A, B and C. Device D is parallel with subsystem ABC. The repair of D depends on its failure. When D fails, then start to repair D. Method 1 cannot model this system. Here we use subdiagram to model the subsystem.

BlockSim Solution

The BlockSim model of this system is shown in the Figure 5 below. By using subdiagram, we can break the limitation in method 1.

Reliability of all blocks follow Weibull distribution with Beta = 1.5 and Eta = 100 hours. Subdiagram block ABC belongs to maintenance group MG_ABC. CM of Block A, B and C are set to "When found failed during an inspection". The CM duration is 20 hour. Upon repair, they are as good as new. There is an inspection for Block A, B and C. The inspection is performed based on events in maintenance group MG_ABC. If "Block fails" event occurs for any of the blocks in the maintenance group MG_ABC, then the inspection is performed. The inspection duration is 30 hours. Upon inspection, the device is restored to as bad as old. CM of Block D is performed upon failure of Block C and the duration is 40 hours. It is as good as new upon repair.

 Block Up/Down plot

The system event log is shown in Figure belwo and is as follows:

1. At 13, Block C fails. Since subsystem ABC doesn't fail, Block C waits for repair opportunity.
2. At 111, Block B fails. Since subsystem ABC doesn't fail, Block B waits for repair opportunity.
3. At 176, Block A fails and brings down the subsystem ABC. Failure of subsystem ABC triggers the inspection of Block A, B and C. 
4. At 197, Block D fails and get repair at 237.
5. At 206, Block A, B and C finish inspection and trigger CMs. CM is performed on Block A, B and C. 
6. At 226, Block A, B and C are restored. 
7. ...