Phase Properties

Operational Phase Properties

Diagram

The diagram property is used to associate an RBD with a phase. You can select and associate any existing RBD with a phase. Note that common components across different RBDs are identified by name. In other words, a component with the exact same name in two RBDs is assumed to be the same component working in two different phases.

Phase Duration

The duration of an operational phase is fixed and needs to be specified. Even though no time units are specified, it is implied that the same unit of measure is used throughout the model and on all inputs. However, this duration may be affected by the choice of action you choose to perform on system failure as specified in the On System Failure property. This is discussed in the On System Failure Options subsection.

Phase Duty Cycle

This property allows for the specification of a common duty cycle value for all the components in the current operational phase. BlockSim applies this duty cycle to all the units in the operational phase. This is in addition to any duty cycle value that may have been specified at the block level. To illustrate this, consider two components A and B. A has an individual duty cycle of 2 and B has the default value of 1. If both of these components are included in a phase with a phase duty cycle of 0.5, then the overall duty cycle value for A is 1 (2 x 0.5), while the overall duty cycle value for B is 0.5 (1 x 0.5). The concept of the duty cycle is discussed in a prior section.

On System Failure Options

This property is used to set a specific action when a system failure occurs in a phase. There are three options:

Continue Simulation.
Start New Simulation.
Go to Maintenance Phase.

These options are discussed next.

Continue Simulation

This is the default option. Under this option, when a system failure occurs, repairs begin as per the repair policy selected and the time to restore the system is part of the operational phase's time. In other words, the repairs continue in the operational phase until the system is up again. If the repairs are not completed before the phase ends, the repairs continue into the next phase. Thus, under this option the duration of an operational phase is not affected by a system failure.

As an example of this option, consider a production line operating in two phases of day and night shifts. A failure occurs in the day shift that renders the production line non-operational. Repair of the production line begins immediately and continues beyond the day shift. The production line is back up after midnight. In this case the repair of the production line exhausts all of the duration of the day shift phase from the time of the failure to the end of the phase. Some part of the night shift phase is also exhausted.

Start New Simulation

This option halts the simulation, which effectively means the end of the mission if the system fails. Specifically, if this option is selected upon system failure, the execution of the current operational phase and all phases that follow the current phase is halted and the mission aborted. This option can be used to model a system whose failure cannot be repaired and the mission has to be aborted if a failure occurs. A good example of this would be the aircraft case discussed previously. A catastrophic failure during cruising would end the mission.

Go to Maintenance Phase

This option takes a system directly to the specified maintenance phase if a system failure occurs. If other phases exist between the current operational phase and the specified maintenance phase (i.e. other phase blocks lie between the two in the RPD), then these phases are skipped and execution continues after the specified maintenance phase. While in this maintenance phase, tasks are carried out on all components included in the linked maintenance template. As an example for this case, consider again the aircraft that has a failure during the taxi phase. If the maintenance phase specified for this phase is the last phase of the RPD, then the failure during taxi phase would lead to the maintenance phase and thereafter end the mission. As another example consider an unmanned submarine used to monitor marine life that undertakes four operational phases and has one maintenance phase. The submarine operates at two different depths, for a period of five days each, for the first two phases. Thereafter it is overhauled (maintenance phase). After this, the submarine goes for another two phases at two different depths, each of three days duration. In this case if a system failure occurs during the first phase (while the submarine is at the first depth), then the submarine is recalled and undergoes the maintenance phase. This would interrupt the first phase and cancel the second phase. After maintenance the submarine would then continue to the last two phases.

Maintenance Phase Properties

Maintenance Template

This specifies the maintenance template to be used in the currently selected maintenance phase.

System Age Threshold

The System Age Threshold (Preventive/Inspection Policy) property provides the ability to add some flexibility to the timing of scheduled preventive maintenance actions and/or inspections. It is used to specify an age interval when a preventive maintenance action or inspection will be performed. This helps in optimizing the resources allocated to repair the system during a maintenance phase by performing preventive maintenance actions or inspections when the system is already down in a maintenance phase. For example, a preventive maintenance action is scheduled for a car (e.g. an oil change, tire rotation, etc.) every 60,000 miles, but a system downing failure of an unrelated component occurs at 55,000 miles. Here the system age threshold will determine whether the preventive maintenance will be performed earlier than scheduled. If the system age threshold is 0.9, the preventive maintenance will be performed since the failure occurred after the system accumulated 91.67% of the time to the scheduled maintenance or is within 8.33%, (60,000 - 55,000)/60,000 = 8.33%, of the system age at which the preventive maintenance was originally scheduled. (A system age threshold value of 0.9 implies that the system accumulated 90% of the time to a scheduled event or is within 10% (1 - 0.9 = 0.1 or 10%) of the scheduled event.) If the system age threshold was 0.95, the preventive maintenance will not be performed at 55,000 miles, since the system failure did not occur within 5% of the system age at which the preventive maintenance was originally scheduled (1-0.95=0.05 or 5%).

Illustrating System Age Threshold

To illustrate System Age Threshold, consider the deterministic scenario represented in Figures 11.14 and 11.15.

Figure 11.14 The RBD for the example illustrating the System Age Threshold option.

Figure 11.15 The phase diagram for the example illustrating the System Age Threshold option.

Table 4. The properties/parameters table associated with the example illustrating the System Age Threshold option. Note that all times are in hours.

In this example, a system has two components: Block A and Block B. The system undertakes a mission that can be divided into two phases. The first phase is an operational phase of duration 1370 hrs with both the components in a parallel configuration. In this phase Block A fails every 750 hrs while Block B fails every 1300 hrs. Corrective maintenance action on Block A in this phase requires 100 hrs to be completed. A preventive maintenance of 20 hrs duration also occurs on Block A every 500 hrs. No maintenance can be carried out on Block B in this phase. The Continue Simulation option is selected for the On System Failure property for this phase. The second phase of the mission is a maintenance phase. In this phase Block A has the same maintenance actions as the those in phase 1. A corrective maintenance of 100 hrs duration is defined for Block B. Phase 2 also has a value of 0.70 set for the System Age Threshold. All maintenance actions during the entire mission of the system have a type II restoration factor of 1.

The system behavior from 0 to 3500 hrs is shown in Figure 11.16 and described next.

Figure 11.16 System behavior for the System Age Threshold example.

Phase 1 begins at time 0 hrs. The duration of this phase is 1370 hrs.
At 500 hrs the first of the scheduled PMs for Block A begins. The duration of these maintenance tasks is 20 hrs. The scheduled maintenance ends at 520 hrs.
At 1000 hrs another PM occurs for Block A based on the set policy. This maintenance ends at 1020 hrs.
At 1300 hrs Block B fails after accumulating an age of 1300 hrs. A system failure does not occur as Block B is in a parallel configuration with Block A in this phase. Repairs for Block B are not defined in this phase. As a result Block B remains in a failed state.
At 1370 hrs phase 1 ends and phase 2 begins. Phase 2 is a maintenance phase. Block B is repairable in this phase and has a CM duration of 100 hrs. As a result, repairs on Block B begin and are completed at 1470 hrs. Block A has the next PM scheduled to occur at 1500 hrs. However, phase 2 has a System Age Threshold for Preventive and Inspection Policies of 0.7. The time remaining to the next PM is 130 hrs (1500-1370=130 hrs). This remaining time over the PM policy time of 500 hrs is 26%. This is within 0.3 (1-0.70=0.3) or 30% corresponding to the threshold value of 0.70. Thus the PM task that is to occur at 1500 hrs is carried out in the maintenance phase from 1370 hrs to 1390 hrs while no PM occurs at 1500 hrs. All maintenance actions are completed by 1470 hrs and phase 2 ends at this time. This completes the first cycle of operation for the phase diagram.
At 1470 hrs phase 1 begins in the second cycle.
At 2000 hrs, the next PM for Block A begins. This maintenance ends at 2020 hrs.
At 2500 hrs, another PM is carried out on Block A and is completed by 2520 hrs.
At 2770 hrs, Block B fails in the second cycle of phase 1 after accumulating an age of 1300 hrs. Since no repair is defined for the block in this phase, it remains in a failed state.
At 2840 hrs, phase 1 completes its duration of 1370 hrs and ends. Phase 2 begins in the second cycle and the corrective maintenance, defined for a duration of 100 hrs for Block B, begins. This repair action ends at 2940 hrs. For Block A the time remaining until the next PM at 3000 hrs is 160 hrs (3000-2840=160 hrs). This remaining time over the PM policy of 500 hrs is 32%. This is not within 30% corresponding to the threshold value of 0.70. Thus the PM due at 3000 hrs is not considered close enough to the beginning of the maintenance phase and is not carried out in this phase. At 2940 hrs all maintenance actions are completed in phase 2 and phase 2 ends. This also completes the second cycle of operation for the phase diagram.
At 2940 hrs phase 1 begins in the third cycle.
At 3000 hrs the scheduled maintenance on Block A occurs. This PM ends at 3020 hrs.
At 3500 hrs the simulation ends.

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