Task Scheduling

The following information applies to the task scheduling properties for preventive tasks, inspection tasks and the inspection portion of on condition tasks.

Click the arrow in the When is this task performed? field to define the criteria for performing the task. The remaining fields in the task scheduling properties will depend on your selection in this field. The task can be performed:

• At certain intervals. If you select this option, you must then specify:

• • Whether the interval is fixed or dynamic.

  • Whether the interval is based on the item's age (taking duty cycle into account) or on calendar time.

• Upon certain events. If you select this option, you can choose for the task to be performed:

• • Whenever the system is down for any reason. No further task scheduling properties are required for this option.

  • Based on events in a maintenance group. The task will be performed when user-specified events occur for either some or all blocks in one or more user-specified maintenance groups. The item that the task is assigned to does not need to be part of the selected maintenance group(s).

  • When a maintenance phase in a phase diagram starts. Please note that if the task is selected to be performed only at the start of a maintenance phase and the item that the task is associated with is not included in the maintenance phase's associated maintenance template, the task will never be performed. No further task scheduling properties are required for this option.

  • Based on events in a flow group (see Flow Groups in the BlockSim documentation). This setting is appropriate only for tasks used in process flow simulation diagrams. The task will be performed when flow either reaches/exceeds its threshold or drops to or below its threshold in one or more user-specified flow groups. The block(s)/item(s) affected by the task may or may not belong to any of the flow groups.

The remaining task scheduling properties for each type of schedule are given next. For any interval that you specify, you will be asked to specify the units used.

• Fixed interval based on either item age or calendar time: Enter the interval when the task will be performed (e.g., if you enter 100, the task will be scheduled for 100, 200, 300, etc.).

• • Item age refers to the accumulated age of the block, which gets adjusted each time the block is repaired (i.e., restored). If the block is repaired at least once during the simulation, this will be different from the elapsed simulation time. For example, if the restoration factor is 1 (i.e., "as good as new") and the assigned interval is 100 days based on item age, then the task will be scheduled to be performed for the first time at 100 days of elapsed simulation time. However, if the block fails at 85 days and it takes 5 days to complete the repair, then the block will be fully restored at 90 days and its accumulated age will be reset to 0 at that point. Therefore, if another failure does not occur in the meantime, the task will be performed for the first time 100 days later at 190 days of elapsed simulation time.

1. • Calendar time refers to the elapsed simulation time. If the assigned interval is 100 days based on calendar time, then the task will be performed for the first time at 100 days of elapsed simulation time, for the second time at 200 days of elapsed simulation time and so on, regardless of whether the block fails and gets repaired correctively between those times.

If the task is performed at fixed intervals, you can select the Override task scheduling properties with a task package option to assign the task to a task package, which is a group of tasks that are performed together at scheduled intervals. (Note that if a task has been assigned to a task package, the task scheduling properties will be disabled and displayed in italics.)

• Dynamic interval based on either item age or calendar time: The task will be performed at variable user-specified intervals. This can be used, for example, to schedule maintenance to be performed with increasing frequency as an item gets older. The Dynamic intervals field shows the number of intervals specified. Click the button (...) to open the Intervals window, which allows you to enter the item age intervals when the task will be performed. The intervals are the actual time between maintenance (i.e., they are not cumulative). Note that the last value entered in the Intervals window will repeat, if necessary, until the end of the simulation time.

• Based on events in a maintenance group: Specify the events that can trigger the task (i.e., a block fails; a corrective, preventive or inspection task starts; a block is restored to operation), and choose the maintenance group(s) in which the event(s) must occur in order to trigger the task.

• Based on events in a flow group: Specify the events that can trigger the task (i.e., flow rate through any connector exceeds or drops below a certain level), and choose the flow group(s) in which the event(s) must occur in order to trigger the task.

Note: For preventive and inspection tasks that are scheduled based on item age, the Perform this task even if the item failed before this task was scheduled to occur? property determines whether actual age or "virtual age" is considered. If you answer Yes, the task will be performed even if the item has already failed. In other words, the item's "clock" is not stopped upon failure, and the item's virtual age is used to trigger the task. (For more about this option, see Preventive Tasks.)