Modeling the Repair of Failure Modes

Each failure mode defined using the Failure Modes tab (with the exception of the Unreliable failure mode type for Action elements) can be specified to be repaired, with a separate repair time distribution.

Note: For the two PM (preventive maintenance) "failure modes", the repair time actually represents the amount of time required to complete the preventive maintenance.

Repair is defined in terms of a repair time distribution, as well as specification of when the repair is to take place.

The repair time distribution is defined at the bottom of the Failure Modes tab:

GoldSim allows you to select from three probability distributions for the repair time:

All three repair distributions are defined by a mean delay time (until repaired) and a standard deviation must be specified for the first two types.

When a repair is triggered, GoldSim automatically samples from the specified repair time distribution to determine when the failure mode is repaired.

The parameters describing the repair distributions can be functions of time, but changes to the parameters only take effect when the next repair begins.

Note: If you specify a zero Mean delay time for the repair, GoldSim will treat this as follows: The time to implement the repair will be the lesser of 1 second or 1% of the length of the next timestep.

You can optionally specify that one or more Resources (e.g., spare parts, personnel) must be available in order to carry out the repair. If Resources are specified for the repair, the repair will not begin until the Resources are available.

It is important to understand exactly what is meant by a "repair". Most failure modes are defined relative to a failure mode control variable (e.g., the total time, operating time, mileage).

For these types of failure modes, a repair means that the failure mode control variable (FMCV) has been reset back to a specific value. Selecting one of these failure modes and clicking the Settings… button in the Failure Modes tab provides access to the Control Variable Settings dialog. Within this dialog, you can define the Repair Definition for the mode:

By default, this value is zero, but it can be specified by the user to be any value.

For two failure modes (Demand>Capacity and Event-triggered failure), there is no failure mode control variable. Hence, repair simply means that the failure mode is no longer making the component inoperable (since there is no failure mode base variable to reset).

Note: Although Exponential/Poisson and Defective Component failures are FMCV-based, they are memoryless, since their hazard function is constant with time. As a result, the value to reset the FMCV to cannot be specified for these failure modes (as it would have no meaning).

As pointed out above, in addition to specifying a repair time distribution, you must also specify when the repair is triggered to take place. That is, you must explicitly specify what triggers a repair to begin.

To automatically repair failures for a specific failure mode, select the failure mode, and then check the Automatically repair failures checkbox. This forces the repair to begin immediately upon failure.

You can also specify that a failure mode can be repaired when a preventive maintenance is carried out, even if it has not yet failed. You do so through the Control Variable Settings dialog (accessed, as mentioned above, by selecting a failure mode and pressing the Settings… button in the Failure Modes tab). From this dialog, you can specify whether or not the mode is repaired upon preventive maintenance:

If this input field is true, the repair of the failure mode begins when the PM mode begins. Otherwise, it is not repaired.

FM_Failed is a locally available property of the failure mode. It is true if the failure mode is in a failed state, and false if the failure mode is in an unfailed state. Hence, by default a Preventive Maintenance event automatically repairs other failure modes that are currently in a failed state, but does not repair other failure modes that are in an unfailed state.

Note: When specifying whether or not a PM repairs a failure mode, another locally available property can also be referenced in the Repair mode if this condition is true field: FM_TimeToFail. This variable represents an estimate of the time to failure (if the mode is already failed, it will be 0). This allows you to specify that the mode is to be repaired even if it has not yet failed, based on your prediction that it will likely fail soon.