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Analysis and Modeling of Grouped and
Opportunistic Preventive Maintenance
When planning preventive maintenance
strategies, considerations of the overall benefit for the whole system
should supersede the optimum plan for each component separately. Maintenance
plans should be addressed in a holistic way to optimize the cost of
ownership of the system. For example, sometimes it is less expensive and
more convenient to perform preventive maintenance on a component in a system
when performing a repair action on another component in the system rather
than at the optimum time for performing the preventive maintenance for that
component. A repairable system reliability and availability modeling and
simulation tool, such as
BlockSim 7,
can help model your system and assess different maintenance strategies to
determine what will be best for the overall system. This article discusses
how to model grouped and opportunistic maintenance strategies in BlockSim.
What is Meant by Grouped and
Opportunistic Maintenance?
Suppose that you take your car to
the mechanic shop to replace a timing belt. You then get a call from the
mechanic asking you whether you would like to have the water pump replaced
(preventively) because it is a good opportunity to do it now rather than
later. The water pump is located close to the timing belt. It might be
cheaper and more convenient to change the water pump now, since most of the
required labor is complete, rather than waiting for the recommended
preventive replacement time for the water pump, taking your car into the
shop again and having parts removed from the engine to be able to access the
water pump. You consider all costs and inconveniences related to the labor
and downtime caused by having to take your car to the mechanic again, in
addition to the increased risk of failure. You also consider utilizing the
additional life you can get from the existing water pump. You might find
that it is in your benefit to accept the mechanic's recommendation. Was this
just a mechanics' gimmick? Or is this a viable preventive maintenance
strategy? It may be more of the latter.
The purpose of grouped and opportunistic
preventive maintenance is to take advantage of the resources, efforts and
time already dedicated to the maintenance of another part in the system in
order to cut cost. It is typically possible and more beneficial when these
parts are located close to each other in the system and no significant
additional work is needed to perform the second maintenance activity. It is
also more commonly applied in cases where significant travel cost is
incurred or major disassembly of the system is required or, when the parts
are not too expensive, they may be disposed of even before they are due for
maintenance. It can also be applied in situations in which waiting beyond
the recommended PM time to actually perform the maintenance (i.e.
waiting for the next opportunity) does not present high risks to the system.
This concept may be expanded by performing the preventive maintenance every
time the system is down, not just when certain parts are being repaired.
Such maintenance strategies are performed
in many areas and industries, including manufacturing, military, automotive,
infrastructure, capital equipment, etc. The approach has many benefits in
addition to being convenient and making fewer interruptions to the system.
It could also reduce the chances of human error and other problems that
arise from frequent intrusions and disruptions to the system.
Example
Let us consider the following
repairable system's reliability block diagram (RBD).
The owner of the system performs repairs
and preventive maintenance actions to extend the system's life. The company
sends a repair crew to perform maintenance (corrective and preventive) and
is concerned about its significant traveling costs ($400 per incident).
The next table displays the failure and
repair properties of the system's components.
|
Parts |
Failure Distribution |
Replacement Labor
Duration |
Part Cost |
|
A |
Exponential (Mean = 900 hrs) |
2 hr |
$100 |
|
B(*) |
Weibull (β = 2, η = 500 hrs) |
1 hr |
$20 |
|
C |
Exponential (Mean = 500 hrs) |
2 hr |
$240 |
Notes:
-
Parts A and C follow an exponential
distribution, which makes preventive maintenance for these parts useless
(more on this can be found
here.)
-
(*):The
duration given is for
the replacement of all four B parts.
The optimum replacement time for each of
the B components can be calculated using the corrective (unplanned) and
preventive (planned) replacement costs and the failure distribution. (This
analysis is not the focus of this article, more on this computation can be
found
here.) The unplanned cost would include part cost and travel cost,
whereas the planned cost would include the part cost (since preventive
maintenance will be performed when the crew is already at the system, as we
will see later in this example). The optimum replacement time for each of
the B components is calculated to be every 112 hr.
The optimum replacement time of 112 hr is
calculated with the goal of optimizing the maintenance cost of the B parts,
independent of the rest of the system. The grouped and opportunistic
maintenance concept, on the other hand, tries to optimize the cost of
maintaining the whole system. To illustrate the application of this concept,
we compare three different maintenance strategies:
-
Strategy 1: Perform preventive
replacements for the B parts upon a fixed time interval calculated based on
the component's optimum replacement time (i.e. every time a B part
accumulates 112 hr of operation, it gets preventively replaced).
-
Strategy 2: Replace all the B parts
whenever any of them fails or whenever part A fails.
-
Strategy 3: Perform preventive
replacements for all of the B parts whenever the system experiences a
downing failure (i.e. when all of the B parts fail, when A fails or
when C fails).
Note: In this example and for the sake of
simplicity, we limited the maintenance strategy options to these three.
Readers are welcome to try other variations or combinations of the plans
listed above.
For this example, we are interested in the
system's operation for 10,000 hr.
Maintenance Strategy 1:
We set up this maintenance strategy
in BlockSim by
creating a Preventive Maintenance Policy and using the fixed
preventive replacement time of 112 hr for each item, as shown next.
This policy is then linked to the B part by
selecting this as the Preventive Maintenance Policy in the
Preventive tab of the block's property window as shown next (a similar
step is to be repeated for all the B parts and in the assessment of
Strategies 2 and 3).
We assume that each time the B parts needs
to be preventively replaced, only one trip is required by the crew. The B
parts are replaced one part at a time to avoid bringing the system down.
Maintenance Strategy 2:
We set up this maintenance strategy
by creating a Preventive Maintenance Policy and using the option of
performing maintenance Upon Maintenance of another Group Item (part
A), as shown next.
The B parts are grouped with part A using
the Item Group # property, by giving the A and all B parts the same
number (using a number other than 0, since 0 means no groups).
In this scenario, the crew proceeds to
replacing the B parts (one part at a time) right after completing the
maintenance of A. No second trip is required.
Maintenance Strategy 3:
We set up this maintenance strategy
by creating a Preventive Maintenance Policy and using the option of
performing maintenance Upon System Down, as shown next.
In this scenario, the crew proceeds to
replacing the B parts (one part at a time) right after completing the
maintenance of the part that brought the system down. A second trip is not
required.
Comparison
We simulate the system for 10,000 hr. In this example, we will use
maintenance cost (parts and travel cost) in addition to downtime cost
($500/hr) as our metric to be used for assessing the maintenance strategies.
|
Maintenance
Strategy |
Total Parts
Failures |
Total Number of
Corrective Replacement Trips |
Total Number of
Preventive Replacements |
Total System
Corrective Replacement Downtime |
Total Corrective
/ Preventive Replacement Trips |
Total Cost |
|
Strategy 1 |
$13,009.33 |
48.28 |
338.54 |
62.33 hr |
386.82 |
$198,900.66 |
|
Strategy 2 |
$10,012.47 |
71.72 |
41.67 |
61.70 hr |
71.72 |
$69,546.75 |
|
Strategy 3 |
$9,645.53 |
87.63 |
31.53 |
63.12 hr |
87.63 |
$76,255.71 |
The values in the table above were taken
from the System Overview and Block Summary
reports obtained as part of
BlockSim 7's simulation
results.
The total cost is calculated by multiplying
the number of "trips" by the travel cost and adding the parts cost and
downtime cost. In the case of Strategy 1, the number of trips is the number
of corrective replacements added to the number of preventive replacement
trips (because in this strategy, the preventive maintenance gets a dedicated
trip). Also in the case of Strategy 1, the number of preventive maintenance
trips is the sum of the number of preventive replacements performed for each
of the B parts (because in this strategy, each B part gets a preventive
replacement scheduled independently from the others). In the case of
Strategies 2 and 3, the number of trips is the number of corrective
replacements only (because in these strategies, the preventive maintenance
is performed along with the corrective maintenance). Strategy 2 emerges as
the most economical strategy for the system overall.
The system's incurred downtime is that of
corrective maintenance only since the preventive replacement of the B parts
can be performed in a sequential fashion and the system won't be downed.
Note: This article does not intend to imply
that the grouped and opportunistic maintenance is always a preferred
maintenance strategy. In other scenarios, different types of strategies
might be more appropriate. Our goal in this article was to illustrate how
such grouped and opportunistic maintenance options can be modeled and
analyzed using
BlockSim 7.
Additional Comments
In other cases, the grouped and opportunistic preventive replacement or
maintenance might be improved with inspection activities to achieve even
more flexibility in maintenance plans and cost savings. For example, in the
scenario of the timing belt and water pump, the mechanic could first check
if the water pump is getting close to failure before recommending the
preventive maintenance. To model such a situation, we need a way to set up
an additional condition (i.e. checking whether a failure is imminent
in the water pump). This can be performed in
BlockSim 7 by
performing on-condition maintenance (on-condition maintenance modeling and
analysis was covered in a Hot Topics article in the
June 2007
issue of Reliability HotWire).
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