One
of a number of
different types of asset replacement
policies.
Replacement
of an asset before Functional Failure ("F")
occurs but after Potential
Failure ("P") has been detected. It is the equivalent of an optimal interval.
Attributes
Listed below are some of the key attributes of a preventive replacement
policy:
Candidate Assets
Listed below are the types of
assets and their attributes that are best suited to this type of
approach:
- Bounded assets
- Assets that are considered bounded
assets and subject to statutory maintenance
requirements.
- Assets
subject to fatigue failure - Assets with a Fatigue
Failure Pattern
- Assets
subject to wear-out failure - Assets with a Wear-out Failure
Pattern
- Assets
subject to Break-in Failure - Assets with an
Initial-Break-in Failure Pattern.
- Critical
assets - Assets for which the financial, business or
service level consequences of
failure are sufficiently severe to justify proactive inspection and
rehabilitation. Critical
assets require more detailed attention with lower thresholds
for action than non-critical assets.
- Assets that
are amenable to Time-Based Maintenance -
- Maintenance
worthy items -
- Assets with a
high utilization index - Asset utilization
index.
- Medium-life
and long-life assets. Long
life assets. has a life of greater than 30-years.
- Assets with a
wide dispersion pattern
- The probability distribution on the survivor curve has
a wide
dispersion so there is little predictability with respect to the
critical year.
- Assets that
suffer covert failure - Covert failure and concealed
conditions require proactive steps to reveal condition.
- Assets with a
left-modal curve - A left modal curve.
Types and Variations
There
are essentially three types of preventive replacement policies, which
can be summarized as follows:
Evaluation
Some of the merits of the preventive replacement approach are
summarized below:
- Economies
- The owners extract the fullest possible
life out of the asset and avoid any opportunity cost of replacing the
asset early (ie., "reaching
life")
- Optimization
- The owners enjoy the benefits of an optimal interval.
- Damage
Mitigation - Mitigation of potential collateral damage.
- Cost Efficiencies
- Reduced costs due to formal tendering process well in advance of
functional failure.
- Reduced Risk
- This approach can potentially lower risk
to the building owners and other stakeholders.
- It lowers the stochastic load and improves the owners ability to anticipate and plan for future cots.
Some of the
limitations and consequences of the preventive replacement approach are
outlined below:
- Stochastic
assets require art as mush as science - This approach is
particularly
challenging because it is difficult to predict the exact point of
functional failure of an asset
- Random
Failure Patterns - Some assets have random failure patterns
that cannot be easily anticipated.
- Sophistication
- This approach requires a highly sophisticated asset management team.
- Capital
Intensive
- The diagnostic technologies required by preventive replacement are
capital intensive and the training of staff is also expensive.
- Onerous
Requirements - It requires a sophisticated asset
management program which is both
labour intensive and capital intensive.
- Detailed
Recordkeeping - This approach relies on detailed recordkeeping of
operating histories.
- Procurement -
Some assets have long lead times to procurement of replacement parts
so waiting until the last minute may be false economy.
- Predictability
- It is difficult to anticipate assets that are stochastic in nature.
- Persuasion
- It may be difficult to persuade the owners on the merits of proactive
replacement.
Management
Listed
below are some of the concepts to incorporate some of the principles of
preventive replacement into the overall replacement strategy.
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Fig. The two
classes of replacement strategies and summary explanation of the five
alternative replacement strategies.
Fig.
Asset replacement policies must align to asset risk profiles in order
to achieve optimization and satisfy ISO 55001 requirements.
Fig. A balanced asset replacement mix helps the organization to achieve optimization in conformity with ISO 55001 principles.
Fig.
Network diagram of different asset replacement policies organized into
the two broad classes of failure replacement and preventive replacement.
Fig. Preventive
replacement mapped onto the P-F curve to illustrate the relationship
with functional failure (F).
 
Fig.
Examples of different types of asset replacement projects - roof
replacement, repiping, pool basin refinishing and sealant renewal.
Fig.
The zone of BER represented conceptually on the P-F curve, typically in
the months or years preceding functional failure ("F") of the asset.
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