Environmental Engineering Reference
In-Depth Information
3
Gradable - In addition to being visually identi-
fiable, the current condition of the featuremust be
able to be graded visually. Theremust be sufficient
visual indictors related to the PF in order to assign
the range of condition values associated with that
PF.
4
Mutually exclusive - Ideally, PFs should be
mutually exclusive. There should be no chance
of mistaking the condition of one PF for another.
However, if necessary, this attribute can be re-
laxed to some degree in order to satisfy the more
important attributes 1, 2 and 3.
Performance features can apply to a single ele-
ment of the asset and therefore be repeated for each
element where relevant or can apply to the whole
asset.
A PF could relate to a number of failure modes
or be associated with a single failure mode. Per-
formance features that are uniquely associated
with a single failure mode are important in the
differentiation of most likely failure mode and
usually will have a high contribution rating link-
ing them to that failure mode. An example of this
is the class of PFs relating to deformation of the
asset structure or cross-section.
The choice of performance feature is partly
determined by what is possible to assess visually
and also by the failure modes that need to be
identified. Performance features must be chosen
that will identify these failure modes directly or
indirectly. Consistency of the method wherever
possible is another factor considered in the iden-
tification of appropriate performance features. It is
necessary to produce a set of performance features
that balance accuracy of inspection with imple-
mentational issues such as the workload associ-
ated with inspecting an individual asset. Too large
a set of performance features, though possibly
producing amore accurate condition index, would
increase the duration of an inspection beyond
reasonable levels.
of the condition rating should allow for the in-
spector to discriminate condition to an appropri-
ate level of detail between the boundary
conditions: excellent (high performance) and very
poor (very low or zero performance). It was deter-
mined that the value range for the condition in-
dexing method should match the existing
condition values used (1-5) to ease any transition
process in implementing a new method of visual
inspection.
A slight change in the definitions associated
with each of the 1-5 grades was needed to reflect
the move from a method based purely on visual
condition to one utilizing visual condition to assess
likely performance. The entire 1-5 range of condi-
tion applies to all performance features inspected
by the revised visual inspection method.
Contribution
Contributions represent the relative influence of
a performance feature on a failure mode. They
are the crucial link between performance and
visual inspection of condition. Contributions
are predefined values obtained from sensitivity
analysis of performance models or, in the absence
of performance models, from collective expert
judgment. Contributions are used to calculate
the failure mode indices for an asset as described
later.
Any value scale could have been chosen to
represent the contribution providing it gave en-
ough differentiation of value to express the rela-
tive influence of all performance features or failure
modes. For the sake of clarity and ease of use it was
decided to use a contribution value range of 0-1
where the total of all contributions identified for
each failure mode was equal to one. This ensures
that all failure modes are weighted by the same
total amount of contribution. It also simplifies the
calculation of the failure mode indices as ex-
plained later.
Themain obstacle in the use of contributions is
obtaining accurate contribution values that reflect
actual performance. The lack of accurate and
extensive performance modelling data led to the
use of expert opinion to obtain contribution value
Condition rating
A condition rating is a numerical value represent-
ing the current condition of a performance feature
determined by visual inspection. The value range
