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the entire life of the development; provided (IEV 191-02-05 [8]). There exist a number
• identification of critical production loss events of availability classifications. Production availability
is the ratio of production to planned production
due to simulated failures of components; (e.g. commonly featured by simulating models
• recommendations for design changes; using Monte Carlo). Inherent availability considers
• guidance for future Opex allocations in the form only the corrective maintenance downtime of a
system. Operational availability is a measure of the
of the frequency and duration of unplanned average availability over a period of time including
maintenance requirements; all sources of downtime (i.e. logistics, mobilisation
• quantification of the impact of operations and etc.).
maintenance philosophies. For a repairable system, commonly used to model
systems in the oil & gas industry, the inherent
The following sections introduce typical concepts availability can be expressed by eq. 1 (Smith, 1997
and terminology used in RAM analyses. [7]).
Reliability definitions and MTBF
terminology A = --------------------------------------------------- (1)
Before discussing the process of RAM analyses, MTBF + MTTR
it is important to first establish some important
definitions and terms. where:
MTBF = Mean Time to Failure (MTTF) + MTTR
Failure and failure mode However, normally the MTTF is much greater than
the MTTR and thus MTBF ≈ MTTF.
A failure can be defined as the non-conformance
to some defined performance criterion, where as a Production availability
failure mode can be defined as the effect by which
a failure is observed on the failed item (Smith, 1997 Production Availability is a measure of the actual
[7]). performance of a production system accounting
for production loss due to planned an unplanned
Reliability outages against the potential production of the
system given no outages have occurred within a
Reliability is the ability of an item to perform a given system life. Thus production availability can
required function under given conditions for a given be expressed as the percentage:
time interval (IEV 191-02-06 [8]). Reliability can
be thought as the probability of conformance to actual production allowing for planned (2)
a specific function over a specific time. Reliability and unplanned downtime
is most commonly defined in association with
the Mean Time between Failures (MTBF), this ------------------------------------------------------------------------------------------ × 100
concept will be further discussed in the subsection potential production with
Availability. no downtime
Maintainability Bath tub curve
Maintainability is the ability of an item under given The failure rate λ(t) is a function that describes the
conditions of use, to be retained in, or restored number of failures that can be expected to take
to, a state in which it can perform a required place over a given period of time. To describe the
function, when maintenance is performed under failure rate the bath tub curve, figure 4 (Amstadter,
given conditions and using stated procedures and 1977 [9]), is used commonly in reliability engineering.
resources (IEV 191-02-07 [8]). Maintainability can be The failures exhibited in the first part of the curve,
thought as the typical time spent fixing a problem, representing a decreasing failure rate, are termed
allowing it to get back into service. Maintainability early failures or infant mortality failures. The middle
is most commonly defined in association with the section, representing a constant failure rate, relates
Mean Time to Repair (MTTR), this concept will be to failures that occur randomly during the normal life
further discussed in the subsection Availability. of the component. The final section, representing
an increasing failure rate, is termed wearout failures
Availability as increased failures occur towards the end of the
components useful life.
Availability is the ability of an item to be in a state to Depending on the failure data available, RAM
perform a required function under given conditions analyses can assume a variety of distributions to
at a given instant of time or over a given time interval, describe the system most accurately. Commonly
assuming that the required external resources are a constant component failure rate is assumed
92 Impiantistica Italiana - Maggio-Giugno 2015
