Hydrocarbon Processing - June 2021 - 81

Maintenance
and Reliability
T. ZOU, Houston, Texas, T.A. Cook
Risk mitigation procedure following
criticality assessment and FMEA
In asset performance management (APM), criticality assessment
and failure mode and effects analysis (FMEA) are
powerful tools that enable the identification of opportunities
to minimize asset lifecycle costs during the development of
equipment strategies.
Here, the author proposes a risk mitigation procedure that
prioritizes tasks based on the criticality assessment and FMEA
results, considers multiple aspects of failure effects and aims to
provide data-driven solutions to maximize the risk mitigation
benefit and return on investment (ROI).
Criticality assessment and FMEA. A criticality assessment
is used to chart the probability of a failure against the severity of
its consequences with regard to organizational impacts, such as
safety, environmental or production consequences. When performed
at the functional location level (in most cases, an individual
component), a criticality assessment identifies the worst
combination of severity and occurrence against a predetermined
risk matrix. The results highlight one or several groups of
" critical " components, allowing remedial efforts to be directed
to mitigate the most prominent risks.
FMEA is a bottom-up, inductive analytical method that is
typically performed at the failure mode level. When previously
assessed criticality results are available, the information can be
used as a reference to speed up FMEA development. Adding
the failure mode detection rankings to the criticality results
yields a risk priority number (RPN). RPNs can be calculated
before and after risk mitigation for a specific failure mode.
In general, multiple failure modes (around 10, on average) are
associated with a given component, and the risk mitigation tasks
identified by the FMEA are usually a much longer list at a more
detailed level, compared to those from the criticality assessment.
Risk mitigation benefit realization. Depending on the findings
of the criticality assessment and the FMEA, one or several
risk mitigation strategies can be applied to each risk item:
* Root cause analysis (RCA): This strategy applies
to failures associated with very high impact, where the
root causes are unknown. Only with the completion
of the RCA can mitigation strategies be developed
for certain risk items.
* Preventive maintenance (PM): This strategy brings the
most benefit to certain wear-out failure modes where it is
difficult to effectively monitor damage conditions.
* Predictive maintenance (PdM): These typically
nondestructive inspections can predict an upcoming
failure (with a probability) and trigger a preventive
maintenance action. Some examples are periodic vibration
analysis and/or oil sample analysis for rotating equipment.
* Operators' rounds: These inspections are conducted by
equipment operators, sometimes referred to as operatordriven
reliability. Compared to PdM, these tasks are
conducted more frequently and involve more visual and
auditory observation and less technology.
* Spare part strategy: Having critical spare parts available
may significantly reduce the mean time to repair (MTTR)
upon an equipment failure, thereby lowering the severity
of the failure. This is especially important for parts with
long lead times.
* Redesign: When the risk cannot be effectively or
economically mitigated by any of the prior strategies,
a redesign may be the last resort. An example is adding
a standby pump to form a redundant pump system
at a process-critical location.
* Risk acceptance: If no risk mitigation strategy, including
redesign, is able to address the risk item effectively, then
the best option is to accept the risk. This is sometimes
referred to as " run to failure. " Risk acceptance often
applies to components with lower criticality and should
be assessed on a case-by-case basis.
Each of the outlined strategies (except for risk acceptance)
has a potential benefit, either financially or with respect to environment,
health and safety (EHS) risk. However, the benefit
is not realized until the risk mitigation tasks are implemented.
In a typical plant-level APM project, a large number of failure
modes may be identified as candidates for risk mitigation. Most
clients face the challenge of limited human resources, including
engineers, technicians and planners, as well as limited budget
for capital projects and spare parts. To best utilize limited resources,
it is important to develop and follow a procedure that
helps prioritize risk mitigation tasks.
Procedure to prioritize risk mitigation tasks. The following
proposed procedure starts after the completion of the criticality
assessment and the FMEA and ends with a list of recommended
tasks for implementation:
Hydrocarbon Processing | JUNE 2021 81
Hydrocarbon Processing - June 2021

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