Hydrocarbon Processing - June 2021 - 82

Maintenance and Reliability
TABLE 1. Information for risk mitigation
Information category
Failure effects
Failure characteristics
PM
Location in criticality assessment and/or FMEA
Severity ranking
Occurrence ranking
Corrective action
(FMEA only)
Required information
* Production impact
* Repair duration (MTTR) and
cost (labor, tools, material)
* Failure mechanism
* MTBF
* PM duration and cost
(labor, tools, material)
Desired information
* Logistic delay
* Rental and backup cost
* Repair effectiveness
* Failure history
* Age
* Expected end of life
* Existing PM
* Maintenance history
* PM frequency
* Shutdown schedule
* PM effectiveness
PdM
Spare part
Redesign
Detectability ranking
(FMEA only)
Recommendation
Recommendation
* Labor and tools cost
* Probability of detection
* Spare cost
* Lead time
* Logistical delay
* Cost of redesign
* Redesign effectiveness
Strategy
RCA
PM/PdM
Operators'
rounds
Spare part
Redesign
TABLE 2. Risk mitigation implementation cost
Risk mitigation implementation cost
* Engineering hours for RCA
* Implementation cost (strategy-dependent)
* PM/PdM development
* PM/PdM training, labor, materials and tools
* Lost production opportunity if shutdown is required
* Operators' training and labor
* Development of operators' checklist
* Development of DCS parameters for condition
monitoring
* Spare part purchase and storage
* Spare part obsolescence
* Redesign engineering hours
* Initial investment
1. Perform failure mode pre-screening
2. Collect information for prioritization
3. Quantify risk mitigation benefits for top-failure modes
4. Perform resource allocation optimization
5. Summarize results and make recommendations.
Failure mode pre-screening. A pre-screening helps narrow
down the risk mitigation candidates efficiently. In general,
failure modes satisfying the following conditions should be selected
for the next step (prioritization):
* Associated with components with high or medium
criticality ranking
* Relatively high RPN scores prior to risk mitigation
(identify a cutoff value based on overall RPN distribution)
* Post vs. prior risk mitigation RPN scores have the largest
percentage reduction.
In addition, consider failure modes that map to a large number
of components (functional locations) in the plant. In this
case, the risk mitigation benefit on one component may have a
large multiplier effect.
82 JUNE 2021 | HydrocarbonProcessing.com
* Existing PdM
* P-F interval1
* Storage cost
* Part shelf life
* Post redesign RPN
Information collection for prioritization. Provided that
the criticality assessment and the FMEA are performed thoroughly,
the majority of the information required for risk mitigation
prioritization is already available. TABLE 1 summarizes the
required and desirable information to be collected for the failure
modes selected.
In TABLE 1, the required information is typically included in
the standard output of the criticality assessment and the FMEA.
By referring to the criticality assessment or FMEA ranking matrix,
the approximate values of the financial loss (severity), the
probability of occurrence or the detectability can be obtained
for a given component or failure mode. To allow easy and accurate
decision-making, the information should be as specific
as possible. For example, a loss of $100,000/occurrence is preferable
to a loss of $50,000/occurrence-$250,000/occurrence,
although the latter can be estimated statistically.
Depending on the level of detail of the criticality assessment
and FMEA results, the desired information may take extra effort
to obtain. However, including that information in the calculation
would increase the confidence level of the prioritization.
Risk mitigation benefit quantification. Depending on the
type of information listed in TABLE 1 that can be collected, the
amount of risk mitigation for a failure mode can be estimated accordingly.
For each type of risk mitigation strategy, the benefit can
be calculated by the difference between the lifecycle cost (LCC)
prior to the risk mitigation and the LCC after the risk mitigation,
plus the cost of implementation of the strategy, as shown in Eq. 1:
Benefit = LCCPrior - (LCCPost
+ Implementation cost)
(1)
For failure modes with a constant failure rate (i.e., random
failure), the calculation of LCC is straightforward. Otherwise, a
software capable of performing random lifecycle simulation can
be used. Other advantages of using this type of software include
its capability to handle random input parameters and address
imperfect repairs or failure detection.
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