Page 622 - Marine Structural Design

598 Pari VRiskAssessment

Faiiure characteristic Failure propagation may be categorized as:
Gradual failure: The progress of failure propagation can be measured by inspection or
condition monitoring techniques.
Aging failure: The failure propagation is agedependent, i.e. a wear-out process.
Sudden failure: Random failure that may not be detected by condition monitoring
measures.
Maintenance method: May hopefully be found by the logic decision applied in Step 5. This
field shall be completed in Step 5.
Recommended tad interval: The identified maintenance action should recommend an
estimated time interval, which shall be performed in Step 5.
Step 4: Selection of Maintenance Methods
Decision logic is designed and used to guide the RCM team through a question-and-answer
process. The input to the decision logic is the dominant failure mode identified in Step 1. The
design of decision logic is based on the principle: preventive maintenance measures should be
specified whenever they exist and are cost-effective against a critical failure.
There are generally three reasons for applying a preventive maintenance task
To detect failures at their early stage in order to have more time to plan and execute
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preventive measures
To prevent equipment to fail with serious consequences
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To discover hidden failures
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Only the critical failure modes shall be subjected to preventive maintenance. The selection of
appropriate maintenance methods is dependent on the following factors:
Failure causes and mechanisms
Failure characteristics
Detection techniques
The basic maintenance methods may be classified as follows:
Scheduled on-condition task
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Scheduled overhaul
Scheduled replacement
Scheduled function test
R~ntoFail~re
Scheduled on-condition task is a scheduled inspection or condition monitoring of an item at
regular intervals to find potential failures. The following criteria shall be met:
Potential failure condition can be clearly defined
Potential failures can be detected by a condition monitoring technique
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There is a reasonable consistent time interval for failure detection and prevention.
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