RELIABILITY ENGINEERING - PREVENTIVE MAINTENANCE

 2.2. RELIABILITY ENGINEERING - PREVENTIVE MAINTENANCE   

By Aleksandar Pudar

Technical Superintendent and Planned Maintenance Supervisor Reederei Nord BV

Co-founder of "Out of Box Maritime Thinker Blog" and Founder of Naro Consilium Group

As the term suggests, preventive maintenance comprises a range of tasks tailored to avert the necessity for corrective or breakdown maintenance while extending the operational life of a vessel's primary and auxiliary equipment. In the context of the marine industry, preventive maintenance programs typically encompass a combination of inspections, cleaning, adjustments, lubrication, and related tasks that contribute significantly to maintaining the reliability of critical marine assets.

Reliability-based preventive maintenance adapts this approach to the marine industry, focusing on tasks that directly prevent failures and extend the operational life of a vessel's assets. By replacing non-essential tasks with targeted maintenance activities, this methodology enhances the reliability and performance of a ship's primary and auxiliary systems.

Developing a reliability-based preventive maintenance program for vessels involves risk assessment logic and work/job selection criteria as its primary tools; these form the basis for evaluating each functionally significant equipment/unit  (FSE/U) - Critical Equipment using all available technical data and the expert knowledge of the crew. The evaluations mainly focus on these items' functional failures and failure causes. The process consists of the following steps:

·         Identification of FSE/Us - Critical Equipment

·         Identification of applicable and practical preventive maintenance tasks using decision tree logic

An FSE/Us - Critical Equipment is equipment whose failure could impact safety and operations or have significant economic consequences in a specific maritime context. Identifying FSE/Us - Critical Equipment relies on analysing the anticipated failure consequences using an analytical approach and sound engineering judgment. This process employs a top-down approach, starting at the system level, then moving to the subsystem level, and finally, where necessary, examining the component level. In addition, iterative processes are used to identify FSE/Us - Critical Equipment by first determining system boundaries and functions, enabling the selection of critical systems for further analysis. This analysis involves a more detailed examination of the system, its functions, and its functional failures.

The procedures for information collection, system analysis, and other related tasks outline a comprehensive set of activities in the FSE/Us - Critical Equipment identification process; these tasks should be applied in the case of complex or new equipment. However, the system analysis tasks can be completed quickly for well-established or simple equipment with well-known functions and failures. Regardless, these considerations should be documented for verification purposes. The depth and rigour of these tasks will vary depending on the equipment's complexity and novelty.

 

Flow 2.1 Development steps - reliability-based preventive maintenance

Table 2.1 Job/Work Set-Up Criteria

 

2.2.1 INFORMATION GATHERING FOR VESSEL SYSTEMS/EQUIPMENT

A comprehensive understanding of the vessel's equipment and systems is essential for an accurate assessment. Before initiating the evaluation, collecting relevant information and updating it as necessary is crucial. Key components to include in the information-gathering process are:

·         Regulatory and operational requirements for the vessel's equipment and related systems

·         Documentation related to the design, construction, and maintenance of the vessel's components

·         Data on system performance, including maintenance records and failure incidents

For a thorough and successful evaluation, it is best to assess the vessel's equipment and systems in a systematic and organised manner. This approach eliminates redundancies and ensures a comprehensive assessment.

 

2.2.2 VESSEL/MARINE FACILITY SYSTEM/EQUIPMENT ANALYSIS

 

The processes outlined in the previous section (Information Gathering for Vessel Systems) establish the framework for identifying functionally significant components and selecting appropriate maintenance tasks for implementation on the vessel. However, it is essential to recognise that these tasks can be customised to suit the specific needs of the marine industry. Therefore, the emphasis placed on each task will vary depending on the unique characteristics and requirements of the sector.

 

2.2.3 IDENTIFYING VESSEL SYSTEMS

 

This task divides the vessel's equipment into different systems, grouping components contributing to specific functions and defining system boundaries. In some cases, it may be essential to further break down these systems into subsystems responsible for critical functions affecting overall system performance. It is important to note that system boundaries may overlap and may not always align with the physical boundaries.

Often, equipment is already divided into systems through industry-specific classification schemes. It is crucial to review and adjust this partitioning, if necessary, to ensure it is focused on functionality. Document the results of this equipment partitioning in a master system index that outlines the systems, components, and boundaries.

 

2.2.4 IDENTIFYING VESSEL SYSTEM FUNCTIONS

This task aims to ascertain the primary and secondary functions carried out by the vessel's systems and subsystems. Utilising functional block diagrams can aid in identifying these functions. The function definition outlines the actions or requirements the system or subsystem must fulfil, often expressed in terms of performance capabilities within specified boundaries. Functions should be identified for all equipment operation modes.

Review design specifications, descriptions, and operating procedures to determine primary and secondary functions, including safety protocols, abnormal operations, and emergency instructions. Functions related to testing or maintenance preparations may be excluded if deemed unimportant, but the reasons for such omissions should be documented. The outcome of this task is a comprehensive list of system functions.

2.2.5 SELECTION OF VESSEL SYSTEMS

This task aims to choose and prioritise systems for the reliability-centred maintenance (RCM) program based on their importance to vessel safety, availability, or cost-efficiency. Methods for selecting and prioritising systems can be categorised into:

·         Qualitative methods, which rely on historical data and collective engineering judgment

·         Quantitative methods, based on criteria such as criticality rating, safety factors, probability of failure, failure rate, life cycle cost, etc., to evaluate the impact of system degradation or failure on vessel safety, performance, and costs. Implementing this approach is more manageable when appropriate models and databases are available.

·         A combination of qualitative and quantitative methods.

The outcome of this task is a list of systems ranked by their criticality. The chosen systems, along with the methods, criteria used, and results, should be documented.

2.2.6 VESSEL SYSTEM FUNCTIONAL FAILURES AND CRITICALITY RANKING

This task aims to identify and prioritise functional degradation or failures of vessel systems. Each system function's functional degradation or failures should be recognised, ranked by criticality, and documented.

As each functional system failure may have varying impacts on safety, availability, or maintenance costs, ranking and prioritisation are necessary. The ranking process should consider the probability of occurrence and the consequences of failure. Qualitative methods based on collective engineering judgment and analysis of operational experience can be employed. Alternatively, quantitative methods such as simplified failure modes and effects analysis (SFMEA) FAult Source Identification Tool (FASIT) and risk analysis may be used.

The ranking is a crucial aspect of RCM analysis. Overly conservative rankings may lead to an excessive preventive maintenance program, while lower rankings could result in increased failures and potential safety risks. In both cases, a non-optimised maintenance program will arise. The outcomes of this task include:

·         A list of system functional degradation or failures and their characteristics.

·         A ranking list of system functional degradation or failures.

2.2.6.1 IDENTIFICATION OF FSE/Us - CRITICAL EQUIPMENT FOR VESSELS

By examining system functions, functional degradation or failures, and their effects, and utilising collective engineering judgment, it is feasible to identify and compile a list of potential FSE/Us - Critical Equipment for the marine industry. As previously noted, these are items whose failures could influence safety, remain undetected during standard vessel operation, have considerable operational consequences, or have significant economic implications. The outcome of this task is a list of candidate FSE/Us - Critical Equipment for the vessel.

2.2.6.2 FSE/Us - CRITICAL EQUIPMENT FAILURE ANALYSIS

Once a Vessel  FSE/Us - Critical Equipment list has been developed, a method such as Failure Modes and Effects Analysis (FMEA) or FAult Source Identification Tool (FASIT) should be employed to identify the necessary information for the logic tree evaluation of each Critical Equipment. The following examples refer to the failure of a pump providing cooling water flow to the Main engine:

·         Function:

The normal characteristic actions of the equipment (e.g., to provide cooling water flow at 0.8-2.3, Cube Meters per minute to the heat exchanger).

·         Functional failure:

How does the equipment fail to perform its function (e.g., the pump fails to provide the required flow)?

·         Failure cause:

Why the functional failure occurs (e.g., bearing failure)?

·         Failure effect:

It is important to consider both the immediate effects and the broader consequences of functional failures, such as inadequate cooling that can lead to overheating and system failure.

The Critical Equipment failure analysis aims to identify functional failures and failure causes. Failures considered not credible, such as those resulting solely from undetected manufacturing faults, unlikely failure mechanisms, or rare external occurrences, should be documented as having been considered. In addition, the reasons for deeming them not credible should be stated.

Before applying the decision logic tree analysis to each Critical Equipment, complete preliminary worksheets that clearly define the equipment, its functions, functional failures, failure causes, failure effects, and any additional relevant data (e.g., manufacturer's part number, a brief description of the item, predicted or measured failure rate, hidden functions, redundancy, etc.). These worksheets should be designed to meet the user's requirements.

From this analysis, the Critical Equipment can be identified (i.e., those with both significant functional effects and a high probability of failure, or those with a medium probability of failure but considered critical or having a notably poor maintenance record).

 

2.2.6.3 MAINTENANCE TASK SELECTION (DECISION LOGIC TREE ANALYSIS)

Identifying applicable and practical preventive maintenance tasks involves providing a logical path for addressing each Critical Equipment's functional failure. The decision logic tree uses a series of sequential "YES/NO" questions to classify or characterise each functional failure. The answers to these questions determine the direction of the analysis flow and help identify the consequences of the Critical Equipment's functional failure, which may differ for each failure cause. Further progression of the analysis will determine if there is an applicable and effective maintenance task that can prevent or mitigate the failure. The resulting tasks and related intervals will form the initially scheduled maintenance program.

 

Note: Conducting the logic tree analysis with inadequate or incomplete Critical Equipment failure information may lead to safety-critical failures due to inappropriate, omitted, or unnecessary maintenance, increased costs due to unnecessary scheduled maintenance activity, or both.

2.2.6.3.1 Levels of Analysis

Two levels are apparent in the decision logic.

1.       The first level (questions 1, 2, 3, and 4) requires an evaluation of each functional degradation/failure to determine the ultimate effect category, such as evident safety, evident operational, evident direct cost, hidden safety, hidden non-safety, or none.

2.       The second level (questions 5, 6, 7, 8, and 9, A to E, as applicable) considers the failure causes for each functional degradation/failure to select the specific type of work/job.

First Level Analysis—Determination of Effects:

The consequence of failure, which could include degradation, is evaluated at the first level using four basic questions.

Note: The analysis should only proceed through the first level if there is a full and complete understanding of the particular functional failure.

Flow 2.2 Reliability decision logic tree (level 1)—effects of functional failures

 

Question 1—Evident or hidden functional failure?

This question aims to differentiate between evident and hidden functional failures in vessel systems and components. Therefore, this question should be asked for each functional failure.

Question 2—Direct adverse effects on maritime safety?

To be direct, the functional failure or resulting secondary damage should achieve its effect by itself, not in combination with other functional failures. An adverse effect on maritime safety implies that damage or loss of vessel equipment, human injury or death, or a combination of these events will likely result from the failure or secondary damage.

Question 3—Hidden functional failure safety effect?

This question considers failures in which the loss of a hidden function (whose failure is unknown to the crew). This type of failure does not directly affect safety, but combined with an additional functional failure, it adversely affects maritime safety.

Note: The crew consists of all qualified staff on duty and directly involved in the vessel's operation.

Question 4—Direct adverse effect on vessel operation?

This question asks if the functional failure could have an adverse effect on vessel operation:

·         Requiring either the imposition of operating restrictions or correction prior to further operation

·         Requiring the crew to use abnormal or emergency procedures

Second Level Analysis—Effects Categories. Applying the decision logic of the first-level questions to each functional failure leads to one of five effect categories, as follows:

Apparent safety effects—Questions 5A to 5E.

This category assumes a work/job (or multiple) is required to ensure safe operation. Therefore, all questions in this category need to be asked. A redesign is mandatory if this category analysis needs to be more relevant and practical work/job results.

Apparent operational effects—Questions 6A to 6D.

A task is desirable if it reduces the risk of failure to an acceptable level. For example, no preventive maintenance task is generated if all answers are in the logic process. On the other hand, if operational penalties are severe, a redesign is desirable.

Apparent direct cost effects—Questions 7A to 7D.

A work/job is desirable if the cost is less than the repair cost. No preventive maintenance work/job is generated if all answers are "NO" in the logic process. If the cost penalties are severe, a redesign may be desirable.

Non Apparent -function safety effects—Questions 8A to 8F.

The Non-Apparent -function safety effect requires ensuring the availability necessary to avoid the safety effect of multiple failures. Therefore, all questions should be asked. The redesign is mandatory if not applicable and practical work/jobs are found.

Non Apprent function non-safety effects—Questions 9A to 9E.

This category indicates that a work/job may be desirable to assure the availability necessary to avoid the direct cost effects of multiple failures. For example, no preventive maintenance work/job is generated if all answers are "NO" in the logic process. On the other hand, if economic penalties are severe, a redesign may be desirable.

2.2.7 WORK/JOB DETERMINATION

Work/Job determination is handled similarly for each of the five effect categories in the marine industry, ensuring applicability to vessels. For Work/Job determination, it is necessary to apply the failure causes for the functional failure to the second level of the logic diagram. Seven possible Work/Job outcome questions in the effect categories have been identified, although additional Work/Job, modified Work/Job, or tailored Work/Job definitions may be required depending on the specific needs of the marine sector.

2.2.8 PARALLELING AND DEFAULT LOGIC

Paralleling and default logic are crucial at level 2 (Figs. 2.3 and 2.4). Regardless of the answer to the first question regarding "lubrication or servicing," the next Work/Job selection question should always be asked. Then, following the hidden or evident safety effects path, all remaining questions should be addressed. A "YES" answer to the first question in the other categories allows for exiting the logic. (At the user's discretion, advancing to subsequent questions after a "YES" answer is obtained is permissible, but only if the cost of the Work/Job is equal to the cost of the prevented failure).

Default Logic.

Default logic is represented in paths outside the safety effect areas by arranging the Work/Job selection logic. In the absence of sufficient information to answer "YES" or "NO" to questions in the second level, default logic dictates that a "NO" answer be given and the following questions be asked. When "NO" answers are generated, the only available choice is the next question, which in most cases leads to a more conservative, stringent, and/or costly route.

Redesign.

The redesign is mandatory for failures that fall into the safety effects category (evident or hidden) and for which no practical and effective Work/Job Cards are available.

Flow 2.3 Reliability decision logic tree (level 2)—effects categories and Work/Job determination

 

Flow 2.4 Reliability decision logic tree (level 2)—effects categories and Work/Job determination

2.2.9 MAINTENANCE WORK/JOB CARD

The terms used for possible maintenance on a vessel are explained as follows:

·         Lubrication/servicing (all categories) – This involves any act of lubricating or servicing to maintain the inherent design capabilities of marine equipment.

·         Operational/visual/automated check (hidden functional failure categories only) – An operational check is a work/job card to determine that an item fulfils its intended purpose on a vessel without requiring quantitative checks. It is a failure-finding work/job card. A visual check is an observation to determine whether an item fulfils its intended purpose and does not require quantitative tolerances; this, again, is a failure-finding work/job card. The visual check could also involve interrogating electronic units that store failure data.

·         Inspection/functional check/condition monitoring (all categories) – An inspection examines equipment/machinery against a specific standard. A functional check is a quantitative check to determine if one or more functions of equipment/machinery perform within specified limits. Condition monitoring is a work/job card, which may be continuous or periodic, to monitor the condition of equipment/machinery in operation against preset parameters.

·         Restoration/Recondition/Overhaul/Repair (all categories) – Restoration is the work necessary to return machinery/equipment to a specific standard. Since restoration may vary from cleaning or replacement of single parts up to a complete overhaul, the scope of each assigned restoration work/job card must be specified.

·         Decomission/Discard/Replace (all categories) – Decomission/Discard/Replace is the removal from service of equipment/machinery at a specified life limit. Decommission/Discard/Replace work/job cards are typically applied to single-cell parts such as cartridges, canisters, cylinders, turbine disks, safe-life structural members, etc.

·         Combination (safety categories) – As this is a safety category question and a work/job card is required, all possible avenues should be analysed for equipment/machinery. A review of the relevant work/jobs is necessary to do this. From this review, the most effective work/job cards should be selected.

·         No work/job card (all categories) – In some situations, it may be decided that no work/job card is required, depending on the effect. Each possible work/job card defined above is based on its applicability and effectiveness criteria for equipment/machinery. Table 2.1 summarises these work/job card selection criteria.

2.2.10 WORK/JOB FREQUENCIES OR INTERVALS

When determining how frequently to complete a task or project, it is important to consider relevant information from past operational experiences. Relevant information may be obtained from one or more of the following sources:

·         Previous experience with similar marine equipment demonstrates that a scheduled maintenance Work/Job has provided substantial evidence of being applicable, effective, and economically worthwhile.

·         Manufacturer/supplier test data indicating that a scheduled maintenance Work/Job will be applicable and practical for the equipment/machinery being evaluated.

·         Reliability data and predictions.

Safety and cost considerations must be considered when setting vessel equipment/machinery maintenance intervals. Scheduled inspections and replacement intervals should coincide whenever possible, and tasks should be grouped to minimise operational impact.

The safety replacement interval can be established from the cumulative failure distribution for the item by selecting a replacement interval that results in an extremely low probability of failure prior to replacement. When a failure does not pose a safety hazard but causes loss of availability, the replacement interval is determined through a trade-off process involving the cost of replacement components, the cost of failure, and the availability requirement of the marine equipment.

Mathematical models exist for determining Work/Job frequencies and intervals, but these models rely on the availability of appropriate data. This data will be vessel equipment specific and relevant to the marine industry, and relevant industry standards and data sheets should be consulted.

Suppose there needs to be more reliable data, prior experience with similar marine equipment, or inadequate similarity between previous and current systems. Then, the Work/Job interval frequency can only be established by experienced marine personnel using sound judgment and operating experience in conjunction with the best available operating data and relevant cost data.

 

References & Bibliography:

 

Burger, D. (1997). Implementing reliability-centered maintenance (RCM). [online] www.wearcheck.com. Available at: https://wearcheck.com/virtual_directories/Literature/Techdoc/WZA006.htm [Accessed 20 Apr. 2023].

Fiix. (2016). Reliability Centered Maintenance: What is RCM? | Fiix. [online] Available at: https://www.fiixsoftware.com/maintenance-strategies/reliability-centered-maintenance/. [Accessed 20 Apr. 2023].

Johnson, L. (2018). Function and Failure Modes - Important Factors in Reliability Centered Maintenance (RCM) Part I. [online] www.fractalsolutions.com. Available at: https://www.fractalsolutions.com/blog/function-and-failure-modes-important-factors-in-rcm [Accessed 24 Apr. 2023].

Johnson, L. (2018). Function and Failure Modes - Important Factors in Reliability Centered Maintenance (RCM) Part II. [online] www.fractalsolutions.com. Available at: https://www.fractalsolutions.com/blog/function-and-failure-modes-important-factors-in-reliability-centered-maintenance-rcm-part-ii [Accessed 25 Apr. 2023].

Rausand, M. and Arnljot Høyland (2004). System reliability theory : models, statistical methods, and applications. Hoboken, Nj: Wiley-Interscience.

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Disclaimer:

Out of Box Maritime Thinker © by Naro Consilium Group 2022 and Aleksandar Pudar assumes no responsibility or liability for any errors or omissions in the content of this paper. The information in this paper is provided on an "as is" basis with no guarantees of completeness, accuracy, usefulness, or timeliness or of the results obtained from using this information. The ideas and strategies should never be used without assessing your company's situation or system or consulting a consultancy professional. The content of this paper is intended to be used and must be used for informational purposes only.