CONDITION-BASED & RISK-BASED MAINTENANCE

 2.3. CONDITION-BASED & RISK-BASED 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

Condition-based maintenance (CBM) is a maintenance approach that involves monitoring the equipment's condition in real-time to detect and prevent possible failures. CBM uses various sensors and diagnostic tools to measure and analyse equipment operating conditions, such as temperature, vibration, and pressure and then makes maintenance decisions based on the equipment's current condition. CBM aims to detect and diagnose problems before they cause equipment failure or downtime.

Risk-based maintenance (RBM) is a strategy that prioritises maintenance activities based on the risk associated with equipment failure. It involves analysing the likelihood and consequences of equipment failure and selecting appropriate maintenance actions to mitigate the risks.

RBM is based on the idea that not all equipment failures are equally important, and some failures can significantly impact operations, safety, and the environment more than others. By focusing maintenance efforts on the most critical equipment and failure modes, RBM can help organisations optimise maintenance spending and improve equipment reliability.

CBM and RBM are two maintenance strategies that can work together effectively. CBM focuses on monitoring the condition of equipment to detect and diagnose potential problems, while RBM prioritises maintenance activities based on the risk associated with equipment failure.

In practice, CBM can provide valuable input to the RBM process by providing real-time data on the condition of the equipment. By continuously monitoring equipment and collecting data on performance, CBM can help identify equipment at a higher risk of failure and prioritise it for maintenance.

Conversely, RBM can help guide the CBM process by providing a framework for prioritising maintenance activities based on the risk associated with equipment failure. By identifying the most critical equipment and failure modes, RBM can help focus CBM efforts on the areas most needed, optimising maintenance spending and improving equipment reliability.

Overall, CBM and RBM are complementary maintenance strategies that can work together to provide a comprehensive approach to equipment maintenance. Organisations can minimise equipment failure, increase operational efficiency, and optimise maintenance spending by utilising real-time condition monitoring and a risk-based approach to maintenance planning.

2.3.1 CBM and RBM - Processes

Equipment maintenance can be approached in two distinct but complementary ways: CBM and RBM. The following is a brief overview of the steps involved in each process:

CBM Process:

·         Equipment Selection: Identify the critical equipment that requires monitoring and maintenance.

·         Sensor Deployment: Install sensors on the selected equipment to measure relevant operating parameters, such as temperature, vibration, and pressure.

·         Data Acquisition: Collect data from the sensors in real-time or at regular intervals.

·         Data Analysis: Analyse the collected data to identify patterns and trends, detect anomalies, and diagnose potential problems.

·         Maintenance Action: Take appropriate maintenance actions based on the results of the data analysis, including inspection, preventive maintenance, and corrective actions.

·         Continuous Improvement: Continuously monitor and evaluate the CBM program's effectiveness, and refine it as necessary to optimise equipment reliability.

RBM Process:

·         Risk Assessment: Identify and evaluate potential risks associated with equipment failure, including the likelihood and consequences of failure.

·         Failure Mode Analysis: Analyse the failure modes of the equipment and prioritise them based on their level of risk and criticality.

·         Maintenance Planning: Develop plans focusing on the most critical equipment and failure modes, including inspection, preventive maintenance, and corrective actions.

·         Implementation: Implement the maintenance plans, including scheduling maintenance activities and allocating resources.

·         Monitoring and Feedback: Continuously monitor equipment performance and adjust maintenance plans based on feedback received.

·         Continuous Improvement: Continuously monitor and evaluate the RBM program's effectiveness and refine it to optimise equipment reliability.

CBM and RBM are proactive maintenance strategies that optimise equipment reliability and reduce maintenance costs. CBM monitors equipment conditions in real time, while RBM prioritises maintenance activities based on the risk associated with equipment failure.

2.3.2 CBM and RBM - Vessel Management

The need for timely and factual data defining the operating condition of critical equipment and the effectiveness of operations such as maintenance and purchasing hinders the effective management of vessel machinery and equipment. CBM and RBM offer solutions to eliminate all factors limiting the performance of machinery and equipment. High maintenance costs directly result from inherent problems throughout the vessel, including poor design standards, outdated management methods, and improper operation, contributing more to high costs than catastrophic equipment failure.

However, the breakdown mentality and myopic view of the root cause of ineffective equipment/machinery performance can restrict vessel owners and operators to corrective maintenance functions or only use CBM and RBM as maintenance functions. Expanding the program to include regular evaluation of all factors that limit overall vessel performance can significantly enhance the benefits derived from CBM and RBM.

CBM and RBM are critical maintenance strategies for tankers and bulkers. These strategies can help eliminate factors limiting equipment and machinery performance, reduce maintenance costs, and enhance overall vessel performance by evaluating all contributing factors, not just those related to maintenance. Here are some detailed examples of how CBM and RBM can be implemented on a vessel:

2.3.2.1 CBM Management on a Vessel:

Example 1: Condition monitoring of a two-stroke Main Engine, such as the MAN ME engine

Engine condition monitoring is critical for ensuring safe and efficient vessel operation. CBM can monitor engine performance, detect potential problems, and prevent equipment failure. For example, one can install temperature and pressure sensors on the engine to keep track of coolant and oil temperatures and pressure. This data can be analysed to identify trends and patterns that indicate potential problems such as leaks, overheating, or worn components. Here is an example of how condition monitoring can be applied to the ME MAN two-stroke main engine:

·         Temperature Monitoring: Temperature sensors can be installed on various engine parts, such as cylinder liners, exhaust gas temperatures, and cooling water temperatures, to monitor the engine's thermal conditions. The collected temperature data can be analysed to identify abnormal temperature changes that may indicate potential issues such as overheating or cooling system blockages.

·         Vibration Monitoring: Vibration sensors can be installed on the engine's main bearing and other vital components to monitor vibration levels. The collected vibration data can be analysed to identify abnormal vibration patterns indicating potential issues such as bearing wear or misalignment.

·         Oil Analysis: Regular oil analysis can be conducted to monitor the engine's oil condition, such as the presence of metal particles, water, and other contaminants. The collected oil data can be analysed to identify any abnormal oil conditions indicating potential issues such as bearing wear or oil system contamination.

·         Pressure Monitoring: Pressure sensors can be installed on the engine's fuel and lubrication systems to monitor the pressure levels. The collected pressure data can be analysed to identify abnormal pressure changes indicating potential issues, such as fuel system blockages or lubrication system failure.

·         Exhaust Emissions Monitoring: Exhaust gas analysers can be installed to monitor the engine's exhaust emissions, such as NOx and SOx. The collected exhaust gas data can be analysed to identify any abnormal emissions indicating potential issues such as incomplete combustion or fuel system blockages.

Example 2: Steam-driven Cargo Pumps

Condition monitoring of steam-driven cargo pumps onboard tanker vessels is critical for ensuring safe and efficient cargo transfer operations. Here is an example of how condition monitoring can be applied to steam-driven cargo pumps:

·         Vibration Monitoring: Vibration sensors can be installed on the cargo pumps to monitor vibration levels. The collected vibration data can be analysed to identify abnormal vibration patterns indicating potential issues such as impeller damage, misalignment, or bearing wear.

·         Temperature Monitoring: Temperature sensors can be installed on the pump bearings and mechanical seals to monitoring the pump's thermal conditions. The collected temperature data can be analysed to identify any abnormal temperature changes that may indicate potential issues such as overheating or insufficient cooling.

·         Pressure Monitoring: Pressure sensors can be installed on the suction and discharge sides of the pump to monitor the pressure levels. The collected pressure data can be analysed to identify abnormal pressure changes indicating potential issues such as clogging or leaks.

·         Flow Monitoring: Flow meters can be installed on the suction and discharge sides of the pump to monitor the flow rates. The collected flow data can be analysed to identify abnormal changes indicating potential issues, such as blockages or worn impellers.

·         Steam Quality Monitoring: Steam quality meters can be installed to monitor the steam quality, such as pressure and temperature, to ensure that the steam-driven cargo pumps receive the required steam quality for their operation.

2.3.2.2 RBM Management on a Vessel:

Example 1: Environmental Risk Assessment - BWTS maintenance

Environmental risk assessment is critical for maintaining the Ballast Water Treatment System (BWTS) onboard tanker vessels. The BWTS is designed to prevent the spread of invasive species and other harmful organisms through the ballast water discharge. Here is an example of how environmental risk assessment can be applied in risk-based maintenance for the BWTS:

·         Identify the potential environmental risks: The first step is to identify the potential environmental risks associated with the BWTS operation, such as inadequate treatment, discharge of untreated ballast water, or discharge of residual biocides.

·         Assess the risks' likelihood and consequences: Each environmental risk's likelihood and consequences are assessed. For example, the likelihood of inadequate treatment can be assessed based on factors such as the system's design, age, and maintenance history. Likewise, the consequences of inadequate treatment can be assessed based on the potential impact on local ecosystems and aquatic life.

·         Prioritise the risks: The identified environmental risks are then prioritised based on their likelihood and consequences. For example, inadequate treatment may be considered a high-priority risk due to its high likelihood and severe consequences.

·         Develop a maintenance plan: Based on the prioritised risks, a maintenance plan is developed to address the most critical environmental risks. The plan may include regular inspections, maintenance, and testing of the BWTS to ensure it works effectively.

·         Implement the maintenance plan: The maintenance plan is implemented, including scheduling maintenance activities and allocating resources. For example, regular inspections and maintenance of the BWTS components may be scheduled, and the system's performance may be tested to ensure it meets the regulatory requirements.

·         Monitor and evaluate the plan: The maintenance plan's effectiveness is continuously monitored and evaluated, and adjustments are made as necessary to optimise environmental performance and reduce the risk of environmental pollution

By prioritising maintenance activities based on the risk level associated with environmental pollution, vessel operators can reduce the risk of invasive species transfer, comply with regulatory requirements, and ensure safe and sustainable operations.

Example 2: Safety Risk Assessment - Emergency Diesel Generator (EDG)

Safety is a critical consideration onboard vessels, and equipment failure can result in potential safety hazards for the crew and the vessel. RBM can identify equipment and systems with the most significant safety risks and prioritise maintenance activities accordingly. Here is an example of how safety risk assessment can be applied in risk-based maintenance for the Emergency Diesel Generator (EDG) onboard a tanker vessel:

·         Identify the potential safety risks: The first step is to identify the potential safety risks associated with the EDG operation, such as engine failure, fuel leaks, or exhaust system failure.

·         Assess the risks' likelihood and consequences: Each safety risk's likelihood and consequences are assessed. For example, it is important to consider the engine's age, operating conditions, and maintenance history to determine the likelihood of engine failure. In addition, it's crucial to assess the potential consequences of engine failure, considering how it could impact vessel safety and the ability to continue operations.

·         Prioritise the risks: The identified safety risks are then prioritised based on their likelihood and consequences. For example, the risk of engine failure may be considered a high-priority risk due to its high likelihood and severe consequences.

·         Develop a maintenance plan: Based on the prioritised risks, a maintenance plan is developed to first address the most critical safety risks. The plan may include regular inspections, maintenance, and testing of the EDG to ensure that it is working effectively.

·         Implement the maintenance plan: The maintenance plan is implemented, including scheduling maintenance activities and allocating resources. For example, regular inspections and maintenance of the EDG components may be scheduled, and the system's performance may be tested to ensure that it is meeting the regulatory requirements.

·         Monitor and evaluate the plan: The maintenance plan's effectiveness is continuously monitored and evaluated, and adjustments are made to optimise safety performance and reduce the risk of safety hazards.

By continuously monitoring equipment conditions and prioritising maintenance activities based on the risk level associated with equipment failure, vessel operators can optimise maintenance spending and reduce the risk of equipment failure, downtime, and costly repairs.

2.3.3. CBM and  RBM - Practical Techniques

Here are some practical techniques that can be used for CBM and RBM:

2.3.3.1 CBM Techniques:

·         Vibration Analysis: An effective method for CBM is vibration analysis. It entails measuring and examining vibration data from rotating machinery, such as turbines, motors, and pumps. The analysis can detect unusual vibration patterns that may signal potential problems, such as bearing wear, unbalance, or misalignment.

·         Thermography: Thermography is a CBM technique that uses infrared cameras to measure equipment temperatures such as electrical panels, motors, and bearings. The data is analysed to identify any abnormal temperature patterns that may indicate potential issues such as overheating or insufficient cooling.

·         Oil Analysis: Oil analysis is a CBM technique that involves regularly analysing the lubricating oil from equipment such as engines, gearboxes, and hydraulic systems. The oil is analysed to identify abnormal conditions, such as metal particles, water, or contaminants, that may indicate potential issues such as wear, contamination, or poor lubrication.

2.3.3.2 RBM Techniques:

·         Failure Modes and Effects Analysis (FMEA): FMEA is an RBM technique that involves identifying and analysing potential failure modes and their effects on equipment, processes, or systems. The data is used to prioritise maintenance activities based on the level of risk associated with each failure mode.

·         Root Cause Analysis (RCA): RCA is an RBM technique that involves identifying and analysing the root cause of equipment failures or incidents. The data is used to develop corrective actions that address the underlying causes of the failures or incidents rather than just addressing the symptoms.

·         Reliability Centered Maintenance (RCM): RCM is an RBM technique that analyses equipment function and performance to identify the necessary maintenance activities to ensure optimal performance and reliability. The data is used to develop plans that prioritise maintenance activities based on the risk associated with each equipment failure mode.

2.3.4 Benefits of CBM and RBM

Tanker vessels can significantly benefit from critical maintenance strategies such as CBM and RBM. These strategies can improve equipment reliability, reduce maintenance costs, enhance safety, comply with regulatory requirements, improve operational performance, and reduce environmental impact.

CBM and RBM enable early detection of equipment issues, allowing for timely maintenance activities to prevent equipment failure and extend equipment life. By prioritising maintenance activities based on the level of risk associated with equipment failures, vessel operators can optimise maintenance spending, reduce the frequency of unnecessary maintenance activities, and minimise unplanned maintenance costs.

CBM and RBM help identify potential safety hazards early, allowing timely corrective actions to mitigate risks and improve safety performance. Additionally, they ensure that equipment is maintained to the required standards and that maintenance activities are documented and tracked, helping vessel operators comply with regulatory requirements.

CBM and RBM improve operational performance by ensuring that equipment operates optimally, reduces downtime and delays, and improve vessel efficiency and productivity. Moreover, RBM prioritises maintenance activities based on the level of risk associated with environmental pollution, allowing for timely maintenance activities that can prevent environmental damage and reduce the risk of non-compliance with environmental regulations.

Implementing CBM and RBM on board tanker vessels can have numerous benefits, including improving equipment reliability, reducing maintenance costs, enhancing safety, compliance with regulatory requirements, improving operational performance, and reducing environmental impact.

 2.3.5 The variances between CBM and RBM maintenance tactics

The critical difference between CBM and RBM is the focus of the maintenance strategy. CBM focuses on monitoring equipment conditions and performing maintenance at the optimal time, while RBM focuses on prioritising maintenance activities based on the level of risk associated with equipment failure. CBM aims to reduce maintenance costs by preventing equipment failure, while RBM aims to optimise maintenance spending by focusing resources on critical equipment.

2.3.6 Conclusion

Tanker vessels, and all other vessels, can greatly benefit from critical maintenance strategies such as CBM and RBM.CBM and RBM allow vessel operators to detect equipment issues early, prioritise maintenance activities based on the level of risk associated with equipment failures, optimise maintenance spending, reduce maintenance costs, improve equipment reliability, enhance safety, comply with regulatory requirements, improve operational performance, and reduce environmental impact.

By implementing CBM and RBM, vessel operators can improve equipment performance and reliability, reduce maintenance expenses, and ensure that their vessels operate safely, efficiently, and sustainably. In addition, vessel operators can mitigate risks associated with equipment failures and environmental pollution, reducing the likelihood of incidents and ensuring compliance with regulatory requirements.

Overall, CBM and RBM are essential maintenance strategies that vessel operators should adopt to maximise the benefits of their investments, minimise risks, and ensure their tanker vessels' safe and efficient operation. By leveraging the practical techniques and examples outlined in this text, vessel operators can optimise their maintenance strategies and achieve operational and environmental goals.

 

References & Bibliography:

 

Ali, A. and Abdelhadi, A. (2022). Condition-Based Monitoring and Maintenance: State of the Art Review. Applied Sciences, 12(2), p.688. doi:https://doi.org/10.3390/app12020688.

Leoni, L., De Carlo, F., Paltrinieri, N., Sgarbossa, F. and BahooToroody, A. (2021). On risk-based maintenance: A comprehensive review of three approaches to track the impact of consequence modelling for predicting maintenance actions. Journal of Loss Prevention in the Process Industries, 72, p.104555. doi:https://doi.org/10.1016/j.jlp.2021.104555.

‌Stamatis, D.H. (2019). Risk management using failure mode and effect analysis (FMEA). Milwaukee, Wisconsin: Asq Quality Press.

Stamatis, D.H. (2015). The ASQ pocket guide to failure mode and effect analysis (FMEA). Milwaukee, Wisconsin: Asq Quality Press.

www.reliableplant.com. (n.d.). Condition-based Maintenance (CBM) Explained | Reliable Plant. [online] Available at: https://www.reliableplant.com/condition-based-maintenance-31823. [Accessed 1 May 2023].

‌6G Controls. (n.d.). Emerson Epro A6740 Module - Reliable Vibration Measurement for Industrial Applications. [online] Available at: https://www.6gcontrols.com/products/emerson-epro-a6740-module-reliable-vibration-measurement-for-industrial-applications/ [Accessed 1 May 2023].

 

 

 

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 first assessing your company's situation or system or consulting a consultancy professional. The content of this paper is intended to be used and must.

 

 

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.