RCM 4: Logic Trees

When the FMEA or COFA is finished, the maintenance tasks or actions must be chosen; to do it we will use a logic tree.

To choose the more appropriate task, the following factors must be considered:

1.    Shall be technically feasible and worth doing, in accordance with the section 5.6.2 of SAE JA 1011. A task is considered as worth doing if it reduces (avoid, eliminate or minimize) the failure consequences. The task must be technically feasible, applicable and effective in the equipment work conditions.

2.    Cost-effective, in accordance with section 5.6.3 of SAE JA 1011, if two or more tasks are technically feasible the most cost-effective task shall be selected.

3.    Probability of failure modes and age, the selection of tasks must take account the fact that the probability of some failure modes shall increase with age or that the probability not change with age, in accordance with section 5.6.1 of SAE JA 1011. To know if the failure mode ratio curve is random or age-related is essential for the process.  

4.    Selection of failure management policy, a failure management policy shall be selected if no specific task is currently being done to anticipate, prevent, or detect the failure, in accordance with the section 5.6.4 of SAE JA 1011.

The next step is to design a logic tree that categorizes the failure consequences and provides a selection of failure management policies:

1.    Evident failure mode with safety or environmental consequences, the task shall assess the risk and implant tasks that reduce the probability of the failure mode to a level that is tolerable to the owner or user of the asset, in accordance with section 5.7.1.1 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time-based task, if it is not feasible then proposes a combination of tasks and, finally, a redesign of equipment.

2.    Hidden failure mode with safety or environmental consequences, the task shall reduce the probability of the hidden failure mode to an extent which reduces the probability of the associated multiple failures to a tolerable level to the owner or user, in accordance with section 5.7.1.2 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time-based task, if it is not feasible then proposes a failure-finding task and, finally, a redesign of equipment.

3.    Evident failure mode with economic consequences, the direct and indirect costs of doing the task shall be less than the costs of the failure mode, in accordance with section 5.7.1.3 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time-based task, if it is not feasible then proposes no scheduled maintenance but a redesign of equipment is recommended.

4.    Hidden failure with economic consequences, the direct and indirect costs of doing the task shall be less than the costs of the multiple failure modes plus the cost of repairing the hidden failure, in accordance with section 5.7.1.4 of SAE JA 1011.

The standard SAE JA 1012 proposes to use an on-condition based task, if an on-condition task is not technically feasible and worth doing the standard proposed a time-based task, if it is not feasible then proposes failure-finding tasks and, finally, proposes no scheduled maintenance but a redesign of equipment is recommended.

COFA methodology takes into consideration the component classification. For the Critical, Commitment, or Economics components an on-condition task, if this task is not applicable or effective then proposes a time directed task, if this task also is not applicable or effective then proposes to initiate a design change or accept the risk.

For the Potentially Critical components an on-condition task, if this task is not applicable or effective then proposes a time directed task if this task also is not applicable or effective then proposes failure finding task, if this task is not applicable or effective then to initiate a design change or accept the risk.

Indicate that the Run-to-Failure components not scheduled maintenance is done.

We have seen that the results of the logic trees can be:

·     On-Condition based tasks, or predictive tasks, they can be done without move equipment, without stopping it or during a planned stop; they give information about the failure with time enough to plan reparations.

·    Time based tasks, or preventive tasks, they require to stop equipment and, usually, move it to a workshop; this task acts overall components, even those that don’t require maintenance.

·     Failure finding tasks, their mission is to find hidden failures, but they assume that find the failure is not immediate.

·     Tasks combination, when to implant only one task is not enough to reduce risk to reasonable values.

·    Run-to-Failure, it is the best solution when the risk is under acceptable values, because it uses all components of life.

·    Redesign, they can include equipment redesign as a process or operation redesign. Before proposing a redesign the complexity and time to implant them, the costs and the risk to no avoid the failure mode probability or consequence must be considered.  

RCM 1: Common sense applied to maintenance

RCM 2: Taxonomy and core principles

RCM 3: FMEA vs COFA

RCM 3: FMEA vs COFA.

After the taxonomy process, that results in a classification of equipment in systems, subsystems and components, we start the RCM analysis based in SAE JA 1011.

 

The first step is to establish comprehensive alphanumeric equipment identification. Then, we can start to give answers to the first five questions of the SAE JA 1011.

The questions are:

1.  What are the functions and associated desired standards of performance of the asset in its present operating context?

Answers must include all the functions of the asset under the operating context and the level of performance desired.

These functions can be divided into Primary Functions or the reason why the organization acquires the asset, and Secondary Functions or other as environmental integrity, safety/structural integrity, control/containment/comfort, appearance, protective devices and systems, economy/efficiency, and superfluous.

FMEA - 1



2.  In what ways can it fail to fulfill its functions?

These are the failed states associated with each function; they can be Partial or Total Failures.  



FMEA - 2

3.  What causes each functional failure?

These are the failures modes reasonably likely to cause each failure. Lists of failure modes can include failure modes that has happened before, that are currently being prevented, or that are credible in the operating context that can be obtained from technical reports or databases.

Recommend databases for mechanical components are the OREDA – Offshore Reliability Data – Handbook by Det Norske Veritas, and the Handbook of Reliability Prediction Procedures for Mechanical Equipment by the Naval Surface Warfare Center – Carderock Division, this last handbook also includes the effects of failures data.

  Other sources are the results of Root Causes Analysis of unidentified failures.

The failure modes can be classified into one of the following categories: Incorrect Operation, Incorrect Assembly, or External Damage.

To define if a failure mode is Evident or Hidden under normal circumstances is crucial to continue with the analysis and define the best task or action.

FMEA - 3

4.  What happens when each failure occurs?

The answers describe what would happen if no specific tasks are done to anticipate, prevent, or detect the failure. The Failure Effects include all the information needed to support its evaluation, such as evidence that the failure has occurred, if it can kill or injure someone, or to have an adverse effect on the environment, to have an adverse effect on production or operations, physical damages, or what must be done to restore the function of the asset. 

  

FMEA - 4

 

5.  In what way does each failure matter?

These are the failure consequences that must take into account if they are consequences of hidden failure modes; and if they have safety and/or environmental consequences or only have economic consequences.

FMEA - 5

  

At this point, we can consider finishing the full FMEA – Failure Modes and Effects Analysis - giving values to Probability (of the failure mode), Detection (of the failure) and Severity (of the failure consequence) and multiplicity them to calculate the RPN - Risk Priority Number-. This is the methodology most commonly used in RCM analysis. The standard SAE J 1739 gives us a methodology to perform an effective FMEA.

FMEA - 6

Another option is to use the COFA – Consequences of Failures Analysis – the methodology proposed by Neil B. Bloom that defines a Component Classification, based in an RCM COFA Logic Tree, that classify the components in Critical, Potentially Critical, Commitment, Economic, or Run-to-Failure.

The COFA methodology is more straightforward and comprehensive than FMEA to give a final component classification to support the choice of the best maintenance tasks or actions, keeping all the SAE JA 1011 standard requirements.

COFA

1st Post - RCM: Common sense applied to maintenance.

2nd Post - RCM: Taxonomy and core principles.  

  



RCM 3: FMEA o COFA.

Después del proceso de taxonomía, que da como resultado una clasificación del equipo en sistema, subsistema y componentes, podemos comenzar el análisis RCM basado en la norma SAE JA 1011.

 El primer paso es establecer un código alfanumérico para la identificación de los equipos, una vez hecho esto comenzamos con las cinco primeras preguntas de la norma:

1. ¿Cuáles son las funciones y los niveles de rendimiento deseados para el active en sus condiciones actuales de operación?

La respuesta a esta pregunta debe incluir todas las funciones del active bajo las condiciones de operación y el nivel de rendimiento deseado.

Estas funciones pueden dividirse en Funciones Primarias, o la razón para la que hemos adquirido el activo, y Funciones Secundarias, o cualquier otra función como protección del medio-ambiente, seguridad / integridad estructural, control / contención / confort, apariencia, dispositivos y sistemas de protección, economía / eficiencia y funciones superfluas.

Análisis FMEA - 1



2. ¿De qué maneras puede fallar en el cumplimiento de sus funciones?

Estos son los estados de fallo asociados a cada función; estos fallos pueden ser Fallos Parciales o Fallos Totales.  

Análisis FMEA - 2

3. ¿Qué causa cada fallo funcional?

Estos son los modos de fallo que, de forma razonable, pueden causar cada fallo. Las listas de modos de fallos pueden incluir a aquellos que ya hayan sucedido, que estén siendo actualmente prevenidos, o que son posibles en las condiciones de utilización y que pueden ser obtenidos de informes técnicos o de bases de datos. Otro recurso puede ser la realización de Análisis de Causa Raíz de fallos no previstos.

Los modos de fallo pueden clasificarse en alguna de estas categorías: Operaciones Incorrectas, Ensamblaje Incorrecto o Daños Externos.

Definir si un modo de fallo es Evidente u Oculto bajo condiciones normales resulta crucial para continuar el análisis y definir las mejores tareas o acciones.

Análisis FMEA - 3

 

4.     ¿Qué sucede cuando ocurre cada fallo?

Las respuestas describen que podría suceder si no realizamos alguna tarea específica para anticipar, prevenir o detectar el fallo. Los Efectos de los Fallos incluyen toda la información necesaria para llevar a cabo su evaluación, así como que evidencias existen de que el fallo ha ocurrido, si puede matar o herir a personas o tiene efectos adversos para el medio-ambiente, tiene efectos adversos para la producción o las operaciones, que daños sufre o que debe realizarse para recuperar las funciones del activo. 

 

Análisis FMEA - 4

5.     ¿De qué manera nos afecta cada fallo?

Estas son las consecuencias de los fallos, que deben tener en cuenta si son consecuencia de modos de fallos ocultos, y si afectan a la seguridad, al medio-ambiente o sólo tienen consecuencias económicas.

Análisis FMEA - 5

En este punto, podemos considerar si realizar un Análisis de Modos de Fallos y Efectos - FMEA – dando valores de Probabilidad (del modo de fallo), Detección (del modo de fallo) y Severidad (de las consecuencias del fallo) y multiplicarlos para obtener en valor de RPN – Número de Prioridad de Riesgo-. Esta es la metodología más utilizada en los estudios de RCM. La norma SAE J 1739 nos proporciona una metodología para realizar FMEA.

Análisis FMEA - 6

 

Otra opción es utilizar la metodología COFA – Análisis de Consecuencias de Fallos – propuesta por Neil B. Bloom que define una Clasificación de Componentes, basada in el árbol de decisión RCM COFA, que clasifica los componentes en Críticos, Potencialmente Críticos, Comprometidos, Económicos o Funcionar hasta Fallar.

Análisis COFA



 La metodología COFA es más sencilla que la basada en FMEA al aportar una clasificación final en base a la cual se deciden las tareas o acciones de mantenimiento, a la vez que mantiene todos los requisitos de la norma SAE JA 1011.

1er Post - RCM: Sentido común aplicado al mantenimiento.

2do Post - RCM: Taxonomía y principios fundamentales.