Tuesday, November 17, 2015

Popular methods of Risk Assessment (Part I)

Failure Mode Effects Analysis (FMEA)

Failure modes and effects analysis (FMEA) is a step-by-step approach for identifying all possible failures in a design, a manufacturing or assembly process, or a product or service.
“Failure modes” means the ways, or modes, in which something might fail. 
Failures are any errors or defects, especially ones that affect the customer, and can be potential or actual.

“Effects analysis” refers to studying the consequences of those failures.
Failures are prioritized according to how serious their consequences are, how frequently they occur and how easily they can be detected. The purpose of the FMEA is to take actions to eliminate or reduce failures, starting with the highest-priority ones.

Failure modes and effects analysis also documents current knowledge and actions about the risks of failures, for use in continuous improvement. 

FMEA includes review of the following:
  • Steps in the process
  • Failure modes (What could go wrong?)
  • Failure causes (Why would the failure happen?)
  • Failure effects (What would be the consequences of each failure?)

When to Use FMEA
  • When a process, product or service is being designed or redesigned, after    quality function deployment
  • When an existing process, product or service is being applied in a new way.
  • Before developing control plans for a new or modified process.
  • Periodically throughout the life of the process, product or service.
  • When analyzing failures of an existing process, product or service.
  • When improvement goals are planned for an existing process, product or service.
Why to use FMEA 
  • Prospective tool
  • Good tool for operators to use
  • Can be used to identify critical steps for validation
  • More objective than Fault Tree Analysis
  • Covers minor risks

FMEA Procedure

1. Assemble a cross-functional team of people with diverse knowledge about the process, product or customer needs. Functions often included are: design, manufacturing, quality, testing, reliability, maintenance, purchasing (and suppliers), sales, marketing (and customers) and customer service.

2. Identify the scope of the FMEA.
  • Is it for concept, system, design, process or service?
  • What are the boundaries? How detailed should we be?
  • Use flowcharts to identify the scope and to make sure every team member understands it in detail.
3.         Identify the functions of your scope. Ask,
  • “What is the purpose of this system, design, process or service?
  •  "What do our customers expect it to do?”

4. Break the scope into separate subsystems, items, parts, assemblies or process steps and identify the function of each.

5. For each function, identify all the ways failure could happen. These are potential failure modes. If necessary, go back and rewrite the function with more detail to be sure the failure modes show a loss of that function.

6.For each failure mode, identify all the consequences on the system, related systems, process, related processes, product, service, customer or regulations. These are potential effects of failure. Ask,
  • “What does the customer experience because of this failure?
  • "What happens when this failure occurs?”

7.  Determination of RPN number      
           Severity: Determine how serious each effect is. This is the severity rating, or S. Severity is usually rated on a scale from 1 to 10, where 1 is insignificant and 10 is catastrophic. If a failure mode has more than one effect, write on the FMEA table only the highest severity rating for that failure mode.

For each failure mode, determine all the potential root causes. Use tools classified as cause analysis tool, as well as the best knowledge and experience of the team. List all possible causes for each failure mode on the FMEA form.
            Occurrence: For each cause, determine the occurrence rating, or O. This rating estimates the probability of failure occurring for that reason during the lifetime of your scope. Occurrence is usually rated on a scale from 1 to 10, where 1 is extremely unlikely and 10 is inevitable. On the FMEA table, list the occurrence rating for each cause.

For each cause, identify current process controls. These are tests, procedures or mechanisms that you now have in place to keep failures from reaching the customer. These controls might prevent the cause from happening, reduce the likelihood that it will happen or detect failure after the cause has already happened but before the customer is affected.
             Detection: For each control, determine the detection rating, or D. This rating estimates how well the controls can detect either the cause or its failure mode after they have happened but before the customer is affected. Detection is usually rated on a scale from 1 to 10, where 1 means the control is absolutely certain to detect the problem and 10 means the control is certain not to detect the problem (or no control exists). On the FMEA table, list the detection rating for each cause.
(Optional for most industries) 

In this failure mode associated with a critical characteristic? (Critical characteristics are measurements or indicators that reflect safety or compliance with government regulations and need special controls.) If so, a column labeled “Classification” receives a Y or N to show whether special controls are needed. Usually, critical characteristics have a severity of 9 or 10 and occurrence and detection ratings above 3.

RPN Calculation:
Calculate the risk priority number, or RPN, which equals S × O × D. Also calculate Criticality by multiplying severity by occurrence, S × O. These numbers provide guidance for ranking potential failures in the order they should be addressed.
Identify recommended actions. These actions may be design or process changes to lower severity or occurrence. They may be additional controls to improve detection. Also note who is responsible for the actions and target completion dates.
As actions are completed, note results and the date on the FMEA form. Also, note new S, O or D ratings and new RPNs.

  • Can be time and resource consuming
  • Mitigation plans must be followed up
  • Not a good tool for analysis of complex systems
  • Compound failure effects cannot be analyzed
  • Incorporating all possible factors requires a thorough knowledge of characteristics and performance of the different components of the system
  • Successful completion requires expertise, experience and good team skills
  • Dealing with data redundancies can be difficult
For more details on Risk Management approach, Please follow below link:

Fault tree analysis

Failure Mode Effects Analysis (FMEA)

A to Z Risk Management with statistical approach

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