L9 - RELIABIL RELIABILITY ITY ENGINEERI ENGINEERING NG •
FAILURE
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CAUSES OF FAILURE
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FAILURE FAILURE RATE - BATH TUB MODEL
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RELIABILITY THEORY
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RELIABILITY EVALUATION TECHNIQUES
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RELIABILITY, AVAILABILITY AND MAINTAINABILITY (RAM)
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MEAN TIME BETWEEN FAILURE (MTBF)
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RELIABILITY MANAGEMENT
RELIABILITY
RELIABILITY IS THE ABILITY TO CONTINUE TO BE FIT FOR THE PURPOSE OR FUNCTION
NO PRODUCTS ARE IMMORTAL
SOME METHODS OF ATTEMPTING TO ASSURE RELIABILITY ARE :•
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USE OF PROVEN DESIGNS SIMPLEST POSSIBLE DESIGNS - AVOID COMPLEXITY USE COMPONENTS WITH KNOWN HIGH PROBABILITY OF SURVIVAL REDUNDANCY (IN CRITICAL AREAS) SPECIFY PROVEN INSTALLATION / OPERATION OPER ATION / MAINTENANCE MAINTENANCE METHODS
FAILURE TYPES TOTAL FAILURE - Complete lack of ability of produce to perform required function
PARTIAL FAILURE - Item does not work as well as expected but continues to perform
GRADUAL FAILURE - Takes place progressively over a period of time
SUDDEN FAILURE - Occurs very quickly
CAUSES OF FA F AILURE MAIN TYPES – •
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WEAKNESS - Usually introduced by poor or wrong Design, Materials, Processes or Operation MISUSE - Represents the application of stress which are outside the usual capability of the component or system
REASONS GIVEN FOR FAILURE OF EQUIPMENT •
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BAD LUCK? FINANCIAL PRESSURES - CUTTING CORNERS TO SAVE MONEY - TIME PRESSURES - FAILURE TO INVEST IN ADEQUATE RESOURCES - FAILURE TO INVEST IN TRAINING POTENTIAL OF FAILURE NOT RECOGNISED AT DESIGN STAGE FAILURE TO SPECIFIFY SYSTEM PROPERLY LACK OF QA / QC IN DESIGN AND MANUFACTURE LATE CHANGES IN DESIGN EXCEEDING DESIGN CRITERIA DURING INSTALLATION
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CHANGES IN OPERATING PRACTICE
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CHANGES IN THE OPERATING ENVIRONMENT
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FAILURE TO INSPECT / MONITOR FOR DETERIORATION
THE BATH TUB CURVE
BATH-TUB CURVE
‘INFANT’ OR EARLY FAILURE PHASE AVOIDED BY :GOOD MANUFACTURING QC FACTORY TESTING WELL DEFINED INSTALLATION PROCEDUTES • • •
‘ADULT’ OR USEFUL LIFE PHASE PERIOD OF NORMAL USEFUL LIFE. LIFE. END OF THIS PERIOD IS DETECTED BY :CONDITION MONITORING TECHNIQUES MEAN TIME BETWEEN FAILURES (HISTORIC INFORMATION) EXPERIENCE • •
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‘WEAR OUT’ PHASE THE ONSET OF FAILURES DUE TO NORMAL PROCESSES OF FATIGUE, WEAR, CORROSION ETC.
RELIABILITY THEORY Reliability theory is the foundation of reliability engineering. For engineering purposes, reliability is defined as: The probability that a device will perform its intended function during a specified period of time under stated conditions. Mathematically, this may be expressed as,
where is the failure probability density function and t is the length of the period (which is assumed to start from time zero).
RELIABILITY ENGINEERING Reliability engineering is concerned with four key elements of this definition: First, reliability is a probability. This means that failure fail ure is regarded as a random phenomenon: it is a recurring event, and we do not express any information on individual failures, the causes of failures, or relationships between failures, except that the likelihood lik elihood for failures to occur varies over time according to the given probability function. Reliability engineering is concerned with meeting the specified probability of success, at a specified statistical confidence level.
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Second, reliability is predicated on "intended function:" Generally, this is taken to mean operation without failure. However, even if no individual i ndividual part of the system fails, but the system as a whole does not do what was w as intended, then it is still charged against the system reliability. reli ability. The system requirements specification is the criterion against which reliability is measured. •
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Third, reliability applies to a specified period of time. In practical terms, this means that a system has a specified chance that it will operate without failure before time t. Reliability Reliabilit y engineering ensures that components and materials will meet the requirements during the specified time. Units other than time may sometimes be used. The automotive industry might specify reliability in terms of miles, the military might specify reliability of a gun for a certain number of rounds fired. A piece of mechanical equipment may have a reliability reliabili ty rating value in terms of cycles of use. Fourth, reliability is restricted to operation under stated conditions. This constraint is necessary because it is impossible to design a system for unlimited conditions. A Mars Rover will have different specified conditions than the family car. The operating environment must be addressed during design and testing.
RELIABILITY BLOCK DIAGRAMMES
Series Configuration Parallel Configuration
Figure 3: Combination of Series and Parallel Configurations
Combination of Series and Parallel Configurations
RISK AND FA F AILURE IDENTIFICA I DENTIFICATION TION METHODS INTUITIVE
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BRAINSTORMING
INDUCTIVE
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FAILURE MODES & EFFECTS CRITICALITY ANALYSIS (FMECA)
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HAZARDS & OPERABILITY STUDIES (HAZOP)
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EVENT TREE ANALYSIS
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MAN-MACHINE SYSTEMS ANALYSIS
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FAULT TREE ANALYSIS (FTA)
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ACCIDENT INVESTIGATION & ANALYSIS
DEDUCTIVE
TOOLS FOR THE QUANTIFICATION OF RISK AND RELIABILITY •
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RELIABILITY NETWORKS & BLOCK DIAGRAMMES FAULT TREE ANALYSIS EVENT TREE ANALYSIS STRESS STRENGTH INTERFERENCE
FMEA APPROACH
EVENT TREE ANALYSIS An event tree analysis (ETA) is a visual representation of all the t he events which can occur in a system. As the number of events increases, the picture fans out like the branches of a tree. Event trees can be used to analyze systems in which all components are continuously operating, or for systems in which some or all of the components are in standby mode - those that involve sequential operation logic and switching. The starting point (referred to as the initiating event) disrupts normal system operation. The event tree displays the sequences of events involving success and/or failure of the system components. The goal of an event tree is to determine determ ine the probability of an event based on the outcomes of each event in the chronological sequence of events leading up to it. By analyzing all possible outcomes using event tree analysis, you can determine the percentage of outcomes which lead to the desired result.
EVENT TREES
This event tree was constructed to analyze the possible outcomes of a system fire. The system has 2 components designed to handle this event: a sprinkler system and an automated call to the fire department. If the fire department is not notified, the fire will be mostly contained by the sprinkler system. If the sprinkler system fails as well, the system will be destroyed. The goal of an event tree is to determine determ ine the probability of an event based on the outcomes of each event in the chronological sequence of events leading up to it. By analyzing all possible outcomes, you can determine the percentage of outcomes which lead to the desired result.
FAULT TREE ANALYSIS A fault tree analysis (FTA) is a deductive, top-down method of analyzing system design and performance. It involves specifying a top event to analyze (such as a fire), followed by identifying all of the associated elements in the system that could cause that top event to occur. Fault trees provide a convenient symbolic representation of the combination of events resulting in the occurrence of the top event. Events and gates in fault tree analysis are represented by symbols. Fault tree analyses are generally performed graphically using a logical structure of AND and OR gates. Sometimes certain elements, or basic events, may need to occur together in order for that top event to occur. In this case, these events would be arranged under an AND gate, meaning that all of the basic events would need to occur to trigger the top event. If the basic events alone would trigger the top event, then they would be grouped under an OR gate. The entire system as well as human interactions would be analyzed when performing a fault tree analysis.
FAULT TREES
FAULT TREE FOR MAJOR LEAK FROM SUBSEA X’MAS TREE
FAULT TREE FOR MAJOR LEAK FROM SUBSEA X’MAS TREE
Case Studies of Uncertainty Analysis in Relia R eliabili bility ty and Risk Assessment ; Dr . Michael V. Fr ank; Saf Saf et y Fact or Associat es es,, Inc
Event Trees ; Pat L. Clemens; Jacobs Sverdrup, Inc.
Fault Tree Analysis ; Dr. John Andrews; Loughborough University
Fault Tree Analysis ; Pat L. Clemens; Jacobs Sverdrup, Inc.
Fault Tree Analysis ; Clif Ericson; AOT
Fault Tree Analysis A nalysis of Computer-Based Computer-Based Systems ; Dr. Joanne Bechta Dugan; University of Virginia
Getting Get ting the the Most From Fr om Mission Success Su ccess Fault Tree Analysis ; Allen Long; Rockwell Collins / Government Systems
Introduction to Probabil Prob abilistic istic Risk Assessment ; Michael V. Frank; Safety Factors Associates
Probabilistic Probabilist ic Risk Assessment Assessmen t; Dr. Todd Paulos; Todd Paulos, Inc.
Use and Abuse of Fault Tree Analysis ;
MEAN TIME BETWEEN FAILURE In general, MTBF (Mean Time Between Failures) is the mean value of the lengths of time between consecutive failures, under stated conditions, for a stated period in the life of a functional unit. A more simplified MTBF definition for Reliability Predictions can be stated as the average time (usually expressed in hours) that a component works without failure.
MTBF for an existing product can be found by studying field failure data. For a new product however, or if significant changes are made to the design, it may be required to estimate or calculate MTBF before any field data is available. In some cases failure fai lure rates for previous products can be used if changes to a design are unlikely to affect reliability. In the case of a new or significantly changed design, an accepted technique is to estimate reliability using a recognized model.
RELIABILITY RELIABILITY - BATH TUB CURVE
What is RAM Analysis? RAM analysis is a numerical analysis technique that quantifies the reliability, availability and maintainability of a complex system, for example, an oil or gas production or process facility. Some definitions may be helpful:Reliability can be defined as the probability that an item will satisfactorily perform its intended function for a specified time. Maintainability can be defined as the probability that an item will be retained in or restored to a specified condition within a given period of time when maintenance is performed in accordance with prescribed procedures and resources. Availability expresses both reliability and maintainability in a single measure. Most studies consider steady state availability. This can be defined as the proportion of deployed time that an item is available for use, when the deployed time considered is very large.
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