Risk Analysis Flashcards
What are the three community risks?
Unavoidable, In Principle Avoidable, Avoidable
Give an example of an unavoidable risk
Falling, Fire
Give an example of an ‘In Principle’ Avoidable risk
Transport accident, Poison by medication
Give an example of an avoidable risk
Drowning, Rock Climbing
What is the formula for calculating risk?
Risk = consequence x frequency
What does ALARP stand for?
As Low As Reasonably Practicable
Name the three stages of an ALARP diagram
Unacceptable, ALARP or tolerable, acceptable
What does HAZOP stand for?
Hazard and Operability Studies
Define HAZOP
Applies a set of guidewords to determine consequences (more than, less than)
What does FMEA stand for?
Failure Mode Effects Analysis
Define FMEA
Step by step procedure for the evaluation of the severity of potential failure modes in a system
Define reliability, R(t)
Probability system remains operating from time 0 to time t, given it was operating at time 0
Define unreliability, F(t)
Probability system failed once or more from time 0 to time t, given it was operating at time 0
R(t) + F(t) = ?
1
Define availability, A(t)
Probability the system is operating at time t, given it was operating at time 0
Define unavailability, Q(t)
Probability the system is not operating at time t, given it was operating at time 0
A(t) + Q(t) = ?
1
For a non repairable component, Q(t) = ?
F(t)
On a bathtub curve the burn in period shows what in terms of failure?
A decrease in failure
On a bathtub curve the useful life period shows what in terms of failure?
A constant failure
On a bathtub curve the wear out period shows what in term of failure?
An increase in failure
List the life cycle of a product?
Concept > design > manufacturing > operation > retirement
Define risk identification
Find and describe risks
Define risk analysis
Understand risks
Define risk evaluation
Determine if risk is acceptable or tolerable
What are the qualitative risk analysis methods?
Checklists, PHA, PFHA, Rapid Ranking, HAZAN/ID
What does PHA stand for?
Preliminary Hazard Analysis
What is PHA?
Tabular form and colour coding
What does PFHA stand for?
Preliminary Functional Hazard Analysis
What is PFHA?
Separates systems into functions
What is Rapid Ranking?
List of risks according to severity and functions
What does HAZAN/ID stand for?
Hazard Analysis/Identification
Define failure
A deviation from normal intent
Define hazard
Potential to cause damage
Define accident
Unplanned event causing damage
Define consequence
Outcome from accident
Define severity
Measure of consequence
Describe bow tie analysis
Like a spider diagram with tope evet in the middle, two threats on the left, two consequences on the right and a hazard in the top
Define Design FMEA
Analysis of product and how failure modes affect the product operation
Define Process FMEA
Analysis of process by which product is built, maintained and used. It examines how failures in manufacturing process can affect product operation
Give an example of a functional approach (coarse-system breakdown)
Valve control system
Give an example of a hardware approach (detailed system breakdown)
Valve, sensor, controller
What is the FMEA procedure?
Define system > construct hierarchal breakdown and functional block diagrams > record assumptions > define system failure modes > list component failure modes > complete FMEA sheet > Determine reliability critical components
List some failure mode examples
No feed to engines
No feed to APU
No feed to heat exchanger
No fuel transfer
List some component failure mode examples
Fails on
Fails off
Fails open
Fails closed
Fails high
Fails low
What does FMECA stand for?
Failure Modes Effects and Criticality Analysis
How is a Risk Priority Number (RPN) calculated ?
Severity x Likelihood of occurrence x Likelihood of detection
What is on the horizontal axis of a criticality matrix?
Severity
What is on the vertical axis of a criticality matrix?
Frequency
How many severity levels are there?
4
How many frequency/occurrence levels are there?
5
What is the formula for criticality?
Criticality = Frequency x Severity
What criticality number are deemed unacceptable?
12+
What assumption is made for fault trees?
The events are independent
List some typical top events
No fuel supply to engines
Safety system unavailable
Standby system unavailable
Explosion
Complete brake system failure
Landing gear fails
List some typical basic events
Pump fails to start
Valve fails closed
Piston stuck
Operator fails to respond
Define a cut set
A list of failure events such that if they occur then so does the top event
Define a minimal cut set
A list of minimal, necessary and sufficient conditions for the occurrence of the top event
Define a path set
A list of components such that if they all work then the system is also in a working state
Define a minimal path set
If any item is removed, the system no longer functions
Symbol for AND?
.
Symbol for OR?
+
Use the distributive law : (A+B).(C+D)
A.C + A.D + B.C + B.D
Use the idempotent law : A+A
A
Use the idempotent law : A.A
A
Use the absorption law : A+A.B
A
What is order culling?
Produce only those minimal cut sets which have k or less failure events
What is probability culling?
Produce only those minimal cut sets which have probability of occurrence greater or equal to a specified cut off value
What is frequency culling?
Produce only those minimal cut sets which have a frequency of occurrence greater or equal to a specified cut off value
What is the symbol for failure rate?
λ
What is the symbol for time of operation?
t
What is the symbol for repair rate?
v or µ
What is the symbol for inspection interval?
θ
What is the symbol for mean time to repair?
T
What is the formula for unscheduled maintenance when system is a steady state?
t -> θ
Therefore formula: λ/(λ+v)
What is a mutually exclusive event?
Events which cannot occur together
Using the addition law: P(AuB)
P(A) + P(B) - P(AnB)
Using the addition law : P(AuBuC)
P(A) + P(B) + P(C) - P(AnB) + P(AnC) + P(BnC) + P(AnBnC)
Using the multiplication law : P(AnB)
P(A) x P(B)
Using the multiplication law: P(AnBnC)
P(A) x P(B) x P(C)
What are the features of a continuously operating system?
A system that can fail in any order
Start point is system operating normally
Success/Failure of each component is then considered
What are the features of a system where the components are in standby mode?
Components operation can be dependent on success/failure of another component
Start point (initiating event) is usually an unsatisfactory event
Monitors all possible outcomes following an initiating event
What does RBD stand for?
Reliability Block Diagrams
What is a farmer curve (X axis?, Y axis?)
A graph with frequency on y axis and consequence on X axis. Above the line is considered unacceptable and below the line is considered acceptable
What is the x and y axis of a bathtub curve?
X axis is the time and y axis is the hazard rate
Why is the culling method used?
For if there are too many minimal cut sets, by culling, the most significant ones remain
What assumption is made for fault trees?
The events are independent
What is failure time?
The time between the occurrence of two failures
What is lower bound?
First two terms in expression
What is upper bound / rare event approximation?
First term in expression
What is min cut set upper bound?
1 - (1-first terms)
How do you calculate F(t)
number failed / total number
How to calculate rate of something when given the mean?
rate = 1/mean
Steady state formula for no repair?
1
