Download - Event tree analysis and risk assessment
Issue 00 Rev.0Course Title 1
PLANT INCIDENT INVESTIGATION
Issue 00 Rev. 0Course Title 2
Day -4
Accident investigation analytical techniques. Event tree analysis Risk analysis Systematic Cause Analysis Technique (SCAT) STEP (Sequential timed events plotting)
Issue 00 Rev. 0Course Title 3
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)An event tree is used to analyse event sequences following after an initiating event. The event sequence is influenced by either success or failure of numerous barriers or safety functions/systems. The event tree analysis is used to quantify the probability of the possible consequences.
Issue 00 Rev. 0Course Title 4
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)
When an accident or process deviation (i.e. an “event”) occurs in a plant, various safety barrier systems (both Engineering and Administration) come into play to prevent the accident from propagating.
These safety systems either fail or succeed.
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)
Event trees are used to follow the potential course of events as it moves through the various safety systems.
The probability of success or failure of each safety intervention is used to determine the overall probability of the final outcome.
Event
Safety systems/ Barriers
Acc.fail
SafeShutdown (safely)
Continuous operation
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)
An Event Tree is used to determine the frequency of occurrence of process shutdowns or runaway systems.
Inductive approachSpecify/Identify an initiating even and work forward.Identifies how a failure can occur and the probability of occurrence
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)Steps to Construct an Event Tree:
A. Identify an initiating event of interest.B. Identify the safety functions designed to deal with the
initiation followed by the impact of the safety systemC. Construct the event tree.D. Describe the resulting accident event sequences.
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)A.Identify an initiating event:
May have been identified during a HAZOP as a potential event that could result in adverse consequences.
Usually involves a major piece of operating equipment or processing step, i.e. a HAZOP “Study Node”.
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)B.Identify safety functions:From PID, process flow sheet, or procedures find what safety systems are in place and what their functions are.These can include things such as automatic controllers, alarms, sensors, operator intervention, etc.On you Event Tree write across the top of the page in the sequence of the safety interventions that logically occur.Give each safety intervention an alphabetic letter notation.
automatic controllers alarms sensors operator
interventionEvent
A B C D
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)Horizontal lines are drawn between functions that applyVertical lines are drawn at each safety function that applies
Success – upward Failure – downward
Indicate result of event Circle – acceptable result Cross-circle – unacceptable result
Safety Function
B
(Event A)
ProbabilityXA
XA (1- XB)
XB
Failure Probability
XA XB
XB is the safety function B failure probability or the unavailability of
XA is the Event failure probability
Compute frequency of failures
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)Follow process through with each step to calculate the frequency of each consequence occurring.Typically three final results
Continuous operationShutdown (safely)Runaway or fail
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)
Example:– Loss of coolant to reactor
Four safety interventions1. High temperature alarm2. Operator noticing the high
temperature during normal inspection
3. Operator re-establishes the coolant flow
4. Operator performs emergency shutdown of reactor
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)
Example:– Loss of coolant to reactor
Assume loss of coolant occurs once per year (occurrence frequency 1/yr)Alarm fails 1% of time placed in demand (failure rate of 0.01 failures/demand)Operator will notice high reactor temperature 3 out of 4 times (0.25 failures/demand)Operator will successfully restart coolant flow 3 out of 4 times (0.25 failures/demand)Operator successfully shuts down reactor 9 out of 10 times (0.10 failures/demand)
Resulting Event Tree Analysis
Event Tree Analyses Example:
03.11.2011Course Title 15
Barriers H.T.Alarm
OP. NoticeH.T.
Op.ReOp.Cooling
Op. MakeS/D Results
Loss ofCooling
Event
One Occurance /yer
X
X
X
Failure Demand 0.01 0.25 0.25 0.1
Continuance Operation
Safe S/D
Runaway
Failure Demand
1
E
A B C D
E (1-A)
EA
0.99
0.01
0.7425
0.2475
0.005625
0.001875
0.001875
0.000625
0.022275
0.02475
0.00016875
0.0001875
0.00005625
0.0000625
0.9
0.0075
0.0025
Safe S/D
Continuance Operation
Failure Demand
Safe S/D
Continuance Operation
0.1
0.09
0.1
0.09
0.1
0.75
0.25
0.25
0.75
0.75
0.25
0.25
0.75
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)Mean time between Shutdown:– Mean Time Between Shutdown, MTBS is calculated from:
MTBS=1/occurrences of shutdowns
Mean Time Between Runaway, MTBR is calculated from:- MTBR=1/ occurrences of runaways
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)
Example – Possible outcomes:–The lettering is used to identify each final outcome.
For instance, ABDE
Indicates that after Initiating event A occurs, that safety
system B failed (high T alarm), that safety system D failed
(the operator was unable to re-start the coolant) and safety
system E failed (the operator was unable to successful shut
down the reactor).
Accident investigation analytical techniques. (Cont.)
8. Event tree analysis:-(Quantitative Risk Analysis)
Example – Determination of MTBS:–For Mean Time Between Shutdowns take the reciprocal of the
sum of all sequences that resulted in a shutdown. (Example gives
1/.225 = 4.4yrs)
For Mean Time Between Runaway do the same thing with all
sequences that resulted in a runaway. (Example gives 1/0.250 =
40yrs)
In Class Example
Construct an Event Tree and determine the MTBS and MTBR for a loss of coolant for the reactor shown in Figure 11-8. Assume loss of coolant occurs once every three years.
Alarm fails 0.1% of time placed in demand
Operator will notice high reactor temperature 3 out of 4 times
Operator will successfully restart coolant flow 4 out of 5 times
Operator successfully shuts down reactor 9 out of 10 times
Solution – Construct Event Tree
Solution Continued – Occurrence Frequency
Event A 0.999 0.8 0.7992Event AD 0.999 0.2 0.9 0.17982Event ADE 0.999 0.2 0.1 0.01998Event AB 0.001 0.75 0.80 0.0006Event ABD 0.001 0.75 0.20 0.9 0.000135Event ABDE 0.001 0.75 0.20 0.10 0.000015Event
ABC 0.001 0.25 0.00025
Solution Continued – Mean Time Between Events
Mean Time Between Shutdowns1
Occurences of Shutdown1 5.56
0.17982 0.000135
Mean Time Between Runaways1
Occurences of Runaways1 49.4
0.01998 0.000015 0.00025
MTBS
MTBS yrs
MTBR
MTBR yrs
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
A Quantitative Risk Assessment (QRA) is a technique offers guidelines to help you conduct your own step-by-step analysis. It is used to analyses and recognize workplace hazards so you can reduce, control and report them.
It s a valuable tool used to demonstrate the risk caused by the activity and to provide the competent authorities with relevant information to enable decisions on the acceptability of risk related to the activates.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Hazard:Something with the potential to cause harm (such as electricity, working on a ladder or with dangerous machinery)
Danger: Likelihood of harm or injury.
Risk:Risk is the likelihood that harm will actually result in particular situation or circumstance . Probability that a hazard will result in an accident .
Controls: The measures or methods that we use to control the risk.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Hazard, Risk and Danger:
Hazard means anything that can cause harm (eg
chemicals, electricity, working from ladders, etc) risk is
the chance, high or low, that somebody will be harmed
by the hazard. Danger is a situation where hazards is
involved.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Risk Assessment Principles:•Risk Assessments shall be performed before any work commences by all projects, departments, units and on all worksites where hazards have been identified or potential hazards are thought to exist.•Risk Assessments should be kept short and simple and MUST be recorded.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Timing Of Risk Assessments:• Risk Assessment should be undertaken prior to:
•Performing any non-routine activity,•Performing a new task,•When new people are involved,•When third party people are involved,•When major changes to the work/system are considered
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
How to assess the risks in your workplace
• The Risk Assessment procedure comprises 6 main key steps:1. Identify task (breakdown the job into tasks)2. Identify Hazard associated with each task3. Identify who/what might be harmed4. Apply existing control measures5. Measure the risk6. Reappraising the residual risk by applyingadditional control measures. (ensuring that residual risks are As Low As Reasonably Practicable -ALARP)
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Unfortunately, there are no standard criteria for the acceptability of risk. However, it may be possible to:
1. Compare the level of risk with existing codes and practices, or
2. Compare the level of risk with existing situations in similar industries, or
3. To set your own criteria of acceptability.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Stage 1 - Task identification
review the job steps with the employee to make sure you have not omitted something.
reviewing the procedures to discussing associated hazards in each task step.
Involve staff, so that you can be sure that what you propose to do will work in practice and won’t introduce any new hazards.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 2 - Identify the hazards:
• Identify hazards for each task (use checklist)• Identify where the hazard is relative to the task• Does checklist cover all hazards• Add to your hazard checklist
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 2 - Identify the hazards:
• Identify hazards for each task (use checklist)• Identify where the hazard is relative to the task• Does checklist cover all hazards• Add to your hazard checklist
Checklist of Hazards
The following list acts as a guide to identifying potential hazards: Slip/trip/Fall Hazards Flammable materials Chemicals/Pollution/Contaminants Moving/Swinging objects Moving parts of machinery/vehicles Ejection of material welding/grinding) Pressure/Vacuum Electricity Working at height (over side) Noise Dust Fumes / Noxious Gases Position Manual handling Poor lighting Low/High temperature Low oxygen environment Radiation Vibration Hydrocarbons Restricted access Tasks with RSI potential Single Point Posture Weak structures Unstable objects Y2K Bug Explosives Crane operations Weather conditions
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 3 - For each hazard you need to be clear about who might be harmed:
(People, Assets, Environment and Company Reputation).
Think about people who may not be at the work place at the time, e.g. cleaners, visitors, contractors, and members of the public, etc. Is there any chance that they might be harmed by the activities?
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 4 – Add existing control:
Having spotted the hazards, you then have to decide what to do about them. The law requires you to do everything As Low As Reasonably Practicable’ to protect people from harm.
Think about what existing controls you have in place and how the work is organized.
List existing control measures against each hazard Then compare this with the good practice and see if there’s
more you should be doing to bring yourself up to standard.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 5 - Measure the risk:
Risk is comprised of both Severity (S) and Likelihood or consequences (L)
often expressed as...
Risk = S x L
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 5 - Measure the risk:
Severity or Consequences:The consequences that could have resulted from the
hazard if things went out of control injury or illness to people, r property damage to Assists, or environmental impact, or company reputation
which is reasonably predictable.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 5 - Measure the risk:
Likelihood or Probability:likelihood is defined as: the chance that a given
event will occur; We can estimate the probability based on the following:
Number of employees exposed;
Frequency and duration of exposure;
Proximity of employees to the danger zone;
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 5 - Measure the risk:
Perception of Risk: Risk is often viewed very differently from individual to
individual base on individual perception. Perception is the way one looks at things. someone's ability
to notice and understand things that are not obvious to other people, based on his/her First impression, Previous experience, and Culture.
Peoples perceptions change as familiarity with hazards increases and the risks evaluation changes.
Perception
A pen in the
the PocketA book on the the disk
A bird on
the tree
41
The first impression of a person
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 5 - Measure the risk:
Likelihood Rating GuidanceLikelihood Rating Guidance:The degree of likelihood is split into five categories:
(A) : VERY UNLIKELY Little or no chance of occurrence, (B) : UNLIKELY Conceivable, occurrence would require multiple failures of systems and controls(C) : POSSIBLE Could happen, easy to assume a feasible scenario where the situation would occur (D) : LIKELY Not certainty but such an event have occurred and represents a credible scenario (E) : VERY LIKELY Almost inevitable
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 5 - Measure the risk:
SeveritySeverity Rating Guidance Rating Guidance: Severity is the degree of harm arising from the
hazard, which is reasonably predictable; Areas affected by risk are:
9.People Injury or illness (P), 10.Assists or Properties damage (A), 11.Environment impact (E),12.Reputation of the Company (R).
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 5 - Measure the risk:
SeveritySeverity Rating Guidance Rating Guidance:Rate Severity meaning
0 Nothing happen Nothing.
1 Negligible Injuries Would require first aid and return to work.
2 Minor Injuries Typically a reversible injury or damage to health needing less than three days away from work to recover.
3 Major Injuries Injury or damage to health requiring extended time off work
4 Fatal Death for one person
5 Multiply Fatality Death for more than one
Never heard ofin
EP industry
A
Has occurred inEP industry
B
Has occurred inAFPC
C DHappens
several times ayear in AFPC
EHappens
several times ayear in the
facility
00
Severity
11
22
33
44
Noinjury
PPeople
Slightinjury
Minorinjury
Majorinjury
SingleSinglefatalityfatality
Nodamage
AAssets
Slightdamage
Minordamage
Local damage
Majordamage
Noeffect
EEnv.
Slighteffect
Minoreffect
Localizedeffect
Majoreffect
Noimpact
RReputation
Slightimpact
Minorimpact
Considerableimpact
Nationalimpact
55 MultipleMultiplefatalitiesfatalities
Extensivedamage
Massiveeffect
Internationalimpact
likelihoodlikelihood
Continuous I
mprovemen
t
Medium ALARP
Serious
46
Level of Risk Meaning Control required
Low Risk is accepted
Operation continue considering existing barriers and control measures are emplace and maintained
MediumALARP*
Further risk reduction measures must be
considered
A full demonstration that all risks are managed to ALARP is required.
High Intolerable Risk Risk unacceptable.
Operation should not be started, Consult specialists
RISK CLASSES CRITERIA
Task Assessment:
ALARP
Risk
Cost/Benefit
Resid
ual
Risk
” A level of risk that is tolerable and cannot be reduced further without the expenditure of costs that are disproportionate to the benefit gained or where the solution is impractical to implement”
Cont
rolle
d Ri
sk
High Risk with low cost
Low R
isk w
ith H
igh co
st
Cost
Risk cost benefit.
Accident investigation analytical techniques. (Cont.)
9. Risk Analysis:-(Qualitative risk Assessment)
Evaluation of Risk:-Step 6 – Reappraisal residual risk:
Risk controlRisk control: Once the risk is decide unacceptable, then additional corrective measures are
required either to reduce the probability of the hazard occurring (by improving the system reliability) or by mitigating the consequences of the hazard.
When making a risk assessment all the aspects of likelihood and consequences should be taken into consideration.
Accid
ent R
ate
Job Steps
Preparation Execution Completion
Lack Of Supervision
When making a risk assessment all the aspects of likelihood and consequences should be taken into consideration.
The Aspect of Risk Management
Severity
Relative Frequency of Occurrencelikelihood
Evolutionary of RiskEx
istin
g C
ontro
lM
easu
res &
SO
P
EMERGENCY PREPAREDNESS
CatastrophicLow
High
ContinuesImprovement
Methods for reducing risk
Increase reliability
Mitigate consequenc
e
By design By procedure
Fail safe principle
Protective systems
Emergency preparedne
s
The Aspect of Risk Management
Elimination Substitution Isolation Tolerate
Engineering Controls Administrative Controls
Provide Special Personal Protective Equipment. Transfer (Contractor / Insurance) .
HIERARCHY OF CONTROL MEASURES
The Aspect of Risk Management
Identify source of Hazard Evaluate Risk
Hierarchy of risk control strategy
Terminate
Take action Substitution or Replace
TransferTolerate
Verify
Contractor/Insurance
Engineering Controls
S.PPE
Administrative Controls
Yes
No
Yes
No
Yes
NoIsolate
YesNo
Risk Assessment Form
No Task Hazard Consequences
ExistingControl
Measures
Risk Analysis (existing controls) Required
AdditionalControl Measures
Risk Analysis (with extra controls)
S L Risk S L Risk
Completed by: Activity:
Checked by:
Line Manager:
Affe
cted
par
ty
Res
idua
l ris
k
Rule No.1
Never trust anyone
Rule No.2
Never assume anything
Class Exercise 1 Horse Hot Stamping
Group Exercises
Issue 00 Rev. 0Course Title 59
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