PART 4 QUANTITATIVE RISK ASSESSMENT

Transcription

1 PART 4 QUANTITATIVE RISK ASSESSMENT Prof. Arshad Ahmad arshad@utm.my

2 Content QRA Procedure Consequence & Impact Analysis Risk Computation Tolerability Criteria Risk Mitigation & Management 2

3 QRA Procedures

4 What is QRA Systematic methodology to assess risks associated any installation Taking into consideration all forms of hazards Uses design information and historical data to estimate frequency of failure Uses modelling software to assess consequence Where/when is QRA needed CIMAH 1997 part of CIMAH safety report EQA 1985 a section under EIA On need basis, e.g. after an incidents etc, required by insurance company

5 Definition of Risks Risk = Severity x Likelihood Extent of Damage Fatality Injuries Losses Analysis based on design and modeling equations Likelihood of event Based of failure frequency of process components Analysis based on manufacturer s and historical data

6 QRA General Methodology Hazard Identification Frequency Analysis Consequence Analysis Risk Estimation and Evaluation Risk Management

7 1 - Hazard Identification ª Purpose: to identify plausible hazard conditions ª Methods Check-list, Preliminary Hazard Review, HAZID, HAZOP etc. Unstructured brainstorming? ª Example: Chemical Process Industries Fire, explosion and toxicity properties of materials Nature of fire, explosion and toxic release process Procedures to reduce fire, explosion and toxic release hazards

8 2 - Frequency Analysis ª Purpose: To estimate the likelihood for a hazard scenario to occur ª Methods Fault Tree Analysis, Event Tree Analysis

9 Fault Tree Analysis Using Fault Tree Analysis, the probability of occurrence for the hazard event is computed Fault Tree is a method by which a particular undesired system failure mode can be expressed in terms of component failure modes and operator actions. The system failure mode to be considered is termed the top event and fault tree is developed in branches below this event showing it causes., connected by using logic gate

10 Event Tree Analysis The system has 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. Event tree analysis provides the consequence spectrum and its associated probability of occurrence. It is a visual representation of all the events that can occur in a system. By analyzing all possible outcomes, the percentage of outcomes (or probability of occurrence), which lead to the desired result can be determined. ETA provides the probability that has been mitigated by the system s barriers.

11 3 - Consequence Analysis Determine the amount / rate of release Source modeling Model the dispersion Effect of Chemicals released Toxic Effect : ERPG1,2,3 Fire: Heat intensity kw/m 2 Explosion: Impulse, peak overpressure, heat intensity Estimate Fatality : Probit Estimate injury / health effect Heat Effect / Toxic Effect Estimate Losses (property damage, business opportunity loss, reputation loss, etc Hazard Analysis Impact Analysis

12 4 - Risk Estimation & Quantification ª Purpose: To assess the extent of damage ª Methods ª Risk Computations ª Criteria ª Individual Risk: IRPA, LSIR ª Societal Risks ª Tolerability Criteria in Malaysia ª LSIR is used as a measure of individual risk ª Residential receptors: 1 x 10-6 fatalities per year ª 1 x 10-5 fatalities per year ª 1 x 10-3 fatalities per year

13 5 Risk Mitigation & Management Risk Avoidance discontinuing the operation producing the risk Risk Retention consequent loss is financed by the company. Risk Transfer legal assignment of the costs of certain potential losses from one party to another. e.g. insurance. Risk Reduction risks are reduced through control measures. 13

14 CONSEQUENCE & IMPACT ANALYSIS

15 Dose-Response Analysis Dose response relationships has been used to estimate the impact of a precurser It is widely used to estimate effect of drugs on biological system It has also been used to estimate fatality or damages caused by undesired events 15

16 Probit Analysis The dose level of the various hazard events against fatality, or other consequences can be conveniently determined using Probit Analysis. It is a graphical and Look-up Table approach to determine probability of fatality The probit variable Y is computed from: Y = k 1 + k 2 ln V Values of constants k 1, k 2 and causative variable V (representing the dose) are given in table Once the probit is obtained, it can be converted into % fatality 16

17 Probit: Toxic Release Causative variable, V = C a T (C is concentration in ppm, T is time in minutes) Probit Parameters Type of Injury a k 1 k 2 Ammonia Death Carbon Monoxide Death Chlorine Death Ethylene Oxide Death Hydrogen Chloride Death Nitrogen Dioxide Death Phosgene Death Propylene Oxide Death Sulfur Dioxide Death Toluene

18 Probit: Fire and Explosion Type of injury or damage Fire Burn deaths from flash fire Burn deaths from pool burning Explosion Deaths from lung haemorrhage Eardrum ruptures Deaths from impact Injuries from impact Injuries from flying fragments Structural damages Glass breakage Causative variable (V) 4 / t e I 3 e /10 4 t I e 4 / 3 /10 4 p o p o J J J p 0 p 0 Probit parameters k 1 k Here, t e is the effective time duration (s), t is the time duration of pool burning (sec), I e is the effective radiation intensity (W/m 2 ), I is the radiation intensity from pool burning (W/m 2 ), t e is the effective time duration (s), p o is peak overpressure (N/m 2 ), J is impulse (Ns/m 2 ), C is concentration (ppm) and T is time interval (min). 18

19 Conversion of Probit to Fatality data % %

20 Thermal Dose for Fire Exposure Thermal Dose (kj/m 2 ) Type of Injury 40 Threshold of pain 100 Sunburn (first degree burn) 150 Blisters (Second-degree burn) % fatal (third degree burn) % fatal (third degree burn) % fatal (third degree burn)

21 Effect of Thermal Radiation on Structures (TNO, 1992) Type of Damage Damage Level 1 (radiation intensity, kw/ m 2 ) Damage Level 2 (radiation intensity, kw/ m 2 ) Steel Wood 15 2 Synthetic Materials 15 2 Glass 4 - Damage Level 1 Surfaces of exposed materials catch fire and structural elements collapse or rupture Damage Level 2 - Surfaces of exposed experience serious decoloration as well as peeling and structural elements undergo substantial deformation

22 Thermal Radiation Limits on Structure (Lees, 1996) Radiation intensity (kw/m 2 ) Limit Description (Design Guidance by Dinneno, 1982) 30 Spontaneous ignition of wood 20 Ignition of No 2 fuel oil in 40 seconds 10 Ignition of No 2 fuel oil in 120 second Cable insulation degrades 12 Plastic melts 37.5 Equipment damage 9 Equip. damage (conservative value used in flare design)

23 Toxic Effect Criteria What Concentration are considered dangerous? PEL, TLV etc designed for workers are overly conservative designed for long-term exposure, not for short-term, emergency condition For design of ERP, the ERPG, SPEGL, AEGL are directly relevant for general public

24 Protective Action Criteria Acute Exposure Guideline Levels (AEGLs) Estimate the concentrations at which most people (including sensitive individuals) begin to experience health effects if they are exposed to a hazardous chemical for a specific length of time In November 2011, the National Advisory Committee for AEGLs was eliminated and the AEGL development process was modified Future development work on the AEGLs will focus on finalizing interim AEGLs through the National Academy of Sciences Derived from extensive reviews of animal and human studies As of early 2012, about 70 substances have final AEGLs and nearly 200 substances have interim AEGLs Three tiers are developed for five exposure periods: 10 minutes, 30 minutes, 60 minutes, 4 hours, and 8 hours

25 Protective Action Criteria Acute Exposure Guideline Levels (AEGLs) AEGL-3 - airborne concentration above which the general population could experience life-threatening health effects or death AEGL-2 - airborne concentration above which the general population could experience irreversible other serious, long-lasting adverse health effects or impaired ability to escape AEGL-1 - airborne concentration above which the general population could experience notable discomfort, irritation, or certain asymptomatic nonsensory effects. Effects are not disabling and are transient and reversible upon cessation of exposure

26 Protective Action Criteria Emergency Response Planning Guidelines (ERPGs) Estimate the concentrations at which most people (excluding sensitive individuals) will begin to experience health effects if they are exposed to a hazardous airborne chemical for 1 hour Developed by the Emergency Response Planning committee of the American Industrial Hygiene Association Derived from extensive reviews of animal and human studies As of early 2012, about 145 chemicals have ERPGs

27 Protective Action Criteria Emergency Response Planning Guidelines (ERPGs) ERPG-3 - maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour without experiencing or developing life-threatening health effects ERPG-2 - maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour without experiencing or developing irreversible or other serious health effects or symptoms which could impair an individual's ability to take protective action ERPG-1 - maximum airborne concentration below which it is believed nearly all individuals could be exposed for up to 1 hour without experiencing other than mild transient health effects or perceiving a clearly defined, objectionable odor

28 ERPG (Selected Chemicals) Chemical ERPG1 (ppm) ERPG2 (ppm) ERPG3 (ppm) Acrylic Acid Ammonia Benzene Chlorine Formaldehyde Methanol Methyl Isocyanate Phenol Styrene Sulfur dioxide Toluene Vinyl acetate

29 Protective Action Criteria Temporary Emergency Exposure Levels (TEELs) Estimate the concentrations at which most people (including sensitive individuals) will begin to experience health effects if they are exposed to a hazardous airborne chemical for a given duration Derived by the U.S. Department of Energy Subcommittee on Consequence Assessment and Protective Actions (SCAPA) according to a specific, standard methodology Development methodology uses available levels of concern and manipulates current data using a peer-reviewed, approved procedure in order to establish the TEELs More than 3,000 chemicals have TEELs

30 Protective Action Criteria Temporary Emergency Exposure Levels (TEELs) TEEL-3 - airborne concentration of a substance above which it is predicted the general population could experience life-threatening adverse health effects or death TEEL-2 - airborne concentration of a substance above which it is predicted the general population could experience irreversible or other serious, long-lasting, adverse health effects or an impaired ability to escape TEEL-1 - airborne concentration of a substance above which it is predicted the general population could experience notable discomfort, irritation, or certain asymptomatic, nonsensory effects. Effects are not disabling and are transient and reversible upon cessation of exposure

31 Sensitive Individuals Protective Action Criteria Summary Publishing Agency Airborne Concentration AEGL ERPG TEEL Yes No Yes National Academy of Sciences AIHA DOE Above Below Above Health Effects Could Without Could Values Published >3,000 Release Duration 10 min, 30 min, 60 min, 4 hr, 8 hr ALOHA Model Default Available 60 min 60 min Only if no AEGL or ERPG Note: Based on Data, ERPG-3/TEEL-3 is more conservative. AEGL-3 shows lower concentration at a closer distance

32 Risk Computation

33 Population at risk Individual Risk Individual risk is the risk of fatality or injury to any identifiable (named) individual who lives within the zone impacted by a hazard, or follows a particular pattern of life, that might subject him or her to the consequences of a hazard. Societal Risk Societal risk is the risk of multiple fatalities or injuries in the society as a whole, and where society would have to carry the burden of a hazard causing a number of deaths, injury, financial, environmental, and other losses.

34 Individual Risk Individual risk is defined formally (by Institution of Chemical Engineering, UK) as the frequency at which an individual may be expected to sustain a given level of harm from the realization of specified hazards. It is usually taken to be the risk of death, and usually expressed as a risk per year. The term individual may be a member of a certain group of workers on a facility, or a member of the public, or anything as defined by the QRA.

35 Individual Risk Cause Probability / year Cause Probability / year All causes (illness) 1.19E-02 Rock climbing 8.00E-03 Cancer 2.80E-03 Canoeing 2.00E-03 Road accidents 1.00E-04 Hang-gliding 1.50E-03 Accidents at home 9.30E-05 Motor cycling 2.40E-04 Fire 1.50E-05 Mining 9.00E-04 Drowning 6.00E-06 Fire fighting 8.00E-04 Excessive cold 8.00E-06 Police 2.00E-04 Lightning 1.00E-07 Accidents at offices 4.50E-06 Individual risk can be calculated as the total risk divided by the population at risk. For example, if a region with a population of one million people experiences on average 5 deaths from flooding per year, the individual risk of being killed by a flood in that region is 5/1,000,000, usually expressed in orders of magnitude as

36 Example: Risk of Fatality in Road Accident What is the risk from driving an automobile? Suppose that there are 1 million accidents per year and there are 30 million people. 1 in 200 results in death. Societal!Risk = 1,000,000!accident! year Individual!Risk = 5000!death/year! 30,000,000!people = 1.67!x!10>4 Life!time!Risk = 1.67!x!10 14! x 1!death! 200!accident = 5,000death! year death! person.year Suppose that the average life of a person is 70 years death! x!70!years = !(1!in!86!people)!death!per!person person.year

37 Location Specific Individual Risk IR = x, y, i pf i i IR x,y,i is the individual risk at location (x,y) due to event i, p i is the probability of fatality due to incident i at location (x,y). This is normally determined by impact analysis techniques such as using Probit Analysis f i is the frequency of incident outcome case i, (per year). This value can be determined by FTA and ETA When there are more than one release events, the cumulative risk at location (x,y) is given by equation n IR x = IR, y i = 1 x, y, i

38 Average Individual Risk / Individual Risk Per Annum The average individual risk is the average of all individual risk estimates over a defined or exposed population. This is useful for example in estimating the average risk of workers in reference with existing population. Average individual risk over exposed population is given by CCPS (1989) as IR AV = x, y IR x, y x, y P P x, y x, y Here, IR AV is the average individual risk in the exposed population (probability of fatality per year) and P x, y is the number of people at location x, y

39 Example of LSIR Contour for Proposed Chemical Plant

40 Societal Risk Societal risk measures the risk to a group of people. It is an estimation of risk in term of both the potential size and likelihood of incidents with multiple consequences. The risk can be represented by Frequency-Number (F-N) Curve.

41 F-N Curves Example: F-N curves showing the number of Fatalities against annual frequency. For natural and man-made hazards

42 Determination of Societal Risk To calculate the number of fatalities resulting from each incident outcome case, the following equation is used: N i = Px, y pf, i x, y Here, N i is number of fatalities resulting from Incident Outcome case i, p f,i is the probability of fatality and P x,y is the number of population. The cumulative frequency is then calculated using the following equation: F = N F i i Here, F N is the frequency of all incident outcome cases affecting N or more people, per year and F i = is the frequency of incident outcome case i per year.

43 Societal Risk: Constructing the FN-Curve The value F(1) is the frequency of accidents with 1 or more fatalities, or in other words the overall frequency of fatal accidents. This is the lefthand point on FN-curves, where the curve meets the vertical axis (usually located at N = 1 with logarithmic scales). F-N curves can be constructed based on historical data in the form of number of events (floods, landslides, plant explosion, etc) and related fatalities They can also be based on different future risk scenarios, in which for a number of events with different magnitudes the number of casualties is estimated

44 Example of Societal Risk Contour (F-N Curve) Frequency (F) one or more Fatalities (per year) 1x10-3 1x10-4 1x10-5 1x10-6 1x10-7 Broadly Acceptable Region ALARP Region Intolerable Region Fatalities (N)

45 Example: Fatality due to Transport Accidents in Europe: Rail and Aviation ( ), Road ( ) Fatality Road Rail Aviation Fatality Road Rail Aviation Total

46 Example: Europe Transportation Accidents First calculate the total number of fatalities for road, railroad and aviation accidents by multiplying the number of events with the fatality class. Also calculate the average number of fatalities per year. Then calculate the cumulative number of events, starting with the lowest one in the table (related to 146 fatalities) and summing them up upwards. Then calculate the cumulative frequency of events per year, by dividing the cumulative number by the number of years. 46

47 Example: Europe Transportation Accidents Number of EVENTS Fatalities Cummulative No. of Events Cummulative Frequency per year Fatality Road Rail Aviation Road Rail Aviation Road Rail Aviation Road Rail Aviation E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E E-02 TOTAL PER YEAR

48 Example: Europe Transportation Accidents Plot these values in the graph indicated at the bottom of the spreadsheet in a log-log manner, with Fatalities (N) or the X- axis, and the cumulative frequency per year on the Y-Axis. Compare the results. What can you conclude on the: Severity of the accident type Frequency of the accident type 48

49 Example: Europe Transportation Accidents Number of Fatality Slope = -1 Line Cumulative Frequency 49

50 Tolerability Criteria

51 Tolerable Risk Risk cannot be eliminated entirely. At some point in the design stage someone needs to decide if the risks are tolerable". One tolerability criteria in the UK is "as low as reasonable practicable" (ALARP) concept formalized in 1974 by United Kingdom Health and Safety at Work Act. Serious consideration must be made to decide on tolerability based on ALARP Page 51

52 Origin of Reasonably Practicable Risk Sacrifice (time, money, trouble) Reasonably practicable is a narrower term than physically possible and implies that a computation must be made in which the quantum of risk is placed in one scale and the sacrifice, whether in money, time or trouble, involved in the measures necessary to avert the risk is placed in the other; and that, if it be shown that there is a gross disproportion between them, the risk being insignificant in relation to the sacrifice, the person upon whom the duty is laid discharges the burden of proving that compliance was not reasonably practicable Edward Vs National Coal Board [1949] (British Court)

53 ALARP Criteria Increasing Individual Risk and Societal Risk UNACCEPTABLE REGION THE ALARP REGION (Risk is undertaken if benefit is desired BROADLY ACCEPTABLE REGION Risk cannot be justified except for extraordinary ground Control measures must be introduced for the risk in this region to drive the risk towards the broadly acceptable region. If risks remain in this region and society desires the benefit of the activities the risk is tolerable only if further risk reduction is impracticable, or requires actions that is grossly disproportionate in time, trouble, effort to the reduction in risks achieved. Level of risks regarded as insignificant and further reduction of risks not likely to be required as resources required to reduce risks is likely to be grossly disproportionate to gained achieved.

54 Tolerability Criteria in Malaysia LSIR is used as a measure of individual risk This means that the risk is not influenced by population Residential receptors: 1 x 10-6 fatalities per year Industrial Receptors: 1 x 10-5 fatalities per year Voluntary risks:: 1 x 10-3 fatalities per year 1 x 10-3 Industrial area Residential area 1 x x 10-5 Proposed refinery

55 Tolerability Criteria (UK) This framework is represented as a three-tier system as shown in figure. It consists of several elements : (1) Upper-bound on individual (and possibly, societal) risk levels, beyond which risks unacceptable. In UK, the guideline and criteria are spelled out in R2P2 (reducing Risk Protecting People) document. (refer to www. hse.gov.uk) (2) Lower-bound on individual (and possibly, societal) risk levels, below which risks are deemed not to warrant regulatory concern. (3) intermediate region between (1) and (2) above, where further individual and societal risk reductions are required to achieve a level deemed "as low as reasonably practicable (ALARP)". Page 55

56 Tolerability Criteria (UK) Dotted line general public Solid line - workers

57 Tolerability criteria (Netherland) 1. Risk to public cannot be more than 1X 10-6 fpy 2. Fatality cannot be more than 10 at risk 1X 10-5 fpy 3. Slope -2 General public only

58 Think about this In real life, a risk is Acceptable when: it falls below an arbitrary defined probability it falls below some level that is already tolerated it falls below an arbitrary defined attributable fraction of total disease burden in the community the cost of reducing the risk would exceed the costs saved the cost of reducing the risk would exceed the costs saved when the costs of suffering are also factored in the opportunity costs would be better spent on other, more pressing, public health problems public health professionals say it is acceptable the general public say it is acceptable (or more likely, do not say it is not) politicians say it is acceptable.

59 Risk Management & Mitigation

60 How can we reduce risks? To reduce risk, we can either reduce severity and/or likelihood Reduce Severity: decrease capacity, operate at low pressure, use less hazardous materials etc. Reduce likelihood: use better technology, multiple layers of protection, redundancy etc. What if we cannot bring it lower than the tolerable limit? ALARP (As Low As Reasonably Practicable) Add Mitigating Measures to reduce impact: dispersion prevention, escalation prevention, damage control, emergency response & evacuation, etc

61 Release Mitigation Release mitigation is defined as lessening the risk of a release incident by acting on the source (at the point of release) either 1. in a preventive way by reducing the likelihood of an event which could generate a hazardous vapour cloud; or 2. in a protective way by reducing the magnitude of the release and/or the exposure of local persons or property. Release mitigation involves 1. Detecting the release as quickly as possible; 2. Stopping the release as quickly as possible; and 3. Invoking a mitigation procedure to reduce the impact of the release on the surroundings. 61

62 Release Mitigation Procedure Toxic Release Model Design Basis Source Model Dispersion Model Prediction of Release Impact yes Operate plant Is Hazard Acceptable? No Revise - Process or plant - Process operation - Emergency response 62

63 Release Mitigation Design Engineering Controls Release prevention Ignition Prevention Dispersion Prevention Escalation prevention Administrative Control Safe Work Procedure at every project stages Emergency Response Management 63

64 Engineering Design Inherent Safety Inventory reduction : Less chemicals inventoried or less in process vessels. Chemical substitution : Substitute a less hazardous chemical for one more hazardous. Process attenuation : Use lower temperatures and pressures. Engineering Design Plant physical integrity : Use better seals or materials of construction. Process integrity : Ensure proper operating conditions and material purity. Process design features for emergency control : Emergency relief systems. Spill containment : Dikes and spill vessels. 64

65 Adding Protective Barriers Early Vapor Detection and Warning Detection by sensors. Detection by personnel. Counter-measures Water sprays. Water curtains. Steam curtains. Air curtains. Firewalls, Blastwalls Bunds, drain, dikes Deliberate ignition of explosive cloud. Dilution. Foams. 65

66 Administrative Control Safe Work Procedure at every project stages Operating policies and procedures. Training for vapor release prevention and control. Audits and inspections. Equipment testing. Maintenance program. Management of modifications and changes to prevent new hazards. Security. Emergency Response Management On-site communications. Emergency shutdown equipment and procedures. Site evacuation. Safe havens. Personal protective equipment. Medical treatment. On-site emergency plans, procedures, training and drills.

67 Risk Communication (Seven Cardinal Rules) 1. Accept and involve the public as a legitimate partner. 2. Plan carefully and evaluate your efforts. 3. Listen to the public s specific concerns (Communication is two way activity) 4. Be honest, frank and open (Trust and credibility are most precious assets) 5. Coordinate and collaborate with other credible sources 6. Meet the needs of Media ( provide risk information tailored to the needs of each type of media) 7. Speak clearly and with compassion ( Use simple non technical language with general public)

68 END OF LECTURE