quantitative risk assessment (qra)

© 2015 – Hervé Baron

HERVE BARON

Engineering Training

Welcome to this presentation.

It shows, based on a practical example, what is a Quantitative Risk Assessment (QRA).

QAR is sometimes called Fire Risk Analysis (FRA) or Fire and Explosion Risk Analysis (FERA).

Comments are most welcome ([email protected]), which I will incorporate for the benefit of all.

Please download this file so that you can see my trainer’s notes in the top left corner – latest Acrobat Pro feature.

Hervé


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Quantitative Risk Analysis (QRA)

Objective: Define the scenarios of likely loss of containment Content: Identify flammable, toxic fluids, isolatable sections Identify possible consequence: fire, explosion, toxic etc. Identify ignition source Input: PFD, HMB, Plot Plan HAZID Report

Step 0:

Failure cases definition

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Commentaires de présentation

Done in the form of a workshop, with attendance of Process and HSE Design Small leaks: flange, valve/pump stuffing box, instrument tapping Medium leak: open purge valve, mechanical failure: impact, vibration etc. Large leak: pipe rupture


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Step 0:

Failure Case Definition

Case : Gas leak from random piping component rupture

Cause: installation error, corrosion,

material defect…

Possible consequence: Dispersion without ignition / jet fire / flash fire / explosion

Section considered: Compressor building


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Step 1:

Identification and characterisation of initiating events

Gas leak inside compressor buidling due to component rupture Hole size (% of component section)

5% 20% Full

Frequency (event/year) 1,11E-01 5,06E-04 6,83E-05

Outflow rate (kg/s) 5,7 90,8 2270,0

Σ risk components *failure rate (from statistics)


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Release Frequency Immediate ignition

ESD & Fire Fighting

Delayed Ignition

Explosion/ Flash-fire

Consequence Event Frequency (ev/y)

0,998 Jet fire ESD & FF 7,779E-030,070

0,002 Jet fire no ESD & FF 1,520E-05

1,11E-01Release/yr

0,949 Dispersion 9,827E-020,930

0,120 Explosion 1,774E-050,028

0,051 0,880 Flash fire 1,301E-04

0,972 Dispersion 5,133E-03

Yes Frequency (event/year)Jet fire ESD & FF 7,779E-03Jet fire no ESD & FF 1,520E-05Explosion 1,774E-05

No Flash Fire 1,301E-04Dispersion 1,034E-01

B04a/b/c/d 5%

Step 2: Event tree analysis


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ESD & Fire Fighting

Delayed Ignition



0,998 Jet fire ESD & FF 7,779E-030,070


1,11E-01Release/yr

0,949 Dispersion 9,827E-020,930

0,120 Explosion 1,774E-050,028

0,051 0,880 Flash fire 1,301E-04




B04a/b/c/d 5%

Probability of immediate ignition for 1-50 kg/s release rate is 7% (from statistical data)

Gas detectors are provided inside the building, that activate isolation and depressurization. It is assumed that they operate 95% of the time.

Probability of explosion vs flash fire (12%) depends on mass of gas and degree of confinement

?

What is the frequency of an explosion?

Probability of delayed ignition (2.8%) takes into account equipment explosion protection (Ex)



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Possible consequences of loss of containment

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Dispersions / Jet fire


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Explosion


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ESD & Fire Fighting

Delayed Ignition



0,998 Jet fire ESD & FF 7,779E-030,070


1,11E-01Release/yr

0,949 Dispersion 9,827E-020,930

0,120 Explosion 1,774E-050,028

0,051 0,880 Flash fire 1,301E-04




B04a/b/c/d 5%



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Overpresssure (bar) 0.2 0.1 0.01

Distance (m) 96 167 1270

Step 3:

Consequence evalutation

CONSEQUENCE CLASS QUANTITATIVE CRITERIA EFFECTS

MINOR ≤0.1 bar locally (within 10m) No effect, no damage

SIGNIFICANT ≤0.1 bar locally (within 50m) Limited damage to plant and operators

SEVERE > 0.1 bar within plant Damage to plant and operators

MAJOR > 0.1 bar on populated areas Damage to plant, operators & public


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Unacceptable risk area– Design change necessary

As Low As Reasonably Practicable – Plant Management measures

Acceptable risk area ?

Final step:

classification of risk


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Unacceptable risk area– Design change necessary

As Low As Reasonably Practicable – Plant Management measures

Acceptable risk area

1.0E-02

1.0E-03

1.0E-04 Unlikely

1.0E-05 Rare

1.0E-06 Minor Significant Severe Major

Final step:


Severity

Prob

abili

ty

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The risk matrix is specified by CPY or by Authorities, for instance in S’poren in France (PPRT).


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Final step:


CONSEQUENCE CLASS QUANTITATIVE CRITERIA EFFECTS MINOR ≤0.1 bar locally (within

10m) No effect, no damage

SIGNIFICANT ≤0.1 bar locally (within 50m)

Limited damage to plant and operators

SEVERE > 0.1 bar within plant Damage to plant and operators MAJOR > 0.1 bar on populated

areas Damage to plant, operators & public

?


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Final step:









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Outcome?


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QRA results: Thermal radiation map (>37.5 kW/m2)

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This is used to specify the fire rating of buildings/structures and PFP


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QRA outcome: Risk Reduction Measures At FEED stage Explosion and fire radiation curves:

Distance between units, e.g., distance between process units and administration buildings, relocation of CCR etc.

Explosion resistance and fire rating of equipment, manned buildings, structures (design for 10-4 per year likelihood: API RP 752 ; ISO19901-3 ; NORSOK Z-013, …)

At Detail Design Stage

Addition of gas detectors Addition of blast/fire protection wall, e.g., between process and utility modules, for

protection of risers ESDV valves Relocation of muster point Relocation of adjacent human occupancy areas (maintenance yard, highway rest area etc.) Recommendation for operations, e.g., increased inspection


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• Failure cases identification and definition • Consequence Analysis • Frequency Analysis • QRA Outcome


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Another example: toxic gas release


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Quantitative Risk Analysis (QRA) Consequence analysis

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Rk: dispersion calculation depend on weather conditions. Worst is minimum wind. Continuous realease case means no ESD (isolation) is initiated. POD = Probability Of Death


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Quantitative Risk Analysis (QRA) Consequence analysis

What conclusion would you draw?

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The impact is determined at manned locations Recommendation: Install H2S detectors at HVAC inlet of Warehouse D and buildings in administration and maintenance areas to initiate automatic shutdown of HVAC upon H2S detection Remark: same dispersion calculations could be done considering ESD activitation, i.e., isolation and reduced inventory, which will show that the risk to workers (located inside buildings) is reduced. Note: the recommendations could be further submitted to confirmation during the QRA when the probability of occurrence will be evaluated.


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Another example: effect of congestion


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• Failure cases identification and definition • Consequence Analysis

VCE: a flammable gas or a flashing liquid released to atmosphere, if not immediately ignited,

disperses to atmosphere creating a cloud which can develop in a Vapour Cloud Explosion (VCE), if the burning velocity of the cloud is increased due to turbulence generated by obstacles present in the cloud. Effects (damages) are associated to levels of overpressure generated by pressure wave.

Explosion strength depends on level of congestion. Congested areas are identified based on arrangement of equipment/group of equipment,

platforms, and structures within each Process Unit. Air coolers / pipe-racks / compressor shelters are considered as roofs underneath which gas

cloud can accumulate. Free areas between group of equipment within Process Unit reduce the size of the congested

areas. Flammable volume/mass is estimated for each unitary congested area.


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Identification of congested areas


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COMPANY Societal Risk Criteria

1.0E-8

1.0E-7

1.0E-6

1.0E-5

1.0E-4

1.0E-3

1.0E-2

1 10 100 1000 Number of Fatalities (N or more)

Freq

uenc

y (/y

r)

Intolerable above line Acceptable below line

ALARP Region


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quantitative risk assessment (qra)

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