W13C - Fire and Explosion(5) - Explosion Risk Analysis

Korea Advanced Institute ofScience and Technology

OSE551 Reliability and Risk Analysis for Offshore Plants

Daejun CHANG ([email protected])

Division of Ocean Systems Engineering

Fire and Explosion- Explosion Risk Analysis

-1- Ocean Systems EngineeringProf. Daejun CHANG

FundamentalsFundamentals


Fire (Explosion) TriangleFire (Explosion) Triangle

Fuel

Ignition source

Air (oxygen)

Fire/Explosion

Sparks, flames, static electricity, heat

Since air always exists for open-air explosion, we focus on the coexistence of the fuel and ignition source.


Concept of Explosion Risk AssessmentConcept of Explosion Risk Assessment

Risk = Consequence x FrequencyConsequence = overpressure Frequency f = fcloud x fign

fcloud : Frequency that the cloud exists at the point.fign: Frequency that the ignition source exists at the point.


Some realSome real--world issuesworld issues

The cloud size is changing with time. Leak Dispersion Cloud formation Dilution by air ESD (process isolation) and EDP (blowdown) changes the leak

rate.

The ignition frequency is changing with time. Ignition frequency depends on the number of equipment,

electrical instrument, hot work etc. Upon detection of the gas, the ESD system stops the electrical

supply to the system (electric isolation).

In consequence, the explosion risk changes with time.


Time DependenceTime Dependence


Leak rate with timeLeak rate with time

Time, s

Leak rate, kg/s

On set of leak

Gas detection &Process isolation

Emergency depressurization(blowdown)


Gas volume with timeGas volume with time

Time, s

Leak rate, kg/sGas volume, m3

On set of leak



Dilution by ventilation


Ignition density with timeIgnition density with time

Time, s

Leak rate, kg/sGas volume, m3Ignition density

On set of leak



Dilution by ventilation


Explosion frequency with timeExplosion frequency with time

Time, s

Explosion frequency Gas volume, m3Ignition density Small because of low ignition density

Small because of low cloud size


Cloud Size EstimationCloud Size Estimation


Cloud size estimationCloud size estimation

Do we have to estimate the cloud size for all leak rates? 8 representative leak rates by NORSOK Standard Z-013:

0.75, 1.5, 3, 6, 12, 24, 48, 96 kg/s

Do we have to simulate all the leak rates? Usually, some of them are simulated and the others

interpolated Simulated: 0.75, 1.5, 3, 6, 12, 24, 48, 96 kg/s Interpolated: 0.75, 1.5, 3, 6, 12, 24, 48, 96 kg/s

What situation do we have to simulate? All the scenarios including ESD and EDP?

Numerous simulation cases Frozen cloud assumption!


Frozen cloud assumptionFrozen cloud assumption

Time, s

Leak rate, kg/sGas volume, m3

On set of leak

Leak rate

The cloud size is just dependent on the leak rate at the moment.That implies the cloud size is independent of its history.Is it justifiable?

Cloud size


Effect of Wind and Leak DirectionEffect of Wind and Leak Direction


Combined effects of leak direction and windCombined effects of leak direction and wind

The leak has direction as well as rate. Leak to the inside vs. Leak to the outsideThe former is the more destructive.

Wind has both magnitude (speed) and direction High wind speed

- High dilution rate- Wider dispersion

Wind direction- The effect of the wind direction depends on the leak position.


An approach to the combined effectsAn approach to the combined effects

As the leak rate, we cannot simulate all the cases depending on Leak rate Leak direction Leak position Wind direction Wind speed



Approaches There are Nleak,pos leak positions. For each position, there are Nleak,dir leak directions. For each leak direction, there are Nleak,rate reference leak rates. For each rate, there are Nwind,dir wind directions. For each wind direction, there are Nwind,spd wind speed.

For example: Total simulation cases = Nleak,pos x Nleak,dir x Nleak,rate x Nwind,dir x Nwind,spd= 4 2 8 3 5 = 960= 4 2 4 3 2 = 192

if interpolation is used based on the frozen cloud assumption



Simulation and interpolationLeak position: Deck 1 (D)

Leak direction: North (N)Wind Direction: North-South (NS)

Frequency 0.071 0.0064 0.005 0.0036 0.0022 0.0008 0.0003 0.0001

Probability Leak Rate

Wind Speed 0.75 1.5 3 6 12 24 48 96

0.03 1.5 DNNS11 DNNS12 DNNS13 DNNS14 DNNS15 DNNS16 DNNS17 DNNS18

0.09 4 DNNS21 DNNS22 DNNS23 DNNS24 DNNS25 DNNS26 DNNS27 DNNS28






Simulation and interpolationLeak position: Deck 1 (D)

Leak direction: North (N)Wind Direction: North-South (NS)

Frequency 0.071 0.0064 0.005 0.0036 0.0022 0.0008 0.0003 0.0001

Probability Leak Rate

Wind Speed 0.75 1.5 3 6 12 24 48 96

0.03 1.5 DNNS11 DNNS12 DNNS13 DNNS14 DNNS15 DNNS16 DNNS17 DNNS18





S: Simulated cases

S S S S

SSSS


Explosion SimulationExplosion Simulation


Is the cloud fixed at the position for which the dispersion analysis is done? The cloud can be moved by the wind. It can also travel on its own momentum. If the leak position is changed, the cloud position will change. The cloud position is possible at any allowable place of the

installation.

Position of gas cloud Small cloud (low category): 5 - 9 positions Large cloud (high category): 2 - 3 positions

Position of gas cloudPosition of gas cloud


Position of gas cloudPosition of gas cloud

Small cloud Large cloud


It is known that the overpressure varies with the ignition position within the cloud.

Ignition point within the gas cloud Small cloud (low category): center Large cloud (high category): 2 - 3 positions

Ignition position within the cloudIgnition position within the cloud


Continuous and Discrete IgnitionContinuous and Discrete Ignition


Ignition densityIgnition density

Without ignition, there is no explosion. Ignition density determines the explosion frequency.

Time, s

Explosion frequency Gas volume, m3Ignition density


Naturally there are two types of ignition sources. Continuous: the ignition source is constantly active. Discrete: the activity of the ignition source is intermittent.

The source can be either of the two (continuous or discrete) Both of the two (continuous and discrete at the same time)

TDIIM Time-Dependent Internal Ignition Model Developed by a JIP program led by DNV

Continuous and discrete ignitionContinuous and discrete ignition


TDIIM (TimeTDIIM (Time--Dependent Internal Ignition Model)Dependent Internal Ignition Model)

Continuous ignition Conditional probability that the gas cloud explodes if it touches the

continuous ignition source. Discrete ignition

Probability that the gas cloud explodes which contains the discrete ignition source


Discrete ignitionDiscrete ignition

Discrete ignition Probability that the gas cloud explodes which contains the discrete

ignition source The ignition source is active and inactive intermittently. The explosion probability is proportional to the contact time between

the cloud and the ignition source. The cloud ultimately explodes if left in contact with the source.

fdis = Prdis,total x Vcloud/Vdeck x Dtfdis: discrete ignition frequencyPrdis,total: sum of all the discrete ignition sourcesVcloud: cloud volumeVdeck: deck or space volumeDt: residence time of the cloud



Discrete ignition Probability that the gas cloud explodes which contains the discrete

ignition source The ignition source is active and inactive intermittently. The explosion probability is proportional to the contact time between

the cloud and the ignition source. The cloud ultimately explodes if left in contact with the source.

fdis = Prdis,total x Vcloud/Vdeck x Dtfdis: discrete ignition frequencyPrdis,total: sum of all the discrete ignition sourcesVcloud: cloud volumeVdeck: deck or space volumeDt: residence time of the cloud


Time, s

Explosion frequency Gas volume, m3Ignition density Category 4 (3,100 m3)

t1 t2 t3 t4


Ignition probability = Prdis,total x Vcloud/Vdeck x DtPrdis,total: Sum of all the discrete ignition sources = (pump + compr + ) * 1600m2Vcloud: cloud volume = 3,100 m3Vdeck: deck or space volume = 12,500 m3Dt: residence time of the cloud = t2 - t1 + t4 - t3


Continuous ignitionContinuous ignition

Continuous ignition Conditional probability that the gas cloud explodes if it touches the

continuous ignition source. As soon as the cloud touches the source, it will explode. Continuous ignition is possible only on the boundary.

fcon = Prcon,total x Qcloud/Vdeck fcon: continuous ignition frequencyPrdis,total: sum of all the continuous ignition sourcesQcloud: cloud volume growth rate (m3/s)Vdeck: deck or space volume


Time, s

Explosion frequency Gas volume, m3Ignition density Category 4 (3,100 m3)

t1 t2 t3 t4

Continuous ignitionContinuous ignition

fcon = Prcon,total x Qcloud/Vdeck fcon: continuous ignition frequency = (pump + compr + ) * 1600m2Prdis,total: sum of all the continuous ignition sourcesQcloud: cloud volume growth rate (m3/s) [V(t2) V(t1)]/(t2-t1)Vdeck: deck or space volume

Qcloud


Frequency CombinationFrequency Combination


Frequency combination for Category 1Frequency combination for Category 1

Position ID Probability Position ID Probability Type ID ProbabilityC 0.5 D 3.41106E-06 2.13192E-07

C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 3.41106E-06 2.13192E-07C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 3.41106E-06 2.13192E-07C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 3.41106E-06 2.13192E-07C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 3.41106E-06 2.13192E-07C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 3.41106E-06 2.13192E-07C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 3.41106E-06 2.13192E-07C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 3.41106E-06 2.13192E-07C 1.48149E-05 2.06506E-06D 3.41106E-06

C 0.5 D 0 0C 0 0D 0

Overpressure,barg

Ignition Position(C:Center, E: Edge)

Ignition Type(D:Discrete, C:Continuous)

E

Cloud PositionCloudClass

No of CloudPositions

IntegratedProbability

E

E

E

E

E

E

E

8

E

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.5

0.13 0.5

0.131

2

3

4

5

6

7

0.13

0.13

0.13

0.13

0.13

0.13

1 8

9

0.13


Frequency combination for Category 5Frequency combination for Category 5


Exceedance CurveExceedance Curve


Overpressure detectionOverpressure detection

The overpressure is function of time and position. Consequently, one exceedance curve is about one detection position. An average can be taken over several detection positions.


Exceedance curveExceedance curve

Cumulative frequency The frequency of the higher over pressure is negligible compared to the

lower frequency.

1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

0 1 2 3 4 5 6 7Drag load, bar

Cum

ulat

ive

freq

uenc

y, /y

r

Level 11.8mLevel 13.4mLevel 19.4mLevel 22.3mLevel 28.6m


MiscellaneousMiscellaneous


Some points not explainedSome points not explained

Ventilation study To simulate air change rate of the installation The initial condition of the dispersion study is the results of the

ventilation study. Cloud volume

Only the volume with concentration higher than the LEL is effective for the explosion.

Windrose data The probability distribution of the wind direction and speed.

FLACS supports these tasks.


ReviewReview


Explosion The most catastrophic accident Inherent to ocean plants handling flammable gas within congested space

Goal To design structure against the explosion with a given frequency

(once in 10,000 years (10-4/yr) or once in 100,000 years (10-5/yr)) Task

To estimate the explosion load with the threshold frequency If the explosion load exceeds the structure strength,

change the design for - Structural strength- Safety systems configuration and reliability- Spatial congestion (or equipment arrangement)- . . .

Explosion Risk Analysis An Example

None of the design changes is easy to implement.None of the design changes is easy to implement.Precise detection in the early stage is the key.Precise detection in the early stage is the key.


Continuous ignition Ignition sources are exist. Inherent to ocean plants handling flammable gas within congested space

Goal To design structure against the explosion with a given frequency

(once in 10,000 years (10-4/yr) or once in 100,000 years (10-5/yr)) Task

To estimate the explosion load with the threshold frequency If the explosion load exceeds the structure strength,

change the design for - Structural strength- Safety systems configuration and reliability- Spatial congestion (or equipment arrangement)- . . .

Continuous Ignition and Discrete Ignition


Leak Gas Cloud Explosion

Factors affecting dispersionFactors affecting dispersion-- Leak rate & directionLeak rate & direction-- Wind speed & directionWind speed & direction-- Spatial congestionSpatial congestion

Factors affecting explosionFactors affecting explosion-- Cloud position within the facilityCloud position within the facility-- Ignition densityIgnition density-- Ignition position within the cloudIgnition position within the cloud-- Spatial congestionSpatial congestion

Affecting safety systemsAffecting safety systems-- Gas detection systemGas detection system-- Emergency shutdown system (ESD)Emergency shutdown system (ESD)-- Power shutoff system isolating ignition sourcesPower shutoff system isolating ignition sources

How many conceivable cases?

Explosion Risk Analysis - Mechanism

Dispersion Explosion


1. Consequence Analysis

- 3D geometry model construction

- CFD simulation for ventilation, dispersion, and explosion

2. Explosion Frequency Estimation

3. Risk Presentation: Explosion overpressure vs. Probability

4. ALARP Demonstration


Main deck

Mezzanine deck

1. Consequence Analysis 3D Model

Open volume = 87.6 %

Open volume = 89.5 %


Ventilation

One wind velocity (4m/s) and 12 directions

Dispersion

Wind direction (3) Wind Speed (5) Leak Rate (8) Leak Position (4) Leak Direction (2) = 960 Scenarios

112 scenarios are simulated and the rest are interpolated.

Explosion

Cloud Size (7) Cloud Position (3~9) Ignition Point (2~4)= 128 scenarios are simulated

1. Consequence Analysis CFD Simulation


OFON Wind rose

0

5

10

15

20

250

30

60

90

120

150

180

210

240

270

300

330

1.546891014

Ventilation Simulation


Ventilation Study Results

The volume fraction of air change rate greater than 12 per hour is 99 %.

Well ventilated

0

10

20

30

40

50

60

70

80

90

100

0 100 200 300 400 500

Air changes per hour

Cum

ulat

ive

freq

uenc

y, /y

r

0

78

1550

30

60

90

120

150

180

210

240

270

300

330


Leak Points for Dispersion Simulation

Seg11 (TEG contactor inlet cooler)

Seg7 (MP compressor suction cooler)

Mezzanine deck

Seg9 (HP compressor suction scrubber)

Seg1 (HP fuel gas scrubber)

Main deck


Example: Dispersion from a Leak

0

500

1,000

1,500

2,000

2,500

3,000

3,500

4,000

0 100 200 300 400 500 600 700 800 900 1000Time, sec.

Equiv

alent

Sto

ichiom

etric

Clou

d, m

30

5

10

15

20

25

30

Leak

Rat

e, k

g/s

Cloud volumeLeak rate

ESDBlowdown

Leak at Segment 1 in Main Deck at 24 kg/s

Wind from the south


Explosion scenario with cloud category 1 (800m3)

Explosion scenario with cloud category 7

(11,350m3)

Gas Cloud and Ignition Position for Explosion Simulation


-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

5.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2Time, sec.

Ove

rpre

ssur

e, b

arg

Main deck floorMezzanine deck floorBlast wall

Example: Explosion of a Cloud


DiscreteGas Age Maintenance Manning Technology Module Adjust Total

Pump 2.10E-07 0.90 0.85 1.00 0.60 25 0.46 2.41E-06Electrical eq. * 2.70E-08 0.90 0.90 1.00 0.60 5089.5 0.49 6.68E-05Other equipment * 2.10E-09 0.90 0.90 1.00 0.60 5089.5 0.49 5.19E-06Other ** 1.70E-08 1.00 1.00 1.00 1.00 2544.8 1.00 4.33E-05Personnel * 4.00E-08 1.00 0.95 0.60 1.00 5089.5 0.57 1.16E-04* per m2 exposed to gas SUM 2.34E-04** per m2 exposed to gas - Only one deck level

ContinuousGas Age Maintenance Manning Technology Module Adjust Total

Hot work (# hours per 365*24h)0.00E+00 - - - - - - 0.00E+00Pump 9.60E-05 0.90 0.85 1.00 0.60 25 0.46 1.10E-03Electrical equipment *2.60E-06 0.90 0.90 1.00 0.60 5089.5 0.49 6.43E-03Other equipment * 2.60E-06 0.90 0.90 1.00 0.60 5089.5 0.49 6.43E-03Other ** 1.30E-06 1.00 1.00 1.00 1.00 2544.8 1.00 3.31E-03Personnel * 3.00E-06 1.00 0.95 0.60 1.00 5089.5 0.57 8.70E-03* per m2 exposed to gas SUM 2.60E-02** per m2 exposed to gas - Only one deck level

2. Explosion Frequency Estimation

Ignition intensities Function of state and number of ignition sources


1.0E-10

1.0E-09

1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

0 1 2 3 4 5 6 7

Drag load, bar

Cum

ulat

ive

freq

uenc

y, /y

r Level 11.8mLevel 13.4mLevel 19.4mLevel 22.3mLevel 28.6m

3. Risk Presentation


ConclusionsConclusions


Conclusions

A lot of assumptions and interpolations Still persuasive Rooms for improvements

Difficult to verify Only the assumption are observable. But, the detailed process is hidden. Quality control is important.

Compared to fire risk analysis Explosion risk analysis is more systematic and quantitative Need to apply a similar approach to fire risk analysis

Documents

W13C - Fire and Explosion(5) - Explosion Risk Analysis