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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
-2- Ocean Systems EngineeringProf. Daejun CHANG
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.
-3- Ocean Systems EngineeringProf. Daejun CHANG
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.
-4- Ocean Systems EngineeringProf. Daejun CHANG
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.
-5- Ocean Systems EngineeringProf. Daejun CHANG
Time DependenceTime Dependence
-6- Ocean Systems EngineeringProf. Daejun CHANG
Leak rate with timeLeak rate with time
Time, s
Leak rate, kg/s
On set of leak
Gas detection &Process isolation
Emergency depressurization(blowdown)
-7- Ocean Systems EngineeringProf. Daejun CHANG
Gas volume with timeGas volume with time
Time, s
Leak rate, kg/sGas volume, m3
On set of leak
Gas detection &Process isolation
Emergency depressurization(blowdown)
Dilution by ventilation
-8- Ocean Systems EngineeringProf. Daejun CHANG
Ignition density with timeIgnition density with time
Time, s
Leak rate, kg/sGas volume, m3Ignition density
On set of leak
Gas detection &Process isolation
Emergency depressurization(blowdown)
Dilution by ventilation
-9- Ocean Systems EngineeringProf. Daejun CHANG
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
-10- Ocean Systems EngineeringProf. Daejun CHANG
Cloud Size EstimationCloud Size Estimation
-11- Ocean Systems EngineeringProf. Daejun CHANG
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!
-12- Ocean Systems EngineeringProf. Daejun CHANG
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
-13- Ocean Systems EngineeringProf. Daejun CHANG
Effect of Wind and Leak DirectionEffect of Wind and Leak Direction
-14- Ocean Systems EngineeringProf. Daejun CHANG
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.
-15- Ocean Systems EngineeringProf. Daejun CHANG
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
-16- Ocean Systems EngineeringProf. Daejun CHANG
An approach to the combined effectsAn approach to the combined effects
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
-17- Ocean Systems EngineeringProf. Daejun CHANG
An approach to the combined effectsAn approach to the combined effects
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
0.05 6 DNNS31 DNNS32 DNNS33 DNNS34 DNNS35 DNNS36 DNNS37 DNNS38
0.02 8 DNNS41 DNNS42 DNNS43 DNNS44 DNNS45 DNNS46 DNNS47 DNNS48
0.01 12 DNNS51 DNNS52 DNNS53 DNNS54 DNNS55 DNNS56 DNNS57 DNNS58
-18- Ocean Systems EngineeringProf. Daejun CHANG
An approach to the combined effectsAn approach to the combined effects
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
0.05 6 DNNS31 DNNS32 DNNS33 DNNS34 DNNS35 DNNS36 DNNS37 DNNS38
0.02 8 DNNS41 DNNS42 DNNS43 DNNS44 DNNS45 DNNS46 DNNS47 DNNS48
0.01 12 DNNS51 DNNS52 DNNS53 DNNS54 DNNS55 DNNS56 DNNS57 DNNS58
S: Simulated cases
S S S S
SSSS
-19- Ocean Systems EngineeringProf. Daejun CHANG
Explosion SimulationExplosion Simulation
-20- Ocean Systems EngineeringProf. Daejun CHANG
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
-21- Ocean Systems EngineeringProf. Daejun CHANG
Position of gas cloudPosition of gas cloud
Small cloud Large cloud
-22- Ocean Systems EngineeringProf. Daejun CHANG
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
-23- Ocean Systems EngineeringProf. Daejun CHANG
Continuous and Discrete IgnitionContinuous and Discrete Ignition
-24- Ocean Systems EngineeringProf. Daejun CHANG
Ignition densityIgnition density
Without ignition, there is no explosion. Ignition density determines the explosion frequency.
Time, s
Explosion frequency Gas volume, m3Ignition density
-25- Ocean Systems EngineeringProf. Daejun CHANG
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
-26- Ocean Systems EngineeringProf. Daejun CHANG
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
-27- Ocean Systems EngineeringProf. Daejun CHANG
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
-28- Ocean Systems EngineeringProf. Daejun CHANG
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
-29- Ocean Systems EngineeringProf. Daejun CHANG
Time, s
Explosion frequency Gas volume, m3Ignition density Category 4 (3,100 m3)
t1 t2 t3 t4
Discrete ignitionDiscrete ignition
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
-30- Ocean Systems EngineeringProf. Daejun CHANG
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
-31- Ocean Systems EngineeringProf. Daejun CHANG
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
-32- Ocean Systems EngineeringProf. Daejun CHANG
Frequency CombinationFrequency Combination
-33- Ocean Systems EngineeringProf. Daejun CHANG
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
-34- Ocean Systems EngineeringProf. Daejun CHANG
Frequency combination for Category 5Frequency combination for Category 5
-35- Ocean Systems EngineeringProf. Daejun CHANG
Exceedance CurveExceedance Curve
-36- Ocean Systems EngineeringProf. Daejun CHANG
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.
-37- Ocean Systems EngineeringProf. Daejun CHANG
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
-38- Ocean Systems EngineeringProf. Daejun CHANG
MiscellaneousMiscellaneous
-39- Ocean Systems EngineeringProf. Daejun CHANG
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.
-40- Ocean Systems EngineeringProf. Daejun CHANG
ReviewReview
-41- Ocean Systems EngineeringProf. Daejun CHANG
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.
-42- Ocean Systems EngineeringProf. Daejun CHANG
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
-43- Ocean Systems EngineeringProf. Daejun CHANG
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
-44- Ocean Systems EngineeringProf. Daejun CHANG
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
-45- Ocean Systems EngineeringProf. Daejun CHANG
Main deck
Mezzanine deck
1. Consequence Analysis 3D Model
Open volume = 87.6 %
Open volume = 89.5 %
-46- Ocean Systems EngineeringProf. Daejun CHANG
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
-47- Ocean Systems EngineeringProf. Daejun CHANG
OFON Wind rose
0
5
10
15
20
250
30
60
90
120
150
180
210
240
270
300
330
1.546891014
Ventilation Simulation
-48- Ocean Systems EngineeringProf. Daejun CHANG
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
-49- Ocean Systems EngineeringProf. Daejun CHANG
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
-50- Ocean Systems EngineeringProf. Daejun CHANG
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
-51- Ocean Systems EngineeringProf. Daejun CHANG
Explosion scenario with cloud category 1 (800m3)
Explosion scenario with cloud category 7
(11,350m3)
Gas Cloud and Ignition Position for Explosion Simulation
-52- Ocean Systems EngineeringProf. Daejun CHANG
-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
-53- Ocean Systems EngineeringProf. Daejun CHANG
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
-54- Ocean Systems EngineeringProf. Daejun CHANG
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
-55- Ocean Systems EngineeringProf. Daejun CHANG
ConclusionsConclusions
-56- Ocean Systems EngineeringProf. Daejun CHANG
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