Probabilistiske brannrisikoanalyser til bruk i design 2013/Brannsikkerhet/7... · Probabilistiske...

Probabilistiske brannrisikoanalyser til bruk i design

Robust brannsikkerhet for offshoreinnretninger

Temadag Petroleumstilsynet 24. oktober 2013

Asmund Huser og Jens J. Garstad DNV GL

© Det Norske Veritas AS. All rights reserved.

Content

Overview

- NORSOK vs FABIG heat flux

- Approaches in DNV and general

Why probabilistic method

DNVs advanced probabilistic method

- Design Accidental Load (DAL)

- Response analysis

- PFP optimzation

Simplified probabilistic analysis

- Fire DAL

Example

Conclusions

2


Standards and guidelines

Norsok S-001:

- Gives heat fluxes

- Opens for probabilistic DAL to find

fire sizes

Guideline for the protection of

pressurized systems exposed to

fire

- Use scenario with Max duration

(worst case)

- Duration set by cut off leak rates

and actual inventory

FABIG Technical Note 11

- Gives heat flux for gas jet, two

phase jet and pool

- Gives probabilistic procedure

3


Comparisons FABIG and NORSOK

Leak rate (kg/s): 0.1 1 10 30

FABIG Gas jet fires 180 250 300 350

Two phase jet fires 200 300 350 400

Pool fires 125 125 250 250

NORSOK Jet fire 250 250 350 350

Pool fire 150 150 150 150

4


Total heat flux (kW/m2) comparison FABIG vs. NORSOK

0

50

100

150

200

250

300

350

400

450

Gas jet firesFABIG

Two phase jetfires FABIG

Jet fireNORSOK

Pool fire FABIG Pool fireNORSOK

0.1 1 10 30 kg/s

5


Fire Design Accidental Load challenges

Definition: Fire DAL is a flux (kW/m2) and duration

- Flux is given in guidelines or from CFD models

- Duration needs to be decided based on actual process

- different strategies:

Approaches to set DAL

- Scenario based:

- Credible scenario picked from QRA

- Worst case scenario picked from QRA

- Probabilistic approach:

- Different x-axis strategies

- Different levels of detail

Different application areas needs different approaches

- For general DAL spec

- PFP optimization on structure

- PFP optimization on piping and equipment

Evolution in approach and knowledge

6

© Det Norske Veritas AS. All rights reserved. Slide 7

Highlights of DNVs advanced procedure

Includes dynamic, 3D effects of all possible fires.

- Each fire described by one number: Max dose received

Fire analysis with same level of detail as NORSOK

explosion analysis

Probabilistic fire analysis is used in optimization of:

- Design Accidental Load (DAL) for fires – also duration

- Flare; ESD, blowdown

- PFP on structure and pipes

- Fire/explosion dividers, and arrangement


Why Probabilistic and CFD?

Optimization of mitigation

Consistent quantitative method

- All areas protected to the same risk level

Fire and explosion risk presented on the same format

- Opposing mitigating measures

Research show large fires can give very high heat

fluxes

- Captures only with CFD


Why CFD and FE fire model?

Research gives (Ragnar Wighus 2008):

- Heat flux most dependent on size of flames

- Heat flux also dependent on amount of equipment

and turbulence generating objects

- Oil (and wood) can give same heat flux as gas

- More PFP gives more heat – vicious cycle

Generalized fire loads can be too conservative wrt

fire protection – not explosion


”Confined fires” measurements, Phase 2, 1998


Test Barrels w/wo PFP

With PFP, 50 kW/m2 higher radiation

With permission from Ragnar Wighus, Sintef NBL

© Det Norske Veritas AS. All rights reserved. Slide 12

Combined analysis: Optimal mitigation

Schematic illustration of optimal passive fire protection. In this example, fire and

explosion escalation frequencies are crossing each other.


Background for development of probabilistic fire analysis

Standards and codes gives deterministic, constant fireloads

- Ex. Jet fires > 2 kg/s, use 350 kW/m2

- ”Average maxvalue” is often conservative

Development of NORSOK Z013-G procedure explosion 1997

DNV started to develop this probabilistic fire method in 2001 with Statoil

- Investigate important effects

- Develop a procedure

- Test it on a real platform

FABIG Newsletter 44, 2005 describes the first procedure

FABIG TN 11 2009 Appendix A describes the procedure

ExpressFire program developed 2006

PFPro developed 2007

Apply procedure in many projects


Some QRA and Safety design methods used in DNV

Guidelines/

standards

Simplified

methodsMore detailed

Load AnalysisProcedures, Loads,

tabulated

Experience

based/ empiric/

integral models

CFD, FE,

MonteCarlo,

advanced event

tree

Explosion max pressures DNV-OS-A101 THOR.xls FLACS

Probabilistic explosion

analysis

NORSOK Z013,

DNV-OS-A101ExpressLite Express

Fire load and sizeFABIG TN 11, Scp

GuidelinePhast KFX

Probabilistic fire

analysisFABIG TN 11

Selected

"Credible

Scenario"

ExpressFire

Structural response

FABIG TN 11,

OGUK FX Guidel.,

API RP 2FB,

EUROCODE

PFPro FAHTS/USFOS

All Risk analysis, safety case NORSOK Z013 SOQRATESRisk Framework/

Neptune

Explosion

Fire

PhastRisk

Offshore

Excel

model


FRA procedure

Initial Fire Risk

Assessment

CFD Fire

Modeling

Fire Risk

Assessment

Leak

Frequencies

Ignition

Probabilities

Inventories

Fire Location

Highest Ranked

Cases

Heat Flux

Fire

Frequencies

Risk

Acceptance

Criteria

Fire Frequencies

Risk Rank Cases

Heat Flux for

cases

Fire Exceedance

Curves

DAL Fire and

Load

Input Assessment Output

Sectio

n 1

DAL Fire and

Load

Structure

temperature and

strain response

Simulation

Temperature and

strain response in

structure

Modify

PFP

Modify

Flare,

ESD and

BD

Minimum PFP

Sectio

n 2

Risk

evaluation

Global

Collapse? Strain or

temperature

Criteria

End

Start

Initial Fire Risk

Assessment

CFD Fire

Modeling

Fire Risk

Assessment

Leak

Frequencies

Ignition

Probabilities

Inventories

Fire Location

Highest Ranked

Cases

Heat Flux

Fire

Frequencies

Risk

Acceptance

Criteria

Fire Frequencies

Risk Rank Cases

Heat Flux for

cases

Fire Exceedance

Curves

DAL Fire and

Load


Sectio

n 1

DAL Fire and

Load

Structure

temperature and

strain response

Simulation

Temperature and

strain response in

structure

Modify

PFP

Modify

Flare,

ESD and

BD

Minimum PFP

Sectio

n 2

Risk

evaluation

Global

Collapse? Strain or

temperature

Criteria

End

Start


Section 1 DAL fire

Initial Fire Risk

Assessment

CFD Fire

Modeling

Fire Risk

Assessment

Leak

Frequencies

Ignition

Probabilities

Inventories

Fire Location

Highest Ranked

Cases

Heat Flux

Fire

Frequencies

Risk

Acceptance

Criteria

Fire Frequencies

Risk Rank Cases

Heat Flux for

cases

Fire Exceedance

Curves

DAL Fire and

Load


Sectio

n 1

DAL Fire and

Load

Structure

temperature and

strain response

Simulation

Temperature and

strain response in

structure

Modify

PFP

Modify

Flare,

ESD and

BD

Minimum PFP

Sectio

n 2

Risk

evaluation

Global

Collapse? Strain or

temperature

Criteria

End

Start

Initial Fire Risk

Assessment

CFD Fire

Modeling

Fire Risk

Assessment

Leak

Frequencies

Ignition

Probabilities

Inventories

Fire Location

Highest Ranked

Cases

Heat Flux

Fire

Frequencies

Risk

Acceptance

Criteria

Fire Frequencies

Risk Rank Cases

Heat Flux for

cases

Fire Exceedance

Curves

DAL Fire and

Load


Sectio

n 1

DAL Fire and

Load

Structure

temperature and

strain response

Simulation

Temperature and

strain response in

structure

Modify

PFP

Modify

Flare,

ESD and

BD

Minimum PFP

Sectio

n 2

Risk

evaluation

Global

Collapse? Strain or

temperature

Criteria

End

Start


Initial Fire Risk Assessment

Coarse screening to find high risk cases

Risk is represented by frequency and mass of HC in segment

Use high risk cases as representative in CFD model


CFD fire modeling

Use parameters from cases with highest risk (composition, etc.)

Obtain effects of varying leak rates

Model impinging and non impinging jets, and pools

- Obtain effects of varying jet direction

- Obtain effects of varying leak location

Gives two main results:

- Gives max heat loads for all fire case as a function of time: Input to fire risk analysis

- Real 3D dynamic heat load fields: Input to Structure response analysis


CFD model captures the fire dynamics

Ventilation and air/fuel flow

Combustion

Congestion

Flame thickness

Scale effects

Radiation

Convection

Deluge


Dynamic temperature plots

N

Z

30 kg/s

10 kg/s

3 kg/s

1. kg/s

0.3 kg/s


Heat flux histories

0

50

100

150

200

250

300

350

0 1 2 3 4 5

Dimensionless time

Q (

kW

/m2)

1

2

3

4

30 kg/s 10 kg/s 3 kg/s 1 kg/s 0.3 kg/s


Heat flux histories

0

100

200

300

400

500

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5

Tbar (-)

Q (

kW

/m2)

Qmax_L1

Qmax_L2

Qmax_L3

Qmax_L4

Qmax_L5


Fire risk analysis principles

Exceedance curve is based on one representative number describing

severity for each case

Rank all fires according to this one number

This number is on x-axis on exceedance plot

Y- axis is the accumulated frequency

Use separate curves for oil and gas

Where total curve crosses 10-4 per year is the DAL fire

The target also decides the x-axis number used

For structure, the total received heat during scenario is used

Can be described by a dose

When the DAL dose is found, it must be converted back to a flux and

duration.

23

1.0E-05

1.0E-04

1.0E-03

10 100 1000

Heat dose received by structure (MJ/m2)

Acc

um

ula

ted

fir

e im

pa

ct f

req

uen

cy (

per

yea

r)


Fire risk analysis

Sorted

Case number and name Phase Size Heat dose (kWs/m

2)

Collapse frequency

(per year)

Cumulative frequency

(per year)

Case 1, LP stages gas Large 20 1.04E-04 3.59E-04

Case 2, HP stage gas Large 51 1.07E-04 2.56E-04

Case 1, LP stages gas Medium 108 5.71E-05 1.48E-04

Case 2, HP stage gas Medium 127 6.44E-05 9.12E-05

Case 1, LP stages gas Small 142 1.56E-05 2.68E-05

Case 2, HP stage gas Small 177 1.12E-05 1.12E-05

Find dose for each case. Integrate in each CFD cell and use max:

- Dose: 𝑫𝑽 = 𝑸 𝒕 𝒅𝒕 (J/m2), where Q(t) (kW/m2) is heat flux

Find fire impact frequency for each case

- Impact freq. = leak frequency*ignition probability*impact probability

Sort cases with increasing heat dose

- from case study:


Plot fire impact frequency vs. dose

1.0E-05

1.0E-04

1.0E-03

10 100 1000

Heat dose received by structure (MJ/m2)

Acc

um

ula

ted

fir

e im

pact

fre

qu

ency

(p

er y

ear)

DAL Dose = Q t

t = Dose/250 kW/m2

DAL:

Q = 250 kW/m2

t = 8.3 min


Pick DAL fire scenario

1.E-06

1.E-05

1.E-04

1.E-03

Large Large Medium Medium Small Small

gas gas gas gas gas gas

Case 1, LP

stages

Case 2, HP

stage

Case 1, LP

stages

Case 2, HP

stage

Case 1, LP

stages

Case 2, HP

stage

Sorted after heat dose received by structure (MJ/m2)

Acc

um

ula

ted

im

pa

ct f

req

uen

cy (

per

yea

r)


Leak profile: Medium is DAL fire

0.1

1

10

100

0 5 10 15 20

Time (min)

Lea

k r

ate

(k

g/s

)

Very large

Large

Medium

Small


Heat flux from DAL fire (medium)

5 kg/s 1.25 kg/s 0.5 kg/s


Max heat flux medium fire (DAL fire)

0

100

200

300

400

500

0 0.5 1 1.5 2 2.5

Tbar (-)

Q (kW

/m2

)

0 6 10 min


Section 2: Structure response analysis


Section 2: Structure response analysis

Initial Fire Risk

Assessment

CFD Fire

Modeling

Fire Risk

Assessment

Leak

Frequencies

Ignition

Probabilities

Inventories

Fire Location

Highest Ranked

Cases

Heat Flux

Fire

Frequencies

Risk

Acceptance

Criteria

Fire Frequencies

Risk Rank Cases

Heat Flux for

cases

Fire Exceedance

Curves

DAL Fire and

Load


Sectio

n 1

DAL Fire and

Load

Structure

temperature and

strain response

Simulation

Temperature and

strain response in

structure

Modify

PFP

Modify

mitigation

measures

Minimum PFP

Sectio

n 2

Risk

evaluation

Global

Collapse? Strain or

temperature

Criteria

End

Start

Initial Fire Risk

Assessment

CFD Fire

Modeling

Fire Risk

Assessment

Leak

Frequencies

Ignition

Probabilities

Inventories

Fire Location

Highest Ranked

Cases

Heat Flux

Fire

Frequencies

Risk

Acceptance

Criteria

Fire Frequencies

Risk Rank Cases

Heat Flux for

cases

Fire Exceedance

Curves

DAL Fire and

Load


Sectio

n 1

DAL Fire and

Load

Structure

temperature and

strain response

Simulation

Temperature and

strain response in

structure

Modify

PFP

Modify

mitigation

measures

Minimum PFP

Sectio

n 2

Risk

evaluation

Global

Collapse? Strain or

temperature

Criteria

End

Start


Two acceptance criteria:

Temperature < 500 C in steel (see Chapter 6 in TN11)

- Can do local heat transfer analyses of single beams

Strain criteria: Global collapse not accepted

- Stress < Yield stress,

- Plastic utilization < 1.

- Use FE model

- Collapse of single beams is accepted if loads are re distributed

© Det Norske Veritas AS. All rights reserved. 33

Structural Response analysis

Context and input

• Calculate Structural response for structure, equipment and

pipe support given the dimensioning fire scenario



Context and input

Interface fire analysis - structure analysis



• Identify the fire case at 10-4 per year. (duration and KFX case

with 3D transient heat flux development)

• Dimensioning fire deveolopment represented by a series of KFX

simulation snap shots with corresponding duration



Context and input

Interface fire analysis - structure analysis

Short summary



• Identify the fire case resulting in the dimensioning fire dose

(duration and transient heat flux development)

• Dimensioning fire case represented by a series of KFX

simulation snap shots with corresponding duration

i. Temperature development in steel calculated using FAHTS

ii. Structural response calculated using USFOS

iii. Fire location moved around to reflect many possilbe leak

locations

iv. Optimize design – ESD and PFP


Dimensioning fire scenario

Steady state simulation for const. leak

rates: (jet or pool)

- 50 kg/s

- 25 kg/s

- …

Dimensioning fire deveolopment represented by

a series of KFX simulation snap shots with

corresponding duration

36

25 kg/s

10 kg/s

5 kg/s

1.25 kg/s

0

5

10

15

20

25

30

0 500 1000 1500

Lea

k R

ate

(kg

/s)

Time (s)

Transient

Steady


Temperature development in steel

37

Example of temperature development calculated with FAHTS representing the

dimensioning fire with KFX simulation results


Structural response/utilization calculated using USFOS

38

Example of utilization calculation using USFOS to determine possilbe yielding or

plastic deformation of structure


Example of fire locations

39

«moving» the simulated fire around to reflect many possible leak locations


Passive fire protection

40

Established in an iterative process, and with many leak locations of the

dimensioning fire load


Optimization in iterative process

• Reduce loads by ESD

isolation and Blowdown

• Passive fire Protection

optimization

Calculate

dimensioning

fire loads

fKFX simulations

Structural

response

Evaluation Update ESD

Update PFP


Simplified FRA assessment

Step 1: Initial fire risk assessment

- List of scenarios with associated fire frequency (leak frequency, ESD/BD failure probabilities

and ignition probabilities)

- List and Locations of fire scenarios (coordinates) and targets

Step 2: Simplified Fire consequence modelling

- Duration based on

- Leak profile calculations with cut-off 0.1 kg/s for jet fires, considering detection, isolation and BD

- Simple pool fire modelling considering size of pool and constant combustion rate per m2 for pool fires

- Apply generic heat flux peak load values from Table 1, NORSOK S-001, Edition 4

- Exposure assessed based on flame lengths calculated either with Phast or simplified

formulaes for jet fire length as function of release rate (depending on congestion and

obstacles)

Step 3: Fire risk assessment

- Coarse assessment by use of Excel calculating heat dose exposing each target from each

scenario case (leak size, ESD/BD failure case)

Step 4: Risk evaluation

- Fire dose exceedance plots and fire DAL heat dose and duration

42


Simplified fire consequence calculations

Leak profile example, after successful isolation and 30 sec delay BD

Flame lengths are estimated f.ex. by use of API RP 521, 1989 method

where Q is the leak rate and Hc is the heat of combustion (J/kg)

43

478.0)(00326.0 cf QxHL


Results

Duration or fire dose exceedance curve

44

1,E-08

1,E-07

1,E-06

1,E-05

1,E-04

1,E-03

0 5 10 15 20 25 30

Ex

ceed

ence

fre

qu

ency

(p

er y

ear)

Duration (minutes)

Separator


Summary – a consistent way of design

Advanced CFD/FE and probabilistic approach gives a more realistic and consistent

fire load than simplified methods

- Includes real fire effects which locally can be worse then averaged

- In general shorter fire durations

- Used for optimization of PFP

Fire exceedence plot is common intermediate result

- Max dose describes size of the fire by one number

- Gives the DAL fire and scenario to be used from CFD

- Gives the escalation probability to QRA

A consistent CFD, FE and probabilistic approach analysis can optimize all

mitigating measures

- Risk for fires and explosions can now be compared on the same detailed foundation

- Easier to make decisions about mitigating measures

- Firewalls and arrangement improved

Discussion

- Develop a NORSOK Z013 Probabilistic fire analysis procedure


46


Safeguarding life, property

and the environment

www.dnv.com

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