Accidental Fires
Design criteria
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
1
NORSOK STANDARD
DESIGN AGAINST ACCIDENTAL LOADS
Design Accidental Actions and associated
performance criteria determined by
Quantified Risk assessment (QRA)
Main safety functions shall not be impaired
– usability of escape-ways
– integrity of shelter areas
– global load bearing capacity
Integrity normally
for et least 1 hour
Beregning av ulykkeslaster for offshore
J. Amdahl,, NTNU
Dept, Marine Technology
stålkonstruksjoner – NFS 15.04.2008
2
NORSOK STANDARD
Risk acceptance criteria
CharacteristicDesign Accidental Actions
corresponding to annual probability of
occurrence of ~10-4 per installation
Overall frequency 5·10-4 all accidents
often used impairment frequency limit
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
3
Fire and Explosion
Design Against Accidental Fires often
based standard fires
HC
(NPD)
 Standard heat fluxes e.g.
1000
°C
100-250 kW/m2
ISO
 Fire duration – e.g. 10 min. –
2hrs
 Exposed areas – ”envelope”
curves
1 Hour
Time
Standard Temperature Curves
J. Amdahl,, NTNU
Dept, Marine Technology
Standard heat fluxes
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
4
Example ”traditional” fire scenario:
Pool fire from process inventories on cellar deck in 55 minutes
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
5
Fire and Explosion
Real fires different from standard fires
 Fire intensity depends upon amount of combustibles,
”fuel” characteristics, environmental conditions etc.
 Heat exposure depends upon location with respect to
fire, ”view of fire”...
 Transient temperature development in the material is a
highly nonlinear process, notably radiation
 The structure often possesses considerable reserve
strength, i.e. load redistribution upon first member
failure
 Advanced analysis tools are required
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
6
Passive Fire Protection
Some Examples
Jet Fire
 Rupture of a gas pipe, gas ignites.
 Pressure drops from 300 bar to “0” within
15min.
Thin wall (Sec. Steel)
Temp [C]
Pressure [bar]
1000
Thick walll (Main
Steel)
100
Pressure
3
0
Time [min]
3
0
Time [min]
Typical temperatures of unprotected
steel
Question:
Does this accidental fire cause structural collapse?
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
7
Passive Fire Protection
Motivation
Facts
 Temperatures inside or close to fire are approx. 1000-1500ºC
 Steel loses strength for temperatures > ~1000 ºC
 Aluminum for temperatures > ~3-400 ºC
 Unprotected structural components inside fire reach 1000 ºC
within minutes:
 Thin walled members : 3 - 5 min
 Thick walled members : 5 -15 min
Conclusion: Non-redundant Structural Components Exposed to “Offshore” Fires
(lasting for more than minutes) need fire protection (f ex. PFP)
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
8
Passive Fire Protection
Some Examples, cont.
Pool Fire
 Oil Leakage, oil flows on deck. Ignites.
 Deck is unprotected
Question:
Will the floor beams on the underside of the deck collapse?
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
9
Example: Advanced fire scenario
HC gas release at riser balcony from production riser.
Initial release, 30 kg/s, constant for 5 min., then linearly
reducing to 0 at 20 min. Wind: 2 m/s.
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
10
Performance Based Design
used for Fire

Define functional requirements, (for example 1 hour
structural integrity).

Identify accidental events with relevant probability
(for example p>10-4)

Compute the fires, the temperature rise in the structure
and the structural performance during the fires.

Use international accepted data for the thermal and
mechanical properties of the materials, (typically using
ISO/Eurocode).
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
11
What is possible to simulate
in 2005?

Combustion of oil and gas (the chemical reaction), where
wind, ventilation, openings, the available Oxygen (O2),
production of soot etc are accounted for.

Combustion of oil and gas when the deluge is activated.

The deluge is modeled with the nozzle locations, the
droplet size, the amount of water, trigger mechanisms for
the deluge (for example radiation level, temperature).

Temperature and radiation on every m2 of the platform

Amount of water [liter per m2 per min] reaching the
different surfaces of equipment and structures
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
12
What is possible to simulate in
2005? cont’d

Temperature development of insulated components
(structures and equipment) including partly protected
components and surfaces with damaged PFP.

“Heat Leakage” through openings in insulation (“how
fast does the heat flow”?)

Mechanical weakening of steel and aluminum
structures and equipment.

Risk for structural collapse, failure of equipment etc.
(Will it fail? How close to failure? What is the margins?)
J. Amdahl,, NTNU
Dept, Marine Technology
Beregning av ulykkeslaster for offshore
stålkonstruksjoner – NFS 15.04.2008
13