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J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS 15.04.2008 1 Accidental Fires Design criteria.

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Presentation on theme: "J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS 15.04.2008 1 Accidental Fires Design criteria."— Presentation transcript:

1 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS Accidental Fires Design criteria

2 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS  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 NORSOK STANDARD DESIGN AGAINST ACCIDENTAL LOADS

3 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS  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 NORSOK STANDARD Risk acceptance criteria

4 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS Design Against Accidental Fires often based standard fires Fire and Explosion 1000 °C Time 1 Hour ISO HC (NPD) Standard Temperature Curves  Standard heat fluxes e.g kW/m2  Fire duration – e.g. 10 min. – 2hrs  Exposed areas – ”envelope” curves Standard heat fluxes

5 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS Example ”traditional” fire scenario: Pool fire from process inventories on cellar deck in 55 minutes

6 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS Real fires different from standard fires Fire and Explosion  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

7 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS Jet Fire  Rupture of a gas pipe, gas ignites.  Pressure drops from 300 bar to “0” within 15min. Some Examples Time [min] Pressure [bar] Time [min] Temp [C] Typical temperatures of unprotected steel Pressure Thin wall (Sec. Steel) Thick walll (Main Steel) Question: Does this accidental fire cause structural collapse? Passive Fire Protection

8 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS Facts  Temperatures inside or close to fire are approx ºC  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 : min  Thick walled members : min Motivation Conclusion: Non-redundant Structural Components Exposed to “Offshore” Fires (lasting for more than minutes) need fire protection (f ex. PFP) Passive Fire Protection

9 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS Pool Fire  Oil Leakage, oil flows on deck. Ignites.  Deck is unprotected Some Examples, cont. Question: Will the floor beams on the underside of the deck collapse? Passive Fire Protection

10 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS 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.

11 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS 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).

12 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS What is possible to simulate in 2005?  Combustion of oil and gas (the chemical reaction), where wind, ventilation, openings, the available Oxygen (O 2 ), 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 m 2 of the platform  Amount of water [liter per m 2 per min] reaching the different surfaces of equipment and structures

13 J. Amdahl,, NTNU Dept, Marine Technology Beregning av ulykkeslaster for offshore stålkonstruksjoner – NFS 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?)


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