Accidental Fires. Design criteria. J. Amdahl,, NTNU Dept, Marine Technology

Transcription

1 Accidental Fires Design criteria 1

2 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 2

3 NORSOK STANDARD Risk acceptance criteria CharacteristicDesign Accidental Actions corresponding to annual probability of occurrence of ~10-4 per installation Overall frequency all accidents often used impairment frequency limit 3

4 Fire and Explosion Design Against Accidental Fires often based standard fires 1000 C HC (NPD) ISO 1 Hour Time Standard heat fluxes e.g kw/m2 Fire duration e.g. 10 min. 2hrs Exposed areas envelope curves Standard Temperature Curves Standard heat fluxes 4

5 Example traditional fire scenario: Pool fire from process inventories on cellar deck in 55 minutes Main deck only 100 Snorre B Fire Event 4b Fire exposure [kw/m 2] 0-55 minutes Cellar deck and underside main deck 5

6 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 6

7 Passive Fire Protection Pressure [bar] 100 Jet Fire Some Examples Rupture of a gas pipe, gas ignites. Pressure drops from 300 bar to 0 within 15min. Temp [C] 1000 Thin wall (Sec. Steel) Thick walll (Main Steel) Pressure 3 0 Time [min] 3 0 Time [min] Typical temperatures of unprotected steel Question: Does this accidental fire cause structural collapse? 7

8 Passive Fire Protection Motivation 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 : 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) 8

9 Passive Fire Protection Pool Fire Some Examples, cont. Oil Leakage, oil flows on deck. Ignites. Deck is unprotected Question: Will the floor beams on the underside of the deck collapse? 9

10 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. 10

11 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). 11

12 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 12

13 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?) 13