Fire on the Sea Surface - Phase III: Ignitability and Sustainability - Threshold Ignition Sources

The objective of these burn tests was to quantify the heat release from different ignition source sizes and to find the minimum heat transfer needed to ignite condensate and crude oil slicks under different environmental conditions.

The variables were:

Oil composition
Air flow rate
Strength of ignition source
 
 

This study on wave effects consisted of:

Literature review.
Contact with experts in the field of hydrodynamics with emphasis on oil spill fate and behaviour at sea.
Assessment of the current position.
 
 

The main elements of this project were:

Meso-scale basin tests (total of 21 tests).
Theoretical study of wave effects.
Theoretical/cone calorimetric study of dependency between oil type and liquid temperature with respect to ignitability.
Recommendations whether "Falling oil tests" are required or not will be concluded based on results by the project.

The tests were carried out on a sub-layer of fresh water at the same initial temperature as the fuel.

The measured parameters were: 

• Time to ignition
• Flame spreading rate
• Radiant heat at a number of locations over the test area
• Temperature in the liquid at a number of points away from the ignition source
• Air temperature at a number of points across and above the slick surface
• Air flow velocity profile above the slick

Specific conclusions from the current study are presented below:

1. The results show that spilled condensate and crude oils can be ignited very easily over a wide range of temperature and air flow rate conditions.
2. Once ignited, the condensate and the light crude oil burned well in most conditions of temperature and air flow rate. The heavier crude oil produced slower flame spreading when it could be ignited.
3. The difference in initial ignitability and subsequent burning appears to be due to differences in the flammable vapour concentration created above the spilled oil. The vapour concentration is a function of:

• Oil composition (the proportion of the most volatile and flammable components with boiling points below 150 °C).
• The prevailing temperature which controls the rate of evaporation of the volatile and flammable components.
• Increasing air flow rate did not extinguish the flames in many cases because the vapour concentration was not adequately dispersed, or diluted, into the air in the region closest to the oil (the boundary layer). Ignitable concentrations of flammable vapour persisted in the boundary layer, even at high air flow rates. 

The main conclusions are:

• The fire risk will be high for condensate and some fresh oils
• The slick thickness will be a major limiting factor for ignition and sustained burning
• Ignition of some petroleum products can be expected in air flow rates exceeding 15 m/s

ACKNOWLEDGEMENTS

These research projects were supported financially by BP Sunbury, Elf, Norsk Hydro Saga Petroleum, Statoil, and UK Health and Safety Executive. The authors would like to thank members of the steering committee for their technical support and input over the course of this programme.
 

Contact person: Kristen Opstad
Telephone: + 47 73 59 10 78
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