Joint industry project ("JIP") on "Blast and Fire Engineering for Topside Structure"

Blast and Fire Engineering for Topside Structures is a joint industry project ("JIP") where SINTEF NBL (Norwegian Fire Research Laboratory) was involved to execute the Test Programme, Confined Jet and Pool Fires. The total joint industry project was financed with approximately NOK 40 000 000,- where the Test Programme, Confined Jet and Pool Fires was financed with approximately NOK 8 000 000,-. This part of the project has been carried out at SINTEF NBL from the beginning of 1994 and was finalized in mid 1996. 11 industrial companies were participating in this "JIP" project.

SINTEF’s task in the Phase II Blast and Fire Engineering for Topside Structures was to study enclosed fires, with the scope to:

"Improve the understanding of confinement on jet and pool fires, obtain extensive quality data for comparison with fire model predictions, and to form the basis of Guidance on hazard consequences of offshore fires in partially confined areas."

24 tests of enclosed hydrocarbon fires were carried out in SINTEF’s large-scale facilities in Trondheim. 5 tests were carried out in a 135 m3 test rig, and the remaining 19 were carried out in a 415 m3 rig. The rig was made from steel, with an inner lining of stainless steel plates, and it was insulated by ceramic fibre between the inner lining and the load bearing structure. It was constructed to withstand several tests lasting until quasi-steady-state conditions were reached inside the enclosure. This means that the temperature rise had slowed down significantly, or reached a near steady-state level. Typical test duration was 15 minutes, but some tests had to be stopped after shorter time, due to intolerable temperatures inside the test enclosure or in the ceiling of the test hall. The rig was instrumented to measure mass-and heat transfer to the enclosure, temperature development of combustion gases, walls, ceiling and objects located inside the enclosure, gas composition, flame characteristics at the vent, radiative and total heat fluxes, velocities and pressures. A number of tests were also dedicated to study the effect of a typical offshore deluge system on fire characteristics.

One gaseous propane jet fire test was carried out in the 415 m3 rig, to make a link to former experiments in smaller scale. In all remaining tests liquid Sleipner condensate was used as fuel. Leakage rates from about 0,3 kg/s (in the 135 m3 rig) and 0,85 –1,04 kg/s (in the 415 m3 rig) were tested in vertical and horizontal jets. Pool fires of 6 m2 and 24 m2 were tested, at the different scales.

The influence on fire severity of global air/fuel ratio, fuel properties and scale were studied. The variation of air/fuel ratio was obtained mainly by varying the vent opening in one end wall of a test rig, which was approximately twice as long as its width and height. The effect of insulation was not tested, as all experiment were carried out in an insulated rig. The effect of early (within 1 minute) and late (after 6 – 15 minutes) activation of the water deluge system was tested, but no systematic variation of water application rate was carried out.

The main findings are reported in two reports, references /1/ and /2/. More detailed information and results are given in individual test reports. Additionally, video recordings from different observation positions exist, and a set of compilation videos from jet- and pool fires has been made.

Among the findings are:

A. Effect of Confinement on the Behaviour of Jet and Pool Fires

• During the initial stages of fire development, confined jet fires and pool fires behave would in the open.
• After a short period, ranging from a few seconds to a few minutes, the development of the fire depends on the degree of ventilation control, specifically on the value of global stoichiometry. (Definition: f = Air/fuel mass rate into the compartment divided by the air mass requirement for stoiciometric combustion).
• A well defined horizontal interface between an upper hot gas/smoke layer and lower cool air layer forms. Depending on the relation of the vent flow and the size and position of the fire source, the conditions for ventilation or fuel controlled burning is eatablished. When the burning approaches ventilation controlled conditions, it is possible for combustion at the interface between these layers to be highly oscillatory and unstable, leading to rapid vigorous combustion and high heat fluxes and temperatures above 1350° C due to soot oxidation when temperatures rise above approximately 1200° C.
• Before steady state conditions are achieved, incident heat fluxes and temperature rise rates can diminish if the fire enters a ventilation controlled regime. Copious amounts of soot are produced from incomplete combustion, particularly when the temperature of the smoke layer is > 900 oC. If the soot is deposited in an area where the temperature is below 900° C, the soot can act like a heat shield insulating the surface of the walls, roof and objects from the radiative flames. If the soot is deposited in an area where temperature is high enough (>1200° C) the burning soot may contribute by increasing the heat load to the walls, roof and objects. How severe this effect is and the exact temperature at which this becomes important can only be estimated. An estimate based on the experimental conditions in this project is that a "fireball" of diameter > 4-6 m is needed to ensure extensive soot oxidation, leading to the increased heat load.
• In general the CO level increases with decreasing f for all large scale fires, but the temperature and residence times are also important as these parameters determine the dominant combustion reaction kinetics. For the small scaled fires (135 m3) the CO level seems to be more constant when f varies. In the tests with temperatures above 1200 ° C in large volumes, the concentration of CO is also significantly higher than in other tests. This indicates more extensive soot oxidation resulting in more CO under these conditions.
• Confined condensate pool fires never reach extremely ventilation controlled conditions. (f never < 0.8). The overall burning rate of condensate pool fires enters a self-limiting regime as ventilation controlled conditions are approached such that f is always greater than ~0.8. The final burning rate is lower than expected when comparing with the burning rate of an open pool fire of the same size.

B. The Thermal Load onto process Vessels, Pipework, Module Walls and Module Decks from Jet and Pool Fires.

• The general finding in the tests is that a heat flux density onto a target (total incident heat flux, both radiative and convective) is up to 200 kW/m2. For certain conditions considerably higher fluxes are seen, in the order of 350 - 400 kW/m2. These fluxes occur simultaneously with high temperatures, above the saturation level of the data logging system 1370 ° C, extensively throughout the compartment.
• The average residence time for the combustion reactants in the confined horizontal jet fire tests was higher than in the vertical jet fire tests. This also occurred in the pool fire tests with a large ventilation opening and with the split vent. These geometries promoted the formation of a single large eddy inside the compartment, as opposed to the normal two eddies (one inside the pool fire or the vertical jet, and one close to the vent area). We speculate that the flow field associated with the single eddy structure promoted higher heat fluxes and temperatures than would otherwise be expected.
• At steady state conditions, incident heat fluxes to the surrounding walls, ceiling and impinged objects are comparable in magnitude to those found for impinging jet fires or pool fires in the open, but can be higher under certain conditions.
  

C. Effectiveness of Active Water Deluge Mitigation Systems.

• The effect of water deluge on ventilation controlled jet fires may not be extinguished if insufficient The effect of water deluge on ventilation controlled jet fires is to extinguish the fire. Fuel time elapses before water deluge activation as the compartment must be 'hot' prior to deluge in order to extinguish the fire. Nevertheless the fire, if present, burns at a much reduced rate and is effectively controlled.
• There are no significant differences between the effects of water deluge on vertical and horizontal jet releases, for the test conditions studied.
• It is possible for the fire to re-ignite after the water deluge is terminated due to the presence of hot gases or surfaces coming into contact with fuel.
• Extinguished jet fires represent a potential explosion hazard if the fuel continues to be released.
• Generally, confined pool fires are not extinguished by water deluge, but the fire is controlled and burns at a much reduced rate.
  

D. Basic data for model validation and for development of simple fire hazard prediction techniques.

An extensive set of quality data for model validation and development of fire hazard prediction techniques is presented in this report, in the technical reports (one individual report for each test) and in video recordings from all tests. The accuracy and usefulness of data is presented in Section 5.4 of the Final Report, /1/, and in the Interpretation report, /2/.

References:

1. G.A. Chamberlain and M.A. Persaud, Shell Research Thornton , UK
R. Wighus and G. Drangsholt , SINTEF NBL as (Norwegian Fire Research Laboratory):
BLAST AND FIRE ENGINEERING FOR TOPSIDE STRUCTURES, Test Programme F3, Confined Jet and Pool fires. FINAL REPORT.
SINTEF report STF25 F95028, May 1997.
 
2. M.A. Persaud, Shell Research Thornton , UK and R. Wighus, SINTEF NBL as (Norwegian Fire Research Laboratory):
BLAST AND FIRE ENGINEERING FOR TOPSIDE STRUCTURES, Test Programme F3, Confined Jet and Pool fires. INTERPRETATION REPORT.
Shell Research and Technology Centre, Thornton RTS Report OP.97.47127, November 1997
 
3. C.A. Selby and B.A Burgan:
Blast and Fire Engineering for Topside Structures - Phase 2, Final Summary Report.
The Steel Construction Institute, UK, SCI Publication number 253, 1998.

Contact person: Ragnar Wighus
Telephone: + 47 73 59 10 78
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