A jet or spray fire is a turbulent diffusion flame resulting from the combustion of a fuel continuously released with some significant momentum in a particular direction or directions. Jet fires can arise from releases of gaseous, flashing liquid (two phase) and pure liquid inventories.
Jet fires represent a significant element of the risk associated with major accidents on offshore installations. The high heat fluxes to impinged or engulfed objects can lead to structural failure or vessel/pipework failure and possible further escalation. The rapid development of a jet fire has important consequences for control and isolation strategies.
The properties of jet fires depend on the fuel composition, release conditions, release rate, release geometry, direction and ambient wind conditions. Low velocity two-phase releases of condensate material can produce lazy, wind affected buoyant, sooty and highly radiative flames similar to pool fires. Sonic releases of natural gas can produce relatively high velocity fires that are much less buoyant, less sooty and hence less radiative.
- To identify areas of uncertainty in the characterisation of jet fires;
- Identify where the jet fire hazard is significant in relation to other hydrocarbon hazards;
- Initiate research to increase knowledge and understanding in ill-defined areas of jet fire evaluation; and
- Promote the use of a consistent methodology for evaluation of jet fire hazards.
Current knowledge of jet fire hazards
The hazards, characteristics and physical properties of hydrocarbon jet fires have been appraised in the Phase 1 reports of the Joint Industry Project on 'Blast and Fire Engineering of Topside Structures':
The main source of detailed information on the characteristics of jet fires covered in the reports on the programme of jet-fire research co-funded by the European Community. This programme studied single fuel natural gas and propane jet fires (Bennett et al, 1990).
Notable recent gains in knowledge have been in the area of unconfined crude oil jet fires and confined jet fires (compartment fires). These areas have been studied in Phase 2 of the JIP on 'Blast and Fire Engineering of Topside Structures' (Selby and Burgan, 1998) and another JIP on releases of 'live' crude oil containing dissolved gas and water.
Unconfined two-phase crude oil jet fires
Phase 2 JIP focussed on horizontal free jet fires of stabilised light crude oil and mixtures of stabilised light crude oil with natural gas viz:
- The free flame releases, of crude oil only, were not able to sustain a stable flame and one of the mixed fuel releases was also unstable.
- All the flames were particularly luminous compared with purely gaseous jet flames and generated large quantities of thick black smoke, mainly towards the tail of the flame.
- All the flames were highly radiative, with maximum time averaged surface emissive powers (SEP's) ranging between 200 kW m-2 to 400 kW m-2.
- The incident total heat fluxes (radiative and convective) measured on the pipe target were significantly higher for the mixed fuel tests than for the crude oil only tests, by a factor two in many cases. Typical values were in the range 50 kW m-2 to 400 kW m-2.
Gosse, 1999 carried out a separate JIP to quantify the hazards posed by realistic releases of 'live' crude oil containing dissolved gas and water.
Small amounts of water had little effect on the characteristics of the fire, larger amounts produced a dramatic reduction in the smoke produced and increasing the water further took the flame to the point of extinguishment.
For high water-cut releases, smoke will be less of a hazard, but very high water-cut releases will not produce stable jet fires.
Work by Chamberlain (Chamberlain 1994 & 1995, OTO 94 011-024) and the Phase 2 JIP (Selby and Burgan, 1998), studied the effect of varying a range of parameters on the fire behaviour. Including size and location of openings (vents), fuel type, release height and pressure for jet fires. Gas temperatures within the compartment, wall temperatures, ceiling temperatures, target temperatures, heat fluxes to the walls, ceiling and target, gas composition of the smoke layer and fuel release rates.
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Jet fire modelling
It was generally accepted in 1991 that the semi-empirical models provided the most accurate and reliable predictions of the physical hazards associated with fires, providing their application is limited to the validation range of the model. (recorded in OTI 92 596 [PDF 6mb]) This conclusion essentially remains valid today. A recent CFD study, carried out for HSE/OSD (OTO 1999 011) by Shell, showed good agreement with medium and large-scale jet-fire experiments.
At present, commercially available semi-empirical models can provide accurate prediction of flame shape, flame size and external radiation flux to external objects but not heat fluxes to impinged objects. The latter must be treated empirically.
Confined jet fires
The general level of understanding of compartment fire behaviour is now sufficiently good to assess most compartment fire hazards with some confidence for modules having simple geometries. In particular jet-fire temperatures, smoke layer temperatures, heat fluxes to surfaces within the module, the extent of external flaming and internal impingement zones can be reasonably well predicted. Estimates for CO concentrations in the smoke layer are also available based on empirical relationships to temperature and flame stoichiometry.
Areas of uncertainty
Confined and Unconfined jet fires
Future improvement in model development of confined jet fires should focus on evaluating the combustion product emissions from module vents.
As fields reach maturity, increasing amounts of water are entrained and formation of an unstable jet fire my result. In unconfined jet fires the flame may be extinguished before the fuel supply has been cut off resulting in an explosion.
There is little information relating to high water cut (e.g. 3:1 or 4:1) releases.
Actual heat transfer from the fire to the vessel is not fully definable, and hence failure conditions and emergency depressurising requirements become uncertain.
Industry practice in assessment of jet fire hazards
Current industry practice is to analyse jet fires for length of the jet fire with respect to distances of plant equipment, buildings, population etc. The extent of impingement into affected area is considered along with the need for PFP, emergency depressurisation and other mitigation options.
It is assumed that personnel are able to survive and escape from exposure to heat fluxes less than 5 kW m-2, but fatality is assumed for higher heat flux values.
The effect of jet fires with high heat fluxes (e.g. flame temperature of 1350 º C, heat flux of 400 kW m-2 are not generally taken into account in safety case assessments. This is because these types of flame are not included in current guidance.
Strategy development issues
- To raise awareness of the effect of jet fires on pressurised systems and the inadequacy of API 521 for fire attack scenarios and the potential for under-sizing of pressure relief systems;
- To raise the awareness of the particular hazards posed by ventilation-controlled jet fires;
- To consider mitigation and control issues relating to the jet fires in the open and confined areas;
- To define the circumstances in which ventilation-controlled jet fires could occur on offshore installations;
- To develop simple tools to assess the hazards posed by ventilation-controlled jet fires;
- To collect data and understanding on the heat flux from high temperature jet flames (ca. 1350 ° C) and the effects of exposure on pressurised storage vessels;
- To develop an understanding of the current capability to model confined (fuel-controlled) jet fires;
- To evaluate such models against suitable large scale test data;
- To develop simple tools to assess the hazards posed by confined jet fires.