2 Aims and principles of fire and explosion hazard management management
Criticality 3 Items whose failure in an explosion may result in module wide escalation, with potential for inventories outside the module contributing to a fire due to
4 Interactions between fire and explosion hazard management management
4.4 Fire and explosion mitigation methods .1 Active fire-fighting systems .1 Active fire-fighting systems
′ = 1 22.4−
V V x ...Equation 4-1 Where,
V is the reduced ventilation rate (m3 s-1) V' is the original ventilation rate (m3 s-1) x is the gas release rate (m3 s-1)
For leak direction co-flowing with the ventilation flow direction;
( )
′ = 1 12.45+
V V x ... Equation 4-2 Where,
V is the increased ventilation rate (m3 s-1) V' is the original ventilation rate (m3 s-1) x is the gas release rate (m3 s-1) Influence of water deluge
Research [4.3] has indicated that the presence of water deluge reduces the ventilation rate in naturally ventilated areas. The data on this are very limited and applies only to deluge rates in the order of 24 l min-1 m-2.
It is suggested that a conservative estimate of this effect would be to reduce the area ventilation rate by 30 % when the area deluge system is operating.
4.4 Fire and explosion mitigation methods
4.4.1 Active fire-fighting systems
For ‘new builds’, this may be achieved by the judicious location of water deluge pipe-work.
However, because of its very nature, such a system will be distributed throughout the whole of the area; it is likely that only a limited degree of protection could be provided.
It is now well established that in well-vented areas, the presence of an area water deluge can reduce the severity of explosions. This would appear to be an argument for the initiation of water deluge, before the ignition of a flammable atmosphere takes place. The could have the benefits of reducing the explosion severity to an extent that the water deluge system in operation to control or mitigate the effects of any subsequent fire, and also prevent damage to automatic isolation and blow-down systems. Reported research on the effects of water sprays [4.5] has provided correlations to estimate the reduction in explosion severity by area deluge.
The correlations also demonstrate the variation in explosion severity with the gas concentration in the flammable atmosphere. The correlations are:
In the absence of water-spray,
( ) ( )
In the presence of water-spray,
( ) ( )
CS is the gas stoichiometric concentration
It is not possible to provide a generic correlation for the absolute reduction in explosion severity, as the domains in which the explosion takes place will vary from one another.
However, it can be stated that the ratio of the unmitigated explosion severity to the mitigated explosion severity increases as the unmitigated explosion severity increases. Thus area deluge mitigation of explosions is most effective where very severe explosions can occur. This means that such a system will be most effective in large, congested well-vented areas.
The mitigation of explosions by area water deluge will only occur when the explosion is accompanied by significant flame acceleration. In practice, this means large, well-vented domains. Where this does not occur, such as in enclosed domains area water deluge will not provide any benefit, and in some cases, could increase the explosion severity.
As has been stated above, the presence of water-deluge will result in the reduction of the natural ventilation rate. This will have the effect of increasing the equilibrium, size of the flammable cloud and also increasing the time required to disperse this flammable cloud. In ageing platforms, there has been a concern that the presence of water-deluge may increase the probability of ignition due to water ingress into electrical equipment.
Thus, in any particular situation, the decision as to whether or not the activation of an area water-deluge is appropriate can only be informed by an assessment of all the above factors.
Water-deluge systems, with or without foam, can be effective in suppressing and extinguishing pool fires. The following correlations for the time to extinguish pool fires have been developed from a research programme of fire trials [4.6]. These trials used diesel as the fuel but the correlations would give a reasonable approximation for the time to extinguish stabilised crude oil fires.
= − +
50
494 376 29
T Y
Y ... Equation 4-5
= − +
E
859 448 80
T Y
Y ... Equation 4-6 where
T50 is the time to reduce the fire size by 50 % (s) TE is the time to extinguish the fire(s)
= × Y C U
and
C is the water spray area cover rate (l min-1 m-2) U is the internal wind speed (m s-1)
These correlations are specific to water sprays with droplets having Sauter mean diameters of 400 to 500 microns. The research report [4.3] indicates that the water droplet diameter can have a significant effect upon the time to extinguish a pool fire. In summary, large droplets are more effective than small droplets, inasmuch as they are less easily displaced by ventilation crosswinds and can penetrate the fire plume more effectively.
The addition of a foaming agent to the water spray system can reduce significantly the time extinguish a pool fire compared to those predicted from Equation 4-5 and Equation 4-6.
One additional factor in foam compound selection is the viscosity of the finished foam. For two
Where water miscible fuels may be encountered, then alcohol resistant foam is necessary.
In locations where the ambient temperature can be below 0 °C for a significant time the foam compound should be ‘freeze-protected.’
A number of characteristics affect the effectiveness of water spray systems (designed for example to NFPA15 [4.6]) against either pool or jet fires. The effects of water systems with respect to different fire types are discussed in more detail in various sub-sections in Section 5.2.
However, area water deluge can provide significant protection against incident thermal radiation from both jet and pool fires.
The research report [4.3] suggested the following correlation for the reduction in incident thermal radiation.
( )
= 100 tanh 1.55
R x ... Equation 4-7 where
R is the percentage reduction in incident thermal radiation
= ×
x f Land
f is the water volume fraction in the atmosphere L is the distance through the water spray or curtain (m)
This indicates that the presence of area deluge could provide significant protection for personnel escaping from the location of a fire. The same applies to the use of water curtains if these are of adequate thickness.
Area deluge systems or water curtains cannot be relied upon to protect personnel from the thermal effects of an explosion. These thermal effects are of too short a duration to prevent any serious risk of failure of equipment or structural elements. Such risks would be associated with the blast and drag effects of an explosion. Obviously, water spray systems can provide no protection against such effects. Dual agent (foam and dry powder) can be effective in the suppression and extinguishment of pool fires. Their effectiveness is probably limited to enclosed areas due to the problem of delivering the dry powder to the base of the fire in open, well-ventilated areas; where effective, dual agents can reduce the fire duration to less than that where water deluge and foam are used.
Any area deluge or local cooling system should be fully operational as soon as possible after the receipt of an initiating signal. The recommendation for the maximum value of this time delay, given in NFPA, should be adhered to. This is because waterspray heads constructed of brass or gunmetal will, when exposed to flame impingement, suffer major damage if water flow has not been established. This level of damage is likely to occur within 60 seconds and seriously degrade the effectiveness of the waterspray system. This could be avoided by the use of waterspray heads constructed of a high melting point material, such as super-duplex stainless steel. However this option would be accompanied by a severe cost penalty.
Consideration should be made as to whether the fire hazard may extend beyond the notional fire area, thus mitigation measures should be able to protect from fire effects from outside (via an adjacent module for example). The application of water systems should then be appropriate to the fire hazard identified and also to the type of protection required, e.g. does the outside area include key escape routes form the primary affected area to the Temporary Refuge.
Where high voltage electrical equipment, or equipment susceptible to damage by exposure to water are present then conventional water deluge or foam systems will not be appropriate fire fighting systems. Historically, such equipment has been protected against fire by the installation of Halon flooding systems. Since the adoption of Montreal protocol, this option is no longer available. A number of drop-in Halon replacement systems have come on to the market but these have not yet seen prolonged general service and thus limited data are available on their effectiveness in ‘real’ fire situations.
Water mist systems appear to be very effective against electrical fires in enclosed areas.
However, there is no general agreement as to whether or not unacceptable levels of damage to such equipment would ensue. At the time of writing this Guidance, more evidence is required to validate this objection to the use of water mist systems.
4.4.2 Fire-proofing systems
Two types of fireproofing materials are in general use on offshore installations. These are:
1. Inert materials;
2. Intumescent materials.
The inert materials provide excellent protection against fire exposure and a resistant to the erosive effects of jet flame impact. They do suffer from the disadvantage of increased load on the structure. It is for this reason that the intumescent materials are generally preferred. The intumescent materials can also provide excellent fire protection. However, there is a concern that the erosive effect of jet flame impact could dislodge the ‘char’ formed and thus reduce the effectiveness of the fireproofing. Where these materials are to be used, the material manufacturer should provide jet fire test data to demonstrate that this is not a problem.
The design standard performance specifications for fireproofing materials are generally based on diffusion flame engulfment rather than jet flame impact. Thus the need for the test data referred to above is reinforced.
In this context it is of course necessary to know whether diffusion flames or jet flames will be encountered. The research report [4.7] does provide some evidence on the likely rainout of liquid from an ignited two-phase release. If the rainout is significant then a pool fire will result.
If not, then a spray fire (equivalent to a jet fire) will result. It is suggested that for ignited two-phase releases;
• If the GOR is low, then at drive pressures above 10 bar absolute a spray fire will result.
• If the GOR is high, than at drive pressures above 5 bar absolute a spray fire will result.
The effectiveness of both types of fireproofing materials can degrade over time. This can be due to mechanical damage of the coating, especially the sealing topcoat. This in turn can lead to water ingress and deterioration of the fireproofing material, together with possible unrevealed corrosion of the substrate. This can be avoided by regular inspection of the fireproofing coatings and repair as necessary. Ideally, the fire-proof coating of any item should be capable of withstanding an explosion blast loading, up to the failure loading of the equipment item or structural element concerned, without suffering any significant degradation of the fire-proof rating. This would retain the protection provided by the fire-proofing against any fire subsequent to the explosion. The design of fire-proofing systems is universally carried out on the basis of the fire loading only.
4.4.3 The temporary refuge
Ideally, the temporary refuge should provide for the protection of personnel against the effects of both fires and explosions. Whilst it is feasible that the temporary refuge could provide such protection against the thermal and smoke effects of fires and against the thermal effects of explosions, there will be a practical limitation on the protection that can be provided against the blast effects of explosions. Thus the objective should be to reduce the explosion blast effects on the temporary refuge to as low as reasonably practicable. This is probably best achieved by maximising the separation distance between the temporary refuge the likely locations of explosions as much as it is reasonably practicable to do.