FOG ATTACK FOR SHIP FIRES
BY COMMANDER JOHN P. FARLEY
There has been considerable debate regarding the tactical employment of water fog for general firefighting purposes. Even today, the controversy between fog and straight-stream advocates continues. It is generally recognized that a direct straight-stream attack is preferred for an incipient or growing, unobstructed fire, whereas an indirect [exterior, combination] attack is preferred for the post-flashover/fully developed fire scenario. There is, however, a wide range of fires that could potentially fall between these two extremes for which neither of the described attack methods would be most advantageous, including the following fire scenarios:
growing/steady state fires where ob-structions shield the seat of the fire from direct agent application,
growing/steady state fires where multiple fire sources are scattered about the fire compartment, and
low-visibility fires where heat and heavy smoke conditions obscure the seat of the fire.
The conditions (heat, smoke, and fire gases) associated with these fire scenarios typically do not prevent initial entry into the fire compartment. However, the time it takes to maneuver within the fire space to locate and directly attack the seat of the fire does present a significant threat, due primarily to the state of the fire. Uncontrolled, these fires could continue to grow rapidly, potentially resulting in flashover conditions.
The Naval Research Laboratory (NRL) initiated a study onboard the Navy`s full-scale fire test ship ex-USS Shadwell to determine the benefits and drawbacks of using an offensive fog attack vs. a traditional straight-stream attack to extinguish a growing/ steady state class A fire threat within the confines of a ship.
One of the first objectives for this study was to develop a realistic fire threat that would provide a great challenge to attack teams attempting a traditional direct attack using a straight stream. The fire threat also had to be repeatable to allow for evaluation and comparison of the test results independently of fire-related variables.
Prefire preparations include ventilating the fire for 10 to 15 minutes to meet preflashover conditions. Ventilation was then secured. The fire threat developed represented a growing/ steady state class A fire that had multiple fire source locations dispersed about the steel fire compartment, which created near-flashover conditions in the fire space. Flames rolling across the overhead and upper layer temperatures in the range of 500° to 600°C (932° to 1,112°F) typified the fire threat used.
The fire compartment volume was ap-proximately 2,600 cubic feet, and the fuel load consisted of three wood cribs, six particleboard panels, and 18 newspaper-filled cardboard boxes. The wood cribs were initiated by n-heptane pool fires. To provide further realism, obstructions were placed between the fire sources and the entry point to the fire compartment. This forced the attack teams to advance well into the fire space before being able to apply water directly on the seat of the fires (see Figures 1 and 2). For each test, the attack team entered the fire compartment via the joiner door (JD) 2-16-0 with a single 3.8-cm (1.5-in.) handline equipped with a 360-lpm (95-gpm) variable fog nozzle and conducted a direct straight-stream attack or an offensive fog attack.
For the offensive fog-attack method, the attack team entered the fire compartment approximately 1.2 to 1.8 m (4 to 6 ft), took a crouched position, set the vari-
nozzle to the medium fog pattern (60 degrees), and discharged the stream upward at a 45-degree angle into the flaming overhead in front of them. A series of two or three short bursts, two to three seconds in duration, was generally sufficient to achieve fire knockdown. After fire knockdown, the attack team adjusted the spray pattern to a straight stream and advanced to the seat of the fire to complete final extinguishment.
For the straight-stream attack, the attack team immediately advanced to the fire sources and applied agent directly on the seat of the fires. A short-burst nozzle technique was also used to help minimize excessive water usage and steam production.
To compare the test results, measures of performance to evaluate heat, steam, and fire threat experienced by the attack team, as well as the water usage were developed. The five measurements determined to best demonstrate the effectiveness of a given attack method were as follows:
wood crib temperatures (thermocouples),
average of overhead temperatures (thermocouples) in the fire space,
upper vs. lower (calorimeter) total heat flux in the fire space,
average of upper layer temperatures (three strong thermocouples) vs. average of lower temperatures (three strong thermocouples) in the fire space, and
cumulative total water usage.
The wood crib thermocouples showed when the fire was knocked down, when it flared up, and when it was finally extinguished. The average overhead temperatures showed the thermal threat existing in the overhead and how well it was controlled. The calorimeter and thermocouple string data demonstrated how much the thermal balance within the fire compartment was disturbed. Based on the results of these quantitative measures and the qualitative observations made during the fire attacks, it was determined that this offensive fog attack, using a medium-angle fog directed 45 degrees upward at the flaming overhead and discharged in short bursts, was the best method for approaching this particular fire scenario.
Control of the overhead fire threat was best shown by the average overhead temperature data. With the offensive fog-attack method, the overhead temperatures were immediately reduced by 200° to 250°C (392° to 482°F) and then continued to cool for the duration of the fire attack. Because the cooling of the combustion gases was accomplished first, flashover potential was mitigated, and a single attack team was able to safely complete final extinguishment within five to 10 minutes after entry into the fire space. In contrast, the data for the traditional straight-stream tactic showed that the overhead temperatures were reduced initially but quickly rebounded to their original values. Since the attack teams were not able to get control of the fire, the conditions for flashover evolved, requiring the attack team to retreat out of the fire space. These conditions usually presented themselves within two minutes.
The offensive fog attack also resulted in the least amount of disturbance to the thermal layer. The disturbances of the thermal balance within the fire compartment were best shown by comparing the total heat flux measured by the calorimeters mounted 0.9 m and 2.4 m (3 ft and 8 ft) above the deck in the fire compartment (see Figure 3). The key indicator of significant disturbances in the thermal balance was the upward spike in the 0.9-m (3-ft) heat flux that approached or met the heat flux for the 2.4-m (8-ft) calorimeter. This indicated total compartment mixing with steam.
During the offensive fog test, the initial attack actually cooled the upper layer enough to result in a 14.2-kW/sq m (1.25 Btu/sq ft-) drop in the 2.4-m (8-ft) heat flux level. While steam was produced, it was described as more of a moist “sweaty” type of steam rather than a hot penetrating steam. In contrast, for the straight-stream test, there were several instances in which the thermal balance was disturbed sufficiently to cause the upward spikes on the 0.9-m (3-ft) heat flux plot (see Figure 3). For all of the straight-stream evolutions, the thermal balance was disturbed sufficiently to impose a serious heat and steam threat to the attack team members. To highlight the significance of this finding, it was noted that none of the attack team members suffered burns during the offensive fog attack tests, whereas many of the straight-stream attacks resulted in burns to the hands, wrists, face, neck, and back.
It is acknowledged that the demands put on manpower and resources and the particular fire scenario will dictate the optimum firefighting attack strategy. It is believed, though, that the offensive fog attack strategy is the best method to safely maintain a rapid, continuous, and aggressive response to a fire when entry to the fire space can be made but direct access to the fire seat cannot be gained. The situations in which a fog attack should be considered include a horizontal approach to a free-burning fire where (1) the overhead gases are burning, (2) the seat of the fire is obstructed and water streams cannot be applied directly to the fire seat, or (3) multiple fire seats are growing within the fire space such that one fire seat could grow out of control while water is being applied to another fire seat.
Another possible situation that may warrant a fog attack would be a fire scenario in which smoke or near-homogeneous thermal conditions obscure a visual or infrared image of the fire but the sounds emanating from the burning fuel bed provide a reasonable indication of an immediate fire threat. n
Farley, J.P.; J.L. Scheffey; C.W. Siegmann III; T.A. Toomey; and F.W. Williams. “1994 Attack Team Workshop: Phase II–Full Scale Offensive Fog Attack Test.”NRL Ltr Rpt 6180/0798.2, Nov. 17, 1994.
Grimwood, P.T. April 1992. Fog Attack, Firefighting Strategy & Tactics–An International View. FMJ International Publications Ltd., April 1992.
n COMMANDER JOHN P. FARLEY is test team leader and ex-USS Shadwell project officer at the U.S. Naval Research Laboratory`s Technology Center for Safety and Survivability in Mobile, Alabama.