IMO (SLF 47/48) Passenger Ship Safety
2.2 RBD Case Story: Large Passenger Vessel
2.2.1 Building Blocks of Risk-Based Design
2.2.1.3 Fire Risk Assessment Background
2.2.1.3 Fire Risk Assessment
ventilation strategies, alternative escape and evacuation arrangements, parks, the- atres, atria, multi-storey restaurants and so on).
For these cases, the performance criteria are associated with safety of human life and mitigation of material damage to the ship for a specified fire event. Thus, parameters describing the hazards associated with fires (such as temperature, heat fluxes, toxic contamination and visibility obscuration) must be predicted with suf- ficient accuracy for large and small spaces of simple and non-standard geometries, e.g. with large horizontal or vertical dimensions (corridors, vehicle decks, staircases, atria, engine rooms). On the other hand, the level of uncertainty in the associated pa- rameters necessitates that variational studies be conducted to ascertain the level of sensitivity of the results. Thus fire-engineering tools must be able to provide quick solutions even for large applications, ranging from few spaces to complete vertical fire zones spanning several decks. In terms of alternative design and arrangements for fire safety as stated in SOLAS Regulation 17, the use of consequence-analysis tools in conjunction with appropriate criteria for evaluating human life safety is es- sential, as explained next.
It is well established that when evaluating the consequences of fire effluent to human life, the crucial criterion for life safety is that the time available for escape from a ship space should be greater than the time required for safe evacuation of occupants. The time available for escape is the interval between the time of ignition and the time for conditions to become untenable such that occupants can no longer take effective action to accomplish their own escape. Untenable conditions during fires may result from:
• Inhalation of asphyxiant gases: these may cause loss of consciousness and ulti- mately death resulting from hypoxic effects, particularly on the central nervous and cardiovascular systems,
• Exposure to radiant and convective heat, and
• Visual obscuration due to smoke.
The above represent the fire hazards and can be imported and distributed in time and space into the evacuation environment (Evi) as explicit semantic information for the agents, much the same as in the case of flooding scenarios. These include concentrations of CO, CO2and O2, as well as Temperature, Radiant Heat Flux and Optical Density directly affecting –at each time step– the awareness and walking speed of the evacuees (agents). In order to estimate the effect of the fire hazards, an approach presented in (Purser 2002) is adopted based on the concept of Fractional Effective Dose (FED, for toxicity and heat) and Fractional Effective Concentration (FEC, for visibility). The FED and FEC are values indicating the human vulnerabil- ity to the cumulative effects of exposure to heat and toxic gases as well as the level of visibility in a space. Their values are calculated for each agent individually and are used to control walking speed and awareness and determine the point at which an agent becomes fatally injured, as mentioned earlier and illustrated in Fig. 2.41.
Once again, by linking a given fire scenario to the ensuing risk directly through pertinent parameters affecting risk prevention/reduction, cost-effective solutions can be identified to “de-risk” passenger ships from fire risk.
Fire Design Scenario
In this case a fire design scenario is selected to demonstrate fire risk assessment and containment by using an alternative design and the principle of “Equivalent Safety”. SOLAS Ch. II-2.24 states that a main vertical fire zone (MVZ) should not be of greater length than 48 m (to coincide with watertight subdivision bulkheads).
In order to incorporate larger public areas, one is allowed to design beyond this length provided that the total area of the public space does not exceed 1,600 m2 (the latter could also be tackled similarly). In July 2002, SOLAS regulation II-2/17 (MSC/Circ.1002) was adopted, which allows designs not strictly complying with the existing prescriptive fire safety regulations to be accepted provided such designs can be shown to be at least as safe as the design made in accordance with the IMO rules. This allows modern passenger vessel designs to go beyond the fire safety limits in order to create more inviting and exciting passenger spaces. The iteration to be followed is illustrated in Fig. 2.51, whilst the scenario being addressed here is described in Table 2.1 and illustrated in Fig. 2.52.
Alternative Design Arrangements
Necessary ?
Yes
No Prescriptive Rules SOLAS II-2
MSC/Circ.1002
Best Design Solution
Basis
Platform Identification of Fire
Design Scenarios Basis Platform
Evacuation simulation Analysis, te Fire/Smoke Tenability
Simulations, tt
tt> te Design
Change/RCO Best solution
NO
YES Satisfactory Design Solution
HAZID
Fig. 2.51 Alternative design and arrangements iteration for fire safety
Table 2.1 Restaurant design parameters – 48 and 60 m restaurant designs
Restaurant design Prescriptive Alternative design
Length 48 m 60 m
Capacity Lower deck 750 pax 690 pax
Upper deck 550 pax 960 pax
Total 1,300 pax 1,650 pax
Total exits’ width Lower deck 9.4 m 11.4 m
Upper deck 10.8 m 10.8 m
Floor area Lower deck 1,154 m2 1,872 m2
Upper deck 1,497 m2 1,748 m2
Fig. 2.52 Alternative design approach – upper and lower Decks of a 60 m restaurant
In the scenario considered, the fire ignites at the forward starboard end of the lower deck as illustrated in Fig. 2.53. Smoke propagates through the forward end of this deck and up through the central opening to the upper deck.
To determine the risk quantitatively, frequency(fi) and consequences(ci)for the scenario in question need to be estimated. In a general case, accident/incident statistics may be used for frequency estimation but in this particular case where an alternative design is considered only consequences will be used to quantify risk. The consequences are quantified on the basis of the number of injuries/fatalities and/or the expected damage to equipment and/or ship. The metric for risk calculation is defined a priory, consistently with an appropriate risk criterion. The number of in- juries/fatalities is obtained from evacuation simulations using Evi in conjunction with output from fire engineering calculations using REUME. The latter is employed to assess the development of fire and smoke in the restaurant arrangement, and the
Fig. 2.53 Alternative design approach – illustration of fire and smoke through the lower and upper decks of the restaurant
Fig. 2.54 Alternative design approach – exits blocked by fire
resulting distribution of fire hazards (quantified in terms of toxicity, visibility and heat) appropriately incorporated into Evi (Fig. 2.54).
The number of fatalities depends on the time to egress from the respective spaces (output from Evi) and the time to reach untenable conditions in these spaces, in terms of toxicity, visibility and heat. According to the risk metric used, appropriate risk acceptability criteria can be defined and hence the risk level evaluated against selected criteria as explained in the foregoing. If the risk level is not acceptable for any one evaluated scenario, potential RCOs will be evaluated based on cost- effectiveness analysis. In the scenario considered, smoke originates at the lower deck, starboard side forward of the restaurant and spreads to upper deck with un- tenable conditions (UC) being reached within 4–5 min. The fire blocks the primary escape route at the forward end of the restaurant and passengers have to use al- ternative (secondary escape) routes. This increases congestion and causes the total egress time to increase. In this respect, the egress time ought to be compared with the time required for conditions to become untenable as a means of assessing the ensuing risk. Table 2.2 and Figs. 2.55, 2.56 and 2.57 show the results deriving from evacuation and smoke spreading simulations.
The results clearly demonstrate unacceptable consequences and hence the need to contain the risk associated with the fire scenario being considered. To this end,
Table 2.2 Alternative design (ALT 1) – consequences analyses
Spaces Egress time Time to reach UC No. of passengers affected
Upper deck 7–8 min 4–5 min 28(∼4%)
Lower deck 8–9 min 4–5 min 50(∼5%)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
450 470 490 510 530 550 570 590
Egress time (s) Cumulative distribution Alternative design
Restaurant “Evacuability”
60m
Prescriptive design 48m
Fig. 2.55 Non-fire scenario – comparison between alternative and prescriptive designs
Restaurant Alternative Design
"Evacuability "
450
Egress time (s) Effect of fire &
smoke scenario Fire Scenario
500 550 600 650 700 750 800 850 900 950
Fig. 2.56 Alternative design – normal vs. fire scenario
a number of RCOs were considered, the most cost-effective of which include the following:
• Increasing the number of smoke extraction fans from 2 to 4
• Increasing the number of inlet air fans from 2 to 4
• Widening escape ways
Following implementation of the aforementioned RCOs, the ensuing conse- quences are now as shown in Table 2.3 and hence acceptable.
Restaurant Prescriptive Design
"Evacuability"
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
450 470 490 510 530 550 570 590 610
Egress time (s)
Cumulative distribution
Effect of fire &
smoke scenario Fire Scenario
Fig. 2.57 Prescriptive design – evacuability – normal vs. fire scenario
Table 2.3 Alternative design (ALT 2) – consequences analyses
Spaces Egress time Time to reach UC No. of passengers affected
Upper deck 5–7 min 8–9 min 0
Lower deck 6–7 min 8–9 min 0