6.1 The quantitative analysis is the most labour intensive
from a fire safety engineering standpoint. It consists of quantifying the design fire
scenarios, developing the performance criteria, verifying the acceptability of the
selected safety margins and evaluating the performance of trial alternative designs
against the prescriptive performance criteria.
6.1.1 The quantification of the design fire scenarios may include calculating
the effects of fire detection, alarm and suppression methods, generating time lines from
initiation of the fire until control or evacuation, and estimating consequences in terms
of fire growth rate, heat fluxes, heat release rates, flame heights, smoke and toxic gas
generation, etc. This information will then be utilised to evaluate the trial
alternative designs selected during the preliminary analysis.
6.1.2 Risk assessment may play an important role in this process.
It should be recognised that risk cannot ever be completely eliminated. Throughout the
entire performance based design process, this fact should be kept in mind. The purpose
of performance design is not to build the fail safe design, but to specify a design with
reasonable confidence that it will perform its intended function(s) when necessary and
in a manner equivalent to or better than the prescriptive fire safety requirements of
SOLAS chapter II-2.
6.2Quantification of design fire scenarios 6.2.1 After choosing an appropriate range of
fire incidents, quantification of the fires should be accomplished for each of the
incidents. Quantification will require specification of all factors that may affect the
type and extent of the fire hazard. The fire scenarios should consider possible future
changes to the fire load and ventilation system in the affected areas. This may include
calculation of heat release rate curves, flame height, length, and tilt, radiant,
conductive, and convective heat fluxes, smoke production rate, pool fire size, duration,
time-lines, etc. References on suggested example correlations and models that may be of
use are listed in appendix C. It should be noted that when using any of these or other
tools, the limitations and assumptions of these models should be well understood and
documented. This becomes very important when deciding on and applying safety margins.
Documentation of the alternative design should explicitly identify the fire models used
in the analysis and their applicability. Reference to the literature alone should not be
considered as adequate documentation. The general procedure for specifying design fires
includes fire scenario development completed during the preliminary analysis, time-line
analysis and consequence estimation which is detailed below.
6.2.2 For each of the identified fire hazards, a range of fire
scenarios should be developed. Because the alternative design approach is based on a
comparison against the regulatory prescribed design, the quantification can often be
simplified. In many cases, it may only be necessary to analyse one or two scenarios if
this provides enough information to evaluate the level of safety of the alternative
design and arrangements against the required prescriptive design.
6.2.3 A time-line should be developed for each of the fire
scenarios beginning with fire initiation. Timelines should include one or more of the
following: ignition, established burning, fire detection, fire alarm, fire
suppression/control system activation, personnel response, fire control, escape times
(to Assembly stations, evacuation stations and lifeboats as necessary), manual fire
response, untenable conditions, etc. The timeline should include fire size throughout
the scenario, as determined by using the various correlations, models and fire data from
the literature or actual fire tests.
6.2.4
Consequences of various fire scenarios should be quantified in fire engineering terms.
This can be accomplished by using existing correlations and calculation procedures for
determining fire characteristics such as heat release rate curves, flame height, length,
tilt, radiant, conductive and convective heat fluxes, etc. In certain cases, live fire
testing and experimentation may be necessary to properly predict the fire
characteristics. Regardless of the calculation procedures utilised, a sensitivity
analysis should be conducted to determine the effects of the uncertainties and
limitations of the input parameters.
6.3Development performance criteria 6.3.1 Performance criteria are quantitative
expressions of the fire safety objectives and functional requirements of the SOLAS
regulations. The required performance of the trial alternative designs are specified
numerically in the form of performance criteria. Performance criteria may include
tenability limits such as smoke obscuration, temperature, height of the smoke and hot
gas layer in a compartment, evacuation time or other criteria necessary to ensure
successful alternative design and arrangements.
6.3.2 Each of the regulations in SOLAS chapter II-2 state the purpose of the
regulation and the functional requirements that the regulation meets. Compliance with
the prescriptive regulations is one way to meet the stated functional requirements. The
performance criteria for the alternative design and arrangements should be determined,
taking into consideration the fire safety objectives, the purpose statements and the
functional requirements of the regulations. The following example is an illustration of
this:
"Example of a performance criterion drawn directly from the
regulations in SOLAS chapter II-2:
Assume that
a design team is developing performance criteria for preventing fire spread through
a bulkhead separating a galley from an accommodation space. They are seeking a
numerical form for this criteria.
(e.1) Regulation II-2/2 contains
the fire safety objective "to contain, control, and suppress fire and explosion in
their compartment of origin."
(e.2) One of the functional
requirements in which this objective is manifest is "separation of accommodation
spaces from the remainder of the ship by thermal and structural boundaries."
(e.3) Regulation II-2/9 contains the prescriptive requirements to
achieve this functional requirement; in particular it requires an "A-60" class
boundary between areas of high fire risk (like a machinery space or galley) and
accommodation spaces.
(e.4) Regulation II-2/3 contains the
definition of an "A" class division, which includes the maximum temperature rise
criteria of 180 oC at any one point, after a 60 minute fire exposure.
(e.5) Therefore, one possible performance criterion for this
analysis is that "no point on the other side of the bulkhead shall rise more than
180°C above ambient t emperature during a 60 minute fire exposure."
6.3.3 If the performance criteria for the alternative design
and arrangements cannot be determined directly from the prescriptive regulations because
of novel or unique features, they may be developed from an evaluation of the intended
performance of a commonly used acceptable prescriptive design, provided that an
equivalent level of fire safety is maintained.
6.3.4 Before evaluating the prescriptive design, the design team should agree
on what specific performance criteria and safety margins should be established.
Depending on the prescriptive requirements to which the approval of alternative design
or arrangements is sought, these performance criteria could fall within one or more of
the following areas:
- Life safety criteria - These criteria address the
survivability of passengers and crew and may represent the effects of heat, smoke,
toxicity, reduced visibility and evacuation time.
- Criteria for damage to
ship structure and related systems - These criteria address the impact that fire and
its effluents might have on the ship structure, mechanical systems, electrical
systems, fire protection systems, evacuation systems, propulsion and
manoeuvrability, etc.
These criteria may represent thermal
effects, fire spread, smoke damage, fire barrier damage, degradation of structural
integrity, etc. - Criteria for damage to the environment - These criteria
address the impact of heat, smoke and released pollutants on the atmosphere and
marine environment.
6.3.5 The design team should consider the impact that one
particular performance criterion might have on other areas that might not be
specifically part of the alternative design. For example, the failure of a fire barrier
may not only affect the life safety of passengers and crew in the adjacent space, but it
may result in structural failure, exposure of essential equipment to heat and smoke, and
the involvement of additional fuel in the fire.
6.3.6 Once all of the performance criteria have been established, the design
team can then proceed with the evaluation of the trial alternative designs (see section
6.4).
6.4Evaluation of trial alternative designs 6.4.1 All of the data and information
generated during the preliminary analysis and specification of design fires should serve
as input to the evaluation process. The evaluation process may differ depending on the
level of evaluation necessary (based on the scope defined during the preliminary
analysis), but should generally follow the process illustrated in figure 6.4.1.
6.4.2 Each selected trial alternative design
should be analysed against the selected design fire scenarios to demonstrate that it
meets the performance criteria with the agreed safety margin, which in turn demonstrates
equivalence to the prescriptive design.
6.4.3
The level of engineering rigor required in any particular analysis will depend on the
level of analysis required to demonstrate equivalency of the proposed alternative design
and arrangements to the prescriptive requirements. Obviously, the more components,
systems, operations and parts of the ship that are affected by a particular alternative
design, the larger the scope of the analysis.
6.4.4 The final alternative design and arrangements should be selected from
the trial alternative designs that meet the selected performance criteria and safety
margins.