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 should 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 recognized 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.
6.2
Quantification 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 D. 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.
6.3
Development 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 temperature 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:
-
.1 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.
-
.2 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.
-
.3 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).
6.4
Evaluation 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.
Figure 6.4.1 Alternative design and arrangements process flowchart