Due to the random nature of fire, many different fire scenarios may occur in a compartment of ship。

Flashover

As the fire continues to grow, the temperature in the compartment of fire origin increases and every part of that compartment is exposed to flame radiation  that leads to an event called flashover。 Flashover is characterized by the rapid transition from  a localized fire to combustion of all exposed fuel surfaces within a compartment。

The undesirability of the flashover event makes it important to know its likelihood and timing。 The criteria governing the occurrence of flashover are two (see also table 1):

1)the temperature of the upper hot gas layer has reached 600°C ;

2)the radiation at the floor has reached 20    kW/

in general from 1 to 3。 A low value i。e。, 1 indicates that the level of uncertainty is low。 A high value i。e。, 3 is an indication of high uncertainty in the calculation of the performance of the fire safety systems。

Probabilistic criteria

Fire protection engineering is a complex science because, in addition to determining the time-dependent physical and chemical processes, uncertainties, such as human behaviour, the conditions of doors and other openings, fire location, arrangements of combustibles, and the reliability and effectiveness of the fire suppression and detection systems, must also be determined in order to develop an overall estimate of ship fire safety。 The time-dependent physical and chemical processes are usually calculated using deterministic approaches。 The impact of uncertainties should be estimated through a rational  way; probabilistic calculation and statistical analysis should be performed。

A combination of deterministic and probabilistic calculations can be used to carry out risk assessments for more compartments of ship system。 Fire risk assessment is a function of the likelihood of unwanted events and the resulting consequences or severity of these events; i。e。, the risk can be evaluated as:

m^2 。

Fires continuing beyond flashover are referred to  as fully developed fires and are characterized by very high

R 

i i

i

(2)

temperatures and heat release rates。

Life safety criteria

The most important objective in fire ship safety  design is the life safety of crew and passengers。 The designer must ensure that the occupants have sufficient time to reach, without harm, a safe refuge area to embarkation。 Deterministic models can be used to calculate the conditions in a compartment during the fire growth state or the pre-flashover stage。 These conditions are critical to determining whether passengers can escape before untenable conditions are reached。 The performance criteria for life safety mainly relate to the carbon monoxide (CO), oxygen (O2), heat flux, air temperature and smoke obscuration levels。

Table 1 gives the tenability upper and lower limits coefficients caused by toxic combustion product, smoke obscuration and heat flux。

Safety factors

Safety factors have been used in most engineering designs to account for uncertainties in calculations。

The addition of safety factors to performance criteria permits the designer to make a conservative assessment while allowing for a smaller margin of error by accounting for uncertainty in the models, the input data and the assumptions。

Deterministic analysis therefore may require the inclusion of safety factors。 From an overview of the literature, the proposed values for safety factors   range,

where:          is the probability of the event random

i

is the expected consequence。

i

In general, a quantitative probabilistic risk assessment analysis includes the development of a number of event scenarios,