Fire risk analysis

The first step in the analysis is to define the context and goals such that stakeholders (designers, tunnel operator, approving authority and fire service etc.) are aware of the risk

Chapter 8 Fire Risk Analysis

the scope of the analysis. In this work a Fault Tree Analysis (FTA) technique is then used to identify the potential fire risk. Figure 8.1 shows the overall approach which involves the use of a probabilistic approach coupled with deterministic approach to establish a design fire for road tunnel smoke control design. A probabilistic approach can be used to identify the fuel load as it relates to the vehicle mix expected in the tunnel and the likely causes of ignition that have the greatest impact on the risk. Numerical modelling to determine the design fire size can then be carried out for different tunnel geometries and ventilation velocities for the identified fuel loads and ignition conditions.

It should be recognised that it is not possible to design a smoke control system for every potential fire that might occur in a tunnel. Depending on the length and usage of a particular tunnel it is feasible that in an extreme case up to several tens or hundreds of vehicles could be involved in a severe collision. The cost and practicality to design for such events is beyond what might be considered a reasonable worst case. Thus the selection of the design fire scenario is best made on the basis of a risk analysis and the peak rate of heat release becomes the critical consequence component in this analysis. In many tunnel smoke control system designs the peak rate of heat release is chosen without proper regard for the effect of the tunnel characteristics on the fire growth or the traffic mix expected to use the tunnel. Often the rate of heat release for a single vehicle fire taken from an experiment published in the literature and it is unlikely to account for the specific conditions within the tunnel being designed, the regulatory environment in place or the likely vehicle usage during the operation of the tunnel (Biollay et al 1999). The selection does not always consider whether such a vehicle is likely to enter the tunnel or whether a multiple vehicle collision scenario might be a more credible event. The type of vehicles allowed access to the tunnel is an important consideration when analysing the fire risk level. As vehicles on the road can vary from motorcycles to heavy goods vehicles or even a petrol tanker, the magnitude of their heat release rate in the event of a fire can vary significantly, restricting certain vehicles from entering the tunnel is therefore an effective means to reduce the tunnel fire risk.

Numerical modelling would aid in the determination of the heat release rate for a specific tunnel design but it is not practical to simulate every possible event through modelling

Chapter 8 Fire Risk Analysis

performing any detailed numerical modelling work to establish the design peak heat release rate it is necessary to know the general fire risk and thus identify the fuel load to be included in the modelling. This is achieved by first performing a fire risk analysis to identify the credible fire scenarios in the tunnel by gathering information on the vehicle population of each vehicle category, the motor vehicle accident rate and the causes of vehicle fire incident; the probability of a faulty vehicle resulting in fire, probability of a careless act resulting in fire, probability of intentional act resulting in fire and probability of vehicle fire due to motor vehicle accident.

Figure 8.1: Approach to estimate fire size in tunnel.

8.2.1

Causes of vehicle fire

According to USFA (1999), the causes of a vehicle fire can be divided into four categories; the result of faulty vehicle, the result of an act of carelessness, the result of arson and the aftermath of a collision. From past international tunnel fire incidents compiled by Carvel and Marlair (2005), it appears that the causes of road tunnel fire will continue to originate

Stage 2: Numerical modelling to establish fire size Stage 1: Risk Analysis to Identify fire risk in tunnel

Deterministic approach Probabilistic approach

Ventilation Condition in Tunnel Tunnel Cross- sectional Area

Fire Risk in Tunnel (Fuel load)

....

Ignition probability

Type of vehicles (fire size from experiment)

Act of carelessness Intentional … Faulty vehicle … Collision . Fire size in Tunnel

Chapter 8 Fire Risk Analysis

engine, overheating of braking systems and sparks are all possible causes of vehicle fire. Careless acts include causes such as dropped lights, naked lights and discarded cigarettes on upholstery. According to Kocsis (2002), intentional acts can be grouped into six categories: a profit motive, animosity crime, crime concealment, vandalism, personality disorders (including suicides) and political objectives such as terrorism. Collision is an incident in which a vehicle impacts into anything that causes damage to itself, other vehicles or the tunnel facilities. Vehicle collisions could involve either single or multiple vehicles of various types.

8.2.2

Heat release rate of vehicles

The heat release rate of vehicle fire plays an important role in the risk analysis as a higher heat release rate would contribute to a higher fire risk level. The literature contains heat release rate data obtained from large-scale vehicle fire experiments conducted in tunnel and non-tunnel environments. Results from these experiments have shown that the heat release rate can vary from 1.24 MW to 202 MW (refer to Table 8.1). Reasons for this variation are due to the vehicle type; experimental geometry and procedure; material and quantity of the fuel package and ventilation conditions.

Full-scale vehicle fire experiments conducted in tunnels often have different cross- sectional areas. The effect of re-radiation will have an effect on the fire size as tunnels with a smaller cross-sectional area tend to yield a higher heat release rate value as compared to tunnels with a larger cross-sectional area (Carvel et al 2004a). Another major influence on the rate of heat release is the ventilation condition in the tunnel. Depending on the critical velocity in the tunnel (the minimum velocity to prevent backlayering of smoke), the design velocities are often different from one tunnel to another. It has been observed from tunnel fire experiments that tunnels with higher airflow tend to fan the fire resulting in higher heat release rates and this burning enhancement likely due to the improved mixing at higher velocities (Ingason et al 1994). The material, quantity and geometry of fuel package used in the experiment will also affect the heat release rate. This is especially true for goods vehicles as the fire size is often dominated by the characteristics of the fuel package that is burning.

Chapter 8 Fire Risk Analysis

The fire test programmes carried out by various researchers have provided valuable information to engineers and tunnel designers on the magnitude of a fire in the tunnel. Depending on the type of vehicle fires, tunnel geometry and ventilation condition, the heat release rate value obtained through these large scale experiments allows fire engineers to make a preliminary estimation of a design fire. A summary of the peak heat release rate obtained from these experimental studies is shown in Table 8.1.

Type of vehicle / Test series Peak HRR (MW)

Testing location

Reference

Motorcycle / Scooter

Scooter 1.24 Laboratory Chen et al (2005)

Motorcar

1.6 Ford Taunus 1.5 Laboratory Mangs & Keski-Rahkonen (1994)

Datsun 160J sedan 1.8 Laboratory Mangs & Keski-Rahkonen (1994)

Datsun 180B sedan 2 Laboratory Mangs & Keski-Rahkonen (1994)

Dodge Caravan Sport (Engine fire) 1.5a Laboratory Santrock (2000) Plymouth Voyaer (Under body fire) 4.8 a Laboratory Santrock (2001) Chevrolet Camaro (Under body fire) 1.2 a Laboratory Santrock (2001a) Chevrolet Camaro (Engine fire) 1.2 a Laboratory Santrock (2002) Ford Explorer (Rear under body fire) 1.35 a Laboratory Santrock (2002a) Ford Explorer (Mid under body fire) 0.5 a Laboratory Santrock (2002b) Honda Accord (Under body fire) 0.8 a Laboratory Santrock (2003) Honda Accord (Engine fire) 1.2 a Laboratory Santrock (2003a)

Austin Maestro 8.5 Canopy Shipp & Spearpoint (1995)

Citroen BX 4.3 Canopy Shipp & Spearpoint (1995)

Renault Espace People Mover u = 0.4 m/s

6 Tunnel EUREKA (1995)

Opel Kadett with u = 6 m/s 4.7 Tunnel Lemair and Kenyon (2006)

Bus

Chapter 8 Fire Risk Analysis

Goods Vehicle

Trailer with 10 ton load of wood and plastic, u = 3 m/s

201.9 Tunnel Ingason & Lonnermark (2005)

Trailer with 6.3 ton load of wood and mattresses , u = 3 m/s

156.6 Tunnel Ingason & Lonnermark (2005)

Trailer with 8.3 ton load of furniture and rubber , u = 3 m/s

118.6 Tunnel Ingason & Lonnermark (2005)

Trailer with 2.9 ton load of plastic cup in cardboard boxes on wood pallets,u = 3 m/s

66.4 Tunnel Ingason & Lonnermark (2005)

Leyland DAF 310ATi Tractor with 2 Ton of furniture, u = 3-6 m/s

125 Tunnel EUREKA (1995)

Simulated small truck with 0.8 Ton of wooden pallets, 4 tyres with tarpaulin, u =0, 4-6 m/s and 6 m/s.

13, 19, 16 Tunnel Lemair and Kenyon (2006)

Simulated track with 2.8 Ton of rubber tyres, wood and plastic cribs, u = 0.7 m/s

17 Tunnel Ingason et al (1994)

Note: a – The fire was extinguished during the experiment. u – Air velocity in the tunnel (m/s)

Table 8.1: Heat release rate from various fire experiments