Developing a quantitative fire risk assessment method for critical infrastructures: The case of road tunnels

Abstract

Road transport is at the heart of critical development challenges for both modern economies and societies. Therefore, it should guarantee a high level of safety to the public. With a view to enhance road network’s safety, it is crucial to focus primarily on its critical infrastructures, one part of which is tunnels. Tunnels are the most sophisticated elements of the road infrastructure. Tunnels are regarded as complex socio-technical systems. Although tunnels benefit the operation of road networks, their use involves the risk that a potential dysfunction of a tunnel can cause serious dysfunction on the broader road network due to its interdependencies. This dysfunction can be particularly extensive in case of a fire accident. Fire is the foremost critical event for road tunnels’ safety. The severity of tunnel fires is related to some special attributes of these infrastructures. Fire safety of tunnels concerned intensely the public opinion after the disastrous trans-Alpine accidents in Europe in the late 90s. Therefore, risk assessment was officially introduced for ensuring tunnels’ level of safety. Despite the significant progress, it is disputable whether just applying any risk assessment method is capable of ensuring preparedness against a fire accident. The review emerges the fact that important parameters for the safe operation of the tunnel system have significant uncertainty. Although these parameters play a key role in tunnel performance, current methods act on a deterministic approach ignoring thus their embedded uncertainties. Faced with these uncertainties, safety analysts make assumptions adopting a “mean” value or a “worst case” scenario. But, the variation to reality because of these assumptions can create serious fallacies regarding the estimated level of tunnel safety. Placed next to the potential risks due to the aforementioned deficiencies, the review indicates another important issue. Although the choice of additional to standard safety measures involves multiple criteria and a ranking of alternatives, current methods lack in dealing with and thus they pose challenges to risk assessment. Therefore, it is imperative to develop more robust risk assessment methods in order to deal with the impact of uncertainty on the tunnel fire safety, which current methods exhibit lack in dealing with. This thesis presents a novel quantitative risk assessment method, named SIREN, aiming at enhancing road tunnels’ operational risk assessment regarding fire accidents. The stochastic-based approach of SIREN mitigates the fallacies arising from the traditional deterministic methods. The structure of the method is as follows. Initially, examining tunnel system’s parameters, the ones that should be treated as stochastic are identified. Subsequently, considering one-dimensional analysis for both estimating tunnel airflows and trapped-users’ evacuation, the potential losses are estimated. Finally, by accumulating the results deriving from the Monte Carlo Simulation, the distribution of the trapped-users losses occurs, illustrating the system’s level of safety. The SIREN method is illustrated through the case of an urban underground road tunnel during rush hour. The outcome highlights a significant proportion of scenarios that exceed the number of losses estimated by the traditional methods. Furthermore, the proposed method offers the possibility of examining the parameters’ criticality, which assists safety analysts in choosing additional safety measures, if needed. In this way, the tunnel’s level of safety is increased to as low as reasonable practicable. Furthermore, this thesis proposes also the EVADE method in order to support the decision-making process towards the selection of fire safety measures for road tunnels. This method provides a systematic decision process through the use of particular and consistent decision criteria, together with considerations of alternative safety measures which are based on the stated subjective preferences of the decision-maker. It can be applied in addition to SIREN or independently. The method incorporates diverse stakeholders’ views while it introduces a list of the most significant criteria that are valuable to judge the appropriateness of selected measures. The relative importance amongst the decision criteria is calculated through the Analytic Hierarchy Process, based on the expert opinion. Meanwhile, by applying the Monte Carlo simulation, the ranking of alternatives is considered reliable since it includes potential uncertainty related to the pairwise comparisons amongst all pairs of decision criteria as well as alternatives. Contrary to current approaches, the alternatives’ ranking comes as a distribution instead of a single number providing the decision-maker richer information for selecting the most suitable measure(s) according to the specific tunnel’s situation. The utilisation of the method is presented through an illustrative case of a typical European tunnel. The results of this research provide a novel approach for enhancing the level of safety of road tunnels and the produced methods can be applied in all types of tunnels.