Socio-economic Analysis and Quantitative Risk Assessment Methodology for Safety Design of Onboard Storage Systems

Catastrophic rupture of onboard hydrogen storage in a fire is a safety concern. Different passive, e.g. fireproofing materials, the thermally activated pressure relief device (TPRD), and active, e.g. initiation of TPRD by fire sensors, safety systems are being developed to reduce hazards from and associated risks of high-pressure hydrogen storage tank rupture in a fire. The probability of such low-frequency highconsequences event is a function of fire resistance rating (FRR), i.e. the time before tank without TPRD ruptures in a fire, the probability of TPRD failure, etc. This safety issue is “confirmed” by observed recently cases of CNG tanks rupture due to blocked or failed to operate TPRD, etc. The increase of FRR by any means decreases the probability of tank rupture in a fire, particularly because of fire extinction by first responders on arrival at an accident scene.
This study of socio-economic effects of safety applies a quantitative risk assessment (QRA) methodology to an example of hydrogen vehicles with passive tank protection system on roads in London.
The risk is defined here through the cost of human loss per fuel cell hydrogen vehicle (FCHV) fire accident and fatality rate per FCHV per year. The first step in the methodology is the consequence analysis based on validated deterministic engineering tools to estimate the main identified hazards: overpressure in the blast wave at different distances and the thermal hazards from a fireball in the case of catastrophic tank rupture in a fire. The population can be exposed to slight injury, serious injury and fatality after an accident. These effects are determined based on criteria by Health and Safety Executive (UK), and a cost metrics is applied to the number of exposed people in these three harm categories to estimate the cost per an accident. The second step in the methodology is either the frequency or the probability analysis. Probabilities of a vehicle fire and failure of the thermally activated pressure relief device are taken from published sources. A vulnerability probit function is employed to calculate the probability of emergency operations’ failure to prevent tank rupture as a function of a storage tank FRR and time of fire brigade arrival. These later results are integrated to estimate the tank rupture frequency and fatality rate. The risk is presented as a function of fire resistance rating.
The QRA methodology allows to calculate the cost of human loss associated with an FCHV fire accident and demonstrates how the increase of FRR of onboard storage, as a safety engineering measure, would improve socio-economics of FCHV deployment and public acceptance of the technology.