The Accident Scenarios

This section describes the possible accident scenarios of the hydrogen objects included in the study. The FMEA method was mainly used to define a list of undesired events that consider all possible breaks or ruptures of items of equipment which would lead to a loss of containment from the hydrogen study objects. The QRA study on hydrogen systems are concerned with flammable properties of liquid, of gas, and the systems have therefore been examined to identify the events which could lead to the release of hydrogen to the atmosphere. The hydrogen can only be released to the atmosphere either as a result of as a result of loss of containment from the storage tanks, through the rupture of pipelines, or handling of failure.

The total number of equipments in a study object where a safety evaluation has to be made can be very large. Since not all equipments contribute significantly to the risk, it is not worthwhile to include all installations in the QRA. According to the European Council Directive (EC Directive) [159], the QRA may be carried out if the hydrogen (as a dangerous substance) is thought to be present at a location (e.g. industrial sites and transportation routes) in amounts that can endanger the environment. The threshold quantity for hydrogen and other dangerous substances are given in the EC Directive "96/82/EC on the control of major-accident hazards involving dangerous substance" ("Seveso-II directive"). It regulates the production, handling and storing of hydrogen as well as other relevant fuels (such as LPG, Ammonia, etc.). It also recommends that the installation that has a larger quantity than the threshold quantity are required to implement a major-accident prevention plan, which includes maintaining risk analysis and implementing a safety management system as well as preparing emergency action plan. For hydrogen, the threshold level amount is 5 tons, for LPG 50 tons, for methanol 500 tons, and for gasoline 2500 tons [236, 159].

Based on the above guidelines the accident scenarios of the hydrogen objects were focused on hydrogen storage and transport, as they have the greatest potential for large releases of hydrogen and consequential damage. Furthermore, the study only considered the societal risk evaluation of the hydrogen objects. Therefore, the QRA study was done only on equipments that contribute most to the societal risk in each installation.

Hydrogen storage and transport mainly consists of tank container and its piping system linking the various pieces of equipments. Each of them, of course, may break or rupture in an infinite number of ways. The final choice of incidents to be modelled took into consideration the following factors: the size of the release, whether the release is instantaneous or continuous, and liquid or vapour phase. The lists of accident scenarios of the study objects are described in the following sections.

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