Accident Scenarios of the Study Objects

4.3.2.1 Hydrogen Storages

The FMEA method has been used to define an initial list of incidents that consider all possible breaks or ruptures of items of equipment which would lead to a loss of containment (called accident scenarios). The considered systems mainly consist of a tank and piping system. Each of them, of course, may break or rupture in an infinite number of ways. For example, a pipe break may be any size from a pinhole to a full bore rupture and may be occurred any position between the pipe ends. This spectrum of failure needs to be reduced to a representative set of failures as defined in the depth of study. In this study, possible pipe failures are represented by either full bore ruptures or holes 20% of the diameter. Failure outcomes such as fires and explosions are considered since hydrogen is flammable. Releases caused by different failures may lead to similar outcomes and these can be combined to reduce the calculational burden. Therefore, the final choice of events to be modelled took into consideration the following factors: the size of the release; whether the release is instantaneous or continuous; and whether the release is liquid or vapour. Based on the above assumptions the following representative set of events was considered in the study:

1. Instantaneous release of the entire hydrogen inventory due to tank rupture, i.e. a catastrophic failure of the tank or vessel.

2. Continuous release of hydrogen due to: (i) Liquid or vapour release through a hole on the tank (may consider equal to the largest pipe diameter); (ii) Vapour release through relief valves; (iii) Vapour release through rupture discs; (iv) Vapour release due to full-bore rupture of the vapour lines; (v) Liquid release due to full-bore rupture of the liquid lines

4.3.2.2 Hydrogen Transportation

4.32.2.1 Road tanker truck

Although a variety of mechanisms may cause a truck accident and cargo release, the greatest relevance with respect to risk analysis can be divided into two categories, i.e. accident-initiated releases and non-accident initiated releases. The accident-initiated releases with a truck represent a great potential for substantial damage and large releases of hydrogen. These include a collision between two vehicles, collisions with fixed objects, and overturn. Vehicular collision between two vehicles and with fixed objects presents the potential for substantial damage and can also represent relatively energetic impact accidents with the potential for significant damage and/or cargo release. Overturned vehicles are most likely during trucking operations where, for some truck designs and cargoes, the vehicle centre of gravity is high, especially on tight curves such as ramps. Meanwhile, the non-accident-initiated releases are characterized by equipment failures not associated with accidents such as leaks of pipes and fittings or failures of relief valves and rupture disks. These mechanisms result in relatively small quantities of cargo being released [ ]. The CCPS [2] quoted that accident-initiated releases tend to dominate the risk of hydrogen in transportation. Therefore, the release frequency of hydrogen from the truck may be estimated from accident rates, and hence does not require a detailed fault tree analysis as in the case of other objects, e.g. tanks.

4.3.2.2.2 Pipeline

In Europe, the major gas companies of Belgium, Denmark, France, Germany, Italy, the Netherlands, the United Kingdom, and Spain have gathered their national statistics and published failure rate data under the European Gas Pipeline Incident Data Group. The European oil companies also publish annual failure rate data under the organization CONCAWE (the oil companies' study group for conservation of clean air and water—Europe) [3]. The databases published by these organizations include a large number of pipelines and incorporate many years of operating experience. Therefore, the general quality of data is good and it can be used with confidence in predicting the likelihood of pipeline failure in risk assessment. When considering all these databases together one broad conclusion comes out of the statistics, despite some variation caused by dissimilarity in the type of data collected— failures occur in roughly equal proportions in three broad categories: (1) Failures caused by external mechanical interference; (2) Failures caused by corrosion defects; (3) Failures caused by miscellaneous factors such as pipe material defects, natural hazards or operator error.

Failures caused by third party external mechanical interference include such causes as being damaged by excavators or other equipment in use by other utility or construction companies, damage during construction of land drainage, etc. The type of failure generally caused by third party mechanical interference is a puncture or split of the pipe or a gouge severely reducing the wall thickness of the pipe. Failure can be immediate or may occur some time later by fatigue. This type of incident is likely to have severe consequences and historically some of the most serious pipeline accidents resulting in ruptures have been caused by such incidents. Pipeline failures by corrosion can be due to internal corrosion or external corrosion. External corrosion failures are due to moisture in the ground and aggressive soils and can take two forms—small pinhole failures caused by pitting and more generalized corrosion leading to a reduction in pipe wall thickness over a plane area. External pitting corrosion leads to small leaks that are often difficult to detect but that gradually grow in size over a period of time. External area or plane defects cause a generalized reduction of wall thickness that can eventually fail catastrophically under pressure, leading to a large scale release. Pipelines can also fail for a variety of other causes. Typical causes are construction defects, pipe material defects, operator error, equipment failure, failure due to internal erosion and failure due to ground slip, flood ground erosion, earthquake, or mining. Failure modes of pipeline

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