Accident scenarios triggered by lightning strike on atmospheric storage tanks
Introduction
Natural events impacting on industrial sites may damage process equipment leading to the release of hazardous substances and to severe technological accidents. These types of cascading events are indicated as Natech (Natural-Technological) scenarios [1]. Natech events triggered by lightning impact on equipment items were recognized as a frequent scenario according to past accident data analysis [2], [3]. Atmospheric storage tanks represent the more vulnerable equipment items with respect to lightning impact. The study of Argyropoulos et al. [4] confirmed that lightning is a major accident initiator for such category of equipment items, and lightning was found as the most frequent cause of loss of containment accidents for storage tank parks in a recent study [5]. Actually, the high ignition probability of flammable substances following lightning impact (as high as 82% in past accident data analysis [2]) resulted in a relevant number of severe fires triggered by lightning.
A previous extensive analysis of past accidents triggered by lightning highlighted that several different final events may follow lightning impact on atmospheric storage tanks containing flammable substances: bund fires, tank fires, and also confined explosions followed by tank fires [2]. Actually, tank features and mitigation barriers may play an important role in the evolution of accident scenarios triggered by lightning. Necci et al. [6] analysed in detail the possibility of tank damage following lightning impact, evidencing the possible release scenarios due to shell perforation caused by direct lightning strike. However, lightning is a strong ignition source, able to trigger accident sequences even without direct damage to vessel shell (e.g. ignition of flammable vapours released by atmospheric vents, rim-seal fires, tank fires, etc.) [2]. Pool or tank fires triggered either by direct shell damage or by ignition of flammable vapours caused by lightning have the potential to trigger cascading effects on nearby equipment, leading to severe accident escalation or domino effects [7].
The extension of Quantitative Risk Analysis (QRA) to the assessment of Natech scenarios has been recently recognized as an important issue to obtain comprehensive data when assessing industrial risk related to major accident hazards [8], [9], [10], [11], [12], [13], [14]. Specific studies recently focused on the development of tools aimed at the inclusion of Natech-related scenarios in risk assessment practice [15], [16], [17], [18], [19]. However, in spite of the relevant frequency of Natech events triggered by lightning, specific methodologies for the detailed assessment of Natech scenarios initiated by lightning impact still need to be developed.
The present study focused on the identification of event sequences and accident scenarios following lightning impact on atmospheric tanks. A methodology was developed to allow the identification and the calculation of conditional probabilities of final outcomes following lightning impact on different categories of atmospheric storage tanks (i.e. atmospheric tanks with fixed or floating roof). Reference event trees, validated using past accident analysis, are provided to describe the specific accident chains identified, accounting for reference protection and mitigation safety barriers usually adopted in current industrial practice. The overall methodology was applied to a case study in order to exemplify the data that may be obtained and to assess their importance in the framework of QRA and of the risk management process of industrial facilities.
Section snippets
Overview
Geometrical features and reference conventional safety barriers used in current industrial practice were defined for the tank categories of interest. The possible impact mode of lighting was then analysed, in order to develop reference accident chains applying event tree analysis (ETA) and to obtain reference event trees (ET). The reference ETs obtained include relevant protection barriers and were validated using detailed data from past accident records. The probability of failure on demand
Event tree analysis (ETA) and reference accident chains
An event tree analysis (ETA) was carried out in order to determine the potential accident sequences following lightning impact and to evaluate the role of protection systems. The protection systems are intended as protective barriers that play their role when the lightning strikes on the storage tank. In case the protective barriers are unavailable, the scenario evolves to the final accident. The unavailability of a protective barrier is presented as the probability of failure on demand (PFD)
Definition of a case-study
In order to understand the potentialities and the results obtained by the application of the ETs developed above, the analysis of a case-study was carried out. A tank farm of an existing oil refinery was considered. The lay-out considered for the case-study is reported in Fig. 5. The tank farm consists of twenty atmospheric storage tanks storing miscellaneous flammable liquids. Geometrical and design features considered for the tanks are reported in Table 5. A fixed foam protection system was
Results and discussion
The application of the methodology presented in Section 3.3 to the case-study defined in Section 4 allowed the determination of the probabilities of all the final outcomes identified. Table 6 reports the results obtained for the probability of direct damage and for the overall probability of failure (all final outcomes considered in Fig. 4 excluding “no consequences”). The overall frequency of failure, fFO, values ranges between 10−6 yr−1 and 10−3 yr−1. In 8 tanks of the 20 analysed, the fFO
Conclusions
A methodology for the assessment of final outcomes following a lightning strike on an atmospheric storage tank containing flammable liquids was developed. In particular, the reference scenarios and the mitigation barriers that may influence event sequences after lightning impact were identified. Reference event trees were obtained and validated using past accident data. The assessment of standard safety barriers applied in industrial practice allowed the quantification of event trees based on
Acknowledgements
The authors gratefully acknowledge financial support received from the EU within the 7th FP iNTeg-Risk project (CP-IP-213345-2).
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