Amos Necci

Accident scenarios triggered by lightning strike on atmospheric storage tanks

Severe Natech accidents may be triggered by lightning strike affecting storage tanks containing relevant inventories of hazardous materials. The present study focused on the identification of event sequences and accident scenarios following lightning impact on atmospheric tanks. 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. An overall methodology was outlined to allow the calculation of the expected frequencies of final scenarios following lightning impact on atmospheric storage tanks, taking into account the expected performance of available safety barriers. The methodology was applied to a case study in order to better understand the data that may be obtained and their importance in the framework of quantitative risk assessment (QRA) and of the risk management of industrial facilities with respect to external hazards due to natural events.

Assessment of domino effect: state of the art and research needs

High-impact low-probability (HILP) accident scenarios in industrial sites are raising a growing concern. Domino effect was responsible of several catastrophic accidents that affected the chemical and process industry, as well as critical infrastructures for energy as oil refineries. However, there is still a poor agreement on assessment procedures to address escalation hazard resulting in domino scenarios. The present study presents a review of the work done in the last 30 years in the field, and a critical analysis of available tools and knowledge gaps concerning domino effect assessment. The analysis of scientific publications concerning domino effect in the process industry resulted in a database of more than 60 documents, addressing three main issues: past accident analysis, models for equipment damage, risk assessment and safety management of domino scenarios. The methods, models and tools developed make now possible the quantitative assessment of domino scenarios in risk analysis and in safety management of industrial sites. Nevertheless, a number of open points still remain, where existing tools may be improved and uncertainty may be reduced.

A model for process equipment damage probability assessment due to lightning

In recent years, severe natural events raised concern about so-called NaTech accident scenarios: technological accidents caused by the impact of a natural event on an industrial facility or infrastructure. Lightning strikes are one of the most important triggers of NaTech scenarios. Moreover, previous studies showed that lightning strikes are among the main causes of loss of containment (LOC) of atmospheric storage vessels containing hazardous materials. Although the lightning hazard is well known, well accepted quantitative procedures to assess the contribution of accidents triggered by lightning to industrial risk are still lacking. In particular, the approaches to the assessment of lightning strike probability and to the damage caused by lightning strike are mainly qualitative or semi-quantitative and are mostly based on expert judgment. In the present study, a quantitative methodology for the assessment of the equipment damage probability due to lightning is presented. The lightning severity was quantified by means of probability distribution functions of two parameters: peak current intensity and lightning charge. Through the application of a Monte Carlo simulation the expected frequency of lightning strikes on the equipment and the equipment damage probability were determined. The results of the equipment damage model were validated by available experimental data on metal perforation in simulated lightning strikes. The results of the validated Monte Carlo simulations were fit to empirical functions obtaining a simplified model suitable for use in a quantitative risk assessment framework.

Release of hazardous substances in flood events: Damage model for horizontal cylindrical vessels

Severe accidents may be triggered by the impact of floods on process and storage equipment containing hazardous substances. The present study analyses the possible damage of horizontal cylindrical equipment, either operating at atmospheric or at higher pressures. A mechanical damage model was developed and validated by available literature data on past accidents. Simplified correlations were then obtained to calculate the critical flooding conditions leading to vessel failure. A fragility model was proposed for the straightforward assessment of equipment damage probability in the framework of the quantitative risk assessment of NaTech scenarios triggered by floods. A case-study was discussed to test the potentialities of the method.

Quantitative assessment of risk due to NaTech scenarios caused by floods

Floods may cause severe damages to chemical and process facilities, triggering major accidents (fires, explosions, and toxic release). Such cascading events are termed as NaTech scenarios. In the present study, a specific methodology for the implementation of Quantitative Risk Assessment (QRA) of NaTech scenarios triggered by floods was further developed and applied to the assessment of different flood events and equipment categories. Specific vulnerability models allowed estimating the failure probability of both atmospheric and pressurized equipment, and the estimation of NaTech-induced release frequencies. A case-study representative of an industrial installation was discussed, comparing the risk due to conventional internal causes to that deriving from NaTech scenarios and identifying possible specific safety barriers. The case-study demonstrated that a significant risk increment may be associated to industrial facilities located in flood-prone areas when flood-triggered NaTech scenarios are considered.

Assessment of Lightning Impact Frequency for Process Equipment

Fires and explosions triggered by lightning strikes are among the most frequent Natech scenarios affecting the chemical and process industry. Although lightning hazard is well known, well accepted quantitative procedures to assess the contribution of accidents caused by lightning to industrial risk are still lacking. In the present study, a quantitative methodology for the assessment of the expected frequency of lightning capture by process equipment is presented. A specific model, based on Monte Carlo simulations, was developed to assess the capture frequency of lightning for equipment with a given geometry. The model allows the assessment of lay-out effects and the reduction of the capture probability due to the presence of other structures or equipment items. The results of the Monte Carlo simulations were also used to develop a simplified cell method allowing a straightforward assessment of the lightning impact probability in a quantitative risk assessment framework. The developed approach allows an in-depth analysis of the hazard due to lightning impact by identifying equipment items with the highest expected frequency of lightning impacts in a given lay-out. The model thus supplies useful data to approach the assessment of the quantitative contribution of lightning-triggered accidents to industrial risk.

Quantitative assessment of risk due to major accidents triggered by lightning

Lightning is one of the most frequent accident causes in storage tank parks, and accidents triggered by lightning are the most frequent Natech event reported in past-accident analysis. In the present study, a methodology for the inclusion of accidents triggered by lightning in Quantitative Risk Assessment (QRA) was developed. A model for the assessment of lightning impact probability on process equipment and specific equipment vulnerability models were coupled to dedicated event trees, allowing the quantification of risk indexes. The methodology developed also allows the assessment of risk reduction by the implementation of different lightning protection strategies. The results obtained represent a step forward towards the introduction of risk-based design of lightning protection systems.

NaTech Scenarios Caused by Flooding: Evaluation of Accident Frequency by the Use of Fragility Models

Natural events impacting on process plants may lead to severe technological accidents. These events are usually defined as NaTech events (Natural Hazard Triggering Technological Disasters). In order to derive the frequencies of accident scenarios associated to NaTech events for QRA (Quantitative Risk Assessment) implementation, a critical issue is the availability of equipment vulnerability models. The aim of the present study was to present a vulnerability model for the assessment of failure probability of atmospheric vessels involved in flooding events. The vulnerability model was based both on the severity of the natural event and on the construction features of the equipment. In order to explore the model features and its potentialities, the application to case-studies was carried out analysing an actual industrial layout. The results obtained confirmed that NaTech scenarios caused by floods may have an important influence on risk assessment and management of industrial facilities.

Risk assessment of mitigated domino scenarios in process facilities

The propagation of accidents among process units may lead to severe cascading events or domino effects with catastrophic consequences. Prevention, mitigation and management of domino scenarios is of utmost importance and may be achieved in industrial facilities through the adoption of multiple safety layers. The present study was aimed at developing an innovative methodology to address the quantitative risk assessment (QRA) of domino scenarios accounting for the presence and role of safety barriers. Based on the expected performance of safety barriers, a dedicated event tree analysis allowed the identification and the assessment of the frequencies of the different end-point events deriving from unmitigated and partially mitigated domino chains. Specific criteria were introduced in consequence analysis to consider the mitigation effects of end-point scenarios deriving from safety barriers. Individual and societal risk indexes were calculated accounting for safety barriers and the mitigated scenarios that may result from their actions. The application of the methodology to case-studies of industrial interest proved the importance of introducing a specific systematic and quantitative analysis of safety barrier performance when addressing escalation leading to domino effect.

Quantitative risk assessment of cascading events triggered by floods

NaTech (natural-technological) events caused by the impact of floods on industrial facilities may lead to major accidents following damages to structures and equipment. In this study, we explored the quantitative assessment of NaTech accidents triggered by floods. A specific methodology was developed, adopting equipment vulnerability models aimed at determining the failure frequency of equipment affected by flooding. A reference case study was analysed, taking into account a flood scenario impacting on an industrial facility. Risk results with and without flood-triggered NaTech scenarios were compared, determining the influence of NaTech scenarios on the overall risk profile.