Development of a methodological framework for the integrated HSE assessment of LNG supply and utilisation chain

This Thesis discusses different approaches, which have been developed to support the Health, Safety, and Environment (HSE) assessment of the natural gas industry. In particular, the Liquefied Natural Gas (LNG) represents the focus of the present PhD activity, as it is the most challenging section of the chain. Operational, technological, and safety issues related to the LNG come from the high flammability of the substance, the extremely low operative temperature (about -162°C), as well as the potential to generate large amount of vapour in case of release (i.e., the volumetric expansion is of about 600 times from liquid to vapour phase). These hazardous issues are encountered in particular during LNG storage and transport activities, which are often located in vulnerable areas. Therefore, the PhD activity has analysed LNG storage and transport systems through different approaches. In the first part of the thesis, the operative hazards associated with LNG storage systems are discussed. These systems face a constant heat exchange from the environment that leads to a thermal stratification of the stored fluid and, consequently a fast pressurisation of the tank. The complex phenomena involved in the stratification process have been rarely investigated in system of industrial interest (i.e. large-scale storage tanks). Therefore, the PhD activity has addressed the relevant heat and mass transfer mechanisms of a 100 m3 LNG storage vessel through a Computational Fluid Dynamic (CFD) model. The effects of the performance of insulation and filling level have been investigated. The results have revealed a strong dependency of the thermal stratification behaviour on the insulation state. In addition, CFD simulations provided detailed information on the thermo-fluidic properties (e.g. temperature, pressure, velocity, direction, etc.) enabling a deep analysis of the behaviours leading to hazardous scenarios, such as a LNG loss of containment. In the second part of the thesis, a focus on accidental scenarios induced by LNG releases was carried out. Large fires and explosions may occur following the accidental LNG release with potential impact other process units, leading cascading events (domino effects). Hence, the PhD activity has investigated the mechanism of LNG hazardous scenarios propagation in industrial assets. A tool for domino effect assessment, based on Agent-based modelling and simulation (DAMS) has been developed and the role of different categories of safety barriers has been addressed. The tool is suitable for the analysis of high complex layouts, being characterised by low computational costs and relying on simple, thus reliable, rules of accidents propagations. Results demonstrate the effective domino risk reduction achieved through the implementation of the protections. The third part of the thesis focuses on LNG transport systems, with reference to the maritime sector. A methodology to perform risk assessment of LNG carriers approaching port areas was developed. Standard features of LNG carriers have been evaluated in order to propose a simplified set of reference scenarios, which are representative of the risk related to the entire system. The approach, based on graph theory, allowed for a cost-benefits analysis of different deployment plans for the safety barriers, accounting also for the accidents’ propagation to the onshore plants. The comprehensive framework proposed in this Thesis represent a useful tool for the risk-based decision-making processes in the LNG field. The multi-scale approach - from the local investigation of storage tank pressurisation; then the analysis of LNG storage tanks in industrial environment during cascading event, and the attempt to overcome the lack of regulations and universally accepted methodologies to perform risk assessment of LNG transport systems – enables the overall HSE dissertation of the entire chain.