Systemic Seismic Risk Analysis of Gas Distribution Networks
Esposito, Simona (2011) Systemic Seismic Risk Analysis of Gas Distribution Networks. [Tesi di dottorato] (Inedito)
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Lifeline is an earthquake engineering term denoting those systems that are necessary for human life and urban contest. Those systems are commonly used to transport water, oil, natural gas and other material. Their disruption due to earthquakes can have a devastating impact on urban context both for human losses both economic stability. Therefore, due to their vulnerabilities, it is important to assess and mitigate seismic risk of lifelines since they are intricately linked with industries, communities and security they serve. The earthquake safety of lifeline systems has attracted great attention in recent years since significant amount of damage was observed during several past earthquakes. This thesis focuses on the seismic performance evaluation of gas distribution networks. The basic function of a gas system is to deliver gas from sources to costumers. A gas distribution system is essentially composed by pipelines, reduction stations, valves and demand nodes. Those systems are essentially located underground. As consequences gas networks are subjected to both transient ground deformation due to seismic waves, which is felt over a wide geographical area, and ground failure due to geotechnical hazards such as liquefaction and landslide, which determine localized ground failure. Moreover since buried pipelines systems generally cover large areas, a sophisticated hazard analysis is required. In particular the quantification of regional hazard is based on a large vector of spatially correlated ground motion intensities and requires the modeling of the joint distribution of intensity measures at all sites of interest. Dependencies among ground motion parameters at different sites imply the estimation of spatial correlation models to be used for the hazard assessment but since each component that characterizes the system may be sensitive to different ground motion parameters, the possibility of the existence of a cross-correlation between these parameters has also to be taken into account. Fragility analysis of gas systems is generally based on empirical data collected throughout past earthquakes. In the case of pipeline components, the usual practice is to evaluate the repair rate as a unit length of pipe, with respect to a parameter representative of ground shaking or ground failure. For processing facilities, that include many subcomponents, a quantitative vulnerability assessment is quite difficult. A possible approach is to consider these facilities as systems and to aggregate the fragility of each component into a global systemic vulnerability through the use of fault tree analysis. Further in order to evaluate the interaction between component response to earthquake and lifeline performance, system performance indicators provide a measure of the impact of the earthquake on the system functionality. Building on the results from past international research projects, existing tools for the vulnerability assessment, and seismic risk analysis of lifelines systems, the SYNER-G project (“Systemic Seismic Vulnerability and Risk Analysis for Buildings, Lifeline Networks and Infrastructures Safety Gain”), has been funded by the European Commission (2009-2012) with the aim to address criticalities. In relation of the objectives of this project, this thesis had the aim to determine methodologies for the probabilistic seismic risk analysis of gas distribution networks and to apply these methods to a real gas system. In literature, there are very few studies on seismic risk analysis of networks that take into account all the aspects of the component of risk (hazard, vulnerability and loss). Moreover there are fewer studies that try to calibrate the analysis on a real system, making the study interesting for network operators. The thesis, in fact, has achieved this goal with special emphasis to the medium and low pressure network of a real system, namely the L’Aquila gas distribution system managed by ENEL Rete Gas s.p.a., for which not only detailed information on the network were retrieved, but also data related to damages occurred on the network followed the 2009 L’Aquila earthquake. A geographic information system (GIS) database was developed containing data on system physical and operational characteristics provided by the network operator to chracterize the seismic behavior of all components. L’Aquila region has been characterized both in terms of transient ground deformation hazard and permanent ground deformation hazard. In particular European and Italian spatial correlation models have been estimated and used for the simulation of probabilistic scenarios earthquakes and the ground failure hazard has been characterized in terms of landslide potential. Probabilistic simulations have been performed to evaluate the system response in terms of performance indicators for different combinations.
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