## 1 Introduction  
Large scale energy infrastructures for natural gas and power play a crucial role for any well-functioning society. These infrastructures are systematically analyzed and controlled in order to understand their operational characteristics and to provide an energy efficient operation and a sufficient level of security of supply. However, ensuring the required level of security of supply is becoming more challenging, especially because of the increasing interconnections among the facilities in both systems. The dependence of power generation on natural gas has increased the vulnerability of electric power systems to interruptions in gas supply, transmission, and distribution. Since the storage of gas on-site is not an option, as it is for coal and fuel oil, the direct gas delivery through pipelines becomes more critical during unexpected events in electricity systems like peak periods or disruptions. Particularly, short term problems caused by pipeline constraints that cause an inability of a generator to receive fuel gas can seriously affect security of power supply [1]. Another issue is the lack of predictability of renewable generation, which might increase the magnitude of imbalances in the gas system. Although the increasing share of renewables will cause a reduction of the power system dependency on natural gas, forecasting the amount of gas needed to serve Gas Fired Power Plants (GFPPs) will become more challenging due to growing penetration of variable resources. Additionally, shale gas production already had a significant impact on the deployment of new infrastructures, especially in the USA, where the installed capacity of GFPPs has increased enormously during the last years and is expected to continue increasing in the coming years [2]. This increase has obviously tightened the dependency of the electricity system on the gas system. This could also be the case in other regions of the world, including Europe, especially under scenarios of abundant shale gas and low carbon policies. Not only is the power system dependent on gas, but also the gas system is dependent on power. A gas network consists of different facilities that depend on electrical power in order to maintain normal operation (e.g., electric driven compressors, liquefied natural gas (LNG) facilities, underground gas storage facilities, valves, regulators, gas meters). The usage of electric drivers in gas facilities is increasing due to advantages regarding environmental impacts and flexibility compared to gas turbines [3]. Moreover, increased availability, better control, improved energy efficiency, and shorter delivery times are other important and attractive advantages of electric drivers. Since the proper functioning of electric drivers requires a reliable power supply, gas system dependency on the power system can be considered critical. Additionally, the present advancement in the Power-to-Gas (P2G) technology, where excess power generation from renewable sources is used to produce hydrogen or synthetic natural gas (SNG) will significantly contribute to the coupling of both systems [4], since the power system will depend on the gas system as an energy storage provider. Summarizing these aspects, it appears that interconnections between gas and electricity systems make the entire energy system vulnerable, since a disruption occurring in one system (e.g., an unexpected failure) may propagate to the other system and may possibly feed back to the system, where the disruption started. Tight relations are increasing the potential risk for catastrophic events, triggered by either intentional or unintentional disruptions of gas or electricity supply and possibly magnified by cascading effects. Analyzing both systems in an integrated manner and developing a combined assessment methodology is needed in order to know whether and how such interdependencies may contribute to the occurrence of large outages and to ensure the proper functioning of the energy supply system.