Currently ongoing research projects examine the storage of hydrogen methane mixtures in existing underground gas storage facilities. By adding a new medium in the gas storage system might cause new risks which need to be examined in detail. The aim of this thesis was to select a risk assessment method and to create a conceptual structure for the case of hydrogen methane mixture storage in porous reservoirs. The first part of this thesis includes a literature review of topics related to the underground hydrogen storage. An overview of a planned underground hydrogen storage system and a brief description of similar projects follow as next. In addition the thesis presents a list of security threats and other potential risks for facilities, wells and reservoir. Common approaches to risk assessment are also given. Furthermore, risk assessment case studies for CO2 underground storage are introduced and similarities for hydrogen storage highlighted. The practical part of the thesis consists of creating a conceptual structure of a bow-tie analysis including the identification of hazards, consequences and barriers. Uncertainties in the system are taken into account through the use of fuzzy logic. At the end of the thesis preliminary results of research projects in the field of hydrogen underground storage are presented.