A transition towards more renewable energy sources such as wind- and solar energy is underway. These sources can be unpredictable with regard to energy production and therefore energy storage has become a major concern. A promising technique, Underground Hydrogen Storage (UHS), converts excess energy into hydrogen and stores it underground, after which, by reversing the process, the stored energy can be used when the imbalance between supply and demand is great. Hydrogen storage in salt caverns is particularly attractive because of their high sealing capacity, low amount of cushion required, the inert nature of rock salt and high possible injection and withdrawal rates. For energy storage purposes, these salt caverns are exposed to different cyclic loading conditions during operation, resulting in variations in stress in and around the cavern. Investigation of the mechanical response of rock salt under cyclic loading conditions is essential for the safety assessment and usability of the cavern. The response of the system to varying loading conditions can be mainly divided into two groups, either time-dependent or stress-dependent. The main focus of this thesis is to record the stress-dependent behaviour of rock salt under varying loading conditions. More specifically, it focuses on nonlinearity that occurs due to viscoplasticity. This phenomenon can be described as the rate dependent
behaviour of a material that occurs when a material exceeds a certain stress level, after which irreversible deformation occurs. This thesis describes the development of a 2D FEM (finite element method) simulator on an unstructured grid that captures the mechanical response. To model the stress-dependent behaviour, the viscoplastic model proposed by Desai is used. This model is
based on the non-associated flow rule and takes into account material dilatancy and compressibility. In addition, the model allows for hardening, the tensile strength of rock salt and variation in yield behaviour with pressure variation. Sonar data from cavern EPE S43 is utilised to test the simulator. This case showed that irreversible deformation occurs as a result of the stress-dependent behaviour of rock salt. For this specific cavern, the total working volume of the cavern decreased roughly by 0.0003% over three operating cycles, indicating that viscoplastic deformation itself does not pose significant risks to the cyclic storage of hydrogen in salt caverns.