Gas loading and unloading processes are critical for the technical and the economic feasibility of Compressed Natural Gas (CNG) transport at sea since they affect in a critical manner the time required for gas loading and gas unloading, the amount of compressed natural gas that can be safely stored and transported, the amount of energy required for the loading and the unloading processes, and the overall cost of the loading and unloading plant (piping, valves, compressors, heat exchangers, etc.). All together they represent critical elements for the definition of the operational costs for the transport of compressed natural gas at sea. With reference to the large dimension composite Pressure Vessels adopted in the GASVESSEL project, we hereby present the results of the application of a multiscale simulation model of the loading and unloading processes that has been developed to support the system design phase based on the typical operational conditions and gas mixtures that can be found in three geographical areas, namely the Barents Sea, the Black Sea and the East Mediterranean Sea. The same simulation model will be used to optimize the operations of the GASVESSEL as well as to define the best sequence in the loading and unloading processes of the composite pressure vessel, optimizing both duration of the loading and unloading processes and the overall energy consumption. The simulation model is parametric and therefore allows to compare a variety of solutions in terms of system components and a variety of loading and unloading scenarios based on the four elementary processes: free gas loading, forced gas loading, free gas unloading, and forced gas unloading. The paper highlights the benefits of using modelling and simulation to support system design and to achieve cost effective and safe operations in the three operational scenarios.
Two systems are traditionally being used to transport and supply natural gas: the transportation of Liquefied Natural Gas (LNG) by ships and the transfer by fixed pipelines. An alternative system is possible by using Pressure Vessels to store Compressed Natural Gas (CNG) at ambient temperature. The H2020 GASVESSEL project  investigated and designed a novel system for CNG transport at sea based on a ship carrying a number of large size composite material Pressure Vessels. A central part of the project focused on the development of a novel, financially viable, storage system to collect, transport and distribute CNG. A significant contribution came from the adoption of a decision support model to optimize the winding and manufacturing process of the innovative lightweight composite material overwrapped Pressure Vessels, to define the optimum size for the ship according to transportation scenarios, and to simulate and benchmark the costs of the novel CNG transport concept against alternative gas transport systems. The inspiring idea for the project was that of enlarging the areas of technical and economic feasibility of CNG transport according to the scheme of Fig. 1.