Concept and features of natural gas hydrate pellet carriers

Abstract

Gas hydrate is a material which contains molecules of gases in the structure like cages formed by molecules of water. Natural gas hydrate (NGH) is in equilibrium state at around 193 K under atmospheric pressure and NGH is expected to be stowed and transported at higher temperature due to its special property so-called "Self-Preservation Effect". NGH may be used as more economical medium for transporting natural gas than LNG temperature of which is around 110 K, because NGH can be transported under self-preservation condition. Furthermore, it is considered that natural gas hydrate pellet (NGHP) is more promising medium than powdery NGH taking into account filling efficiency in a cargo space and the high self-preservation property of pellets. Mitsui Engineering and Shipbuilding Co., Ltd., the National Maritime Research Institute and Osaka University carried out the collaborative three-year research project on the sea-borne transportation of NGHPs with the financial support of the Japan Railway Construction, Transport and Technology Agency. In this research project, it was experimentally confirmed that Methane Hydrate Pellets (MHPs), which are supposed to have similar properties to those of NGHPs, can be stowed and transported at around 253 K in view of self preservation property. The authors were in charge of conceptual design of "NGHP carriers", i.e., specialized bulk carries having gas-tight and insulated cargo spaces, including the identification of safety measures for such ships. The conceptual design was carried out under the philosophy of "Zero Emission of Natural Gas" to prevent greenhouse effect due to natural gas. In this paper, we present the concept and some features of NGHP carriers such as necessity of special safety measures, in particular for preventing of explosion, and cargo handling systems, in particular for unloading of NGHPs.