¹Department of Maritime Safety, National Maritime Research Institute

Mitaka, Tokyo, JAPAN

shirota@nmri.go.jp

　

²Mitsui Engineering and Shipbuilding Co., Ltd., Chuo, Tokyo, JAPAN

³Mitsui Engineering and Shipbuilding Co., Ltd., Ichihara, Chiba, JAPAN

⁴Osaka University, Toyonaka, Osaka, JAPAN

　

　

Almost all natural gas consumed in Japan is liquefied at a low temperature below -150℃, and is transported by liquefied natural gas (hereinafter called LNG) carriers. This mode of transportation requires substantial energy consumption for producing LNG. If the 'self-preservation' property of gas hydrate can be utilized economically in addition to its high-density gas containing property, it is possible to store and transport stranded natural gas at higher temperature and lower pressure compared to the conventional LNG method. The authors are pursuing the hopeful prospect that natural gas hydrate (hereinafter called NGH) could be a medium for natural gas transportation, and are examining the properties of hydrate, as well as handling and quality control issues for NGH cargo forms. The authors also found that marine transport of natural gas in the form of natural gas hydrate pellets (NGHPs) would be available, and started the three-year research project last year under the financial support by the Corporation for Advanced Transport & Technology. The project consists of (1) the investigation on microscopic properties of a single NGHP by using laser Raman spectroscopy and plasma replica methods, (2) the evaluation of thermal and mechanical properties of NGHPs, and (3) the conceptual design of an NGHP carrier, i.e., a specially constructed bulk carrier fitted with insulated cargo holds. The authors conducted preliminary self-preservation experiments of pelletized methane hydrate. Both at -5℃ and -10℃, only approximately 5% of the theoretically contained gas remained in the MHPs' sets after a lapse of 3 weeks from the base time. On the other hand, at -15℃/-20℃, approximately 0.5 to 0.75 of the theoretically contained gas remained in the MHPs' sets even after a lapse of three weeks from the base time. It is qualitatively assumed that the dissociation rate of the MHPs' set used in the experiment decreased monotonically as the temperature around the set dropped. The MHPs' set dissociated rather slowly at -20℃ in the experiments, so it is assumed that the MHP's self-preservation is maintained on the time scale including seaborne transportation. Namely, there is assumed to be every possibility of transporting NGHPs by ship from the viewpoint of hydrate's self-preservation.

　

　

The thermodynamic equilibrium temperature below which methane hydrate remains stable at 1 atm methane pressure is about -80℃elsius. In recent years, however, it has been reported that methane hydrate remains metastable under some conditions outside its stability region by Yakushev et al. (1992) and Stern et al. (2001). According to the above groups, methane hydrate may remain metastable at temperatures 50℃ to 75℃ above its 1 atm dissociation temperature. Or, Gudmundsson et al. (1994) confirmed metastability of methane-ethane-propane hydrate plus ice mixtures, whose composition was selected to represent natural gas from North Sea fields. The metastability of hydrate is usually called 'self-preservation'. Japan has never had abundant indigenous energy resources, and has imported virtually all of its energy supplies. Since the exhaust gas of combusted methane is relatively clean compared to conventional fossil fuels such as coal and oil, natural gas consumption in Japan is expected to rise in the future. According to the government forecast of long-term energy supply and demand, natural gas consumption in Japan will reach about 1.7 times the present consumption in more than 15 years (Max, 2000).

　

Although a great deal of natural gas is globally transported through pipelines from gas fields to consumer markets, almost all natural gas consumed in Japan is presently liquefied at extremely low temperature below -150℃, and is stored/transported by LNG carrier. However, the energy loss due to liquefaction is rather high in this method; it is reported that the energy loss due to liquefaction accounts for 8.8% of the energy content of natural gas (Tamura et al., 1999). If the 'self-preservation' property of hydrate can be utilized economically in addition to its high-density gas containing property, it is possible to store and transport stranded natural gas at higher temperatures and lower pressures compared to the conventional LNG method. This method has the potential not only for prevention of the energy loss due to liquefaction, but also for reduction of equipment costs for liquefaction and storage.

　

　

Among some reports upon the self-preservation property of gas hydrate which have been submitted thus far, principle papers are interpreted as follows. Yakushev et al. (1992) conducted methane hydrate dissociation experiments at atmospheric pressure by using several samples in different conditions. They observed various dissociation behaviors and preservation periods depending upon humidity, surface/mass ratio in hydrate sample, temperature, light radiation, etc. Preservation periods of some samples ranged from several months to a few years. They suggested that this self-preservation property of gas hydrate occurs because thin ice films, impermeable to gas molecules, form on hydrate surfaces during depressurization and interrupt further dissociation of the hydrate.

　

Gudmundsson et al. (1994) made mixed-gas hydrate at pressures from 2MPa to 6MPa and at temperatures from 0℃ to 20℃, from a 92:5:3 methane:ethane:propane mixture, which should form structure II that is stable to much warmer temperature and lower pressure than structure I methane hydrate. The amount of water converted to hydrate was 27% to 44% in these experiments. Temperature was decreased adiabatically to -18℃ at high pressure, and hydrate samples were kept frozen in a freezer for 24 hours. They examined metastability of the hydrate after rapid depressurization to atmospheric pressure, confirming that the samples remained meta-stable at -18℃, -10℃ and -5℃ at atmospheric pressure and dissociated only slightly in 7 days to 10 days. Based on the experimental results, Gudmundsson et al. (1996) examined natural gas transportation by hydrate from an economical viewpoint. They compared a NGH transportation chain (including production plant, hydrate carriers and re-gasification plant) to an equivalent conventional LNG transportation chain, on the assumption of natural gas transportation of 3.5 billions cubic meters over 5,500 kilometers. As a result, the capital cost of the NGH chain was estimated to be 24% lower than the capital cost of an equivalent LNG chain. In the latest study of Gudmundsson et al. (2000), an NGH slurry (mixture of natural gas hydrate and crude oil) process was also examined using floating production storage and offloading (FPSO) vessel for stranded natural gas utilization.

　

Stern et al. (2001) conducted thorough experiments on dissociation regions of methane hydrate at atmospheric pressure over the temperature range from -78℃ to 17℃elsius. According to Stern et al., there are three distinct dissociation behaviors when the hydrate was removed from its stable field by rapid depressurization. Of them, the regime between -31℃ and -2℃ has an anomalously slow dissociation rate of hydrate. At the temperature of -5℃, the dissociation percentages within 24 hours and one month after dissociation start were 7% and 50% respectively (confirmed as shown in Stern et al. (2002)), which were the best results of self-preservation. In the latest study of Stern et al. (2002), they stated it is highly probable that ice 'shielding' effects provided by partial dissociation along hydrate grain surfaces are not the primary mechanism for the anomalous preservation behavior observed in rapidly depressurized samples, in the light of SEM imaging of hydrate sample materials and their experimental results on both structure I and structure II gas hydrates.

　

Also in Japan, Shirota et al. (2002) examined experimentally the influence of dissociation temperature upon pure methane hydrate dissociation between -7.5℃ and 0℃elsius. They obtained relatively extremely slow dissociation data within temperature range between -7.5℃ and -3℃, which coincided with the report by Stern et al. qualitatively but differed from them quantitatively. They also estimated the period at which all samples finished dissociating at approximately 120 days, and concluded that the estimate seemed to be very promising for practical application of self-preservation property to natural gas storage and transportation. Although some ideas for the mechanism of the hydrate's self-preservation effect have been proposed as stated above, the details of the mechanism are still poorly understood.

　

　

Mitsui Engineering & Shipbuilding Co., Ltd. (MES), National Maritime Research Institute (NMRI) and Osaka University had a hopeful prospect that NGH could be a medium for natural gas transportation, examining the properties of hydrate, handling, and quality control for some NGH cargo forms (Takaoki et al., 2002; Nakajima et al., 2002). The authors also found that seaborne transportation of natural gas in the form of natural gas hydrate pellets (NGHPS) would be available, and have started the three-year collaborating research project 'Research on Transportation of Natural Gas Using Gas Hydrate Pellets' under the financial support from the Corporation for Advanced Transport & Technology (CATT) since July 2001. Partial responsibilities of each research group in the project are as follows: MES mainly oversees the conceptual design of a NGHP carrier that is specially constructed with thermally insulated cargo holds. Transportation plans and design of the carrier are considered, including ports of loading/unloading, total amount of transportation per year, number of ships, and capacity. Other elemental systems of the NGHP carrier that are considered include: propulsion systems, cargo handling systems, residual cargo processing systems, explosion prevention systems, evolved gas processing systems, cargo holds, and ballast tanks.

　

At NMRI, thermal/mechanical properties of NGHPs in bulk are examined. Specific heat and therrnal conductivity are measured and evaluated in addition to NGHPs' self-preservation property. Heat transfer and temperature in cargo holds is analyzed by numerical simulation taking into account dissociation heat and phase change effects, and NGHPs' dissociation is estimated based upon experimental data including their compressed conditions. In parallel, safety measures for NGHP carriers are also considered; regulations for transport of dangerous goods are investigated, and necessary safety functions/measures are clarified. The research at Osaka University is focused upon both crystal morphology and surface structure of a single NGHP. In the former, the relation between crystal morphology and stability of the single NGHP due to phase change resulted from contained gas other than methane is investigated using laser Raman spectroscopy. In the latter, dissociation rate and surface structure change of the single NGHP are examined using plasma replica method.