A new concept storing LNG in a hard rock lined cavern have been developed and tested for several years in Korea. In design and construction of underground storage for LNG, it is very important to evaluate the characteristics of the rock mass under cryogenic conditions. In this study, laboratory experiments for the measurement of the mechanical properties such as uniaxial compressive strength, tensile strength, fracture toughness were performed in the temperature range of -60°C to 20°C. And an expansion pressure and cryo-suction related to ice ring formation were also measured. Experiment results show that uniaxial compressive strength, tensile strength, fracture toughness increased with decreasing temperature, but elastic modulus changed little with decreasing temperature. The expansion pressure was approximately 2.43 MPa at the phase change temperature. In the cryosuction test, the growth rate of the ice in joints was 1.0 mm/day to 1.1 mm/day. The results of this study can be utilized for the evaluation of technical feasibility of the developed underground LNG storing concept.
Some attempts have been made to store LNG underground in unlined containment but were not successful. Underground storage failures were due to thermal stresses generating cracks in the host soil and thermal cracks contributed to induce gas leakage and to an increase in heat flux between LNG and the ground (Kim, Amantini & Chanfreau, 2003). In order to overcome to the problems, a new concept storing LNG in a hard rock lined cavern have been developed and tested for several years. In this concept, groundwater in rock mass around cavern has to be fully drained until early stage of construction and operation to avoid possible adverse effect of groundwater near cavern walls. And then rock mass should be re-saturated to form an Ice- ring (Figure 1), which is frozen zone including ice in several joints within rock mass. To verify the technical all aspects for the concept, a pilot plant has been constructed and tested in Daejeon since 2003 by storing liquefied nitrogen, LN2(Figure 2), which is an alternative of LNG for safe operation. When considering the stability of underground storage for LNG, it is necessary to study the formation mechanism and extent of the Ice-ring which the frozen including ice in several joints within the rock mass. However, little research exists on Ice-ring formation and the thermo-mechanical behaviour of rock exposed to cryogenic conditions. In this paper, in order to investigate mechanical and thermo-mechanical characteristics of rock caused by cooling, several tests were conducted on granite sampled from a LNG pilot plant. Thermo-mechanical tests, such as the uniaxial compressive test and the Brazilian test, were conducted in the temperature range of -60°C to 20°C. And In order to analyze the effect of the phase change of water to ice, the expansion pressure caused by the phase change of water to ice were measured. The cyro-suction test was performed to investigate the flow characteristics after ice formation.
2. ROCK SPECIMENS
The rock specimen used in the experiment is biotitic granite, which is sampled from LNG pilot plant. Mechanical and thermal properties of tested rocks are summarized in table 1.