Japan Oil, Gas and Metal National Corporation (JOGMEC) has constructed the world's largest underground LPG storage caverns in Kurashiki of Okayama prefecture and Namikata of Ehime prefecture from 2002. Kurashiki site is composed by four 488m-640m length storage caverns with the volume and areal dimension of 18m(W)×24m(H) to storage 400,000 tons propane. Comparatively, Namikata site was designed as a 430m butane/propane dual propose cavern and two 485m propane caverns to contribute 450,000 tons capacity. Both the constructions were completed in 2010 and passed the air-tightness test in 2012. After certification, the two storage cavern have received LPG and launched their operation to contribute more than half of the 1,500,000 tons national stockpile target. From an economical viewpoint, both Namikata and Kurashiki sites adopt unlined underground storage caverns to preserve the LPG (vapor pressure 0.8MPa at 20℃). Therefore, the storage caverns are located in sufficient depth (Kurashiki site: EL.-160m, Namikata site: EL.-150m) to maintain the permanent inward groundwater flow and ensure the tightness. Despite of economy, the underground storage caverns also have merits on: (1) Large storage capacity only by small area for surface facilities. (2) Minimizing the risk of mega disaster (e.g. earthquake, typhoon, tsunami, etc.) and providing protection on fire and explosion. (3) Environmental protection by the excellent tightness. Even there are construction experiences of underground LPG storage caverns in the worldwide and also underground crude storage caverns in Japan. However, the hydraulic containment type underground LPG storage cavern has to be designed and constructed with detail consideration on the local hydro-geological and geological characteristics. Especially, in order to maintain the stable groundwater level during cavern excavation, the horizontal and vertical boreholes have been designed to enclose the storage caverns with high water pressure. Therefore, in order to ensure the water curtain efficiency during cavern excavation, a new hydraulic evaluation system has been developed by the integration of the original 3-dimensional hydrogeological modelling method together with the high accuracy hydraulic behaviors monitoring system and the detail hydrogeological/geological investigations. Additionally, for the high permeable fractured zone in Kurashiki site, numerous grouting tests were conducted for obtaining well grouting efficiency and reducing the rock permeability with geological considerations. On the other hand, considering the development of excavation disturbed zone (EDZ) under high water injection pressure in great depth, the numerical rock mechanical analysis has been conducted and rock mechanical behaviors have been monitored to determine the cavern shape and support patterns for ensuring the stability. After the cavern excavation and facilities were completed, the storage caverns were pressurized to a specified test pressure around 1MPaG and evaluated the variation of cavern pressure in 72 hours' duration as tightness test. In order to evaluate the tightness with high accuracy, extreme high precision pressure and temperature measurement instruments were utilized, also the layout of temperature measurement instruments were determined by 3-dimensional computational fluid dynamics (CFD) simulation. The achievement of the first hydraulic containment type underground LPG storage caverns has proven the domestics geotechnical design and construction on the large underground cavern, also contributes precious experience on design/construction and operation for the future underground energy storage caverns and civil structures in rock mass.

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Japan Oil, Gas and Metals National Corporation (JOGMEC) has constructed the world’s largest level underground LPG storage cavern in Namikata (450,000 tons) and Kurashiki (400,000 tons). In order to ensure the air-tightness performance of storage caverns, the authors developed a new observational design/construction system for the construction of the underground LPG storage cavern and the hydraulic containment system. The developed observational design/construction system integrates the geological/hydro-geological data to evaluate the hydraulic behaviors and mechanical stability during excavation. In this study, the authors applied the developed observational design/construction system to evaluate the hydraulic behaviors during the site-scale hydraulic test in Namikata underground LPG storage site. The pore pressure distribution and seepage of tunnel walls were compared with simulation results for evaluation. As the result, it has been confirmed that the hydraulic potential has been recovered in the inner area of water curtain and the amount of seepage inflow has been kept under the criterion.

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Japan Oil, Gas and Metals National Corporation (JOGMEC) has constructed the world’s largest hydraulic containment type underground LPG storage caverns in Kurashiki from 2002. The construction was completed and the operation was launched at 2013.For the hydraulic containment type underground LPG storage caverns, despite seepage rate is control in the cavern excavation stage, the stable water pressure is also demanded to ensure the air-tightness of storage cavern. Therefore, the authors have designed the water curtain system to enclose the storage cavern to provide artificial water pressure continuously and also have applied grouting works at the cavern vicinity to reduce the seepage. Considering the rock mechanical stability at the fractured zone in the cavern vicinity, the authors have applied pre-grout prior to the cavern excavation to derive better efficiency by high grouting pressure.After the grouted zone was excavated, the authors measured the seepage and surrounding pore pressure to evaluate the efficiency of water curtain and the ground improvement. The derived geological and hydrogeological data are also applied on the following excavation stages to determine the construction method and the improvement criterion for pre-grout based on the observational construction method. After excavation was completed, the authors have compared the measured seepage and pore pressure with estimated values. If the seepage or pore pressure is unsatisfactory with the criterion, we applied post-grout as countermeasure.Finally, we compressed the water curtain system and evaluate the water curtain efficiency from groundwater behaviors.

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Compared with conventional caverns for traffic tunnels and hydraulic power stations, the construction of hydraulic containment type underground rock storage caverns should maintain the hydraulic potential from the cavern excavation throughout the operation. Therefore, the authors have applied a new observational water curtain system construction method to the hydraulic containment type storage caverns at Kurashiki national LPG stockpiling base in order to control the hydraulic behaviors during the cavern excavation. In this study, concerning the hydrogeological characteristics of Kurashiki site, the authors designed the artificial water curtain system and grouting works surrounding the cavern wall as two countermeasures for hydraulic behaviors control. For the water curtain system, the authors designed the vertical/horizontal boreholes to enclose the storage caverns and applied high water pressure from boreholes prior to the cavern excavation. Meanwhile, the authors designed 10m thick grouting zone at the cavern vicinity and conducted in-field grouting tests to decide the proper grouting methods for better grouting efficiency. For the purpose of the hydraulic behaviors control, the authors applied the 3-dimensional groundwater simulation model to estimate the hydraulic behaviors (e.g. pore-pressure drop, seepage) during the cavern excavation and compare with the monitoring data for evaluation. The evaluation results were applied for deciding the necessity of additional water curtain boreholes and grouting works. Further, in order to evaluate the hydraulic behaviors with high precision, the authors established and upgraded the 3-dimensional groundwater simulation model based on investigated and measured permeability data in the excavation. After the construction was completed, the caverns were pressurized to a specified test pressure in order to examine the air-tightness. The result has validated the performance of the constructed water curtain system and also verified the applicability of our observational water curtain system construction method.

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Hydraulic containment type underground LPG storage cavern applies the artificial water curtain system to preserve the LPG by permanent water flow toward storage caverns. Therefore, the storage caverns have received high water pressure during cavern excavation and influenced on the rock stability. In this paper, the authors have concerned the development of excavation disturbed zone (EDZ) under high water pressure and applied the FEM rock mechanical analysis to predict the deformation by high water pressure. Based on the analytical results, we designed a 10m grouting zone to cover the EDZ for rock permeability reduction. For the special geology structure (e.g. fault and fractured zone), we designed high strength rock bolt and applied fiber reinforced shotcrete based on the observational construction method. After cavern excavation was completed, we have submerged the water curtain tunnel and compressed the water curtain system to examine the rock mechanical stability of storage cavern. As the results, there was no deformation observed while water curtain pressure reached to 1.2MPa. Consequently, the water curtain compression test has achieved to validate the rock mechanical stability of the underground LPG storage cavern.

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Hydraulic containment type underground LPG storage caverns preserve liquid petroleum gas productions with the relative high storage pressure than conventional underground crude storage caverns. Therefore despite of the conventional evaluation on rock mechanical stability, the air-tightness evaluation is essential for the risk assessment on the underground LPG storage cavern.However, the conventional air-tightness evaluation methods apply the groundwater simulation and rock mechanical analysis respectively and have difficulty to represent the pore-pressure and gas pressure while fracture occurred at the cavern vicinity. In this study, the authors have developed a new hydro-mechanical coupling analysis method to evaluate both the rock mechanical stability and air-tightness of underground LPG storage caverns by the bonded-particle model. For air-tightness assessment on underground LPG storage caverns, the authors have concerned the capillary pressure in the gas-water dual phase flow and attempted to encode it into the analysis method for discussing the condition of gas leakage. Further, considering the fractures in the cavern vicinity are the primary leakage path, the authors have discussed the relationship between capillary pressure with rock permeability and fracture density and achieved to define the capillary pressure theoretically.For verification, the authors have conducted an in-field air-tightness test to compress an air-chamber to 1.0MPa, meanwhile the authors have applied the developed hydro-mechanical coupling analysis method to evaluate the air-tightness of the air-chamber.As the result, the leakage was detected while the air-chamber was compressed to 0.778MPa. For the evaluation results, the developed method achieved to represent the gas leakage and distributions of unsaturated zone, meanwhile, the represented pore pressure characteristics also verified the applicability of developed hydro-mechanical coupling analysis method.

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The air-tightness test for the propane storage cavern was conducted in June 2012 at Kurashiki national LPG stockpiling base, using the hydraulic containment type underground rock storage cavern tank system, which was the first attempt for preserving high pressure liquefied petroleum gas in Japan. The aim of this paper is to report the advanced air-tightness evaluation system we developed in order to evaluate the air-tightness of the cavern rigorously, and the result of evaluating the cavern air-tightness at Kurashiki base at the test pressure 950kPaG by the system.Air-tightness of an underground rock cavern is evaluated by measured cavern pressure variation throughout the test period under the condition the cavern is pressurized by compressed air up to the test pressure and all the lines between the cavern and the ground are closed. The measurement value of cavern pressure should be corrected from various factors affect to the cavern pressure, or variation of temperature and gas-phase volume in the cavern caused by seepage into the cavern and its pumping-up, and the air dissolution into seepage, etc. For evaluating the air-tightness of the 800,000 m³ world’s largest storage cavern at Kurashiki base rigorously, the advanced air-tightness evaluation system was designed, which included the field suitable temperature measurement instrument with uncertainty of ±0.01°C, the appropriate layout of temperature sensors in the cavern, and the management procedure for the field installation. As the test result evaluated by the advanced air-tightness evaluation system, the corrected cavern pressure variation at the test pressure 950kPaG during the 72-hour test period was 0.002kPa and it was substantially within the criterion0.5kPa derived from the instrument uncertainties. It indicates the air-tightness of the cavern was validated very rigorously and verified the applicability of our advanced air-tightness evaluation system.

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