Namikata and Kurashiki underground LPG storage caverns apply natural groundwater with artificial hydraulic containment system to preserve the LPG at high pressure and normal temperature (e.g. Propane, 0.73MPaG at 20°C). The water curtain galleries and water curtain boreholes were designed to maintain the surrounding pore pressure higher than the storage pressure and to ensure the groundwater flow toward the storage caverns constantly. Therefore, the constitution of storage caverns under high water pressure has to be discussed throughout the construction phase, commissioning and operation phase. Pore pressure sensors, acoustic emission sensors, vibration sensors and accelerometers were installed in the caverns to ensure the rock mechanical stability. These monitoring and measurement are launched from the water curtain compression test, the access tunnel water filling and the cavern air tightness test, and are continued even in the operation phase. Based on the measurement results, the storage caverns have been evaluated the integrity.

1 Introduction

Namikata and Kurashiki are two of the national LPG stockpile bases in Japan. Both of the two sites utilize underground storage caverns to confine the LPG gas by relative higher water pressure at depth (Figure 1). Grouting works were applied to homogenize and to reduce the rock permeability and seepage at the cavern vicinity. Furthermore, water curtain system composed by an array of boreholes and water curtain tunnel, was constructed to enclose the storage caverns and to ensure the air-tightness by permanent groundwater flow toward the storage caverns

Figure 2 illustrates the layout of Kurashiki base. The Kurashiki base is composed by four 488~640m length underground storage caverns at EL.-160m within dimension of 24m(height) and 18m(width) to contribute 800,000m3 stockpile capacity. After the cavern excavation was completed, the water curtain system was submerged and confirmed the rock mechanical stability by maximum operation hydraulic pressure. Then, the access tunnel was also submerged and the storage caverns were compressed to maximum design pressure to examine the air-tightness.

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