Abstract

The aim of this paper is to report the cavern gas-tightness test for the propane storage cavern at NAMIKATA national LPG stockpiling base, using the underground rock cavern tank system. The cavern gas-tightness test was characterized by the advanced gas-tightness evaluation system we developed, and the groundwater management by the appropriate cavern pressurization before the gas-tightness test based on the analysis of the hydrological response to the cavern pressurization and the groundwater simulation.

Gas-tightness of an underground rock cavern is evaluated by measured cavern pressure variation throughout test period under the condition the cavern is pressurized by compressed air up to the test pressure and all the line between the cavern and the ground are closed. The measurement value of cavern pressure is 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 gas-tightness of the large cavern at NAMIKATA base rigorously, the advanced measurement instruments for the influential factors were designed, and the management procedure for their field installation was established. As a result, the cavern gas-tightness was validated very rigorously by the system at the test pressure 970kPaG.

Also, in order to maintain the hydrodynamic containment required to the cavern gas-tightness, the groundwater management was conducted by applying the pressurization rate and pressurizing suspension periods based on the analysis of the hydrological response to the cavern pressurization. The hydrological response contained transient behaviors was observed partly at the pore pressure cells located around the cavern, and it was found that the response could be characterized in accordance with their geological feature. Consequently, the hydrodynamic containment could be maintained during the cavern pressurization and the test period.

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