Evaluation of drainage system around a lined pilot cavern for underground cryogenic LNG storage

A lined pilot cavern for underground cryogenic LNG (liquefied natural gas) storage was constructed in granite in Daejeon, Korea in 2003 and commissioned in 2004. As the hydrostatic pressure of groundwater and thermal stress due to the formation of ice lenses may damage the containment system, rock drainage around the pilot cavern is needed to maintain the stability of the containment system. Once the drainage works were completed, the level of groundwater around the pilot cavern was controlled using drainage holes to form an ice ring as a second barrier to prevent the leakage of LNG from the cavern. In order to establish the drainage system for the pilot cavern, 15 boreholes were drilled into the rock. Fractures in the rock mass around the pilot cavern were characterized to determine the most appropriate orientation for the drainage holes. The major joints acting as conduits for inflow water were designed for efficient drainage. After the 15 drainage holes were drilled and their efficiency tested during the dry season in April 2003, it was found that there was a problem with the inflow of water through the main joint along the right-hand wall of the cavern, indicating the system was less efficient on the right-hand side. Hence, three more boreholes were drilled in the correct direction on the right-hand wall of the cavern. A second drainage test was conducted during the rainy season in August. This showed some seepage into the bottom of the pilot cavern. In order to reduce this seepage, two more additional drainage holes were drilled and grouted below the concrete invert. Although the drainage system was very efficient, weak points in the system were found by testing and changes were made to the system to improve its efficiency.