ABSTRACT:

Incoherent results of in-situ stress measurements and their implications in designing a crude oil storage caverns project are presented in this paper. Core-discing at depths corresponding to abnormally higher stress magnitude values were observed at the project site. It is brought to the notice that an optimistic assessment of in-situ stresses may result into costly design surprises during the excavation stage. Consideration of incoherency in the computed in-situ stress magnitudes and orientation resulted into lowering of the caverns level farther 5m from the surface than the design requirements as per the storage pressure. The paper also briefly outlines the project settings, fundamentals of crude oil storage projects in unlined mined caverns and in-situ stress measurements using hydro-frac technique. Impacts of in-situ stress regime on the caverns design is evaluated by using numerical modeling for which plain strain numerical models are prepared using a general purpose finite difference code, FLAC3D.

1 PROJECT DESCRIPTION

M/S South Asia LPG, Visakhapatnam (India) has constructed the first mined unlined caverns for LPG storage in South-East Asia. India. The project site is located at 17°41_N and 83°18_E in southern part of India. The caverns with 1,23,600m3 capacity are storing maximum 60,000MT of LPG and they are located at a depth between 162m to 186m below ground surface. The major axis of the caverns is in the N30°E direction. Geologically, the project caverns are located in Khondalite group of metamorphic Archaean rocks. The main joint set family strikes in the N85°E direction and has a near vertical dip. The joints are tight (separation less than 1 mm), less persistent, widely spaced (spacing more than 1 m) and are clear of infillings. This rock mass structure provides an environment similar to Continuously Homogenous Isotropic Linear Elastic (CHILE) material.

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