Abstract:

The present study includes the stability analysis of large unlined rock cavern for the storage of crude oil. The analysis was performed by simulating staged excavation and rock support for different rock classes, taking into account in-situ stress condition and depth of overburden as observed during site investigation. Deformation values obtained from FEM analysis using Hoek-Brown criteria were compared with observed values for different stages as the construction progressed. Details of the investigations, rock mass parameters, analysis methods and monitoring are presented in the paper. Some three-dimensional analysis of the components of the storage caverns are also presented.

1. INTRODUCTION

Storage of hydrocarbons in underground caverns is an established technology which has been successfully adopted in many countries. The principle of storage essentially employs ground water pressure for containing the product within an unlined rock cavern. A first step in such projects is to locate suitable sites based on an investigation methodology involving geological, geophysical, Geotechnical and geog hydrological studies along with proximity to oil and gas transport networks. With the basic principle of storage through hydraulic confinement, rock caverns are planned at a depth such that there is sufficient hydrostatic pressure to counter the vapour pressure of the stored product. In order to further ensure the net water flow from the rock mass towards the cavern, a water curtain system is provided consisting of galleries and bore holes located above the crown of the cavern. The paper presents a case study on underground crude oil storage project. Located under a hill, with a maximum & minimum elevation of 130 m & 25m respectively above mean sea level, the proposed storage site was investigated by a detailed campaign involving geological mapping, geotechnical investigations through core drilling, laboratory testing, in-situ stress measurements, ego physical surveys and hydro -geological tests. The cavern layout is U-shaped in plan with an approximate „D" shaped cross section and a horizontal roof level (EL –30) maintained through the cavern length of 360–840 m having an average cross section of 20 m wide and 30 m high. Invert of cavern was sloping down (1:250) from intake to pump pits to minimize the interface between product and water and to make possible for circulation of crude in the cavern. Pillar width between two caverns is 30 m to maintain the stability of cavern as well as to avoid short circuiting of crude flow. Initially rock support was designed using the typical rock support chart proposed by Grim stand et al. (2002). The rock caverns are then numerically analyzed with this support system to check stability of the cavern in terms of wedge stability, deformation and stresses. However, due to an increased span and chances of a simultaneous blast at the intersection, it is more vulnerable to the problem of instability. Therefore, the intersections in the cavern are analyzed using a linear three dimensional boundary element code, Examine3D (Rock Science 2007), to study the effect of a larger span of the excavation in all directions.

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