The first underground rock caverns for hydrocarbon storage in South-East Asia has recently been completed in Singapore. The storage capacity of 5 twin-caverns is 1.47 million cubic meters with total volume of rock excavation is about 3 million cubic meters. Geologically, it was excavated in low-angled bedded meta-sedimentary rocks, namely Jurong Formation, of late Triassic to early Jurassic age. The dimensions of cavern are 20 m in width, 27 m in height and 340 m in length. Extensive rock tri-axial tests were carried out to determine intact strength properties. Consequently equivalent rock mass properties were calculated based on GSI (Geology Strength Index) and D (Excavation Damage Factor) those were recorded from rock face mapping. In situ horizontal stresses of rock mass were measured before and during rock cavern excavation. The stability analysis was carried out based on those parameters by using 2D FEM program such as Phase2 and UDEC. The calculated vertical displacements of cavern roof and wall by FEM analysis were compared with those values monitored during excavation. The calculated axial forces were far below design capacity of rock bolts. The strong rock mass strength and high horizontal to vertical stress ratio enhanced safe working condition throughout the excavation period. Thus wide span caverns could be successfully excavated in meta-sedimentary rocks.

1. Introduction

South East Asia's first underground hydrocarbon storage rock caverns, namely Jurong Rock Cavern Project (JRC), had recently completed excavation works in Jurong Island within the south western region of main Singapore Islands as shown in Fig. 1 [1]. Two Access Shafts (AS 1 & AS 3) had been constructed through the reclaimed land and terminated at the fresh bedrock layer where the start of the main storage rock caverns had been excavated using dill and blast methods under the seabed as shown in Fig. 2.

1.1. Project background

A total of five caverns of storage capacity 1.47 mil cubic meters volume had been excavated under Phase 1. There are two levels of excavation namely Level 0 at 132 m below ground level and Level 1 at 100 m below ground level. Storage rock caverns are excavated at level 0 whereas operation tunnels for working access / operations and other water curtain galleries for water injection into surrounding rock mass are excavated at level 1 (Fig. 2 & 3).

As the caverns are unlined, the stored hydrocarbon will be directly in contact with rock mass. The tightness of the product is based on the hydraulic containment principle. The hydraulic potential of the water in the rock mass shall be higher than that of caverns which is essential to prevent migration of product into the rock mass. To achieve higher hydraulic potential in rock mass, water curtains, which ae composed of gallery and an array of boreholes are crated between each cavern (Fig. 3).

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