Large underground unlined rock caverns for storage of crude oil or liquid hydrocarbons are very often designed to operate with a gaseous phase maintained to a pressure range above atmospheric pressure (>1 bar abs.). To maintain the pressure of the gaseous phase in a cavern above atmospheric pressure it is necessary to inject a compensation gas (gas in) during the cavern emptying phase. The reverse operation (gas out) must be carried out during product filling to avoid overpressurization in the cavern. The management of this compensation gas requires a dedicated industrial unit for production, recovery, and treatment. In case of large underground caverns with net capacity of several million tons, the construction and operation costs for such a unit become prohibitive. To limit the need for compensation gas, the permissible pressure range in operation must be extended. Whereas the maximum gas pressure is mostly limited by the tightness issue, the minimum pressure is mainly governed by stability aspects. In this paper, the possibility of lowering the minimum pressure in operation below the atmospheric pressure is investigated, focusing on the cavern stability aspects. During construction at atmospheric pressure, for the large cavern section and rock mass quality expected, long-term stability can be achieved by conventional spraying of shotcrete and rockbolt installation. During operation, minimum pressure in the storage cavern close to vacuum is investigated and its impact on reinforcement need is reviewed using results of two distinct approaches: homogeneous continuum as well as wedge stability. The particularities regarding the required reinforcement for caverns operated at atmospheric pressure are also investigated.
Underground rock caverns excavated for the storage of crude oil have capacities of several millions of cubic metres. These caverns are typically excavated in hard rocks, allowing the excavation of large section supported with rock bolts and shotcrete.
To reduce the construction cost of a vapour recovery unit and the associated cost of gas compensation during operation of underground rock caverns for storing liquid hydrocarbons, the minimum operating pressure anticipated in the cavern is extended below the atmospheric pressure. As the lower pressure is applied around the cavern wall during operation only, it is important to distinguish between the reinforcement needs during construction and during operation to optimize the construction schedule of the project. Indeed, only the support required to achieve stability during construction remains on the critical path, whereas the installation of the support needed to achieve stability in operation can be delayed and rejected from the critical path.