added a research item
Efforts to decrease CO2 emissions in the Swedish steelmaking process involve the use of industrial quantities of hydrogen gas supplied from large-scale Lined Rock Cavern (LRC) storages in order to eliminate the use of fossil fuels. This storage must be placed at sufficient depth so that the overburden is able to resist the uplifting gas pressure from inside the cavern. Both the high reliability requirement and addressing the uncertainties related to the rock mass properties make it difficult to design for such structures. In this work, a reliability-based design methodology for the LRC depth specification using the Response surface (RS) method is presented. Geologic conditions of Sweden, i.e. hard rock, are considered and the analytical solution for the resistance to uplift includes the tensile strength of the failure surface in addition to the overburden weight pressure. The highest uncertainties are assumed to be related to the rock mass parameters and both the cavern radius and the maximum operational pressure are chosen to be the same as for the LRC in Skallen, in southwestern Sweden. Four random variables with varying correlation are used to estimate the acceptable cavern depth and the results are reasonable compared to previous experience. The efficiency of the RS method for the considered problem is observed both for required number of samples and accuracy, showing suitability to be used with more complex, difficult to evaluate, problems such as Finite Element models.
Efforts to substitute the use of fossil fuels in industry by hydrogen gas requires the storage of large volumes of gas with a reliable pressure vessel design. The Hydrogen Breakthrough Ironmaking Technology (HYBRIT) initiative aims to make the whole steel making process in Sweden fossil-free with the storage of industrial scale quantities of hydrogen in underground Lined Rock Cavers (LRCs). The LRC concept is a relatively new design methodology that can be further developed with respect to safety and economic efficiency and reliability-based design methods provide one option to comply with codes and regulations. High reliability is required for the storage of hydrogen gas and the computational time becomes unpractical for the evaluation of a complex system such as the LRC. In this paper, the efficiency of Subset Simulation (SuS) regarding accuracy, precision and required number of samples is studied for the calculation of probability of failure against fatigue of the steel lining. It can be observed that by increasing the number of samples per level and increasing the conditional probability of failure the precision increases as well as the total number of samples. The accuracy of the SuS is checked with respect to Monte Carlo simulation (MCS) showing good agreement and with greater precision for fewer number of samples. A case study is performed for the geologic conditions of Sweden showing that the considered failure mode is unlikely for high stresses and good rock mass quality.
The objective of HYBRIT RP1 is to explore and assess pathways to fossil-free energy-mining-iron-steel value chains and thereby provide a basis for industrial development activities and the necessary future transformative change in this field. A large-scale storage capacity for hydrogen gas is an important component of the proposed HYBRIT concept. Underground storage in lined rock caverns provides a reasonable option: a large-scale demonstration plant for storage of natural gas was constructed in Sweden in 2002 and has operated safely since then. Considering that this lined rock cavern facility was constructed for natural gas, the present report investigates the current research needs to allow for underground storage of hydrogen gas in such a facility. This will serve as a basis for the research in Work Package 2.3 of HYBRIT RP1. Studying the experiences from decades of Swedish and international research and practice on the construction of underground gas storage facilities, the conclusion is that the lined rock cavern concept seems a reasonable way forward. In terms of rock engineering research, there are currently no critical research issues; however, a development of a previously proposed risk-based design framework for lined rock caverns may further strengthen the ability to manage risks related to underground gas storage facilities. The report identifies several potential research questions on this topic to be further studied: development of a risk-based design approach using subset simulation, the optimization potential of the concrete thickness in the lining, and the effect of spatial variation of rock mass properties on a location’s suitability for the storage facility. Additionally, the report identifies the potential effect of hydrogen embrittlement on the steel lining as a critical research issue to ensure safe storage of hydrogen gas in lined rock caverns. However, as this issue is not related to rock engineering, but a material issue, it will not be covered further in Work Package 2.3.