Rapid Depresurizations: Can they lead to irreversible damage?

International audienceRapid gas depressurization leads to gas cooling that is followed by slow gas warming when the cavern is kept idle. The decrease in the temperature of gas depends upon the relative withdrawal rate (in %/day), and cavern size and shape. Gas cooling may result in the onset of tensile stresses at cavern walls and roofs that may generate fractures or cracks. However, in most cases, the depth of penetration of these fractures is small, and they are perpendicular to the cavern wall. The distance between two parallel fractures becomes larger when fractures penetrate deeper into the rock mass, as some fractures stop growing. Fractures form a polygonal pattern. Salt slabs are created, with boundaries formed by the opened fractures. As long as the depth of penetration of the fracture remains small, these slabs remain strongly bonded to the rock mass, and it is believed that, in many cases, their weights are not large enough to allow them to break off the cavern wall

Utilisation des ondes de surface pour l'auscultation des structures en génie civil : application à la caractérisation des fissures de surface

There is a increasing need for non destructive testing applied to concrete civil works. The use of surface waves, especially Rayleigh waves, fulfill many requirements to assess near surface of concrete structures because they are particularly energetic and sensitive to the surface state. The present work proposes and evaluates a method based on Rayleigh waves for the characterisation of surface cracks. The diffraction of R-waves by a surface crack is modelled with the Indirect Boundary Element Method (lBEM) to obtain the synthetic respons to an impulse input. The results make it possible to understand the diffraction pattern and to design a spectral analysis method for the determination of the crack depth . For the validation of the method on observed data, an experimental setup is developed and a data processing method is elaborated to eliminate source and receiver effects. The comparison of theoretical and experimental data on artificial cracks of different depths in concrete slabs shows that the method is reliable for these simple geometries. The method is then applied to special cases (water filled cracks, cracks with a point contact at different depths) and compared to the results obtained by the body waves transmission time method. The two methods show to be complementary, with a low experimental additional cost. They are finally carried out on a real cracks where it is confirmed they provide complementary information for the characterisation of cracks.Dans le domaine du génie civil, la demande en matière de contrôle non destructif est croissante. Les ondes de surface, et de Rayleigh en particulier, présentent différentes propriétés intéressantes pour l'auscultation de la surface des structures en béton. L'objectif de ce travail est de proposer et d'évaluer une méthode de caractérisation des fissures de surface par les ondes de Rayleigh . La diffraction des ondes de Rayleigh par une fissure de surface est modélisée par la méthode indirecte d'éléments de frontière (IBEM pour Indirect Boundary Element Method). Les résultats permettent une analyse fine des différents phénomènes de diffraction et l'élaboration d'une méthode spectrale de détermination de la profondeur des fissures . Des dispositifs et procédures de traitement prenant en compte les effets de la source et des capteurs permettent de l'appliquer expérimentalement. La comparaison des données numériques et expérimentales sur des fissures artificielles de différentes profondeurs valide la méthode. Elle est alors appliquée à des cas particuliers (fissures remplies d'eau , fissures présentant des contacts entre les deux lèvres) et comparée à une méthode temporelle utilisant les ondes de volume. Les résultats montrent clairement la complémentarité des deux méthodes pour un faible surcoût de mise en oeuvre. Leur application à des cas de fissuration réelle sur ouvrage d'art confirme leur complémentarité pour de véritables conditions d'auscultation

Effects of a rapid depressurization in a salt cavern

International audienceRapid gas depressurization leads to gas cooling followed by slow gas warming when the cavern is kept idle. Gas temperature drop depends upon withdrawal rate and cavern size. Thermal tensile stresses, resulting from gas cooling, may generate frac-tures at the wall and roof of a salt cavern. These fractures are perpendicular to the cavern wall; in most cases their depth of penetration is small. The distance between two parallel fractures becomes larger when fractures penetrate deeper in the rock mass, as some fractures stop growing. These conclusions can be supported by numerical computations based on fracture mechanics. Salt slabs are created. These slabs remain strongly bounded to the rock mass and it is believed that in many cases their weight is not large enough to allow them to break off the cavern wall. However, depth of penetration of the fractures must be computed to prove that they cannot be a concern from the point of view of cavern tightness

Twelve-year monitoring of the idle Etrez salt cavern

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Blowout from a hydrogen storage cavern

International audienceTo prevent catastrophic climate change, Europe and the world must rapidly shift to low carbon and renewable energies. Hydrogen as an energy vector, provides viable solutions to replace polluting and carbon-emitting fossil fuels. Gaseous hydrogen can be stored in underground storage and coupled with the existing natural gas pipe networks. Storage in salt caverns was recognized to be the best suited technology to meet new energy system challenges. Hydrogen storage caverns are currently operated in the UK and in Texas. A preliminary risk analysis dedicated to underground hydrogen salt cavern highlights the importance of containment losses (leaks) but also of the formation of a gas cloud following a blowout whose ignition may generate dangerous phenomena such as jet fire, Unconfined Vapor Cloud Explosion (UVCE) or flashfire as well. A blowout is one of the major accidental scenarios likely to occur during the operation of a hydrogen underground storage in salt cavern. Blowout is an uncontrolled release of gas from well after pressure control systems have failed. Several examples of blowouts in gas storage caverns have been described in the literature, such as that in an ethane storage at Fort Saskatchewan, Canada (Alberta Energy and Utilities Board, 2002) or in a natural gas storage at Moss Bluff, Texas (Rittenhour and Heath, 2012), see Réveillère et al., 2017. This paper presents the modeling of the subterraneous and aerial parts of a blowout from a hydrogen storage cavern. In the first part of this article, the method presented in Bérest et al. (2013) is used to predict the duration of the eruption and the evolution of key thermodynamics parameters such as hydrogen temperature, pressure, velocity and density. Then these results are used to compute dispersion in the atmosphere of the hydrogen jet outflowing from the wellhead and to evaluate the effects of potential resulting phenomena on surrounding assets

Blowout from a hydrogen storage cavern

International audienceTo prevent catastrophic climate change, Europe and the world must rapidly shift to low carbon and renewable energies. Hydrogen as an energy vector, provides viable solutions to replace polluting and carbon-emitting fossil fuels. Gaseous hydrogen can be stored in underground storage and coupled with the existing natural gas pipe networks. Storage in salt caverns was recognized to be the best suited technology to meet new energy system challenges. Hydrogen storage caverns are currently operated in the UK and in Texas. A preliminary risk analysis dedicated to underground hydrogen salt cavern highlights the importance of containment losses (leaks) but also of the formation of a gas cloud following a blowout whose ignition may generate dangerous phenomena such as jet fire, Unconfined Vapor Cloud Explosion (UVCE) or flashfire as well. A blowout is one of the major accidental scenarios likely to occur during the operation of a hydrogen underground storage in salt cavern. Blowout is an uncontrolled release of gas from well after pressure control systems have failed. Several examples of blowouts in gas storage caverns have been described in the literature, such as that in an ethane storage at Fort Saskatchewan, Canada (Alberta Energy and Utilities Board, 2002) or in a natural gas storage at Moss Bluff, Texas (Rittenhour and Heath, 2012), see Réveillère et al., 2017. This paper presents the modeling of the subterraneous and aerial parts of a blowout from a hydrogen storage cavern. In the first part of this article, the method presented in Bérest et al. (2013) is used to predict the duration of the eruption and the evolution of key thermodynamics parameters such as hydrogen temperature, pressure, velocity and density. Then these results are used to compute dispersion in the atmosphere of the hydrogen jet outflowing from the wellhead and to evaluate the effects of potential resulting phenomena on surrounding assets

Blowout Prediction on a Salt Cavern Selected for a Hydrogen Storage Pilot

International audienceTo prevent climate change, Europe and the world must shift to low-carbon and renewable energies. Hydrogen, as an energy vector, provides viable solutions for replacing polluting and carbon-emitting fossil fuels. Gaseous hydrogen can be stored underground and coupled with existing natural gas pipe networks. Salt cavern storage is the best suited technology to meet the challenges of new energy systems. Hydrogen storage caverns are currently operated in the UK and Texas. A preliminary risk analysis dedicated to underground hydrogen salt caverns highlighted the importance of containment losses (leaks) and the formation of gas clouds following blowouts, whose ignition may generate dangerous phenomena such as jet fires, unconfined vapor cloud explosions (UVCEs), or flashfires. A blowout is not a frequent accident in gas storage caverns. A safety valve is often set at a 30 m depth below ground level; it is automatically triggered following a pressure drop at the wellhead. Nevertheless, a blowout remains to be one of the significant accidental scenarios likely to occur during hydrogen underground storage in salt caverns. In this paper, we present modelling the subterraneous and aerial parts of a blowout on an EZ53 salt cavern fully filled with hydrogen

Very Slow Creep Tests on Salt Samples

The objective of this paper is to assess the creep law of natural salt in a small deviatoric stress range. In this range, creep is suspected to be much faster than what is predicted by most constitutive laws used in the cavern and mining industries. Five 2-year, multistage creep tests were performed with creep-testing devices set in a gallery of the Altaussee mine in Austria to take advantage of the very stable temperature and humidity conditions in this salt mine. Each stage was 8-month long. Dead loads were applied, and vertical displacements were measured through gages that had a resolution of 12.5 nm. Loading steps were 0.2, 0.4, and 0.6 MPa, which are much smaller than the loads that are usually applied during creep tests (5–20 MPa). Five salt samples were used: two samples were cored from the Avery Island salt mine in Louisiana, United States; two samples were cored from the Gorleben salt mine in Germany; and one sample was cored from a deep borehole at Hauterives in Drôme, France. During these tests, transient creep is relatively long (6–10 months). Measured steady-state strain rates (ε˙ = 10 −13 –10 −12  s −1 ) are much faster (by 7–8 orders of magnitude) than those extrapolated from relatively high-stress tests (σ = 5–20 MPa). When compared to n = 5 within the high-stress domain for Gorleben and Avery Island salts, a power-law stress exponent within the low-stress domain appears to be close to n = 1. These results suggest that the pressure solution may be the dominant deformation mechanism in the steady-state regime reached by the tested samples and will have important consequences for the computation of caverns or mines behavior. This project was funded by the Solution-Mining Research Institute

Salt Creep: Transition Between the Low and High Stress Domains

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