Hard-rock caverns for hydrocarbon storage are a very particular type of underground facility controlled not only by local administrative and regulatory constraints that can be very specific, but also by the possible risks of geotechnical or geomechanical failure. Each such facility throughout the world is a prototype that needs to be adapted to very different site conditions. The straight forward application of design methodologies from relatively similar fields (mines, tunnels) can be very useful, but risks holding back the scientific and technical improvements needed by an engineering industry in full development. Here we look at various case histories and at problems that have been raised where recent attempts at standardization would appear to have been ill adapted, and where storage designers, through experience and judgment, may still provide innovative engineering solutions.
Although a rock cavern is neither an underground mine nor a civil engineering structure (You et al. 2003), one finds that the relevant authorities may, on the one hand, tend to impose constraints possibly based on the sometimes poor evolution of abandoned workings and post-mining lessons and, on the other hand, even when the cavern is not in the specific scope or field of application of new design codes such as Eurocode, tend to impose the use of these codes in the cavern’s design. Here we examine certain pressures arising from those who consider, with some justification, that more control is needed in the granting of hydrocarbon storage permits. We also consider several specifics in rock mechanics for storage design, along with new original methodologies (put forward by Geostock) based on nearly 50 years of feedback and lessons learnt from around the world (You, 2013). One should also bear in mind that the poor experiences of others are useful to all but the short sighted, and that rapid policies can hinder and jeopardise what we consider as much needed engineering improvements and so-called best practices.
Mega wedges represent adverse conditions during the excavation work of large underground caverns for storing hydrocarbons. Even in “Very Good” rock (assessed using an empirical approach such as the Q system or the RMR), the potentiality of wedges involving atop heading and one or several benches is a recurrent problem that cannot be neglected in unlined rock caverns, only stabilized using rock bolts. The empirical approaches have not been developed to define the necessary support in this very specific context which requires detecting and stabilizing such very large wedges (mega wedges)using a deterministic approach, based on structural geology. Although minimization of the risk and impact of mega wedges is considered by the cavern basic design and the adaptation of the layout and support, a deterministic re-assessment of the areas where mega wedges may occur shall be done during the excavation phase at a more precise scale, on realistic structural conditions. Established on real cases in granite, this paper presents a practical and balanced methodology that is used from the design to the excavation and support phases. For that, as a first step, structural geology aims at identifying location, geometry and potentiality of mega wedges and then rock mechanics helps to characterize the shearing capability of the identified mega wedge and its putative failure. In addition to the support recommendation, the phasing of the support installation with respect to the excavation work to achieve stable condition of the mega wedges is discussed. Results are promising in term of applicability, even if a better estimation of the shear strength parameters of the discontinuities involved in the failure mechanism can be necessary for local fine tuning.