Storage caverns in salt in sizes up to 1 million m' have been made available in the past primarily in saltdomes. Planning is executed using calculation methods derived from mining experiences and laboratory investigations on salt core material. In the meantime the construction of storage caverns even in stratified salt formations is becoming a necessity, as the dissemination of such salt deposits is far larger than that of the saltdomes. The computation procedure used for dimensioning and assessing the stability of saltdome caverns are now only conditionally applicable. Examples of realized and proposed projects show the development of the stability assessment for caverns in stratified salt. Stress rearrangements connected with the construction and operation of storage caverns in the rock material are evaluated using theoretic simulation models and are more realistically investigated using numerical computation - such as the finite elements method.

INTRODUCTION

The installation of caverns in salt formations for the underground storage of hydrocarbons has become increasingly widespread since the beginning of the 50's. Significant for the decision to construct an underground storage were its low investment costs compared with the conventional tank storage. For the seasonal balance between rate resp. production and consumption of primary energy relatively large storage units were necessary. For this purpose conventional tank storages whose economicability lies especially in high turnover rates are less advantageous.

As long as solely economic criteria are decisive, it is only too understandable that for the selection of the most suitable storage formation the salt masses developed into mighty saltdomes penetrating right into the proximity of the surface can be particularly considered. The solution- mining process has long since been mastered, although a definite geometrical form could not first of all be established due to the lack of suitable measurement procedures for checking the cavity growth. Now such measuring procedures exist.

Very soon, however, it became all too apparent that it was necessary to calculate the creep strength of caverns in order to ensure their serviceability for many years in the case of everincreasing individual volumes by appropriate planning of the cavern configuration as well as of their distribution resp. arrangement. The dimensioning of safety pillars was attributed decisive significance.

Individual cases became known, whereby caverns suffered considerable volume losses after a short operation time due to unusual convergence. One was faced with the task of dimensioning and arranging the planned cavities among one another in such a way as to be able to achieve maximum effective volumes on the one hand and on the other hand to ensure that the convergence measured by the expected lifespan of the cavern remained within a tolerable size order. A further condition was the optimal utilization of the salt deposit, be it of the deposit itself or even of the available, usable area above ground.

In the meantime cavern sizes of 0.5 - 1.0 × 106 m3 have been established in numerous storage facilities or are in construction and caverns of 1.6 × 106 m3 are in the planning stage. The dimensions of an existing 585 000 m3 cavern are reproduced in a vertical section in Fig. 1.

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