A group of vertical cylindrical rock caverns is the basic idea of the POLYTANK concept. Vertical caverns can sometimes offer a good solution to underground storages in rocks with horizontal discontinuities. The small roof area of vertical caverns compared to horizontal tunnels gives low cost for reinforcement. This is of great importance particularly in weak rocks. The POLYTANK system implies a group of storage caverns which is kept well together. This is favourable for easy and rapid handling of the product to be stored. POLYTANK is particularly aimed for storing of oil, liquefied gases, hot water, and various chemicals.

INTRODUCTION

Underground storage of liquids has grown rapidly with the increasing demand of oil in the world. Before World War II no suitable underground storage methods existed, but from about 1950 methods for storage on waterbed have been developed and in the last few years become more frequently used as they are cheap and can be adopted also for very large capacities. Recently storage in underground caverns have become even more important all over the world as it offers far better protection against fire, explosion, sabotage and military threats. The environmental advantages are numerous with underground storage. The concept of POLYTANK offers many advantages also for seasonal energy storage in rock caverns, e.g. of spill-heat heated or sunheated water.

THE POLYTANK CONCEPT

In most cases oil caverns in rock have been performed as horizontal tunnels of large areas, 600 m2 or more. Such tunnels will usually offer a cheap and comparatively rapid solution as they can be excavated with established tunnelling methods. In rock of good quality and with limited requirements of reinforcement, horizontal tunnel constructions are usually preferred. In many cases, however, tunnels or large rooms with horizontal axes are less suitable. Constructions in soft rock often require extensive reinforcement of rock. In areas with limited underground space it will be favourable to extend the storage downwards in a vertical direction. Here the POLYTANK system offers advantages. Hence, the POLYTANK system is very versatile as it can be used to solve design and construction problems. It will also improve and simplify the maintenance and operation of the the storage as will be shown.

CONSTRUCTION PRINCIPLES

A POLYTANK storage consists of a group of cylindrical caverns with vertical axes, Fig. 1. The shape of the cross-section of the caverns and the arrangement of the caverns are dependent on the state of stress in the earth crust and the rock mass quality. For the storage of oil the group is favourably designed with a central shaft, where pumps and most of the control devices are placed. The storage caverns are placed in one or several hexagons or rings around the shaft. They can be given the same shape and size and will preferably be placed on the same level below ground water table. Excavation of the storage is made by a sloping access tunnel around the storage. From suitable levels of the access tunnel, branch tunnels are excavated to the caverns, Fig. 2

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