Recent Developments In Underground Storage Techniques

Changes in the energy situation, as well as safety aspects, have caused new and more stringent demands to be made for the storage of oil. It will become necessary to put an increasing share of the oil storage underground. However, conventional methods for storage of oil, on waterbed in unlined caverns, are not sufficient for all types of products. Vertical, cylindrical caverns can provide better storage conditions for oil products. Simple calculations show that from a rock mechanical point of view, such caverns are preferable because the cylindrical wall and the dome or cone-shaped roof provides more favourable stress conditions at the contour.

Anderungen der Energisituation wie auch Sicherheitaspekten haben neue und strengere Anspruche fuer öllagerung mitgebracht. Dadurch hat es sich notwendig gezeigt einen steigernden Anteil von den öllagern untertage zu bringen. Konventionelle Methoden fuer lagerung auf Wasserbett ohne Auskleidung sind aber nicht zufrieden fUr alle Typen von Ölprodukten. Verticale cylindrische Cisternen können bessere Lagerungsbedingungen fuer alle Produkte bringen. Einfache elastische FEM-Berechnungen zeigen auch besseren Spannungsverhaltnisse auf die Wand und Firste in Verhaltnis zu konventioneller Tunnelausformung.

Le changement dans la situation de l''energie a implique des demandes nouvelles et plus rigoureuses pour le stockage d''huile. Mais les methodes con- Ventionelles pour le stockage souterrain d''huile en couche d''eau dans les cavernes sans revetment etanche, ne sont pas suffisantes pour tous les types d''huile. Les cavernes verticales peuvent offrir de meilleures conditions de stockage pour tous les produits et cette formation est de stabilite meilleure pour las parois rocheuses et le toit rocheux que la formation conventionnelle des tunnels horizontaux.

The first underground storages for petroleum products on waterbed were made in Sweden about 40 years ago when storing on waterbed in Underground tanks was first used, according to the so-called Sentab method. In principle, these storages Were made as shown in fig. 1. The bottle-shaped steel tank with backup concrete, was kept filled to the barrier level by pumping water to and from the bottom of the cavern. This method was succeeded 30 years ago by a method for storage on waterbed in unlined rock caverns invented by H Edholm. The method is very simple and consequently cheap. As it is also very reliable, with regard to safety of the stored product and the environment, the method

Fig. 1. UNDERGROUND STORAGE ON WATERBED FOR PETROLEUM PRODUCTS ACCORDING TO THE "SENTAB" METHOD is competitive to this very day for storage of most petroleum types and petroleum products. It also provides good storage conditions for liquified gas. The common principle design is shown in fig. 2. Up till now, the storage caverns have usually been constructed horizontally, contrary to the Sentab method, with tunnel-shaped caverns with a span up to 20m, depth to 35m and length up to several hundred m. The tunnel-shape was chosen as a consequence of the development of efficient tunnel excavation equipment during the last decades. This design has proven to give safe and cheap operation of the storage. But, experience also shows that storing on large extended waterbeds is not suitable for some petroleum types e.g. heavy crude types inclined to give sludge deposit problems. The large waterbed is unsuitable for storing products sensitive to water such as jet fuels. In fact, jet fuel can be adversely affected by bacteria and fungi, mainly in the oil/water surface, and in such a way as to be disastrous for safety in flight. New problems can be added to the aforementioned ones due to the petroleum supply and transportation patterns which will apply in the near future. The shortage of light petroleums will bring more heavy ones onto the

1. ROCK CAVERN

2. WATFRFILLED SHAFT

3. PUMP PIT

4. OIL (or liquefied gas)

5. OIL/WATER INTERFACE

6. WATER-BED

7. GASFILLED SPACE

8. INCOMING OIL PIPE

9. CONCRETE BARRIER

10. OIL PUMP HYDRAULIC PRESSURES Ywater=1,O C

11. OIL DISCHARGE PIPE

12. LEAKAGE WATER PUMP

13. DISCHARGE PIPE LEAKAGE WATER

14. DRILLED HOLES FOR WATER INJECTION GW.

Fig. 2. 011storage on waterbed In unlined rock cavem market, even very heavy ones with high pour point and considerable tendencies to sludge deposits. In a few years the IMCO codes for crude oil washing (COW) will be effective for all countries. The crude carriers will then transfer all sludge deposits into the storage tanks (presently often dumped into the sea on the return trip to the export terminal). These two features, combined, will make horizontal caverns unfit for storage of heavy crudes. Although the horizontal cavern construction has usually functioned well, there are some obvious drawbacks with this design. The cost for reinforcing fair and weak rock is considerable and such work can interfere badly with the excavation time schedule.