Man is just beginning to anticipate the dizzying wealth hidden in the seafloor as research and production of offshore oil are increasingly developing in deeper waters. Today 150 to 200 m (500 to 600 ft) deep fields are no longer considered as exceptional. Oil companies are now contemplating production in blocks situated in various seas with depths down to 500 to 600 m (1,500 to 2,000 ft). We can expect confidently that within the next 10 years the interest of the oil industry will shift from the present relatively shallow waters to sea depths going down to, let us say, 2,000 m (6, 000 ft), giving birth to important new developments in oil research and production.
Temporary underwater storage of crude oil on the field will be one of the numerous problems to be solved, perhaps even more need fully than for shallow seas.
The Sea Tank Co. performed an active role in the development of gravity underwater storage tanks and platforms to be laid on the continental shelf. 1,2. Now we are extending our field of internet by preparing designs of deep and very deep storage tanks to be used in a range of depth from 200 m (600 it) to 1,000 m (3,000 ft).
A first approach of the problem makes it clear that the following principles have to be respected in order to design deep tanks.
The structure is to be a gravity structure. Its stability on the seabed will be secured by its dead weight without any piling or anchorage. This condition can be easily fulfilled since the influence of wave forces is very small when the depth is larger than 200 m (600 ft). The main force to be withstood is the uplift force due to the difference of density between the stored oil and the sea water.
The tank must not have to withstand external hydrostatic pressure. That means that the stored oil is to be in acupressure with the external sea water. Communication is to be provided between oil and sea. The tank will work following the water-oil displacement principle.
There should be no rigid connections between the tank and the sea surface, for exploitation as well as for immersion, because of the great depth taken into account. This point is important since it leads to immersion procedures of the tanks that have to be quite different from those usually adopted for shallower seas. 1,2. In this latter case, the presence of emerging columns or towers acting as floats makes it easy to control the immersion process
Last, but not least, the design of such a tank must allow safe, simple, and quick offshore operations. This is particularly important for the immersion operations that must be performed in open sea.