The present invention relates to a storage system for hydrocarbons on a offshore platform comprising a number of substantially completely submerged storage cells for hydro¬ carbons, wherein hydrocarbons are stored on top of a water bed.

The object of the present invention is to provide a storage system with reasonable costs, si ulataneously as the safety against incidental oil leakages to the ambient sea is radially reduced. A further object of the invention is to provide a system whereby the need for permanently installed, movable units are substantially reduced.

In accordance with the present invention these objects are obtained with an oil storage system having a plurality of storage cells for storing hydrocarbons, and which stands .in open communication with at least one other cell which practically speaking only contains water, since the said second cell freely communicates with the ambient sea outside the storage system. There is provided a conveyor- or pipe connection between the lowermost part to the uppermost part of the water cells in the oil storage system, simultaneously as the lowermost part of the water cells freely communicates with the ambient sea, for instance by means of a pipe line or an opening in the cell wall. From the upper end of the water cell one can with advantage connect a pipe system which extends towards the sea surface. The lowermost part of the pipe should be positioned above the level for the outlet for the communication piping from the oil storage cells. At said uppermost end of the pipe one may further with advantage arrange a tank positioned at the sea surface, from which tank oil may be removed by pumping for instance by means of a submersible pump. The media which in this fashion is pumped away may with advantage be pumped further through a separate separation unit for complete removal of oil residuals before the water is pumped out in the sea.

The water cell has two principal functions, namely: - to serve as an oil separation/water rinsing vessel for oil which incidentally has come into this cell through the unobstructed, open communication between the storage cells. Water will pass to and from in dependence upon whether oil

is pumped into or out from the oil storage cells. Oil in this water will in such case get sufficient time to float up to the upper part of the water cell and up through the pipe to the tank. It shall in this connection be remarked that the through flowing of water in the water cell will be very slow, such that oil particles may be separated, - serve as an oil catcher whereby one can secure that oil will •not escape out into the sea.

Through the solution in accordance with the present invention is obtained that all equipment having movable elements such as valves, manifolds, pumps and the like can be positioned above the sea surface and thus not be submerged or positioned in dry spaces below the sea surface, such as is known in connection with earlier solutions.

This involves that the pipe system in practice for instance can be cast into the concrete walls of a platform, either directly or into vertical pilasters or the like. Thereby is reduced a risk for pipe failures for instance caused by blows or corrosion. Especially suitable and advantagous will be the solution for use in connection with mono-tower production well platforms where the tower must be filled with water.

The system can also be utilized in connection with an oil storage submerged down to the sea bottom, for instance in conjunction to a fundament for a tension leg platform. In such embodiment the pumps and manifolds would be positioned in the floating structure and connected to the oil storage to separate riser pipes.

The present invention shall in the following be described further with reference to the attached drawing which schemati¬ cally is showing the principal features of the present invention, somewhat simplified, since only one single oil storage cell is shown for the sake of clarity.

The Figure is schematically showing a simplified oil storage system in accordance with the invention. The system comprises one oil storage cell 1 designed for storage of oil. As shown the oil storage cell 1 contains oil 2 which floats on top of a water bed 3. The oil storage cell 1 stands in open

communication with a water cell 4 through a pipe system 5. The end of the pipe system 5 which is positioned in the oil storage cell 1 is located at the lower end of same and at the lower part of the water bed 3. The water cell 4 is designed for substantially containing water and stands in free communication with the ambient sea via an opening or a pipe 6. This opening 6 is located at the lower end of the water cell 4. From the upper end of the water cell 4 and at a level somewhat above the end of the pipe 5, extends a pipe 7 upwards towardes the deck of the platform. In the vicinity below the ambient water surface 8 the pipe 7 extends over to a tank 9 which has a such size and a such cross-section that a pump 10 can be sent down into the same and/or be permanently installed in said tank 9.

From the upper end of the oil storage cell 1 extends a pipe system 11 which reaches above the sea surface 8 and which in ths zone above the same is enlarged into a tank 12. The tank 12 has a volume and a cross-section which makes it possible to lower and/or permanently installing a pump 13 for pumping oil from the oil storage cell 1. Several tanks 12 may naturally be coupled together through a manifold 14.

In the Figure only one single oil storage cell 1 is shown. It shall, however, be noted that it rests within the frame of th invention to utilize a series of oil storage cells 1 and that the oil/water level in the same at any time can be different from each other. For this purpose the manifold 14 is equipped with valves for one each of the oil storage cells 1. Likewise the manifold can be controlled with valves if oil shall be pumped into or out 1rom one of the cells 1.

If the system is for instance used in connection with a Condeep platform, for instance having a caisson with twentyfour cells and four towers, for instance nineteen cells may be oil storage cells 1, while one may be a water cell 4. Lines 7, 11 can extend up into one or more of the towers on the platform.

The oil storage system in accordance with the invention is further equipped with instrument enabling measurement of the level for the dividing plane or separation plane between oil 2 and water 3 in the oil storage cell 1. This system 18 indicates

stored oil at any time enabling control in regard loading/ eloa- ding storage.

In the water cell 4 is further provided an alarm system 19 which shall be further described in detail in the following. It is in addition possible to take water samples at the opening 6 in connection with the escape of water from the storage.

Since the water cell 4 for all practical purposes will be filled with water, it will not exist any substantial pressure difference in the water inside the tank 9 and the outside located sea level. It will further be relatively constant. The difference in level between the sea surface outside and the liquid level in -the tank 12 will, however, vary in dependence of the volume of the oil 2 in the oil storage cells relative to the water volume 3, because water is heavier than oil.

If oil is pumped out of the oil storage cells 1 via the tank 12, water will be drawn into the water cell 4 via the opening 6 and the pipe line 5, whereby the dividing or border plane between oil and water designated with the number 17, will move upwards. If on the contrary oil is transferred from the oil storage cell 1 via the line 15,11, water will be pressed out of the oil storage cell 1 in opposite direction. 'By such pumping of oil into the cell, water mixed with oil will be pressed through the line 5 into the water cell 4. In this cell the water will, however, remain for a sufficiently long time such that possible oil particles due to the low flowing velocity in the water cell 4, possess possibility to ascend upwards and will accu mulate in the dome in the water cell and possibly descend up through the pipe 7. Oil can in this fashion be removed from the water cell 4 through the tank 9.

In order to operate the system in a safe -manner and without causing oil contamination or pollusion, one needs a control- and alarm system in the oil storage cells 1. This system indicates the level for the dividing plane 17 between oil and water and will give the operator of the system an alarm if too high or too low water level 17 occurs in the oil storage cell 1. Such measuring meters shall be present in all storage cells, and may consist of conventional pressure gauges.

Also in the water cell 4 are located instruments 19 which measure a possible dividing plane between oil and water. A such alarm system has two objects, namely:

- to give alarm when water enters the water cell 4, and

- to stop further supply of water into the storage cells 1, if the oil penetration supersedes a certain quantity.

With this system one will therefore firstly receive a warning from the oil storage cells 1, and thereafter a warning from the water cell 4 if the first warning has not been followed by some action. Alternatively the system can auto¬ matically stop further supply of oil into the oil storage cells 1.

Oil which possibly has come into the tank 9 can suitably be pumped over into one of the oil storage cells l or to a treatment tank by means of the pump 10. If the water cell 4 is partly filled with oil, the same will flow back into the oil storage cells 1 by means of the pump 13.

As a measure of safety one may let the pump 10 operate between certain intervals and control whether oil has reached the tank 9. This will increase the safety.

The system 7, 9 including the pump 10, may be such dimensioned that the pumps 10 has somewhat greater capacity than the oil production volume. Thereby the pump 9 will remove the production volume and also supply some extra water through the opening 6. Through this solution no water will escape from the oil storage, but has to be rinsed in known manner.

This system 7 and 9 in accordance with the invention can also be utilized for injecting chemicals in controlled fashion into the storage cells 1, for instance in order to combat acid water or the like.