In the oil-and gas industry there is today different solutions for storage of oil, condensate or other liquified petroleum products.

NO 165 972 describes an oil storage system which is based on separation of oil and water in one or more storage tanks which are connected to a cleaning tank (water cell). Each storage tank is connected to a cleaning tank. The storage system is intended for use in permanent installations which are founded on the seabed. One of the objectives of the invention is to reduce the need for perma- nently installed, movable units.

US 4 685 409 describes a storage system which is used on a floating plat- form. The principle is that walls (bulkheads) are used in the construction to con- nect the top of a compartment with the bottom of another compartment. Thereby a total volume is obtained which consists of a number of compartments con- nected with each other in series. One end compartment is connected to sea in the bottom. The other end compartment is connected to a pipe for filling/off- loading of hydrocarbons in the top. The storage system solution has, however, limitations and weaknesses which result in loss of storage volume, mixing of oil/water over big areas which reduce the separation effect and increase the risk of pollution into sea, and will also influence the stability of the storage facility.

The market has been dominated by an oil storage system with air and oil/condensate in the tanks, as used in e. g. land tanks or in tankers. This type of storage requires addition of nitrogen to the air at the top of the tanks to eliminate the risk of explosion. This system is simple, but for storage facilities placed in the sea (floating or fixed to the seabed) the weight of such storage facilities will vary considerable during filling or offloading. For a floating storage the variation of weight due to filling/offloading of oil/condensate has to be compensated by bal- last water to maintain constant draught. Possibly the platform can float with a

varying draught. For storing facilities fixed to the seabed measures must be taken that the minimum weight is high enough at all times. This often leads to high costs by use of solid ballast, and the maximum weight is often governing for the foundation design. An alternative system is used for storage facilities immersed in water (sea), where oil/condensate and water are stored together in the same tank (wet oil storage), and the difference in specific weight causes separation and the oil/condensate is floating on the top of a water cushion. This system can be used in floating platforms with storage facilities and will require less use of ballast to maintain constant draught, because only the difference in spe- cific weight between oil/condensate and water (about 0.15-0.3 t/m3) has to be compensated for. Alternatively the platform in this case also can be left with a varying draught, which will vary less than by use of the first men- tioned system. For storage facilities resting on the seabed a wet storage will reduce the difference between minimum and maximum weight and ballast can be saved because the storage facilities are never empty.

At the border between oil and water a mixed layer will occur. The thickness of this layer varies with the specific weight of oil/condensate, but in practice it leads to a reduced effective (net) volume of the oil storage compared to the gross volume. Storage facilities consist of one or more tanks. The bigger the total area of the storage tanks is, the more volume is lost. It is therefore agreed that storage tanks with a small foundation space and a big height are the optimum with regard to effective volume.

An objective of the present invention is to provide a storage tank wherein the mentioned effective (net) volume of the oil storage is optimised with regard to the gross volume of the storage. This means that the area of the mixed layer of oil/condensate is as small as possible.

Another objective of the present invention is that the storage facilities have good hydrodynamic properties and is stable in the sea.

A further objective is that the storage tank according to the present invention shall be flexible with regard to the constructive design of the stor- age system, e. g. with regard to mechanical equipment such as pumps, pipes, etc. This is achieved according to the invention with a storage sys- tem for oil and condensate primarily for use in a floating or fixed platform construction, which is characterised in that the storage tank is separated

into a chosen number of compartments which are in liquid communication with each other through at least one pipe which extends from a recess/sump in the bottom of a compartment and to an elevation in the top of another compartment.

Preferred embodiments of the storage system are further explained in claims 2,3 and 4.

The storage system will be explained in the following by means of a preferred embodiment thereof, wherein - fig. 1 shows a floating storage system according to the invention where the storage tank is filled with oil, i. e. a full storage; - fig. 2 shows the storage system according to fig. 1 which is half full ; - fig. 3 shows the empty storage system above; - fig. 4 shows alternative designs of serial connections of storage cells in connection with a construction with thin walls, and - fig. 5 shows alternative designs of serial connections of storage cells in connection with a construction with thick walls, With reference to the figures a storage system for oil and condensate is shown in connection with a floating production platform. The storage system consists of a storage tank 1 and a floating body/shaft 16 which connects the storage tank 1 with a platform deck 17. The storage tank 1 is further divided into five compartments/chambers 7,8,9,10,11. The compartments 7,8,9,10,11 are further connected to each other in series by means of one or more pipes 12, 13,14 and 15 which extend from the bottom of e. g. the compartment 7 to the top of compartment 8, in such a way that liquid communication between the com- partments 7,8 takes place through the pipe 12. The pipes 12,13,14,15 extend from a cavity/bottom 7b, 8b, 9b, 10b in the bottom of compartments 7,8,9,10, 11, respectively, to an elevation 8c, 9c, 1 Oc, 11 c at the top of an adjacent com- partment 8,9,10,11. The upper ends of each pipe 12,13,14,15 is above the elevation for the lower side of the top plate of the storage facilities, and the bot- tom end of each pipe 12,13,14,15 is below the elevation for the surface of the bottom plate of the storage facilities. This means that no effective volume in the storage"is lost".

Figure 4 shows serial connection of storage cells with thin walls (e. g. steel or other metallic construction). The figure shows alternative designs of pipes 12,

13, 14,15 which extend from a cavi/bottom 7b, 8b, 9b, 10b in the bottom of a chamber 7,8,9,10,11 to an elevation at the top of the adjacent chamber 8,9, 10,11 An alternative shape of a pipe from chamber 10 to chamber 11 is shown as pipe 14'which leads from bottom 10b and which at the half height of the stor- age compartment goes through the wall of the storage compartment and contin- ues as pipe 15'which ends in elevation 11 a in storage facilities 11.

Figure 5 shows serial connection of storage cells in a construction with thick walls which e. g. can consist of concrete/sandwich. Alternate shapes of the pipes 12, 13 and 14, 15 and bottom 7b, 8b, 9b, 1 Ob and elevation 8a, 9a, loua, 11 a, respectively, are here shown in a manner corresponding to figure 4.

A filling/oFFloading system 2 will extend from the platform deck 17 and into the upper part of compartment 7. Compartment 11 will further be connected via a communication system 3 to sea. The communication system 3 comprises a buffer tank pipe 6 which extends from the lower part of compartment 11 and into the upper part of a buffer/cleaning tank 4. The buffer/cleaning tank 4 is provided with a sea water inlet/outlet 5 in the bottom.

With reference to fig 1 the storage tank is shown filled with oil, i. e. full storage. In this case the storage tank 1 only have a mixed layer oil/condensate and water in the lower part of the compartment 11. Over the mixed layer 11 only oil will be present and the rest of compartments 7,8,9, 10 will also be filled with oil. Below the border layer in compartment 11 and in the buffer/cleaning tank 4 sea water will be present.

Fig. 2 shows the storage system according to fig 1 in a condition where the storage tank 1 is half full. Oil has now been pumped out of the compartment 7 through filling/offloading system 2, and the oil volume taken out has been replaced with water taken in through the seawater in- take/discharge 5.

Fig. 3 shows the storage system according to fig. 1 and 2 where the oil has been tapped off with exception of a small rest volume at the top of compartment 7.

Fig. 1,2 and 3 illustrates the manner of operation of the oil storage facilities, and it is evident from the figure and the above description that the main principle of the invention is that each storage tank is divided into a number of compartments which are connected to each other in series. The

top of one compartment is connected to the bottom of the next, and the border between water and condensate will occur only in one compartment at a time (some water could be caught in the transitions between the com- partments, but the total volume of this is neglectable). Said in a simple way the compartments are placed above each other without this being done physically. In this way one is liberated from the earlier principle of construct- ing high storage tanks with small areas to maximise the net storage volume and also with regard to the stability of the storage facilities in the sea. The present invention therefore provides more freedom for design of the storage concept as such. It is not necessary to build a high storage construction to get as much effective storage volume as possible.

In addition to the fact that the present invention contributes to maxi- mising of the effective storage volume, the principle according to the pre- sent invention also contributes to other favourable effects. A wet storage which is connected to sea at the bottom and which is filled with oil/condensate, has in the bottom a neutral inside pressure (the same pres- sure as in the water outside), but in the top there is an inside overpressure which is determined by the height of the storage facilities oil/condensate and the specific weight. Low oil storage facilities can therefore be built and an effective reduction in the maximum differential pressure at the top of the storage occurs, compared to that of high storage facilities.

In the case of floating storage facilities will, as mentioned earlier, the mixed layer between oil/condensate and water affect the stability of the storage facilities in the sea. The stability will depend on the area of the mixed layer. A large mixed area will give a negative contribution to the stability of the storage facilities and vice versa. With the floating storage system according to the inven- tion the total area of the mixed layer is reduced to a minimum which thereby gives a positive contribution with regard to stability.