Subsea storage system

1. A method for compensating bathymetric pressure inside a subsea tank or a tank in a system of a plurality of subsea tanks, said subsea tank or subsea tanks having an inner diameter, containing a volume of a liquid working fluid (FL) following a change or decrease of the volume of said liquid working fluid (FL), the method comprising:

compensating said change or decrease with an equivalent volume of a liquid bathymetric compensation fluid (FC) represented by seawater, said liquid working fluid (FL) being mutually miscible with said bathymetric compensation fluid (FC),

wherein the liquid working fluid (FL) and seawater are separated from each other by a separation layer of a separation fluid (FS), which is liquid under operative conditions and immiscible with said liquid working fluid (FL) and with said bathymetric compensation fluid (FC).

2. The method of claim 1 , wherein said separation fluid (FS) has an intermediate density between the density of said liquid working fluid (FL) and the density of said bathymetric compensation fluid (FC).

3. The method of claim 1 , wherein said separation fluid comprises a first separation fluid (FS 1 ) and a second separation fluid (FS 2 ), which are immiscible with said liquid working fluid (FL) or with said bathymetric compensation fluid (FC) with which they are respectively in contact.

4. The method of claim 3 , wherein the density of said first separation fluid (dFS 1 ) is different from the density of said second separation fluid (dFS 2 ).

5. The method of claim 3 , wherein said first separation fluid (FS 1 ) and said second separation fluid (FS 2 ) both have intermediate density between the density of said liquid working fluid (FL) and the density of said bathymetric compensation fluid (FC).

6. The method of claim 3 , wherein said separation fluid (FS), said first separation fluid (FS 1 ) and said second separation fluid (FS 2 ) are selected from the group consisting of: chloroalkanes, fluoroalkanes, and mixtures thereof, chloroparaffins and mixtures thereof, chlorosilanes, fluorosilanes, and mixtures thereof.

7. The method of claim 3 , wherein to at least one of said liquid working fluid (FL), separation fluid (FS), first separation fluid (FS 1 ), second separation fluid (FS 2 ), and bathymetric compensation fluid (FC) filling bodies made of an appropriate inert, plastic or metallic material, having higher density than that of the liquid working fluid (FL), separating fluid and bathymetric compensation fluid (FC) are added.

8. The method of claim 1 , wherein said liquid working fluid (FL) has higher or lower density than the bathymetric compensation fluid (FC).

9. The method of claim 1 , wherein the density of the bathymetric compensation fluid (FC) is increased or decreased by adding an appropriate additive.

10. The method of claim 1 , wherein said separation layer inside said subsea tank has a thickness between approximately 0.5 and 0.2 times the inner diameter of the subsea tank.

11. The method of claim 1 , wherein the density of the separation fluid (FS) is higher than the density of the liquid working fluid (FL) and higher than the density of the bathymetric compensation fluid (FC) or the density of the separation fluid is lower than the density of the liquid working fluid (FL) and lower than the density of the bathymetric compensation fluid (FC) and wherein the liquid working fluid (FL), the separation fluid (FS) and the bathymetric compensation fluid (FC) are never present inside one of the subsea tanks of the system.

12. The method of claim 1 , wherein said subsea tank or said subsea tank in a system of a plurality of subsea tanks is in fluid connection with a service tank containing said liquid working fluid (FL) from which said liquid working fluid (FL) is withdrawn and reinjected into said subsea tank or is withdrawn from said subsea tank and reinjected into said service tank.

13. A subsea tank for storing a liquid working fluid (FL) according to the method of claim 1 , said subsea tank having a height/inner diameter ratio of approximately ≥7 and comprising separator walls.

14. The subsea tank of claim 13 , wherein said separation fluid (FS) comprises a first separation fluid (FS 1 ) and a second separation fluid (FS 2 ), wherein said first and second separation fluids (FS 21 , FS 2 ) are immiscible with said liquid working fluid (FL) or with said bathymetric compensation fluid (FC) with which they are respectively in contact, and said bathymetric compensation fluid (FC) and said liquid working fluid (FL) are mutually miscible.

15. The subsea tank of claim 14 , wherein said subsea tank works as a compensator in fluid connection with a service tank containing said liquid working fluid (FL), which is withdrawn or reinjected into said service tank.