Tank

A tank for storage of hydrocarbon liquids on the seabed, the tank comprising a main storage area having a first compartment for storage of the hydrocarbon liquids, an outer side wall, and an inner side wall defining an hollow wall surrounding the first compartment, the hollow wall is configured to define a plurality of second compartments, the plurality of second compartments being adapted to be selectively displaced between flooded conditions and purged conditions to control submersion of the tank onto the seabed and to control ascension of the tank from the seabed, wherein the tank is adapted to:

• (a) fully flood the first compartment of the storage area and flood at least partially at least one compartment of the plurality of second compartments to submerge the tank onto the seabed, and to inject air into another at least one compartment of the plurality of second compartments to control submersion of the tank onto the seabed, and
• (b) inject air into the at least one compartment of the plurality of second compartments that is at least partially flooded to allow controlled evacuation of water therefrom to control ascension of the tank from the seabed.

A method for retrieving a tank for storage of hydrocarbon liquids from a seabed, the tank comprising a main storage area having a first compartment fully flooded, an outer side wall, and an inner side wall defining a hollow wall surrounding the compartment of the main storage area, the hollow wall configured to define a plurality of compartments adapted to be selectively displaced between flooded conditions and purged conditions to control descent of the tank, the method comprising the step of:

• inserting air in the flooded compartments of the second compartments and allowing ascend of the tank to the water surface, wherein the inserting of air into the compartments of the second compartments is done in a controlled manner to avoid uncontrolled ascent caused by unrestrained air space expansion, as external hydrostatic pressure reduces with reduced submergence.

A tank for storage of hydrocarbon liquids on the seabed, the tank comprising a main storage area having a compartment, an outer side wall, and an inner side wall defining an hollow wall surrounding the compartment of the main storage area, the hollow wall being configured to be selectively displaced between flooded conditions and purged conditions to control descent and/or ascent of the tank, wherein the outer side wall comprises a first outer layer and a second inner layer that is distance inboard with respect to the first outer layer to define a first space between the first inner and outer layers, the first space being permanently sealed, wherein the first space comprises a water layer for reducing the rate of heat loss to the surrounding sea and facilitating thermal management of the hydrocarbon liquids stored in the tank.