A ballast system for a marine structure, said ballast system comprising a ballast tank which is in fluid communication via a first inlet conduit with a tank arrangement for temporarily holding bilge fluid and/or ballast fluid in a marine structure, said first inlet conduit comprising a valve, wherein said tank arrangement comprises: a closed storage space defined by at least one tank wall; at least one fluid inlet, in fluid communication with said first inlet conduit, for introducing said bilge fluid and/or ballast fluid in said storage space; and at least one fluid outlet permitting removal of at least parts of said bilge fluid and/or ballast fluid from said storage space; said tank arrangement can be operated to separate air entering said storage space together with said bilge fluid and/or said ballast fluid; wherein said at least one fluid outlet is at least partly formed by a caisson (140, 170) adapted for receiving a submersible pump at a submersible pump position in said storage space, said submersible pump comprising a high side and a low side, characterized in that said caisson (140, 170) comprises at least one circulation conduit having an inlet and an outlet, adapted to enable circulation of at least parts of said bilge fluid and/or ballast fluid from said high side of said submersible pump, when said semisubmersible pump is in said submersible pump position, to said low side of said submersible pump, when said submersible pump is in said submersible pump position, said tank arrangement comprises means for removing air (160, 200) from said storage space, which air removing means is arranged to said circulation conduit, said means for removing air (160, 200) from said storage space comprising a priming ejector (161, 200) arranged to operate by means of at least a part of said circulating flow in said circulation conduit as motive power. a closed storage space defined by at least one tank wall; at least one fluid inlet, in fluid communication with said first inlet conduit, for introducing said bilge fluid and/or ballast fluid in said storage space; and at least one fluid outlet permitting removal of at least parts of said bilge fluid and/or ballast fluid from said storage space; said tank arrangement can be operated to separate air entering said storage space together with said bilge fluid and/or said ballast fluid; wherein said at least one fluid outlet is at least partly formed by a caisson (140, 170) adapted for receiving a submersible pump at a submersible pump position in said storage space, said submersible pump comprising a high side and a low side, characterized in that said caisson (140, 170) comprises at least one circulation conduit having an inlet and an outlet, adapted to enable circulation of at least parts of said bilge fluid and/or ballast fluid from said high side of said submersible pump, when said semisubmersible pump is in said submersible pump position, to said low side of said submersible pump, when said submersible pump is in said submersible pump position, said tank arrangement comprises means for removing air (160, 200) from said storage space, which air removing means is arranged to said circulation conduit, said means for removing air (160, 200) from said storage space comprising a priming ejector (161, 200) arranged to operate by means of at least a part of said circulating flow in said circulation conduit as motive power.