A method of performing an operation on a vessel assembly, said vessel assembly comprising - a vessel comprising a top wall, a bottom wall and an upright wall extending from the bottom wall to the top wall, said top wall, bottom wall and upright wall defining a lumen, - a superstructure on top of the vessel, said super structure comprising - a bear i ng, said bear i ng comprising an upper race member, a lower race member and roller bodies between said upper race member and said lower race member, said lower race member being attached to the super structure, and - a locking nut for locking the bear i ng, said locking nut comprising an inner thread, - a rotor, said rotor comprising - an upright, rotatable shaft, said shaft comprising i) a top shaft section extending above the top wall and ii) a bottom shaft section (131") in said lumen, said top shaft section - being provided with said upper race member, all owi ng the rotor to rotate along a shaft axis of the upright, rotatable shaft in line with the axis of rotation of the bear i ng, and - comprising an out er thread for engagi ng the inner thread of the locking nut to lock the upper race member at a desired location on the top shaft section; and - a plurality of rotor elements attached to said bottom shaft section (131"); wherein the rotor is stopped and the operation is performed; characterized in that - the top shaft section comprises at a location above the upper race member a first member chosen from a male member and a female member for engaging a device, said device having - a top end, and - a bottom end, said bottom end comprising a complementary second member for engagi ng the top shaft sect i on, said complementary second member bei ng chosen from a female member and a male rrerrber; wherein - the device is connected to the top shaft section of the shaft, - the rotor is lifted by lifting the shaft at its top end section using the device, and - the locking nut is rotat ed upwardly along the top shaft section of the rotor, following whi ch the operation is performed. - a vessel comprising a top wall, a bottom wall and an upright wall extending from the bottom wall to the top wall, said top wall, bottom wall and upright wall defining a lumen, - a superstructure on top of the vessel, said super structure comprising - a bear i ng, said bear i ng comprising an upper race member, a lower race member and roller bodies between said upper race member and said lower race member, said lower race member being attached to the super structure, and - a locking nut for locking the bear i ng, said locking nut comprising an inner thread, - a bear i ng, said bear i ng comprising an upper race member, a lower race member and roller bodies between said upper race member and said lower race member, said lower race member being attached to the super structure, and - a locking nut for locking the bear i ng, said locking nut comprising an inner thread, - a rotor, said rotor comprising - an upright, rotatable shaft, said shaft comprising i) a top shaft section extending above the top wall and ii) a bottom shaft section (131") in said lumen, said top shaft section - being provided with said upper race member, all owi ng the rotor to rotate along a shaft axis of the upright, rotatable shaft in line with the axis of rotation of the bear i ng, and - comprising an out er thread for engagi ng the inner thread of the locking nut to lock the upper race member at a desired location on the top shaft section; and - a plurality of rotor elements attached to said bottom shaft section (131"); - an upright, rotatable shaft, said shaft comprising i) a top shaft section extending above the top wall and ii) a bottom shaft section (131") in said lumen, said top shaft section - being provided with said upper race member, all owi ng the rotor to rotate along a shaft axis of the upright, rotatable shaft in line with the axis of rotation of the bear i ng, and - comprising an out er thread for engagi ng the inner thread of the locking nut to lock the upper race member at a desired location on the top shaft section; and - being provided with said upper race member, all owi ng the rotor to rotate along a shaft axis of the upright, rotatable shaft in line with the axis of rotation of the bear i ng, and - comprising an out er thread for engagi ng the inner thread of the locking nut to lock the upper race member at a desired location on the top shaft section; - a plurality of rotor elements attached to said bottom shaft section (131"); - the top shaft section comprises at a location above the upper race member a first member chosen from a male member and a female member for engaging a device, said device having - a top end, and - a bottom end, said bottom end comprising a complementary second member for engagi ng the top shaft sect i on, said complementary second member bei ng chosen from a female member and a male rrerrber; - a top end, and - a bottom end, said bottom end comprising a complementary second member for engagi ng the top shaft sect i on, said complementary second member bei ng chosen from a female member and a male rrerrber; - the device is connected to the top shaft section of the shaft, - the rotor is lifted by lifting the shaft at its top end section using the device, and - the locking nut is rotat ed upwardly along the top shaft section of the rotor,