Abstract: To render residues such as thickened sludge and sedimented crude oil or refinery products which collect at the bottom of a tank into pumpable flowable condition, crude oil is taken from an upward region of the tank, pumped upwardly to a suction pump (609)-pressure pump (605) unit (610) located, for example, on top of a floating roof of the tank, the pressurized crude oil taken from the top portion of the tank being reintroduced through a preferably rotating liquefaction lance (13) to a region in the vicinity of the sedimented sludge. The pressure is converted into hydrodynamic energy. Preferably, a plurality of such lances are used, arranged with respect to each other to form eddies or vorteces, to establish flow patterns within the tank, thereby liquefying the sludge or residues and rendering them into pumpable flowable condition for removal from the tank.