Disclosed is a pig assembly for maintaining a multi-diameter functional line with a large range of diameter variation for conveying fluid. The pig assembly includes a central shaft having a first shaft end and a second shaft end and at least one cylindrical pig body coaxially mounted on the central shaft. Attached to the at least one pig body are a plurality of arms capable of moving between an extended position radiating from the at least one pig body and a retracted position within the at least one pig body. Each of the pluralities of arms includes a spring to enable each arm of the plurality of arms to move into the extended position and to store potential energy when the arm is in the retracted position, and a roller to enable rolling contact between the pig assembly and an interior of a functional line during movement of the pig assembly within the functional line. The pig assembly further includes at least one set of a plurality of scraper elements located along the central shaft capable of rotating about the central shaft to move between an opened position and a collapsed position. A method for maintaining a multi-diameter functional line using the pig assembly is also disclosed.