A riser assembly includes a main body which encloses a main body main passage extending from a first end to a second end which is generally parallel to a longitudinal axis, the main body being mounted in a riser so that that the main body main passage forms a part of a main passage of the riser. A first port and a second port extend through the main body to connect the main body main passage with an exterior of the main body. A tubular extends along the main body main passage. A first sealing assembly is arranged in the main body between the first port and the second port. The first sealing assembly provides a seal between the main body and the tubular. At least two diversion lines extend from the first port to the second port. A pump is arranged within each diversion line.