A wellhead adapter assembly is disclosed. A wellbore adapter is designed for connection to a drillstring, with a riser fluidly connected to the wellbore adapter. At least one tubular arm section is rotatably and fluidly connected to the riser. A positioning system is attached to the at least one tubular arm. The positioning system is adapted to maintain the at least one tubular arm in a desired position relative to the riser, despite relative movement between the drillstring and a floating structure such as a drilling rig.