An apparatus for tying back a subsea well assembly to a surface platform, the subsea well assembly having an outer tieback conduit mounted on top of a wellhead housing, the apparatus comprising: a mandrel having an axis, external threads, and adapted to be lowered through the outer tieback conduit; an upward facing lip on an external lower end portion of the mandrel; a ring having internal threads engaged with the external threads of the mandrel, wherein the ring is a backup ring or locking ring; a sleeve on an outside diameter of the ring, the sleeve having a downward facing load bearing surface located above the upward facing lip on the mandrel; a first locking member carried by the sleeve, the first locking member having a plurality of teeth biased in a radially outward direction and adapted to engage with a corresponding first locking member profile located on an inner diameter of the outer tieback conduit as the mandrel is lowered into the outer tieback conduit; and an annular seal assembly carried between the load bearing surface of the sleeve and the upward facing lip of the mandrel; wherein when the mandrel is rotated relative to the ring, the sleeve, and the first locking member, the ring advances axially relative to the first locking member to a backup position between the mandrel and the first locking member and in engagement with an inner diameter of the first locking member, thereby locking the first locking member in a radially outward position; and the rotation of the mandrel causes the mandrel to move axially upward relative to the sleeve, deforming the annular seal assembly between the upward facing lip of the mandrel and the load bearing surface of the sleeve, thereby creating a seal between the apparatus and the wellhead housing. a mandrel having an axis, external threads, and adapted to be lowered through the outer tieback conduit; an upward facing lip on an external lower end portion of the mandrel; a ring having internal threads engaged with the external threads of the mandrel, wherein the ring is a backup ring or locking ring; a sleeve on an outside diameter of the ring, the sleeve having a downward facing load bearing surface located above the upward facing lip on the mandrel; a first locking member carried by the sleeve, the first locking member having a plurality of teeth biased in a radially outward direction and adapted to engage with a corresponding first locking member profile located on an inner diameter of the outer tieback conduit as the mandrel is lowered into the outer tieback conduit; and an annular seal assembly carried between the load bearing surface of the sleeve and the upward facing lip of the mandrel; wherein when the mandrel is rotated relative to the ring, the sleeve, and the first locking member, the ring advances axially relative to the first locking member to a backup position between the mandrel and the first locking member and in engagement with an inner diameter of the first locking member, thereby locking the first locking member in a radially outward position; and the rotation of the mandrel causes the mandrel to move axially upward relative to the sleeve, deforming the annular seal assembly between the upward facing lip of the mandrel and the load bearing surface of the sleeve, thereby creating a seal between the apparatus and the wellhead housing.