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 . <EOS>