Abstract: A double walled insulated tubing string (46) adapted to be hung from a well head (58) has an inner tubing string (21) formed of plural inner tubes and a radially outer tubing string (22) formed of plural outer tubes, the strings being independent of one another apart from axially fixing the strings relative to one another at the top end of the tubing string, although axial fixation may also be provided at the lower end of the tubular strings if desired. An annular insulating gap (30) between the tubing strings (21, 22) is hermetically sealed at both opposing ends and may be evacuated. Inner and outer tubing strings may be run into a well using conventional oil well drilling technology and there is no mechanical connection between the inner and outer tubing strings (21, 22), especially at connections of the respective inner and outer lengths of tube.