Method and apparatus for the narrow gap welding of thick tubes

A method is disclosed for the narrow gap welding of thick tubes. A thick, large-sized tube (1) is laid approximately in a horizontal direction, and a thick, small-sized tube (2) is disposed in such a manner as to be substantially intersected with the thick, large-sized tube (1) so that the small-sized tube (2) is jointed to the upper face or lower face of the large-sized tube (1) and a gap (4a) established between the small-sized tube (2) and the large-sized tube (1) has a narrow I form. The small-sized and large-sized tubes (1, 2) are welded together approximately in a downhand position by moving a welding head (9), which is capable of being moved concentrically with the small-sized tube (2), along the outer periphery of the small-sized tube (2), and detecting the extension of a welding wire (5) and controlling the vertical movement of the welding head (9) so as to keep the extension substantially constant. An apparatus for automatically carrying out the method is also disclosed.