A method, an apparatus (150), and a prefabricated replacement tube (108) are provided for repairing a defect (112) in a tube (102), such as in a waterwall (100). In the method, a replacement tube (108) is prefabricated having a wall (116), a central opening (114) extending through a portion thereof, and a predetermined length. A length of the tube (102), including the defect (112), equal to the predetermined length is removed. The replacement tube (108) is positioned between stubs (126, 128) of the tube (102), and a rotatable torch (152) of an automated welding apparatus (150) inserted through the opening (114) to align with a joint between an end (118, 120) of the replacement tube and a stub (126, 128). The torch (152) is rotated to join it to the stub (126, 128) along the entire joint. The torch (152) is then removed, turned 180 degrees, and reinserted to weld the remaining joint. Optionally, the replacement tube (108) is held in position before welding by slip rings (140, 142) which are slid down to cover the joints.