Robot for Upward Welding on Ship's Bottom Shell Plate
NKK is developing a welding robot that can weld on a ship's bottom shell plate from underneath the bottom of the ship while in dry dock. The butt welding of the ship's bottom shell plates has conventionally been done from the inside downwardly. The inside of the ship's bottom presents many obstacles such as piping systems and reinforcing members, making it difficult to use welding robots. Thus, skilled welders have been required to perform this work by hand. NKK has devised a way to prevent the welding metal from dripping by using a copper shoe under the welding pool, thereby making it possible to weld from the outside of the shell. Consequently, automation of the welding process and cost reduction are now possible. The new robot inserts two welding torches horizontally, one above the other, into the gap between the two plates to be welded. The torches are weaved sideways to fill the gap between the plates, and the whole device moves gradually along the gap to complete the butt welding. The torches are fitted with laser sensors, which allow the robot to monitor the gap width and welding direction and select the optimum welding conditions from a database. During NKK's tests, the trial robot successfully welded two plates 16mm in thickness with a gap of 13mm, at the rate of 15cm per minute. The welding torches are mounted, together with the power source and control equipment, on a 4-wheeled carriage. The welding machine is fixed to the end of an extendible boom, which is raised to bring the machine against the shell plate. NKK has been developing the system under a contract from the Ship and Ocean Foundation supported by the Nippon Foundation. The company plans to conduct further tests to improve the accuracy of welding control, by exploring the conditions for different thickness plates and widths of gap between the shell plates, with a view to commercializing the system within this fiscal year. |
