The hyperbaric welding technology and system for tie-in and contingency repair welding of subsea pipelines, as used in The North Sea by Hydro and Statoil since the 1980s is presented. It is based on TIG U-butt welding remotely controlled from a surface vessel, with the assistance of divers for installation set-up work inside the welding habitat on the sea bed. Statoil, with this technology, has been selected by Nord Stream AG as supplier of the hyperbaric welding services on the new Nord Stream pipeline in the Baltic Sea.
To cover the repair contingency for pipelines at sea depths beyond the depth limit where divers can operate, Statoil is now developing a fully remote controlled repair welding system. The MIG welding process is selected based on better performance under higher pressure, and it is found to be less sensitive for variations in the ambient pressure. This new system, based on MIG fillet welding of an outer sleeve connecting the pipe ends, is also briefly described. Finally, the results of initial experiments on MIG butt welding against copper backing, is presented.