The Effects on Process Performance of Reducing the Pressure From 36 to 1Bar in Hyperbaric MIG Welding

Technology for remotely controlled (diverless) repair welding of subsea pipelines from 170 to 1000m water depth is being developed by StatoilHydro. The repair technology is based on a sleeve concept combined with MIG welding and the development is currently nearing completion. Technology for diver-assisted remotely controlled welding down to about 200m has been used in the North Sea for about twenty years. In order to reduce the use of divers, the deep water diverless technology is also being considered for use in shallow waters. The present work has been performed to investigate whether the deepwater welding procedure may also be used in shallow waters, and which modifications for the lower pressure conditions need to be made. Test welding has been performed in the pressure range from 36 to 1bar corresponding to 350 to 0m sea water depth to study the effect of ambient pressure upon the welding process behaviour and weld bead appearance and geometry. For the 12 o’clock welding position tested, welding parameters developed for deep water conditions also worked well for shallow water conditions down to about 2bar. It was also evident that the electrode polarity, which is negative for the deep water procedure, had to be changed to electrode positive for the lowest pressures, which coincides with conventional 1-atm MIG welding. Mechanical property testing and microstructure examinations revealed satisfactory results using the modified welding procedure.