Historically, the manual metal arc welding (MMA) process has been used for welding of full encirclement split tees on hot tap connections for gas transmission pipelines. The National Grid high pressure gas transmission network currently consists of pipelines up to and including 1219 mm outside diameter. The large diameter pipes require split tee assemblies up to 80 mm thick. The arc time to complete welding can be considerable and requires multiple welders to complete welding in one continuous operation.
The qualification of a mechanised gas shielded flux cored arc welding (GSFCAW) procedure for welding the longitudinal seams on large diameter tee connections would realise significant operational and cost benefits over the MMA method. The equipment for mechanised field welding is readily available for a large number of applications across many industries. Recent advances in the technology suggest that a suitable mechanised procedure can be developed for the longitudinal weld seams of split tee assemblies.
The primary aim of this project was to qualify a mechanised GSFCAW process and set of procedures in line with the National Grid specification for welding longitudinal seams of split tee assemblies. A comprehensive welding and test schedule was performed using 50 mm tee material. Welding was performed using the Firefly welding system in three positions, flat, overhead and horizontal to cover the full range of welding positions required for tee connections with either horizontal or vertical off-take branches.
Based on the mechanical testing and non-destructive examination (NDE) results, the combination of process and consumables used in this project have been qualified in accordance with the National Grid specification.
A number of quality issues were observed during welding and recommendations to address these have been identified. The travel speeds achieved using the GSFCAW process are up to twice those recorded when welding a similar size fitting using MMA. Even after considering the remedial work required to rectify quality issues, the overall welding times recorded using the GSFCAW process were lower than those recorded on a similar size fitting welded on site using the MMA process.