Abstract
The present investigation was initiated with the objective to study the root bead penetration profiles in hyperbaric GTAW welding ofX70 pipelines. Such profiles may vary substantially depending on the wire and base metal chemical composition. Root beads were deposited with a pressure corresponding to 75 meter sea water depth, and with a systematic increase of lOA in the pulse current until burnthrough took place. The results obtained showed that the penetration profiles were different between the two wires included in the welding program. The largest penetration width was found for wire B with the highest sulphur content (0.013% S), with a maximum width of about 8 mm width on the pipe inside for 160 A. At pulse current levels of 120 to 170 A, the difference between the two wires was about I mm. With current beyond 180 A, the profiles approached similar values, followed by burnthroughs for both wires at 190 A. However, the weldingparameters were already ""too hot"" at 180 A. These results are probably caused by Marangoni convection in the weld pool. High content ofsurface active elements (e.g., sulphur) are known to shift the flow pattern, providing deeper penetration. In practice, small variations in bead penetration profile may have large consequences during offshore tie-in welding. Cost enhancing repair operations may be required iflater non destructive inspection reveals poor root bead quality. Such actions require mobilisation of huge resources.
