In the present investigation, 2 different wires for hyperbaric (underwater) GTA (gas tungsten arc) welding of X70 pipelines have been tested with respect to their weld metal mechanical properties. Welding of full coupons at different pressures (seawater depths of 16, 75 and 200 msw) was done with subsequent weld metal chemical analyses, hardness measurements, tensile testing and Charpy V notch testing as well as microstructure characterization. It is shown that both wires satisfied strength requirements set to X70 grade, representing a weld metal overmatch situation. Both wires gave sufficient impact toughness, but the toughness of the Ni-Mo containing weld was reduced with increasing seawater depth. This observation was strongly linked to the positioning of the Charpy V notch, and crack growth in a brittle, partially transformed region as a consequence of reheating by subsequent stringer beads. The embrittling microstructure consisted of high carbon MA (martensite-austenite constituents islands) decorating prior austenite grain boundaries. This microstructure was less pronounced when welding with the high Ni wire, which may explain why no similar toughness drop was found.
Up to now, subsea pipelines of grades X60 and X65 have Mainly been used in the Norwegian continental shelf installations. These are 10 to 42 in outer diam and their wall thickness ranges from 14 mm to 40 mm. Offshore tie-ins using qualified welding procedures have been made at 40 to 218 msw (meter sea water). X70 has been used only in one case, the Europipe (Aune et al., 2005). Forthcoming installations will include several X70 pipelines. One example is the Langeled pipeline, which will be the longest subsea pipeline in the world and will transport gas from Nyhamna on the West Coast of Norway via Sleipner in the North Sea to Easington in the U.K. Prior to subsea installations of pipelines, a test programme on welding consumables is required.