|Theme||Visible||Selectable||Appearance||Zoom Range (now: 0)|
Akselsen, Odd M. (Norwegian University of Science and Technology (NTNU)) | Ren, Xiaobo (SINTEF Materials and chemistry,) | Nyhus, Bård (Norwegian University of Science and Technology (NTNU)) | Alvaro, Antonio (Norwegian University of Science and Technology (NTNU))
Pipelines for transport of oil and gas in Arctic areas are subjected to some extreme challenges; among these being low temperatures. Thus, the steel behaviour with respect to the ductile to brittle transition will be important. Moreover, when the design temperature falls down to -50 to -60°C, the toughness of the weld metal may become a critical factor. In the present investigation, submerged arc welding was performed using two different wires (Wires 1 and 2), using 23.7 mm base plate corresponding to API X80 quality. The test programme included tensile and notched tensile testing, Charpy V notch testing, and finally, SENB05 (bending with a/W = 0.5) and SENT02 (tension with a/W = 0.2). The tensile test results confirmed that the base metal and weld metal yield and ultimate strength increases with falling temperature. The Charpy V results showed high values for Wire A with all individual values above 50 J. The fusion line (FL), FL+2 mm and FL+5 mm had even higher toughness than the weld metal. The CTOD testing confirmed the trend from Charpy V. Wire A gave good weld metal results (SENB05 > 0.3 mm), while wire B possessed low toughness (≤ 0.11 mm). Constraint effects are evident by comparing the results obtained from SENB05 and SENT02 weld metal testing.
Akselsen, Odd M. (Materials and Chemistry, SINTEF, Trondheim, Norway) | Fostervoll, Hans (Materials and Chemistry, SINTEF, Trondheim, Norway) | Hårsvær, Ansgar (Materials and Chemistry, SINTEF, Trondheim, Norway) | Aune, Ragnhild (Materials and Chemistry, SINTEF, Trondheim, Norway)
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.