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Fabrication and installation of offshore steel structures in the Arctic region will face some major challenges. Many of these challenges are well known and brought from the North Sea and the Norwegian offshore fields. Exploration in the Norwegian territory of the Arctic has taken place in the southwestern Barents Sea, i.e., in the area free of ice. So far, Snøhvit and Goliat fields have complete installations, Johan Castberg is now under planning. Therefore, there will be a gradual approach towards temperatures lower than −20°C (the lowest temperature in the current NORSOK standard is −14°C), which may represent a major challenge for the materials and structural integrity. The design temperature for Goliat is −20°C, while Johan Castberg will possibly be somewhat lower. Due to the continuous decrease in temperature the further north the field is, welded structures need focus concerning their low temperature properties. Although the initial base metal toughness may be excellent, a severe toughness deterioration occurs normally as result of fabrication welding. The present investigation summarizes results achieved in the steel part of the Norwegian project ”Arctic Materials” concerning the low temperature fatigue properties in terms of crack growth, fracture toughness of steel weldments, the toughness scatter and its treatment, constraint corrections, effect of residual stresses and finally, the stress-strain behavior. The results are currently the basis for establishment of design guidelines for steel structures for the Arctic region.
In Norway, research projects on materials behavior at low temperatures have been in progress since 2008 due to an expected increased oil and gas activity in the Barents Sea (e.g., Akselsen et al, 2011; Østby et al, 2011; Mohseni et al, 2012; Welsch et al, 2012; Østby et al, 2012a, 2012b; Jørgensen et al, 2013; Mohseni et al, 2013; Østby et al, 2013; Akselsen and Østby, 2014; Haugen et al, 2014; Mohseni et al, 2014; Wiklund et al, 2014; Hjeltereie, 2015; Kane et al, 2015). In the southwest area of the Barents Sea, north-northwest of the city of Hammerfest, the Snøhvit and Goliat fields are completed and in production. While Snøhvit consists of subsea production units only, the Goliat topside structure fabrication had design temperature of −20°C. This is below the minimum temperature set in existing NORSOK standards (NORSOK, 2008, 2011, 2014), which covers temperatures down to −14°C. Lower minimum design temperatures require project specific evaluations. The operator ENI accounted for this during fabrication and installation. At present, the Johan Castberg oilfield, is located about 100 kilometers north of the Snohvit-field, is under planning. Havis oilfield is another one, to be developed together with Johan Castberg due to the short distance between the two. Several other promising discoveries, e.g., the Gotha/Alta fields and many more, make the situation quite attractive. When moving further north, the temperature falls below −20°C, which means that the low temperature behavior of the structural steel becomes critical. Thus, the situation calls upon the importance of available adequate standards and guidelines for selection and design of steels for structural application in these areas. Such guidelines are now under development in the ongoing Norwegian project (Horn and Hauge, 2011, Horn et al, 2012; Østby et al, 2013; Horn et al, 2016, 2017).
Assessing the Acceptability of Flaws in Metallic Structures show a DBTT around 0-20 o C. The pre-cracked specimens had typically "Micro-fracture behavior induced by MA constituent (island arrest toughness, estimated from the instrumented Charpy tests, show "Cleavage initiation in the resulting in initiation controlled brittle fracture (bainitic and Trans., controlled fracture (ICCGHAZ) (Østby 2013). "HAZ Toughness of Microalloyed materials, the arrest DBTT of the ICCGHAZ phase is lower than the "The effect of vanadium and niobium on the properties and and which of the two methods (Eq. 1 or 2) is most applicable to weld For example, many of the F a4kN values affected zone in low carbon microalloyed steels", ISIJ Int., Vol.41, pp obtained and used in this paper have been taken from "an early" point "Effect of morphology of martensiteaustenite the specimen dominate the crack driving forces. Mohseni, P, Solberg, JK, Karlsen, M, Akselsen, OM and Østby, E (2012)."Investigation of mechanism of cleavage fracture initiation in As mentioned above, there is a need to explore further the robustness of "Application of combined EBSD and 3D-SEM technique on fusion line, is not likely for Arctic temperatures. "Mathematical theory of heat distribution during relationships in intercritical heat affected zone of low carbon
Brandt, Kristin (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Solberg, Jan Ketil (Dept. Material Sci. Eng., Norwegian University of Science and Technology) | Akselsen, Odd Magne (SINTEF Materials and Chemistry, SINTEF) | Østby, Erling (SINTEF Materials and Chemistry, SINTEF)
Akselsen, Odd M. (SINTEF Materials and Chemistry) | Lange, Hans I. (Norwegian University of Science and Technology (NTNU)) | Ren, Xiaobo (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry)
For steel structures to be installed in the Arctic region, the risk of brittle fracture represents a primary concern due to the ductile to brittle usually transition taking place at sub-zero temperatures. Therefore, the present investigation addressed the heat affected zone and weld metal toughness of two extra low carbon steels of 420 MPa yield strength grade, supplied in 20 and 50 mm thickness. The testing included tensile, Charpy V and CTOD. The results obtained showed that the Charpy V toughness was relatively high at -600C, but that some low values may occur for the fusion line position. The fracture toughness at -600C, based on SENB05 (a/t=0.5) geometry, appeared to be low for both weld metal and fusion line positions. More specific measures may be taken into account in welding procedure qualification of the current steels, such as using lower crack length (e.g., a/t=0.2), tension instead of bending (SENT testing) or a full engineering critical assessment.
The oil and gas industry has been gradually moving towards the north. In Norwegian waters, the Goliat field was recently set in production by ENI. The design temperature for this field was -200C, which is somewhat lower than previously experienced, and below the lowest design temperature in the NORSOK standard (2014), which is currently -140C. Not far from the Goliat, Johan Castberg may be the next field of exploration, and is now under evaluation by Statoil. When going further north and east, the ice edge is approached, and the design temperature may fall down to -300C, or even below. This represents huge challenges to the materials which are to be used. Normally, e.g. structural steels and pipelines may easily satisfy toughness requirements at such low temperature. However, welding tends to be very harmful to low temperature fracture toughness. Recent results have demonstrated that the toughness may be on the borderline for both the heat affected zone and the weld metal (e.g., Akselsen et al, 2015; Akselsen & Østby, 2014; Akselsen et al, 2012; Akselsen et al, 2011), indicating that required robust solutions are not yet available for the most challenging part of the Arctic region, unless some constraint loss corrections are applicable.