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The fabrication of structures for Arctic applications is expected to face major challenges when it comes to the fracture toughness of the heat affected zone and the weld metal. Although the initial base metal toughness may be excellent, a severe toughness deterioration normally occurs as result of the rapid heating and cooling cycles in welding. The present investigation addresses tensile behavior and toughness properties of 32 and 50 mm thick 420 MPa plates, including tensile testing at both room temperature and −60°C, and Charpy V impact toughness and CTOD fracture toughness at −60°C. The welds were deposited by gas shielded flux cored arc welding using a heat input of 2-2.4 kJ/mm. The results showed a dramatic reduction in the fracture toughness after welding, i.e., from CTOD level above 2.5 mm to below 0.25 mm for the 50 mm plate, and from ~ 2 mm to the lowest value of 0.12 mm for the 32mm plate. The Charpy V toughness appeared to be good for the 50 mm, both for the heat affected zone and the weld metal, while the 32 mm plate suffered from low values in the weld metal root area. The results for the 50 mm thick plate are very promising, particularly for use in the temperature range down to −20 to −40°C.
The oil and gas industry is moving north due to the large oil and gas reserves. For example, a preliminary assessment by the US Geological Survey suggests the Arctic seabed may hold as much as about 30% of the world's undiscovered gas and 13% of the world's undiscovered oil (Gautier et al, 2009), mostly offshore under less than 500 meters of water. In these areas, the temperature may occasionally fall below −30 to −40°C, which represents new challenges to the materials. Normally, 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. Both the heat affected zone (HAZ) and the weld metal may fail in providing sufficient toughness (e.g., Akselsen et al, 2011; Østby et al, 2011; Akselsen et al, 2012; Akselsen and Østby, 2014).
In the North Sea oil and gas installations, steel castings have been used for many decades. Here, high strength steel castings offer the chance to manufacture complex heavy-lift and fatigue-critical components for larger offshore structures without increasing the weight of the components or platforms. However, when the activities are moving north to colder climates, current existing castings may fail to meet the toughness requirements, and there is very limited information available on behaviour of weldments of castings under such extreme conditions. Therefore, the present investigation was carried out addressing the low temperature toughness of high nickel (~1.5% Ni) steel casting with 460 MPa yield strength. Preliminary welding trials were performed with flux-cored arc welding (FCAW) with an overmatch in weld metal strength. Both Charpy V notch impact and CTOD fracture mechanical testing were included at ?60°C. The results show that the Charpy V notch toughness is excellent at -60°C (> 100 J). The fusion line CTOD fracture toughness showed low values for the SENB05 samples, while SENB02 gave higher values. For both geometries, the lowest values were connected with pop-in events. The weld metal fracture toughness was satisfactory with the lowest value of 0.28 mm.
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).