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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).
Alvaro, Antonio (SINTEF Materials and Chemistry) | Akselsen, Odd M. (SINTEF Materials and Chemistry, The Norwegian University of Science and Technology (NTNU)) | Ren, Xiaobo (SINTEF Materials and Chemistry) | Perillo, Giovanni (SINTEF Materials and Chemistry) | Nyhus, Bård (SINTEF Materials and Chemistry)
This paper focuses on the investigation, assessment and comparison of a 420 MPa structural steel Charpy (CVN and pre-cracked) and fatigue crack growth rate test at different temperature spanning from room temperature to −120°C. Since weldments constitute the most probable location for fatigue-related failures, the material have been weld simulated in order to isolate and represent its Coarse Grained Heat Affected Zone. Results are analyzed and compared and an attempt to relate the Fatigue Ductile to Brittle transition (FDBT) and the static Ductile to brittle transition (DBT) temperatures is attempted in order to exploit the possibility to avoid or limit the most expensive and time consuming crack growth rate testing.
In the last years, a great push for oil and gas explorations in the Arctic regions (Gautier, Bird, Charpentier, Grantz, Houseknecht, Klett, Moore, Pitman, Schenk and Schuenemeyer, 2009) together with the increase possibility of an alternative and more direct Asia-North Europe connection kept the interest of oil and gas and maritime industry high. The development of oil and gas fields in the arctic brings to the table several challenges due to the cold and harsh climate; when it comes to the use of structural ferritic steels, particular concerns relate to their low-temperature properties. More precisely, when it comes to structural integrity of offshore structures built with ferritic steels, Ductile to Brittle Transition (DBT) and Fatigue Ductile to Brittle Transition (FDBT) needs to be carefully assessed in order to avoid unexpected catastrophic failures.
It is long known that, as ferritic steels operates at lower temperatures, they undergo a transition from a ductile shear-dominated to a brittle cleavage dominated fracture mode. This phenomenon is known as Ductile to Brittle Transition (DBT) and it is commonly quantified through the typical fracture mechanics parameters, i.e. CTOD (Crack Tip Opening Displacement), Charpy impact energy Cv, KIc or J-integral values. A schematic is presented in Fig 1.
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).
Nyhus, Bård (SINTEFMaterials and Chemistry) | Dumoulin, Stephane (SINTEFMaterials and Chemistry) | Nordhagen, Håkon (SINTEFMaterials and Chemistry) | Midling, Ole Terje (Marine Aluminium AS) | Myhr, Ole Runar (Hydro Aluminium) | Furu, Trond (Norsk Hydro ASA) | Lundberg, Steinar (Hydal Aluminium Profiler AS)
Aluminium is known as a safe and suitable material for offshore installations. Factors that favour aluminium are low weight, no need for surface treatment and low maintenance costs. Though aluminium has a high strength-to-weight ratio, it suffers from strength reduction in heat affected zones when welded. The strength of the soft zones is often dimensioning in design, and the ability to predict the strength reduction is important for fully utilizing the potential of aluminium as a structural material. In the current study, the cross weld strength of EN AW 6082-T6 and EN AW 5083-H321 as a function of wall thickness at room temperature and at −60°C (”arctic temperature”) was tested. The main objectives were to verify that the materials and the weldments are not deteriorated at low temperatures, and to check if using additional reduction factors for the heat affected zones for plates and extrusions thicker than 15 mm as specified in the design standard EN 1999-1-1 is correct. The results show that there is no reduction in strength for low temperatures, nor for plates and extrusions thicker than 15 mm. Based on the results in this study, changes in EN 1999-1-1 are recommended.
Unlike body-centred cubic (BCC) metals, the yield and strength temperature sensitivity of face-centred cubic (FCC) materials, such as aluminium (Al) alloys, is negligible when lowering the temperature below room temperature (Hertzberg 1996). Because of the high specific strength, good corrosion resistance and good mechanical properties at low temperature, Al-alloys are often used for low temperature conditions such as cryogenic applications (e.g. Liquefied Natural Gas (LNG) tanks and space/aeronautics). Thus, the low temperature characterization found in the literature focus on test temperatures far below −60°C.
In BCC materials, such as steels, the dislocation width is narrow and the Peierls stress increases rapidly with decreasing temperature, thus the yield stress will increase strongly with decreasing temperature. An important consequence of this for BCC materials is that the yield stress can rise to such high levels that only a very limited plastic zone ahead of a crack will occur before unstable (brittle) fracture results. This material brittleness will not occur in FCC alloys (Aluminium), and a ductile fracture mode will prevail.
Alvaro, Antonio (SINTEF Materials and Chemistry) | Akselsen, Odd M. (SINTEF Materials and Chemistry) | Ren, Xiaobo (The Norwegian University of Science and Technology (NTNU)) | Nyhus, Bård (SINTEF Materials and Chemistry)
The development of oil and gas fields in the arctic brings to the table several challenges in the use of structural steels, particularly concerning their low-temperature properties. Among others, also fatigue behavior needs to be accounted for when using structural steels for arctic applications. As for static fracture, ferritic steels experience a fatigue ductile to brittle transition (FDBT) when temperature is decreased below a certain temperature. This may result in higher crack growth rate and, consequently, unpredicted fatigue-related failure. In order to shed some more light on this phenomenon, fatigue crack growth tests have been performed on a 420 MPa structural steel weld simulated coarse grained heat affected zone (CGHAZ) at different temperatures: room temperature, -30, -60, -90 and -120 °C, with -60 °C considered as a possible design temperature relevant for the most extreme arctic areas. Post-mortem fracture surface investigations have been also conducted in order to confirm the expected switch in fatigue crack growth mechanisms as temperature is lowered below the FDBT temperature. Finally, two analytical equations, valid for temperature ranges above the FDBT, were established based on the experimental results to relate yield strength and temperature variation of the Paris law constants . These are used to quantify the temperature impact on the designed fatigue life, and the results are compared to the actual design rules (BS 9710).
Exploration of oil and gas in the Arctic regions is increasing due to the large share of the remaining resources (estimates indicate that about 13% of the remaining oil and 1 gas resources is located in the northern regions (Gautier, Bird, Charpentier, Grantz, Houseknecht, Klett, Moore, Pitman, Schenk and Schuenemeyer, 2009) and the possibility for an alternative and direct Asia-Europe connection route keep both oil and gas and maritime industry interest growing. However, the harsh and cold climate characteristic of the arctic regions imposes several challenges when it comes to materials integrity. The combination of long and repeated ice loading together with operating temperatures which are typically lower than the ones at which the offshore industry is used to work with, demands for new research-based development in order to avoid catastrophic leakage and failures. It is well known, in fact, that as ferritic steels is subjected to sub-zero temperature, they undergo a transition from stable, ductile fracture to unstable, brittle fracture. While for pure materials, the transition may occur very suddenly at a particular temperature, for many materials used in practice the transition occurs over a range of temperatures. This causes difficulties when trying to define a single transition temperature and no universally recognized and specific criterion has been established. Similarly, a fatigue ductile to brittle transition (FDBT) can be observed in ferritic steels. Fig. 1 summarizes the qualitative fatigue crack growth behavior variation for ferritic steels as temperature is lowered.
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.
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.
Bjaaland, Helena (Norwegian University of Science and Technology (NTNU)) | Akselsen, Odd M. (Norwegian University of Science and Technology (NTNU)) | Olden, Vigdis (SINTEF) | Nyhus, Bård (SINTEF) | Karlsen, Morten (SINTEF) | Hjelen, Jarle (Norwegian University of Science and Technology (NTNU))
This work investigates the metallurgical changes occurring during welding of clad pipes. Welded samples with and without a Ni-interlayer between the clad and base metal (BM) were investigated by light optical microscope, electron microprobe analysis and microhardness measurements. The investigations have focused on the microstructure and properties of the clad and BM close to the weld, in addition to the root and hotpass. The results indicate that the presence of a Ni-interlayer prevents carbon diffusion across the BM-clad interface during exposure to elevated temperatures, thus preventing the formation of a hard and crack susceptible microstructure.
Ren, Xiaobo (Department of Engineering Design and Materials, Norwegian University of Science and Technology (NTNU)) | Ås, Sigmund K. (Department of Applied Mechanics and Corrosion, SINTEF Materials and Chemistry) | Nyhus, Bård (Department of Applied Mechanics and Corrosion, SINTEF Materials and Chemistry) | Akselsen, Odd M. (Department of Engineering Design and Materials, Norwegian University of Science and Technology (NTNU), and Department of Applied Mechanics and Corrosion, SINTEF Materials and Chemistry)