ABSTRACT The fatigue behavior of a PbSnSb alloy used in subsea power cable sheathing was studied using small- and full-scale experiments. The aim of the work was to understand the transferability between the scales and suitable testing methods. Creep phenomena are addressed by considering the cyclic strain rate as well as the small-scale loading mode. The fatigue test results show significant difference between different loading modes and scales. It is also evident that fatigue- creep interaction is highly important. INTRODUCTION High voltage subsea power cables operating require an impermeable water barrier to prevent humidity in the isolation system. In the case of Mass-Impregnated (MI) or Cross-Linked High-Density Polyethylene (XLPE) insulation systems as normally used for high voltage submarine power cables, humidity can compromise the systems electrical integrity. Lead-based alloys are normally used as a water barrier due to ease of extrudability and ductility. Lead-based alloys are also associated with favorable low-cycle fatigue properties when compared to higher strength materials such as copper or aluminum. However, due to its combination of very poor high cycle fatigue properties and low creep resistance, multiple cyclic scenarios can challenge the fatigue-creep life of the sheathing. Critical scenarios include cable installation and offshore jointing. These operations imply a temporary dynamic suspension between the seabed and a floating vessel where the cable will be subjected to bending due to wave motion on the cable and vessel. Additional bending fatigue damage can be introduced from vortex-induced vibrations (VIV) in the temporary catenary or free-spans along the cable route. Another case arises from the insulation temperature fluctuations during operation. Thermal expansion and contraction of the oil in MI insulation or the XLPE will cyclically strain the lead sheath in radial direction. The high relative temperature for lead-alloys at room temperature (∼0.5 T m ) and low creep resistance for typical cable sheathing alloys cause a significant strain rate dependency under loading. These properties also depend on the alloying elements and the microstructure.

ABSTRACT 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. INTRODUCTION 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).

ABSTRACT 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. INTRODUCTION 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).

ABSTRACT 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. INTRODUCTION 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 C v , K Ic or J-integral values. A schematic is presented in Fig 1.

Abstract 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). INTRODUCTION 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.

Abstract The exploration of oil and gas fields in the arctic brings several challenges in the use of structural steels concerning their low-temperature properties. Among others, fatigue behavior needs also to be considered for arctic applications, despite little attention to fatigue at low temperature has been given so far. This paper summarizes a set of fatigue crack growth rate tests performed both at room temperature and at -60 °C, with the latter representing the possible design temperature relevant for the most extreme arctic areas. Accordingly, the material under investigation is a 420 MPa structural steel, one of the probable candidate materials to be used for structural purpose here. Since weldments are the most susceptible to fatigue failures, the fatigue crack growth measurements have been performed not only on parent metal, but they have been extended also to weld thermal simulated Coarse Grained Heat Affected Zone (CGHAZ) and Intercritically Reheated Coarse Grained Heat Affected Zone (ICCGHAZ). The resulting fatigue crack growth curves are compared to the fatigue assessment curves indicated in BS 9710:2013. Data indicates that, for all the material under investigation, the fatigue properties are improved at -60 °C when compared to room temperature

Abstract 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.

Abstract Over the past decade, it has been a continuously growing interest in exploration of oil and gas in the arctic region. The harsh, cold climate imposes challenging tasks which concern the structural integrity of steels and their weldments. Specific knowledge of metals behavior in such conditions is therefore mandatory in order to provide sufficient robustness. Within this framework, the present paper focuses on the fatigue properties of steels with the intention to provide a comprehensive review of the open literature about the effect of low temperature on the different aspects of the fatigue life of steels and their weldments. The main objective is therefore to provide a reliable basis for suggestions of necessary testing of low temperature fatigue in steels.