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 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 -60°C, but that some low values may occur for the fusion line position. The fracture toughness at -60°C, 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. INTRODUCTION 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 -20°C, which is somewhat lower than previously experienced, and below the lowest design temperature in the NORSOK standard (2014), which is currently -14°C. 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 -30°C, 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.

Abstract This state-of-the-art paper is devoted to testing and evaluation of microstructural crack arrest. Testing and analysis of crack arrest have developed in the last decades, enhancing our understanding of the mechanisms behind crack arrest in a continuum mechanics perspective. Understanding crack arrest is important when operations are moving towards Arctic regions as low temperatures are detrimental to most steel's fracture toughness. Large-scale testing is expensive and unpractical, and current methods fail to reflect the microstructural and micromechanical features of the fracture process. In order to increase the effectiveness of characterizing crack arrest properties, small-scale tests, as well as numerical methods, have been developed. The mechanical basis and mechanisms behind crack arrest are presented. Global and micro-arrest is considered. Key methods for understanding, evaluating and obtaining arrest parameters are presented: statistical treatment of experimental results, barrier models for separating fracture and arrest sequences, and numerical tools for determining arrest behaviour. Brief presentations of the main mechanisms of crack arrest are presented with focus on the micromechanisms of arrest. The effect of grain boundaries, lattice orientation and second-phase particles upon propagation controlled cleavage are discussed, as well as their role in the arrest mechanism. Developments in arrest testing and evaluation are presented. Experimentally and numerically obtained results are linked to relevant mechanisms and theory, exhibiting the predictability and importance of crack arrest properties, and the understanding of the governing mechanisms behind crack arrest. The potential for increased understanding of the brittle fracture arrest phenomenon associated with new methods for nanomechanical testing of the material properties inside individual grains, and over grain boundaries, as well as the rapidly improving capabilities of atomistic modelling of deformation and fracture, is presented to pave the way for the future research within this field. Areas where further research could enhance our knowledge of crack arrest are listed. Introduction Crack arrest is considering running cracks that are halted due to increasing resistance to crack propagation and/or reduced crack driving force. The former may be due to microstructural barriers or thermal gradients in the material. The latter may occur under partly displacement controlled loading, where the crack extension may increase the compliance of the structure and reduce the local crack driving force, or as a result of dynamic effects caused by impact loading or stress oscillations in the structure. This paper is mainly concerned with aspects related to the material's resistance to crack propagation, i.e. the arrest toughness. Further, crack propagation is assumed to be dominated by cleavage fracture, i.e. ductile fracture and fatigue are not considered. The relative importance of these factors depends on the scale of which the arrest is considered. Further, the arrest can also be considered for different scenarios ranging from arrest of single grain sized microcracks up to arrest of macroscopic cracks on in the centimeter to meter range. In the first group the arrest happens locally, probably highly influenced by local microstructural features like grain boundary orientation, and would rather be categorized as avoidance of cleavage initiation on the macroscopic scale. In the latter group the problem is more of a conventional engineering fracture mechanics issue, ideally assessed through knowledge or measurements of the macroscopic arrest toughness, K ia . Ultimately, the two groups are part of the same problem, and there is a research aim to establish quantitative relations at different scales in orderto arrive at a general treatment of the problem.

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 A data management system for material assessment at low temperature is developed in the context of large test programmes run in a current research project called Arctic Materials II. A generic database platform is proposed for efficient access to material data and to analyses of various materials such as steels/weldments, aluminium alloys, composites and polymers/coatings. Safe and flexible data processing accessible through a customized web interface is obtained by unifying test results, test methods, and procedures (e.g. tensile testing, fracture mechanics testing) and predictive models of fracture. The framework provides a solid support for the development of more robust solutions to account for the influence of, e.g., temperature, constraint effects, residual stresses, crack arrest, probabilistic scatter, and scaling on the material response. Its aim is also to contribute to an improved design guideline for materials requirements in Arctic conditions (down to -60°C)

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 It is well known that metallic materials, such as ferritic steels, the yield and tensile strength increases with decreasing temperature. For some steels, the level and length of the Lüders band also increases with decreasing temperature. Taking into account these effects of temperature in the constitutive equations for steels can be of great importance for higher utilization of steel, requirement of fracture toughness and mismatch conditions of welds. A detailed study addressing this issue is necessary in order to have tools and methods to take these effects into account. This is both concerning the definition and suggestions of procedures to perform tensile testing below room temperature, and improved constitutive models that correctly describe the effects of temperature. This paper will present a model that can describe temperature dependent stress-strain curves based on the tensile testing of base materials and weld thermal simulated microstructures (CGHAZ and ICCGHAZ) from a 420 MPa steel at 0, −30, −60 and -90°C. The dependency of yield stress, tensile strength, Lüders strain as well as strain hardening on temperature will be discussed. For situations where tensile test data is not available, a correction of yield stress and Lüders band development at low temperature based on the yield stress at room temperature is proposed. The effect of temperature dependent material properties on brittle fracture is studied using cohesive zone model.

Abstract The state of the art in laser and laser-arc hybrid welding of thick steel components is reviewed, particularly with respect to possible applications in the oil and gas industry. The most relevant information comes from the shipbuilding industry, where the CO 2 laser-GMAW process was taken in use about 15–20 years ago. The different aspects of these welding techniques are briefly discussed, including different laser-arc hybrid techniques and mechanical properties of welds. A brief review of numerical welding simulation techniques is performed with main focus on the use of the WeldsimS software, which allows predictions of heat flow, microstructure and residual stresses after welding.

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.

ABSTRACT In this paper, the distribution of residual stresses of X80 pipe girth weld was investigated by both experimental measurements and numerical simulations. The incremental hole-drilling method was used to quantify the residual stresses. The distributions of residual stresses at locations near the fusion line on the outer surface and the weld start position on inner surface of the pipe have been measured. Two-dimensional axi-symmetric models have been utilized to simulate the pipe, and the welding procedure was simulated by WeldsimS code, in which the phase transformation phenomenon in steel was considered. Results show that the predicted distribution of residual stresses deviate to some degree from measured results. However, the trends of the distribution are quite similar. The reasons for these deviations are also discussed. INTRODUCTION The recent trend of exploring oil and gas in the Arctic region has attracted much research interests due to environmental and safety concerns. The reliability of structures and materials is one of the most important issues to consider. In an ongoing research project, many candidate materials have been investigated for potential utilization in oil and gas transportation. For the pipe materials, X65 and X80 steels are studied. Welding of pipeline is critical issue for safe transportation of oil and gas. There are many topics to be investigated for welding of the pipe, e.g. the weldability, low-temperature fracture toughness of the weld, microstructure evolution as well as residual stresses. Welding residual stresses are unavoidable in the fabrication of pipeline as well as in service. It has been demonstrated that residual stresses have significant influence on the crack-tip constraint (Ren et al., 2009), ductile crack growth resistance (Ren et al., 2010), cleavage fracture behavior (Ren at al., 2011) and the structural integrity. Therefore, for integrity assessments of engineering component, it is important to accurately depict the residual stress field.

ABSTRACT: In this paper, a numerical investigation of the difference between hybrid laser-arc welding and conventional multi-pass GMAW is described with respect to residual stresses and their potential effect on the structural integrity under Arctic conditions. A two-dimensional axisymmetric model has been used to simulate welding of X70 pipe. Welding simulations were performed in the WeldsimS FE code, which models the thermo-mechanical process from viscoplastic strains, including strains due to phase transformations. A one-pass hybrid laser-arc welding process and a four-pass Gas Metal Arc Welding (GMAW) process were simulated on the same geometry. The aim of the present study is to compare the two methods and to evaluate the potential applicability of the hybrid laser arc welding in the Arctic region. The main focus in this study is to investigate the welding bead profile, welding residual stresses, temperature field and microstructures. INTRODUCTION Over the past decade, the petroleum resources in the North Sea have shown a steady decline in production. Oil and gas exploration have therefore moved into Arctic regions, where there is little or no infrastructure and harsh weather conditions. More cost effective exploration and production facilities are required in order to exploit these resources, and the vulnerable biological environment means that highly robust materials and structures are required. The fabrication technique of pipeline is highly relevant both with regard to development costs and safety. It has been shown that the productivity of pipeline construction is determined by the root pass welding speed and numbers of the follow-up fill passes (Yapp and Kong, 2008). At the same time, it is vital that the welds meet the demands, for example, low temperature toughness. Hybrid laser-arc welding refers to the coupling of at least two welding processes into one single process (Booth et al. 2003), as illustrated in Fig.1.