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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), Department of Applied Mechanics and Corrosion, SINTEF Materials and Chemistry)
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.
The majority of current dry storage systems used for spent nuclear fuel consist of a welded 304 stainless steel container placed within a passively-ventilated concrete or steel overpack. In service, atmospheric salts, a portion of which will be chloride bearing, will be deposited on the surface of these containers. As the canister surface cools over time, these salts will deliquesce to form potentially corrosive chloride-rich brines. Because austenitic stainless steels are prone to chloride-induced stress corrosion cracking (CISCC), the concern has been raised that SCC may significantly impact long-term canister performance. While the susceptibility of austenitic stainless steels to CISCC is well known, uncertainties exist in terms of the residual stress states that will exist at the container welds. A full-scale cylindrical mock-up was produced, and the residual stresses associated with the weldments in that structure characterized. Results to date indicate that residual stresses will be large and tensile in both the axial and hoop directions, extending through the thickness of the container wall.
Following initial cooling in pools, spent nuclear fuel (SNF) is transferred to dry storage casks for longer-term storage at the reactor sites. The storage cask systems are predominantly welded stainless steel containers enclosed within a ventilated concrete or steel overpack. These cask systems are intended as interim storage until a permanent disposal site is developed, and until recently, were licensed for up to 20 years, and renewals also up to 20 years. In 2011, 10 CFR 72.42(a) was modified to allow for initial license periods of up to 40 years, and also, license extensions of up to 40 years. However, as the United States does not currently have a final disposal pathway for SNF, these containers may be required to perform their waste isolation function for many decades beyond the original design intent. Several recent analyses1-4 have identified and prioritized concerns with respect to the safety performance of long-term interim storage. In each of these studies, the potential for canister failure by chloride-induced stress corrosion cracking (CISCC) was identified as the major concern with respect to canister performance.
In those analyses, the potential for SCC of welded stainless steel interim storage containers for SNF was also identified as a high priority data gap. Uncertainties exist both in the understanding of the environmental conditions on the surface of the storage canisters and in the textural, microstructural, and electrochemical properties of the storage containers themselves. The canister surface environment is currently being evaluated by researchers at Sandia National Laboratories (SNL) and the Electric Power Research Institute (EPRI) 5-8 ; however, little has been done to assess canister material properties and their impact on corrosion. Of specific interest are weld zones on the canisters, because the welding process modifies the microstructure of the stainless steel as well as its resistance to localized corrosion. In addition, welding introduces high tensile residual stresses that can drive the initiation and growth of SCC cracks. In order to meet the need for additional data on the canister material properties, a full-diameter cylindrical mockup of a dual-certified 304/304L (UNS S30400/S30403) stainless steel (SS) storage canister was produced using the same manufacturing procedures as fielded SNF fuel interim storage canisters. The weld and base metal zones on this mockup will be characterized to determine residual stresses, metal properties and susceptibility to SCC.
It is widely recognized that the application of different welding processes, involving different heat input, lead to different residual stresses on the weld joints. A very few experimental tests and numerical simulations have been carried out to improve the knowledge of the impact of residual stresses on typical welded joints for offshore pipeline industry. In this paper is reported the outcome of a dedicated study on the subject, in which the same pipe joint, a carbon steel grade API 5L X65, has been welded with two different welding procedures and then subjected to experimental tests to determine surface residual stresses through the hole-drilling strain gage method. The two welding procedure under testing were both representative of welding procedures typically applied during the installation of SCR in deepwater, in offshore vessel firing line or during prefabrication of multiple joints, but involving different welding processes combinations and different heat input values. A numerical simulation of the two welding procedures has been also carried out through SYSWELD®, a well-known commercial tool. Capabilities of the numerical model to provide a satisfactory prediction of the final residual stresses have been investigated. Both 2D and 3D approaches available in the numerical model are analyzed with a comparison of results between simulations and experimental measurements. Advanced numerical tools could enhance engineering leverage for fracture mechanics phenomenon comprehension, experiment design optimization and technology improvements.
In offshore Oil & Gas industry a typical example of components highly subjected to cyclic loads are Steel Catenary Risers, a common solution connecting a subsea pipeline to deepwater floating production units. SCRs are short subsea pipelines connecting other pipelines laid on the seabed with a floating or fixed production structure in a catenary shape; they are commonly subjected to fatigue loads, particularly in the touchdown zones, due to floating units movements, Vortex Induced Vibrations (VIV) due to waves and sea currents.
Understanding welding-induced residual stresses can help the pipeline industry to produce and install safer, more reliable and potentially more cost-efficient pipeline designs. The weld residual stresses are often found to be in the tensile stress state in parts of the weld which is well-known to have a destructive effect on the fatigue life and fracture properties of the pipeline. With the latest development within Computational Welding Mechanics, it is today it is possible to give accurate estimations of the residual stresses and distortions due to welding and the results can be used to optimise and determine the structural life time of the pipeline. However, it is vital that the welding simulation methods are validated against reliable measured data, so that welding-induced residual stresses distribution can be accurately predicted using computational methods. During the welding there are several factors that might affect the influence of welding residual stresses, such as the materials properties of the base and filler material, the welding process parameters, number of passes, joint groove parameters, geometry effects such as the thickness and t/D ratio. The different factors contribute all to the weld residual stress in different levels and it is therefore crucial to understand how this will influence residual stress level. This paper presents a review of the latest achievements within Computational Welding Mechanics (CWM) simulations to determine the weld residual stresses in pipeline girth welds and the influence the weld residual stress has on the pipeline integrity.