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This paper reports an approach towards studying the susceptibility of materials to hydrogen embrittlement of several yield strength levels of ASTM A320 L7 & L43 grade carbon alloy material used for bolting . The test method used is based on ASTM E1820 using pre-cracked compact tension (CT) specimens in a seawater environment and under cathodic protection. The procedure has been to monitor the crack growth and the crack growth rate (CGR) at various constant stress intensity (K-levels) and at varying cathodic protection potentials. In this way these two parameters can be seen as representative of the susceptibility to embrittlement cracking of the materials at the specific stress intensity levels and as a function of the varying CP levels.The results indicate the strong effect that both stress intensity K-values and applied potential have on crack growth rates in environment and on the resulting susceptibility of the material to embrittlement. On the practical operational level the results also highlight the importance of actively monitoring and controlling cathodic protection (CP) potential levels as a means of increasing the allowable margin of error of the intersection between material quality design factors and the effect of the challenging environments of Oil & Gas subsea drilling and production operations.
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Hydrogen is considered an alternative energy source for fossil fuels, and consequently the requirements for materials used in hydrogen applications have been increased. These materials need to have high resistance against hydrogen embrittlement (HE). HE, that affects several metallic materials, is a complex phenomenon characterized by a degradation of the mechanical properties, in particular ductility.
The high strength and corrosion resistance of nickel-chromium alloys such as Alloy 718 and nickel-iron-chromium alloys such as Alloys 945 and 945X make them particularly good candidates for use in demanding environments in the upstream oil and gas industry. These materials generally perform well where resistance to sulphide stress cracking and chloride stress corrosion cracking is required. However whilst these alloys are considered ‘NACE compliant' environmentally-assisted failures can still occur.It is generally accepted that for hydrogen cracks to initiate there must be a critical combination of stress susceptible microstructure and hydrogen concentration. In this project the effect of microstructure is explored by heat treating Alloy 718 945 and 945X to standard and non-standard conditions. Tensile specimens were slow-strain-rate-tested in air and under CP to explore sensitivity to hydrogen embrittlement. Finally the effect of a severe stress concentration in the form of a sharp notch was used to determine whether there is an enhanced susceptibility to hydrogen embrittlement due to the presence of local stress raisers. The results are compared with tests undertaken by other authors under various hydrogen-charging conditions.
An unexpected failure of 316L Stainless Steel instrument tubing occurred in a high pressure Hydroprocessing unit resulting in a shutdown of the unit. The tubing system consisted of a compression type fitting commonly used in instrument systems and had only been in service for 3 years when the failure occurred. The failed tubing samples were removed for metallurgical analysis and determination of damage mechanism.
Metallurgical analysis and finite element analysis of the tubing identified excessive cold working leading to hydrogen embrittlement as the primary mode of failure. This paper details the investigation into the failure to arrive at the root cause and the preventive measures adopted to assess the installed population of tubing in similar service.
Flexible pipes are frequently used both as flowlines and risers in the oil and gas industry. A flexible pipe has a complex structure consisting of layers of polymer and metallic materials. The armor wire layers – shielded with polymer materials from seawater on the outside and well fluid on the inside – are the load and pressure bearing parts. Due to diffusion from the well fluid and/or damage of the outer polymer layer, the annulus can be water-filled, and armor wire can corrode. In this work, the susceptibility to hydrogen embrittlement (HE) with the presence of atomic hydrogen due to cathodic polarization has been investigated for six different tensile armor wire materials. Samples were exposed to Slow Strain Rate testing (SSRT) in 3.5% NaCl solution and cathodic polarization to -1.1 and -1.4 VAg/AgCl at room temperature. Reference samples without hydrogen charging were tested in air for comparison. Stress-strain curves, reduction in area (RA) and the microstructure of the fracture surfaces were investigated. The HE susceptibility tended to increase with the carbon content, strength and hardness and the materials tended to be more brittle when charged to -1.4 VAg/AgCl than -1.1VAg/AgCl.
Hydrogen induced stress cracking (HISC) resistance of super duplex stainless steels is basically controlled by the material’s microstructure. Friction stir welding is a low heat input joining process that has the potential to maintain the base metal original resistance to HISC since it does not significantly alter the proportion of ferrite and austenite. This work evaluated the susceptibility to HISC under cathodic protection of friction stir welded super duplex stainless steel UNS S32760. Microstructure evaluation revealed a recommended proportion of ferrite and austenite phases as well a refinement of the overall stir zone microstructure. Fracture toughness tests in synthetic sea water under cathodic protection of -895 mVsce indicated that the microstructure of the stir zone is actually less sensitive to HISC than the microstructure of the base metal.Keywords: Friction Stir Welding (FSW) Super Duplex Stainless Steel (SDSS) UNSS32760 Stir Zone (SZ) Hydrogen Induced Stress Cracking (HISC)
A summary of hydrogen bakeout history to remove hydrogen from a component. Existing recommendations regarding hydrogen bakeout in codes and standards. Results from an industry survey of energy producers. Proposed methodology for selecting bakeout parameters.
In 2018 the apparent failure of two subsea jumper end-fittings was detected by remotely operated vehicle (ROV) inspection prompting their retrieval from the seabed for analysis. The components were manufactured from UNS S32760 a 25 wt% Cr super duplex stainless steel (SDSS) and connected to the flexible pipe by swaging. The end fittings were exposed to the cathodic protection system presenting extensive accumulation of calcareous deposits upon inspection. A single longitudinal crack was observed on each end-fitting propagating from end-to-end of the ferrule i.e. the swaged section of the part. The visual appearance of the fracture surface suggested at first glance cleavage fracture and thus the possibility of hydrogen induced stress cracking (HISC).This paper summarizes the results of an extensive investigation aimed at elucidating the root-cause of the failure. Given the suspicion of a hydrogen embrittlement phenomenon the scope of the work was designed to address the three HISC causative factors namely a susceptible alloy microstructure a critical tensile stress level and the presence of atomic hydrogen. The experimental techniques included scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) electron backscatter diffraction (EBSD) analysis mechanical and corrosion examinations as well as slow strain rate testing (SSRT).