Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.
In this work, the corrosion fatigue behaviour of steel armours used in the flexible pipes, in aqueous solutions initially containing different concentrations of Fe 2þ , was investigated by four-point bending testing under saturated 1 bar CO 2 condition. Corrosion fatigue results were supported with ex situ measurements of Fe 2þ and pH. Characterisation of the corrosion scales and crack formations was performed using microscopic and diffraction techniques. Fatigue results showed two times better fatigue life, at the stress ranges of 250 MPa, for samples tested in solutions containing the concentration of Fe 2þ marginally above the solubility limit of FeCO 3 compared to the samples tested in highly supersaturated solution of Fe 2þ. Results revealed that the impact of the alternating stresses on the corrosion behaviour of samples reduces with lowering the applied stresses. At the stress range of 100 MPa, fatigue samples experienced the same corrosion rate as samples that were not subjected to dynamic loading.Keywords: Carbon steel, Carbon dioxide, Corrosion fatigue, Supersaturation, Iron carbonate IntroductionFlexible pipes, which are used in the oil and gas industry, are an attractive alternative to rigid pipes due to their shorter installation times and longer durability. The typical structure of flexible pipes is composed of polymeric and steel layers, which are not bonded together, to ensure the flexibility of the pipe. 1 The fatigue life of a flexible pipe is determined by the fatigue life of the steel armours, which is placed in the annular space between the inner liner and outer sheath. It was reported 2 that the fatigue life of tensile armour depends on the applied stresses, which are generated due to waves and water current, and the corrosive operating environment. The annulus region might be water filled either due to diffusion and condensation of water from the bore or as a result of a breakage of the outer sheath allowing the entry of sea water. In addition, the severity of the corrosion environment may be increased due to diffusion of small molecules such as methane (CH 4 ), carbon dioxide (CO 2 ) and hydrogen sulphide (H 2 S) from the pipe bore through the inner liner into the annulus.1-3 However, due to a low ratio of free water volume (V) to steel surface area (S), typically v0.1 mL cm 22 , 3 a rapid, high supersaturation of Fe 2þ in the liquid is possible. As a consequence, low corrosion rates, usually v10 mm year 21 , are achieved. 3,4The corrosion behaviour of steel in aqueous CO 2 solution is influenced by the formation of protective films, which might decrease their corrosion rate. 5,6 Typical corrosion scales, formed in such an environment, consist of iron carbonate (siderite, FeCO 3 ) and undissolved cementite (Fe 3 C).7,8 A large number of environmental variables, such as pH, 9-11 temperature, 9,12,13 partial pressure 11 and flowrate, 14-17 influence the corrosion behaviour of steel. Further, material characteristics such as microstructure, chemical composition and heat treatment condition [...
Fatigue design of armour wires in flexible risers is reviewed, with particular emphasis on effects of corrosive environments in pipe annulus. Test procedures for corrosion fatigue testing of armour wire and assessment of fatigue design criteria are described.
This paper presents the outcome of investigations on the effects of H2S consumption in the annulus of a flexible pipe. Low-molecular gases, such as CH4, H2S, H2O and CO2, permeate slowly from the bore through the inner liner into the annular space between the inner liner and outer sheath of a flexible pipe. This space is densely packed with carbon steel armour wires leaving a very limited free volume. In the presence of water, a corrosive environment for the armour wires is generated and a risk of sour service cracking is introduced. H2S concentration in the annulus is traditionally calculated by balancing the inflow through inner liner and the outflow through outer sheath and vent valve. In order to assure H2S resistance of the armour wires towards calculated H2S concentrations, pipes for sour service are typically designed with lower strength wire grades of larger dimensions compared to the possibilities of sweet service pipes. Over the last decade, more and more offshore data has been obtained indicating considerably less H2S in the annulus than predicted by the traditional annulus models. This observation has triggered in-depth investigations of the complex corrosive H2S environment inside a flexible pipe annulus exposed to sour service conditions. An extensive small-scale test program has been conducted and showed that at permeation rates typical for flexible pipes, the consumption of H2S in the corrosion processes occurring in the annular space lowers the concentration and hence criticality of the H2S so significantly that it leaves the traditional models overly conservative to an extreme extent. Using this knowledge of consumption of the corrosive gases in the annulus has become an increasingly important topic with the focus on deeper waters, cost savings and service life extensions without compromising flexible pipe integrity. Based on experimental data obtained, a new annulus model for prediction of H2S pressure in annulus has been derived. Data is presented in this paper to illustrate the methodology for an annulus prediction where the consumption of H2S is included. The data presented covers laboratory tests with variations and effects of gas flux, H2S concentration and total pressure. A full-scale validation, led to an Independent Verification Agency certification of the model. With the introduction of this new annulus prediction model, a wider range of wire products becomes available for the pipe designers. Lower weight pipes with stronger armour wires render optimizations for both cost savings and applications at deeper waters possible.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.