A kinetic model of the physicochemical processes occurring during the corrosion of steel covered by a thin water film exposed to a
CO2
atmosphere is proposed. This model explains theoretically the unusual physicochemical behaviors generally observed in such confined corrosive media:
Fe(II)
supersaturations, low corrosion rates, high pH values, and their respective dependence on the electrolyte film thickness. More quantitatively, the model predictions agree well with pH and corrosion rate measurements performed in environments similar to the annulus space of flexible pipelines.
Using an accurate thin-layer cell, the electrochemical impedance corresponding to the diffusion of ferro-cyanide ions was measured on a large-disk electrode covered by thin films of electrolyte. Experimental
impedance diagrams obtained in a large frequency range (100 kHz−5 mHz) were in good agreement with
those expected in the case of a spatially restricted linear diffusion impedance. For electrolyte film thicknesses
lower than 100 μm, an increase of the low-frequency limit of the real part of the impedance and a decrease
of the high-frequency phase angle were however observed experimentally as the electrolyte film thickness
decreases. This specific behavior, in disagreement with the classical linear diffusion theory, was explained
by the potential distribution existing in the thin-layer cell and was modeled using an original transmission
line. In a very low-frequency range (lower than 1 mHz), an additional time constant was also evidenced. In
consistency with previous work of Gabrielli et al. on microelectrodes, this feature was ascribed to the radial
contribution of diffusion processes existing within a thin-layer cell.
The integrity of BP's West of Shetland (WoS) flexible riser systems has been actively managed using a risk-based approach since they were installed in the late 90s. A failure mode that was identified as part of this process was the fatigue of armour wires in those risers with flooded annuli. Annulus flooding WoS was caused by external sheath breaches that occurred during installation and to mitigate the risks of both corrosion and corrosion fatigue, BP sealed the breach locations, displaced the sea water in the annuli with a corrosion inhibitor, and implemented a programme of regular fatigue life reassessments for the affected risers.In 2008, informed by the integrity management process BP put in place, the decision was taken to replace one of the WoS high pressure gas risers which had a flooded annulus. To complete the integrity management cycle, the decommissioned riser was dissected and the various metallic and polymer layers inspected. In addition, armour wire samples were taken from the fatigue critical region of the riser, and fatigue tested to establish S-N curves in an environment representative of the annulus. This paper describes the dissection process and the general condition of the pipe following decommissioning. It shows that generally the condition of both the polymer and metallic layers was good and the introduction of the corrosion inhibitor into the annulus of the pipe had a positive effect. The fatigue testing programme is also presented, and the S-N curves generated using the wires from the decommissioned riser are compared against curves for as-manufactured wire. This indicates a significant level of conservatism in current fatigue life prediction methods.The inventory of flexible risers in the major offshore basins that have flooded annuli is large and the fatigue performance of armour wires in this situation can be a key integrity management risk. Currently assessment of this risk is based on calculation and small scale testing alone, rather than actual operational experience. This paper seeks to address that gap in industry knowledge and to start building a database of actual fatigue performance.
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