Flexible pipes used in oil and gas production are composed of densely packed steel wires enclosed in an annulus confined by inner and outer thermoplastic sheaths (pressure barriers). Hydrocarbons, water, CO2 and H2S from the bore diffuse through the inner sheath and form a corrosive environment in the confined space between the sheaths. A large steel surface to water volume ratio leads to rapid accumulation of dissolved corrosion products and precipitation of protective iron carbonate (FeCO3) films. Low corrosion rates (<0.01 mm/y) are usually experienced in the field and have been reported in a number of experiments where fresh abraded steel surfaces are exposed in environments with high concentration of dissolved corrosion products.

This paper presents results from experiments where the protective properties of iron carbonate films were studied when the armour steel was exposed to oxygenated water for some time, i.e. simulation of accidental ingress of aerated seawater into the annulus. The protective iron carbonate film broke down during the O2 exposure and was converted to a porous film of iron oxides and iron carbide. The porous film gave poor protection and the corrosion rate increased from less than 0.01 mm/y to more than 1 mm/y during the O2 exposure. The presence of the porous film disturbed the reformation of protective iron carbonate films when outer sheath repair was simulated by removing the O2 source. It was shown that reformation of protective films was strongly dependent on the duration of the O2 exposure and on the corrosion history of the steel surface prior to the O2 exposure. The experiments were performed at atmospheric pressure, at 25 and 60 °C and with a CO2 partial pressure of 0.2 bar. The paper discusses the experimental results, the experimental approach and the challenges of simulating the annulus conditions in small scale lab experiments.

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