Abstract

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. Hydrocarbons, water, CO2, and H2S from the bore permeate through the thermoplastic sheaths and form a corrosive environment in the confined space between the sheaths. Generally, low corrosion rates (<0.01 mm/y) are experienced. This is attributed to the low water volume to steel surface ratio (V/S ratio) causing rapid accumulation of dissolved corrosion products and precipitation of protective corrosion product films.

The relationship between the supersaturation of dissolved FeCO3 (SRFeCO3), the precipitation rate of FeCO3, and the protective properties of the precipitated corrosion product film was studied in a series of experiments performed at different V/S ratios and different temperatures (10 °C, 25 °C and 60 °C). The experiments showed a large effect of temperature and duration of exposure.

The paper discusses how SRFeCO3 affects the stability of the corrosion product films and how SRFeCO3 can be used to estimate the equilibrium pressure of H2S in the annulus.

Introduction

Unbonded flexible pipes used in oil and gas production have a layered structure with densely packed carbon steel pressure and tensile armor wires in an annulus enclosed by the inner liner and outer sheath, as illustrated in Figure 1. A detailed description of the design and operation of flexible pipes is presented elsewhere.1

Water, CO2, H2S, and light hydrocarbons from the bore fluids permeate through the inner liner, resulting in a corrosive environment in the confined space between the polymer sheaths. Low corrosion rate (<0.01 mm/y) is experienced under normal operation.2-8 This is attributed to the confined annulus volume (V) and large steel surface area (S), where the aqueous phase quickly becomes supersaturated with dissolved corrosion products, enabling formation of protective FeCO3 on the wires surface. The V/S ratio is typically below 0.1 ml/cm2.

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