Flexible Pipes - Corrosion Fatigue Testing under Representative Annulus Conditions

Abstract The fatigue life of armor wires in flexible pipes and the shape of generated SN-curves are affected by the corrosion rate, the steel surface condition and the annulus/test environment. An experimental program was performed to develop a test protocol including a pre-fatigue phase where protective corrosion product films are formed and a corrosion fatigue phase where the corrosion rate should be about or less than 0.01 mm/y. This is the maximum expected (industrial standard) corrosion rate under normal operation. To optimize the pre-fatigue phase, a series of corrosion experiments were performed at different CO2 partial pressures, pH levels, concentrations of dissolved corrosion products, bicarbonate additions and partial pressure of H2S. The target concentration (supersaturation) of dissolved corrosion products to be used during the corrosion fatigue exposure was studied in combined corrosion and FeCO3 precipitation experiments. These experiments, which were performed in condensed water (1.65 g/l NaCl) at 0.02 MPa CO2 partial pressure, showed that a high supersaturation (>100) of dissolved FeCO3 was required to maintain a low corrosion rate (< 0.01 mm/y).