Abstract
Flexible risers are used to transport oil and gas from the seabed to Floating Production Storage and Offloading system (FPSOs). They can suffer from a failure mechanism known as corrosion fatigue, which is brought about by flooding of the pipe annulus initiated by either damage to the outer sheath (which causes ingress of seawater into the annulus) or by the transport of condensed water through the inner sheath caused by the high pressure inside the pipe. These two wet environments combine with corrosive gases such as CO2 and H2S, which coupled with the cyclic wave loading on the flexible riser, can lead to corrosion fatigue failure. Corrosion fatigue tests have been carried out to explore the effects of these two environments and iron confinement on the failure mechanism, with a V/S ratio of 0.2, which is the ratio of the free volume (V) to surface area of the steel wires (S).
Iron and sulphide measurements demonstrate the effects of chloride on the dissolution of iron and the effects of iron confinement on the amount of sulphide in solution. The results for the environments with a V/S ratio of 0.2 shows a more than two-fold increase in the amount of iron in solution for the seawater environment compared to deionised water, whereas, the sulphide peaks are comparable. The environments with no additional iron demonstrate a peak sulphide concentration up to four times greater than that for iron confinement, with up to a 20 times decrease in the soluble iron concentration.
X-ray Diffraction (XRD) results for the surface films demonstrate the formation of mackinawite (FeS) and iron carbonate. The presence of chloride in solution suppresses the formation of mackinawite with the deionised water environment demonstrating the highest peak intensity for FeS formation. The effects of the environment on the fracture surface and fatigue crack path have also been explored.
