51315-5605-Corrosion Fatigue Behaviour of High Strength Steel Wire in Various Aqueous Environments

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Product Number: 51315-5605-SG

ISBN: 5605 2015 CP

Author: Peter Barnes

Publication Date: 2015

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Flexible risers are used to transport oil and gas from the seabed to 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 with a V/S ratio of 0.2 on the failure mechanism.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 a up to a 20 times decrease in the soluble iron concentration.Corrosion rate tests using the Linear Polarisation Resistance (LPR) method show up to four orders of magnitude decrease in corrosion rate for the tests carried out with a V/S of 0.2 compared to those carried out with no additional iron.X-ray Diffraction (XRD) results for the surface films demonstrate the formation of mackinawite (FeS) with some evidence for the formation sulphated green rust GR(SO42-) in the seawater environment. The deionised water environment demonstrated the highest peak intensity for FeS formation. The effects of the environment on the fracture surface and fatigue crack path have also been explored.