51312-01173-Long Term Corrosion Testing and Monitoring of Oxygen and Corrosion Potential in an Umbilical Design

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Product Number: 51312-01173-SG

ISBN: 01173 2012 CP

Author: Thierry Cassagne

Publication Date: 2012

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Subsea umbilicals are used for control and operation of subsea oil and gas production facilities. An umbilical normally consists of steel tubes electrical cables fiber optic cables weight or strength elements and fillers. Super duplex stainless steel tube material has been used for almost 20 years within the umbilical industry. However a recent failure on two orbital welds of 25Cr super duplex of an umbilical installed in the South China Sea has been reported. From these results a limit of 20°C above which the use of thermoplastic coating to avoid crevice corrosion on 25Cr super duplex stainless steel has been recommended by some end users of umbilicals. However these limits are based on results obtained for super duplex stainless steel in aerated natural seawater and very little is known on the micro-environment formed by the confined seawater between metallic tubes and polymer fillers of an umbilical. This work reports for the first time corrosion potential and oxygen content measurements in the confined zone between the metallic tubing and the polymer matrix of the umbilical. These measurements were performed using micro-electrodes on a 2 meter long real umbilical at 30°C in heated natural seawater. From the measurements it is shown that the oxygen content in the confined zone is rapidly consumed probably due to the passive current on the stainless steel tubes and then remained below 2 ppm over one year exposure period. From the open-circuit potential performed in the confined zone it is clear that the OCP remained below -150 mV/AgAgCl indicating no electrochemical effect of the biofilm in the confined zone. Visual and metallographic inspections of the tubes after one year exposure confirmed the results obtained by the microelectrodes and clearly indicate that no corrosion initiations were observed on superduplex stainless steel (including base material and welds) under these experimental conditions. Additional measurements on super duplex stainless steel tubes exposed at 30°C in natural seawater were performed at different dissolved oxygen content. From the results it is shown that the oxygen level is a key parameter for the potential ennoblement and consequently for the initiation and propagation of crevice corrosion.