51315-5537-Coatings in Combination with Cathodic Protection—Field Experiences after 20+ Years

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Product Number: 51315-5537-SG

ISBN: 5537 2015 CP

Author: Ole Knudsen

Publication Date: 2015

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By application of coatings on subsea structures the amount of anodes to be installed for cathodic protection (CP) can be decreased. The number of anodes is mainly defined by the lifetime of the structure and the bare metal surface to be protected. An equation is used to estimate the breakdown of the coating as function of time. This equation was developed for projects with a typical lifetime of 20-25 years and without any experience from structures that have been exposed for long lives. The objective with this study was to evaluate coating degradation on subsea offshore installations after long term exposure of coatings on offshore structures and to suggest a revised coating breakdown equation for use in CP design.Four different installations each with more than 20 years exposure time have been investigated; two subsea templates and two jackets. One of the templates has been taken to shore and was investigated there. For the three other installations the condition of the coating has been evaluated based on subsea survey reports. The condition of the coating on the investigated structures shows that the coating breakdown models used in NORSOK M-503 and DNV RP-B-401 are very conservative. However the anode consumption on the installations is higher than the coating degradation would predict. Hence it seems that other parameters in the CP design rules are non-conservative. One possible explanation is that the anode auto-corrosion is underestimated so that too high anode capacities are used in the CP design.