The long-term degradation of decommissioned pipelines left in situ is an important topic in decommissioning projects. These constitute a long-term health and safety challenge in the form of snagging risk to other users of the sea. An accurate forecast of the long-term behavior of the pipelines in the marine environment would allow to make an informed decision regarding the feasibility of leave in place option for pipeline decommissioning.

This paper aims to summarize the effect that individual environmental factors in marine corrosion have on the corrosion rate and to discuss in detail a chosen corrosion model that could be used to predict the long-term corrosion of in situ decommissioned subsea carbon steel pipelines in the marine environment.

In addition, the long-term degradation will be predicted with the chosen corrosion model and the results will be compared for a range of subsea pipelines to be decommissioned in the North Sea and the Gulf of Thailand. With the chosen corrosion loss model, considering the higher average annual seabed temperature in the Gulf of Thailand in comparison with the North Sea, the predicted long-term corrosion rate of unprotected carbon steel is 0.053 mm/y for the North Sea and 0.069 mm/y for the Gulf of Thailand.

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