Corrosion Resistance Of Stainless Steel Pipelines In Seawater During Pre-Comissioning Phase

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In Oil & Gas industries, Cr-Ni-Mo stainless steels and Ni-Cr-Mo alloys with Pitting Resistance Equivalent Number (PREN) lower than 40 could be selected for Rigid Production subsea risers, pipeline, and associated structure’s piping, according to a CO2/H2S corrosion assessment that considers all steady and transient conditions foreseen to operate the reservoir.

However, there are frequently some localized corrosion concerns for these materials during the installation and pre-commissioning of the line, when the internal surface of the line could be in contact temporarily with untreated seawater. These concerns systematically lead to discard the selection of these materials and to select, for conservative purpose, UNS N06625 that is admitted immune to localized corrosion in ambient seawater in international standard and operator’s specification, whatever the outcomes of the CO2/H2 corrosion assessment.

Product Number: 51323-19137-SG

Author: Charles Leballeur, Tran Quang Tung, Nicolas Larché

Publication Date: 2023

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In-house operator specifications and international standards report some recommendations on the line fill water chemical treatment that should be implemented when installing and pre-commissioning subsea Corrosion Resistant Alloys pipelines. The main objective of this paper is to clarify some uncertainties in these recommendations and to better quantify actual corrosion risks.

UNS S31603, UNS N08904 and UNS N08825 type materials have been exposed in natural seawater from the bay of Brest (France) at 5 and 15°C and under the conditions representative of storage/pre-commissioning periods, i.e. in closed compartment conditions at 27°C. The pitting, crevice and under-deposit corrosion susceptibility of the different alloys have been assessed under these specific conditions.

The results of this study allow to complete the information from operator specifications and the standard recommendations used for the material selection of corrosion resistant alloys pipeline. According to the results obtained in this paper, some practices from these recommendations are susceptible to be over conservative and could thus be optimized. Indeed, tests performed have shown that the pitting and under- deposit corrosion risk seems to be very low in natural renewed seawater. This risk is expected to be even lower in close comparted media due to the fast decrease of dissolved oxygen concentration even at 27°C.