Corrosion of metallic well and process materials can be caused by water that is co-produced with oil and gas. The corrosion processes involve electrochemical reactions that are dependent on the composition of the produced water. Variation of the produced water composition during oil and gas production can be investigated using electrolyte simulation tools that model the chemical reactions (chemical equilibria) that control the composition of the produced water. This paper describes how electrolyte simulation was used to investigate the failure of 2205 duplex stainless steel (UNS S31803) process pipework on the Shearwater installation, operated by Shell Exploration and Production, in the Central North Sea.

The failure of the 2205 duplex stainless steel (22Cr) involved stress corrosion cracking (SCC) initiated from both the outside and inside of the pipework. Cracks initiated from the outside were attributable to a known chloride SCC failure mode of 22Cr. The mechanism involved a high degree of evaporation of seawater on the pipework surface. However, the mechanism that initiated cracks from the inside was unusual because oxygen was not present. Electrolyte simulation tools were used to establish the chemical environment associated with the cracks initiated from the inside. It was demonstrated that evaporation of produced water, resulting from a 90 bar to 15 bar pressure reduction at ~136°C, had created brines with very high calcium and magnesium chloride concentrations. The resulting "exotic" brines had not been anticipated and were outside the range of environments for which the process materials had been qualified. Subsequent laboratory materials testing replicated the field failure in the presence of these brines.

This study highlights the need for materials selection to include rigorous evaluation of steady state and transient process environments using both electrolyte and hydrocarbon simulation tools. Special attention should be given to projects that apply materials and processing concepts in novel combinations. The failure illustrated that surface wetting of pipework can initiate corrosion failure even at very low water volume fraction (< 0.5 vol%).

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