INTRODUCTION

ABSTRACT

The use of solid corrosion resistant alloys for the construction of pipelines to transport highly corrosive sour fluids is very expensive and the use of internally clad or lined pipe is quite common. In high salinity sour environment the use of UNS N06625 as clad material is the safe selection but still very expensive. UNS N08825 is the usual alternative in such conditions but the opinions of corrosion engineers and the available data do not agree on its resistance in these conditions. In this paper is summarized the work done for the selection of the cladding material for subsea pipelines, 22" and 8" OD, about 50 km of total length for each size. The publicly available data and opinions of different authors are analyzed together with different approaches followed in selecting this material for different applications in the oil and gas industry. These are compared to field experience and laboratory test results performed for this application. Corrosion resistance limits and acceptance criteria for localized corrosion testing are also discussed.

The produced fluids in the recent developments of oil and gas fields are characterized by high contents of CO2 and H2S and in several cases also NaC1 is very high. In these conditions the corrosion engineer has to face the problem of selecting the materials for the fabrication of the various equipment. The cost impact of such a selection for the economy of the project can be very important. The tendency of the industry towards low cost developments puts even more stress on the requirement to find the best alternatives that would permit a safe production of the field keeping the cost of the development as low as possible. This is reflected also on the materials selected. On the other side there is the requirement for low risk operations minimizing the impact on safety to people, problems to the production and pollution of the environment. Furthermore the low cost has to be intended not only relative to capital expenditures but as life cycle cost that includes also the operating expenditures and the impact of eventual failures. In case of material failures in service the costs relative to such failure can be very high, considering both repair cost and lost/delayed production. In the case of offshore developments these costs can be extremely high.

In the light of the above considerations, even if there is a widespread tendency of using carbon steel in aggressive environment there are some applications where the use of corrosion resistant alloys (CRA) becomes imperative. In this case the requirements for an accurate selection of the material becomes very important.

Alloy 825 (UNS N08825) is widely used in oil & gas production systems for a variety of applications because of its excellent resistance to CO2-and-H2S-corrosion and its high resistance to stress corrosion cracking. However, it has only limited resistance to chloride-induced crevice and pitting corrosion. Hence, if both high concentration of chlorides and H2S are encountered in oil & gas production, nickel base alloy like UNS N06625 could be used. The resistance of UNS N08825 to oxygen containing fluids and natural water is outside the scope of this paper.

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