Corrosion Performance of Welded CRA-Lined Pipes for Flowlines

Abstract

ABSTRACT This paper presents the results of corrosion and mechanical testing of welded high-nickel, corrosion-resistant-alloy (CRA)-lined pipes manufactured using various lining processes. The corrosion resistance of CRA-lined pipes for use as a flowline material in sour service was evaluated. The effects of the manufacturing process, CRA-lining material and welding on the corrosion performance of CRA-lined pipes were also determined. INTRODUCTION Many recently discovered and developed oil/gas fields are deep and sour. In these severe sour environments, the use of steel pipes with inhibition is ineffective or infeasible to combat corrosion and cracking. Therefore, to combat corrosion and environmental-cracking problems that may occur in these hot and sour environments, CRA'S have been increasingly used. The high initial cost of nickel-base CRA materials often makes the economic-justification for use of solid CRA pipes difficult to achieve. Therefore, a steel pipe with only a CRA-lining has been developed by many manufacturers [1-3] as a cost-effective alternative to solid pipes. The CRA-lined pipe is a double-wall, bimetallic pipe in which the inner layer is made of a high-corrosion-resistant CRA and the outer layer made of low-cost steel. Some CRA-lined pipes have metallurgical bonding between the lining and the steel [1,2], while some have only a mechanical bond between the CRA layer and the steel layer [3]. The metallurgically-bonded pipes (referred to as clad pipes) are usually manufactured by processes such as coextrusion, or hot-rolled bonding, explosive bonding, or centrifugal casting. The mechanically-bonded pipes are manufactured by thermo-hydraulic gripping of CRA and steel pipes. The manufacturing processes of CRA-lined pipes differ from solid CRA pipe processes. Flowlines transport production fluids from the wellhead to treating and processing facilities. They typically are constructed by girth welding pipe segments end-to-end. In sour fields, the fluids transported through flowlines may contain highly-corrosive gases such as hydrogen-sulfide (H2S) and carbon-dioxide (C02) as well as highly corrosive brines. The objective of this study was to evaluate the corrosion resistance and suitability of CRA-lined pipe for use as a flowline in sour service. The primary questions to be answered included:whether CRA-lined pipe performs in sour service as well as a solid CRA pipe;whether a difference in performance exists between manufacturing processes;whether the CRA-lining materials perform differently from each other; andwhether a weldment performs in sour service as well as the parent pipe. Three UNS N06625-lined and three UNS N08825-lined X60/X65 pipes were tested in this study. These pipes were manufactured by coextrusion of the CRA and steel pipe, coextrusion of CRA powder metallurgy cladding and steel pipe substrate, and thermo-hydraulic gripping of the CRA and steel pipe. Table 1 briefly summarizes the manufacturing procedures for each of the pipes investigated in this study. A solid pipe of UNS N06985 was also included in the test program so that the corrosion performance of CRA-lined pipe and solid CRA pipe could be compared. Table 2 lists the standard chemical compositions of UNS N06625, UNS N08825 and UNS N06985.