Corrosion-Fatigue of Ti 29 Alloy in a Sour Brine Environment

Abstract

Corrosion-fatigue is a significant design consideration in deepwater floating production systems. Mechanical loading is accentuated due to the compliant nature of these structures, and sour service conditions can also occur either due to the nature of the crude production or due to seawater flooding of the reservoir to enhance production yield. Consequently, over the past ten years a significant amount of corrosion-fatigue data have been generated on the influence of sour brine environments on conventional steels (X 65 and X 70), and more recently, on new higher strength steels specifically developed for deepwater applications. Although corrosion-fatigue data have also been generated for Ti-alloys in seawater, little or no data are available for Ti alloys in sour brine environment. The goal of this study, sponsored by the US DOE through the RPSEA Project, was to fill this knowledge gap by generating corrosion-fatigue data on a Ti Grade 29 alloy in sour brine with low-oxygen representative of the environment inside risers and stress joints. Corrosion-fatigue crack growth rates were initially obtained at a constant crack-tip driving force, ΔK, to assess the influence of cyclic loading frequency. These results were used to determine optimum frequencies for subsequent fatigue crack growth rate testing as a function of ΔK, thereby providing results that can be used in engineering critical assessments to establish NDE inspection limits. In addition, classical corrosion fatigue S-N fatigue data, which are typically utilized in fatigue design, were also generated on Ti-29 using full-thickness strip fatigue specimens extracted from the pipe wall. All data were generated on base material. The Ti-29 data are also compared to those from a companion study on high strength steels. At high ΔK, the baseline air fatigue crack growth rates in Ti-29 exhibited rates that were 3X-4X greater than those in the steels due to the effect of the lower modulus and lower ductility in Ti-29 compared to those in the steels. In contrast, at low ΔK, the rates in Ti-29 in air were equal to or less than those in the steels of comparable strength levels. In sour brine at ΔK values of 20 MPa√m and above, the rates in sour brine were up to 2X-4X greater than those in air; however, at low ΔK the rates in sour brine merged with those in air. Consequently, at high ΔK, the higher baseline rates in air plus the increase of 2X-4X in the sour brine environment resulted in corrosion-fatigue crack growth rates in Ti-29 that approached those of the steels. However, at low ΔK in sour brine, a reduction in the local crack driving force in Ti-29, believed to be due to roughness-induced crack closure, resulted in Ti-29 rates that were comparable to the air crack growth rates in steels. The S-N fatigue lives of Ti-29 in sour brine were reduced by a factor of about 2X or less compared to those in air. These S-N fatigue lives in sour brine were 8X-10X better than those in the steels in the sour brine. Thus, for sour-service applications in the intermediate- and high-cycle fatigue regimes, Ti-29 has significantly better sour corrosion fatigue performance than that of steels with comparable strength levels.