Sour Service Corrosion Fatigue Testing of Flowline Welds

Abstract

An examination of the corrosion-fatigue behavior of production quality welds in X65-type pipes was performed. Due to the low cycle operational nature of the production flowline system, the fatigue test frequency was substantially lower (0.01Hz vs. 0.33Hz) than typically utilized during corrosion-fatigue testing. Also the tests were performed at higher stress ranges than previous sour service fatigue tests, which to date have targeted riser fatigue loading regimes. Stress-life (S-N) samples were removed from segments of pipe with outside diameters of 10.75 inch (wall thickness of 1.30 inch) and 9.625 inch (wall thickness of 1.26 inch) containing fully inspected, production-quality circumferential welds. Environments examined included laboratory air conditions as well as deoxygenated brine supplemented by a gas mix of H2S and N2. For all environmental tests performed, the dissolved oxygen levels were maintained at less than 10 ppb during all testing. The measured fatigue life decrease in the curved pipe segments was in the range of 8–110 times due to the combined effect of the material and fluid property variables examined. The results of this work clearly illustrated the impact of sour-service corrosion fatigue, in welded carbon steel pipes, to the multitude of variables involved. Nevertheless, the foregoing experimental work clearly demonstrated the importance of performing environmental relevant testing when considering material and process selection for offshore applications.

Corrosion-Fatigue Performance of High-Strength Riser Steels in Seawater and Sour Brine Environments

“…Fatigue testing in the air environment used relatively high cyclic loading frequencies (2)(3)(4)(5)(6)(7)(8) to expedite the data generation. This is an acceptable practice since air is a relatively benign environment for these steels; thus fatigue lives are insensitive to cyclic loading frequency.…”

“…Consequently, the available full-scale data measured in air are not adequate for design of risers that transport sour production, since sour environments can significantly degrade the fatigue performance compared to that in an air environment. For example, in conventional riser steels, reductions in the fatigue life by factors of 10 to 40 times have been observed in sour brine environments, compared to the fatigue life in laboratory air [1][2][3][4][5][6][7]. Moreover, the extent of degradation is dependent on the cyclic loading frequency and typically increases with decreasing cyclic frequency due to the time-dependent interaction of the riser materials and the environment [8][9][10][11].…”

Testing methodologies for corrosion fatigue

“…By a test set-up, the temperature can be controlled by a cooling unit (see below). Sour environments (pH value lower than 7) speed up the corrosion process [7]. The pH of sea water is approximately 8.3 and globally constant [8].…”

“…In the work of McMaster et al [7] a test setup is built to evaluate the corrosion fatigue behavior of production quality welds in X65 grade steel pipes. The tests take place in a sour brine environment consisting of water, CO2 and H2S.…”

“…The environmental primary chamber (a) and complete sour brine environmental setup (b) [7] Shroud Third chamber…”