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10313 Effect of Sour Brine Environment on the S-N Fatigue Life of Grade 29 Titanium Pipe Welds

Picture for 10313 Effect of Sour Brine Environment on the S-N Fatigue Life of Grade 29 Titanium Pipe Welds
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Product Number: 51300-10313-SG
ISBN: 10313 2010 CP
Author: Ronald W. Schutz, Carl F. Baxter and Chris S. Caldwell
Publication Date: 2010
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Based on its exceptional corrosion and fatigue resistance, high strength-to-density ratio, and low elastic modulus, Grade 29 titanium represents an attractive candidate material for dynamic offshore steel catenary riser components such as taper stress joints (TSJs), and touch-down zones (TDZ) sections, and deepwater intervention and production risers for HPHT and XHPHT service. Traditional steel alloy riser components are known to exhibit substantial S-N fatigue life knockdown (i.e., corrosion fatigue) when exposed to sour-rich well fluids, making safe fatigue design life difficult or even unachievable. Similar to its documented corrosion fatigue resistance in hot seawater, Grade 29 titanium welded pipe joint specimens tested in this lab study exhibited neither statistically significant corrosion, hydrogen absorption, nor reduction in S-N fatigue life when exposed to a sour NaCl brine at 150°C (302°F). A test frequency effect was observed, which probably stems from sustained-load strain occurring during the time when peak stresses exceed the alloy’s proportional limit in the loading cycle. As such, normalization of air and sour test frequencies at the lower value would have further diminished any difference between the two population means in this study, suggesting little or no sour knockdown for Grade 29 Ti pipe welds in this lower cycle regime study. Additional S-N testing in the higher cycle regime at normalized, low frequencies is recommended to establish a design curve.

Keywords: titanium alloys, corrosion-fatigue, sour brine, sulfides, welds, S-N fatigue
Based on its exceptional corrosion and fatigue resistance, high strength-to-density ratio, and low elastic modulus, Grade 29 titanium represents an attractive candidate material for dynamic offshore steel catenary riser components such as taper stress joints (TSJs), and touch-down zones (TDZ) sections, and deepwater intervention and production risers for HPHT and XHPHT service. Traditional steel alloy riser components are known to exhibit substantial S-N fatigue life knockdown (i.e., corrosion fatigue) when exposed to sour-rich well fluids, making safe fatigue design life difficult or even unachievable. Similar to its documented corrosion fatigue resistance in hot seawater, Grade 29 titanium welded pipe joint specimens tested in this lab study exhibited neither statistically significant corrosion, hydrogen absorption, nor reduction in S-N fatigue life when exposed to a sour NaCl brine at 150°C (302°F). A test frequency effect was observed, which probably stems from sustained-load strain occurring during the time when peak stresses exceed the alloy’s proportional limit in the loading cycle. As such, normalization of air and sour test frequencies at the lower value would have further diminished any difference between the two population means in this study, suggesting little or no sour knockdown for Grade 29 Ti pipe welds in this lower cycle regime study. Additional S-N testing in the higher cycle regime at normalized, low frequencies is recommended to establish a design curve.

Keywords: titanium alloys, corrosion-fatigue, sour brine, sulfides, welds, S-N fatigue
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