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

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 unachievable in many situations.  Although titanium alloys exhibit elevated resistance and insensitivity to sulfur, sulfides, and H₂S, and to stress corrosion cracking and corrosion-fatigue in seawater as high as 150°C (302°F), no database has yet been established to validate Grade 29 titanium’s fatigue resistance in hot sour brine environments. This paper is intended to document a preliminary, statistically-meaningful S-N fatigue database on Grade 29 titanium pipe welds in air and in a sour (10 psia H₂S / 100 psia CO₂) NaCl-rich brine at 150°C (302°F) to assess the extent of sour knockdown for fatigue design purposes.  The results reveal no significant degradation in  S-N life in this hot sour environment, which was supported by specimen fracture characteristics and hydrogen uptake measurements.