Preliminary Determination of Hot Sour Brine Environmental Effect on Fatigue Crack Growth Rate in Grade 29 Titanium Pipe Base and Weld Metal

Grade 29 Titanium is an established, prime candidate material for critical dynamic offshore riser components, particularly for deepwater and/or corrosive, sour HPHT field developments, stemming from its unique, synergistic combination of properties.  These include an elevated strength-to-density, low elastic modulus, and exceptional resistance to corrosion and corrosion fatigue in seawater and sour environments even at high temperatures. Recently published laboratory S-N testing of Grade 29 titanium GTA-welded riser pipe specimens in air vs. sour brine at 150C (302F) indicate no significant knockdown in fatigue life under sour conditions, in contrast with steels. The next question regarding riser fatigue/fracture mechanics design, the subject of this paper, focuses on the influence of sour environments on Grade 29 titanium’s fatigue crack growth (FCG) resistance.  Utilizing “frequency scans” to measure Stage 2 FCG rates of Gr29 Ti pipe base and weld metal at various loading frequencies,  comparative FCG rates in air and sour brine at 150C (302F) are measured at a fixed delta K and (closure-corrected) stress ratio value. These results will establish frequency and environmental dependence on FCG rate, and preliminary correction factors on Grade 29 Ti Stage 2 FCG rate design for sour service use.