11119 Preliminary Determination of Hot Sour Brine Environmental Effect on Fatigue Crack Growth Rate in Grade 29 Titanium Pipe Base and Weld Metal

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Product Number: 51300-11119-SG

ISBN: 11119 2011 CP

Author: Ronald W. Schutz, Ramgopal Thodla, Carl F. Baxter, and Christopher S. Caldwell

Publication Date: 2011

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Grade 29 Titanium is an established, prime candidate material for critical dynamic offshore riser components; particularly for deepwater and/or corrosive, sour High Pressure/High Temperature (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 Gas Tungsten Arc-welded riser pipe specimens in air vs. sour brine at 150°C (302°F) revealed no significant knockdown in fatigue life under sour conditions, in contrast with steels. The next pertinent question relating to riser fatigue/fracture mechanics design must address the influence of sour environments on Grade 29 titanium’s fatigue crack growth (FCG) resistance. The laboratory study described in this paper utilizes “frequency scans” to compare Stage 2 FCG rates of Grade 29 Ti pipe base and weld metal at various loading frequencies in air versus sour brine at 150°C (302°F), as measured at a fixed stress intensity factor range ?K value and high, closure-corrected stress ratio. Test results indicate no discernable effect of the hot sour environment and loading frequency on FCG rate of Grade 29 Ti pipe base and weld metal.

Key words: titanium alloys, corrosion-fatigue, sour brine, sulfides, welds, fatigue crack growth, offshore risers