Fatigue Life Performance of Titanium Grade 29 Welds in Tapered Stress Joints.

Design of a steel catenary riser (SCR) requires the use of specialized connection hardware at the floater hang-off to accommodate the high bending moments produced by vessel motion. Strength and fatigue reliability of this connection hardware is imperative, especially for high pressure and high temperature applications involving corrosive production fluids. One type of SCR connector is a metallic tapered stress joint which is typically mounted on a porch basket assembly. Because of its inherent density, strength- and stiffness characteristics. steel may not be well suited for this application as it typically results in excessively long and heavy stress joints. Titanium Gr. 29 has been identified as a good material candidate for harsh stress joint applications due to its attractive mechanical characteristic including lower stiffness, high strength and fatigue performance and high resistance to corrosive fluids. Industry has successfully used this technology in approximately fifty SCR applications as described in the vendor Project Database (2016). Large titanium stress joints (TSJs) for deep-water applications may not be fabricated as a single piece due to titanium billet volume limitations, thus making an intermediate girth weld necessary to satisfy the length requirement. Fracture and fatigue performance of these welds in the presence of cathodic potentials in seawater and galvanic potentials in sour production fluids, which may produce hydrogen embrittlement effects, must be characterized to ensure long term structural integrity. Although corrosion-fatigue data have been generated for Ti alloys in seawater and sour brine environments, data are limited to small scale welded specimens or larger full thickness base metal specimens. To date, no data are available for either base metal or welded Ti Alloys under applied or galvanic currents. This paper describes a joint industry project (JTP) to qualify titanium stress joints welds for ultra-deep water applications under harsh sendee conditions. Total fatigue life results for Ti 29 1G full thickness welded specimens in air. seawater under cathodic potential and sour brine environments under galvanic potentials are summarized along with general observations and design guidance. An improved automatic gas tungsten arc welding (GTAW) process developed during the test program to better manage the maximum defect size in the girth welds and surmount current ultrasonic inspection limitations is also described. [ABSTRACT FROM AUTHOR]