Abstract
The need for galling resistance in corrosion resistant stabs of hydraulic lines determined the selection of Nitronic 60 alloy as the material to be joined to 316 SS tubing by autogenous orbital GTAW process. These lines, which are operating in about 500 Completion Risers Joints for more than 3 years exposed to a mixture of seawater, mud, oil and eventually products of acidification in the annular space, have shown good performance except for an unusual case of cracking of the Nitronic stab.
After 14 months laid in the Macae Yard without operation, and without cathodic protection, a specific Riser Joint was hydrostatically tested and two of the eighth hydraulic lines failed. Although a complete metallurgical investigation was not possible, since the equipment went back to operation after changing the lines, a metallographic analysis of the two cracked lines allowed the determination of stress corrosion cracking as the failure mechanism. This paper illustrates the metallurgical results obtained by scanning electron microscopy and by optical microscopy, as well as the use of a microprobe as an auxiliary technique.
The failure was associated to the long time in stagnant conditions and an inspection program was adopted to avoid other failures. Stabs from other Risers, not exposed to the same stagnant conditions were also characterized and are presented.
