SUPER DUPLEX/DUPLEX STAINLESS STEEL UMBILICAL TUBE RESISTANCE TO HISC

Super duplex/duplex stainless steel is susceptible to Hydrogen Induced Stress Cracking (HISC) whilst under the influence of Cathodic Protection (CP) systems. Field failure experiences have been reported on super duplex stainless steel components in subsea applications. However, HISC is a non ductile mode of failure caused by combinations of particular conditions associated with hydrogen charging, metallurgical factors of the alloy, stress/strain level and service conditions. DNV RP F112 [4] has been introduced recently and is a guideline for the design of duplex stainless steel components for offshore applications operating under CP systems. Super duplex/duplex stainless steel tubes have been used as components in DUCO umbilical systems for more than 15 years without any failure feedback. A combination of controlling the material microstructure, welding operation design, component stress design, and surface condition design have contributed to this record. This paper describes the design methodology applied by DUCO for the super duplex/duplex umbilical tubes for optimum mitigation against HISC. The influences of the microstructure, stress/strain condition, surface condition and cathodic potential on the seamless super duplex stainless steel (UNS 32750) umbilical tubes resistance to HISC are discussed. Results of laboratory tests using Slow Strain Rate Testing (SSRT), stepwise uni-axial and constant load methods, with reference to offshore oil field application best practice, are presented.