Ferritic-austenitic (duplex) stainless steel components are used for oil and gas production duties due to their high strength and corrosion resistance. The material is routinely used for short flowlines, as well as for welded hubs and flanges. Cathodic protection (CP) is employed, via sacrificial aluminium based anodes, which protects ferritic steel parts from seawater corrosion. Whilst CP has proven successful in preventing corrosion, failures have occurred due to the ingress of electrolytically evolved hydrogen. Duplex stainless steel joints become susceptible to environmental cracking under a combination of high stress, hydrogen content, and susceptible microstructures; critical combinations of which may result in hydrogen induced stress cracking (HISC).
Successful operation of duplex equipment, in avoidance of HISC, necessitates a good understanding of the total in-service stresses (including from loading applied in service and from residual stresses from manufacture, fabrication, installation and commissioning). One of the key components of understanding the in-service stress at welds is knowledge of the residual stress distribution, following welding. The focus of this paper is to provide an overview of the typical residual stress levels in a welded superduplex stainless steel (SDSS) subsea joint, using neutron diffraction and finite element modelling. The results are presented in the context of current recommended practice, for example DNV RP-F112.
