Duplex stainless steel has been used on subsea facilities since the mid 80-ties. The experiences with these materials have been relative good and only a few failures have been reported. However, BP and Shell experience some serious cracking of duplex steel in the mid 90-ties and in beginning of the century. The root cause of these failures was identified to be Hydrogen Induced Stress Cracking, HISC, where the hydrogen source was the cathodic protection system of the subsea facility. These and other similar failures resulted establishment of Joint Industry Projects, JIPs with financial and technical contribution from leading oil companies, contractors, material suppliers and research institutions as TWI, SINTEF and DNVGL. The objective of the JIPs was to establish practical usage limits for duplex stainless steels. The JIPs resulted in a recommended practice “DNV-RP-F112 - Design of duplex stainless subsea equipment exposed to cathodic protection.”
This document minimized the failure rate of duplex steel components used subsea. However, since duplex steels components have been used on subsea facilities long before the guidelines and recommendations were issued, there are lot of components presently in use that may be overloaded compared to guidelines and recommendations. As a part of life time extension of one of Statoil’s long time producing fields, a HISC re-calculation of spools connecting SPSs to infield pipelines showed that many of the spools were exposed to stresses above the recommended stresses given in DNV-RP-F112. Since these recommendations were primarily based on testing at ambient seabed temperature (4°C), Statoil, together with SINTEF, started in 2016 a project where the aim was to evaluate the resistance against HISC as an effect of the operation temperature.
The results of this project show that the critical net section stress/AYS (HISC resistance) increases with increasing temperature. Based on this, the before mentioned spools can be considered safe even though the spools are exposed to stresses above the recommendations in DNV-RP-F112.
Further, the investigations show that the guidelines and recommendations given in DNV-RP-F112 may be conservative for temperatures above 4°C. It is therefore recommended to perform more testing to confirm and incorporate the findings from the present investigation in future revision of DNV-RP-F112.
