Between 1975 and 1995 roughly half a million kilometers of duplex stainless steel (DSS) pipe had been installed in the North Sea, subsea, with insulation coating and cathodic protection (CP) applied. In contrast to the previous 20 year of good experience, between 1996 and 2004 a cluster of subsea failures of new and relatively newly installed DSS assets occurred. These failures were attributed to HISC as a consequence of CP. The paper reviews the available literature detailing a number of case histories and presents some additional anecdotal information not previously reported. Some similarities between these failures and a cluster of HISC failures of martensitic stainless steel pipelines that occurred shortly after the first DSS failures are detailed. Current methods of mitigation such as those detailed in design codes, the use of hot iso-statically pressed production methods, surface treatments and modified alloys with improved HISC resistance are discussed. The paper considers issues surrounding analysis of the problem and changes in practice that may further reduce the risk of HISC failure in the future.
As long ago as 1973, design codes1 considered the possibility of hydrogen embrittlement due to CP. Between 1986 and 19952-4 the failure of DSS fasteners subjected to CP were reported. These were associated with high ferrite levels in the steel (approximately 70%) combined with precipitation hardening at 475°C to give the high levels of strength desired for fastener applications.
At the same time, the susceptibility of DSS welds to hydrogen embrittlement had been reported5. Just like the fastener failures, the hydrogen cracking of welds was associated with high ferrite levels (70%), highly restrained joints and in the case of welds, high levels of diffusible hydrogen.
In 1987 a UK operator investigated the crack susceptibility of 22% Cr (UNS S31803) DSS. This related to 2 × 7km lengths of 6" (150mm) × 12.7 mm wt 2205 duplex stainless steel flow lines for a project in the North Sea6. Interestingly in 1989, this work resulted in the deployment of Schottky diodes on the pipeline and the use of very high integrity insulation coating system. The diodes were placed in series with the CP connections on to the two flow lines. The diodes limited the potential of the pipes in seawater to between -450 mV and -800mV (Ag/AgCl), such that the pipe was in the passive range, but more positive than the potential for significant hydrogen evolution. This was the first time that such a system had been used in the North Sea and was some 6 years prior to the first HISC failure of DSS.