Hydrogen Induced Stress Cracking (HISC) is a problem that duplex and super duplex material may experience when cathodic protected in seawater. The hydrogen of the seawater may diffuse into the material and may lead to cracking.
DNVGL-RP-F112 describes diffusion rates, acceptance criteria for both stress and strain levels. HISC is often one of issues that limits the capacities of subsea piping.
DNVGL-RP-F112’s strain criteria states that effects of the hydrostatic pressure test can be considered when assessing the submerged strain relevant to HISC. The hydrostatic test often yields the material on the inside of piping components such as tees and bends. In the areas where yielding has happened there will be a residual compressive stress. This is beneficial for the HISC assessment, since compression will reduce the strain in tension. The challenge is to report the correct strains. We at Stressman have developed a method, a script, that account for the initial yielding and recalculates the principal strains in the operational submerged states. The HISC strain in each node of the FEA can be assessed, and the post-processing time is heavily reduced. Our method has been verified by hand calculations, extraction of data from FEA, simplified models. DNVGL has been to our offices to go through the method as well.
We do not share this script, but we utilize it so that our clients will benefit from it.
We have seen many engineers who over-dimension the piping components based on results from linear FEAs. Don’t do this next time, contact us instead.