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In the near future, natural gas pipelines (type X-52, X-60, X-65 and X-70) may also be used for transport of Hydrogen or even Carbon Dioxide. In case of Hydrogen conveyance, Hydrogen might be mixed with Natural Gas (parallel gas transport, using a membrane to seperate the gases at the outlet) or transported solely. Discarded natural gas pipelines may also be completely retrofitted for Hydrogen or Carbon Dioxide transport.
With regard to Hydrogen, a service life concern could be HISC (Hydrogen Initiated Stress Cracking) of steel. Hydrogen embrittlement or HISC results from combining of diffusing Hydrogen atoms into molecular Hydrogen - or the formation of molecular Methane - in internal metal voids of nanoscopic size. The generated pressure - in combination with intrinsic circumferential stress in the material - can exceed the restrain pressure of steel, especially near the loading surface. Whether HISC is an issue is largely dependent on the sort of steel, internal pressure and temperature. If HISC takes place, the strength of the pipeline will reduce, Hydrogen will escape and in a worst case, even buckling could occur.
To enhance the lifetime of these pipelines or to improve the reliability of these pipelines, nowadays several organic and inorganic material combinations (in a composite or multilayer configuration) are being researched. One suggestion for large diameter X-60 pipelines is the application of an internal Fusion Bonded Epoxy (FBE) coating.
In this case study a diffusion-chemical-mechanical simulation will be carried out for a retrofitted Natural Gas pipeline, in order to transport Hydrogen. The material under evaluation consists of (1) an inside Fusion Bonded Epoxy coating, (2) structural high strength steel and (3) an outside PE (Polyethylene) weathering coating. Key issues are:
Will HISC reduce by application of an internal Epoxy (eventually reinforced) coating?
What is the durability of the coating under internal pressure, interfacial adhesion and if a metal crack due to HISC would appear under the coating (severe conditions or long times)?
Figure: Hydrogen concentration profile of a retrofitted X-60 pipeline (subsequently: 1 micrometer coating, 10 millimeter steel, 3 millimeter PE coating). |