Repurposed natural gas pipelines

Repurposing implies converting an existing natural gas pipeline into a dedicated hydrogen pipeline. The main elements of the conversion process include nitrogen purging to remove undesirable parts, replacement of compressors, a thorough inspection of the pipeline and the integrity of its components, and replacements of valves and other leak-prone parts, and reconfiguring or replacing gas meters. Due to differences in chemical properties, hydrogen can accelerate pipe degradation through a process known as hydrogen embrittlement, whereby hydrogen induces cracks in the steel. A range of solutions exists to combat this: regularly monitor the integrity of the pipeline, e.g. through in-line inspections (ILI) and pigging; apply a hydrogen barrier coating to protect the pipeline; lower the pipeline pressure until the required threshold value for safe operation is met; and minimise pressure swings. The optimal approach will depend on transport capacity requirements, status of the existing pipeline (e.g. existing fractures) and trade-offs between capital and operating expenditure. There are still challenges on the repurposing of offshore gas pipelines, as the monitoring of the pipeline with the current technology is difficult, and sometimes there is no detailed documentation on the pipeline operation over past years. There is no standard for offshore hydrogen pipelines, unlike the ASME B31.12 for onshore hydrogen pipelines.