Re-Qualification of Existing Subsea Pipelines for CO2 and H2 Transport, Structural Integrity Challenges

There is currently a strong interest to explore the use of existing pipeline subsea infrastructures to transport carbon dioxide (CO2) in dense phase or hydrogen gas (H2) as a part of the new energy mix. Change of product will per applicable pipeline design codes require a re-qualification to ensure that the new premises for change in operation is properly assessed and confirmed acceptable with respect to pipeline safety, reliability, operability and transport capacity. Per DNV-ST-F101, Sec.11.E, a re-qualification is a re-assessment of the pipeline design under changed design conditions. For CO2 pipelines, a re-qualification work process is presented in the updated DNV-RP-F104 ‘Design and Operation of carbon dioxide pipelines’ and some industry experience exist for onshore pipelines. ASME B31.12 offers guidance for design of new H2 pipelines. However, for repurposing of existing pipeline infrastructure for hydrogen gas transport, limited guidance exists, especially for offshore pipelines. In this work, it has been explored if it is feasible to apply the same work processes used for CO2 transport as a basis for re-qualification of hydrogen transport. The paper will on a high-level discuss and present a possible work process to document acceptable system integrity under changed operational conditions from natural gas to CO2 and H2 with focus on the structural integrity of the pipeline. The paper will also highlight some of the special challenges related to the two possible new applications. For hydrogen pipelines this is especially related to potential impact of the hydrogen on the mechanical properties of the line pipe material and associated welds. Special challenges for CO2 pipelines are related to risk of very high corrosion rate and documentation of capability to arrest running ductile fracture. This paper will conclude with a discussion of the main gaps that need to be sorted out before fully robust general procedures to re-qualify pipeline for CO2 or H2 transport are available.