Qualification of Flexible Pipes for Gaseous Hydrogen Transportation
Authors: Gabet Christelle , Bienvenut Florian , Michaux Aline , Valcy Stael-John , Tronc Frédéric , Kjenner Svein 
Published: April 2023

Abstract
Renewables in the offshore space are integrated with technologies to deliver at-scale solutions for offshore green Hydrogen production and sustainable renewable energy. The system in the frame of the Deep Purple project consists of offshore wind turbines and offshore Hydrogen technologies for the production, storage and transportation of energy in the form of pressurized green Hydrogen. It can also be used to supply off-grid energy in remote locations, and to provide green Hydrogen for decarbonizing industrial processes.
Within this context, subsea pipelines are needed to transport gaseous Hydrogen. Nevertheless, Hydrogen is well known for its ability to cause steel embrittlement and is also a non-usual gas within standard offshore Oil & Gas sector. For these reasons, caution has to be taken and suited qualification testing needs to be conducted to propose a reliable transportation solution.
Flexible pipes represent an effective and versatile solution for conveying gaseous Hydrogen subsea from fixed or floating turbines or systems, in particular under dynamic conditions. Their intrinsic low allowable minimum bending radius, and tolerance to accommodate large relative motions, make this field proven solution in Oil & Gas infrastructures a perfect choice for floating offshore systems in shallow water areas targeted for deployment of offshore wind turbines with Hydrogen technologies. Furthermore, the efficient installation time and cost enable to envisage an extended use for the offshore green Hydrogen transportation systems. Finally, by construction, flexible pipes have their high strength steel wires in between two polymer sheathes, in a less severe, so-called annulus, environment. With this kind of solution, a very low part (&lt; 1% of the flow) of the carried Hydrogen will permeate through the external polymer sheath but the high strength steels will be submitted to lowered Hydrogen pressures compared to bore, and might be used without any embrittlement risk.
Within the frame of the present study, many materials used in flexible pipes for Oil & Gas application have been tested in presence of gaseous Hydrogen, in annulus (a few tens of bar) or in bore (a few hundred bar) conditions: different carbon steel grades, used as wires and connexion parts, 316L austenitic stainless steel and several polymer materials (polyethylene, thermoplastic elastomer, polyamide). It includes the following tests: Slow Strain Rate Tensile (SSRT), toughness, fatigue, crack propagation, four points bending, Rapid Gas Decompression (RGD) and ageing.
Results allow to select materials compliant with the Hydrogen transport application and to acquire the data needed to perform the detailed engineering of the flexible pipe. Furthermore, they enable to establish the flexible pipe as a qualified and reliable solution for Hydrogen transportation, tolerating advantageously elevated hydrogen pressure.
This work is likely one of the first to be discussing about the influence of Hydrogen on materials used in flexible pipes for subsea transportation and to highlight a qualified solution for such an Hydrogen carrier application.