Investigation of Hydrogen Embrittlement Susceptibility of High Strength Steels Used in Offshore Mooring Chains in Nigeria

This study investigates the hydrogen embrittlement (HE) susceptibility of R4 and R5 grade high strength steels used in the manufacture of offshore mooring chains deployed in Nigerian deepwater oilfields. Offshore mooring chain failures have resulted in platform drift and hydrocarbon release incidents globally, and the combined effects of cathodic protection-induced hydrogen generation and sulphide stress mechanisms in Nigeria's deepwater produced water environment make HE a critical integrity concern. R4 and R5 grade samples are obtained and characterised by tensile testing, Vickers hardness, optical microscopy, and EBSD crystallographic analysis. Slow strain rate testing (SSRT) is conducted in synthetic seawater under free corrosion and cathodic polarisation conditions of minus 900 and minus 1050 mVSCE to quantify HE susceptibility via the embrittlement index. Hydrogen permeation experiments using the Devanathan-Stachurski cell are conducted to measure hydrogen diffusivity and subsurface hydrogen concentration at each polarisation potential. Fractographic analysis of SSRT fracture surfaces is performed by SEM. Findings show that R5 grade steel is significantly more susceptible to HE than R4 grade under cathodic polarisation at minus 1050 mVSCE, exhibiting a 38 percent reduction in fracture energy in SSRT. Hydrogen permeation data confirm higher hydrogen flux in R5, correlated with its higher yield strength and lower austenite stability. Keywords: hydrogen embrittlement, high strength steel, mooring chain, cathodic protection, offshore Nigeria.