Hydrogen induced cracking (HIC) tests of flexible pipe pressure and tensile armor wires were carried out in a H2S-containing environment in order to assess the resistance to cracking. Tests were carried out in substitute ocean water solution, constantly bubbled with 1%H2S/N2 mixture for different duration in order to investigate crack growth. After each test specimens were ultrasonic inspected for internal cracks. Specimen was sectioned at any region with crack indications and the cross section was observed for internal cracks. HIC parameters were measured and associated with microstructure and hardness profile. Results showed that HIC susceptibility is related to the hardness. Regions with higher hardness had a higher tendency for crack initiation.
Flexible pipes may be exposed to high pressures during deep-water operation. They are made up of several different layers. Pressure armor layer is designed to withstand the hoop stress that is caused by the inner fluid pressure, while the tensile armor layer is designed to resist axial loads. They are wound around an internal polymer sheath to isolate it from production gases and fluids. However, external sheath damage can expose these layers to sea water, while gases contained in the crude oil environment can reach the annular space due to permeability through the polymer sheath, exposing the steel armor layers to corrosive environments that may contain H2S1,2,3. Depending on the mechanical properties and microstructure of the steel, the absorption of hydrogen generated by corrosion in a H2S-containing environment can lead to the failure of these layers. Despite this, some authors suggest that the rate of which H2S enters the annulus by permeation is much lower than the ability of the steel wires in the annulus to consume H2S3.
HIC is often used as a generic way to define the fracture of a material at subcritical stress levels assisted by hydrogen. Cracks generally originate from defects (that act as hydrogen traps) inside the metal when the diffusing hydrogen concentration exceeds a set threshold. It differs from others hydrogen assisted cracking mechanisms due to the absence of external load.