Field life extension through controlling the combined material degradation of fatigue and hydrogen (HyF-Lex)

Life extension of existing oil & gas fields is highly prioritized within the OG21 national technology strategy for the petroleum industry. In order to minimize the environmental hazard and avoid significant costs due to downtime and repair, degradation mechanisms as corrosion, fatigue, and hydrogen embrittlement are important factors to control and management of ageing infrastructure is essential. Fatigue causes material failure by repeatedly applied loads (cyclic loading and stress). The cyclic loading can cause catastrophic failures in aggressive environments like offshore O&G production, not least due to the cathodic protection methods that create hydrogen gas. Absorbed hydrogen is often found to reduce the lifetime of components. However, how this occurs is still up to debate. Hydrogen embrittlement and its effect on fatigue behaviour are one of the most challenging phenomena to predict and model in steels. The challenges are that the hydrogen effect changes based on the testing conditions, and the degradation to the component initiates locally, and is usually not detectable before the final leakage or component fracture. These failures can have severe environmental impacts. The Hyflex project started with the study of the hydrogen evolution and absorption into the steel using an electrochemical permeation cell where a very thin foil of steel separates two electrochemical cells. This is an extremely sensitive method, and the corresponding mathematical model involves a large number of unknown parameters. A new fitting procedure using the so-called Akaike Information Criterion was used to solve the fitting uncertainties to obtain an accurate mathematical model for the hydrogen absorption. The HyFlex project has also developed a new testing rig where the fatigue life of samples under cathodic polarization was measured. The hydrogen effect was found to enhance the crack growth rate. It was also found that the frequency of the load alteration altered the magnitude of the hydrogen effect. The tested steel samples were further analyzed with high-resolution microscopy techniques. This project has made the post-analysis of the dislocation evolution along the fracture surface more agile and easier by developing a technique able to capture dislocations in the SEM. This has typically been an investigation performed by a destructive characterization method using a Transmission Electron Microscope (TEM). The new Scanning Electron Microscope (SEM) methodology developed by HyFlex significantly reduced the analysis time. The enhanced crack growth rate due to hydrogen was linked to pinning of dislocations by hydrogen. Pinning of dislocation close to the crack tip would reduce the plastic response of the crack, reducing the fracture toughness of the system. In the modelling work package, the plasticity in the vicinity of a crack was simulated using a Discrete Dislocation Dynamics (DDD) model. Suggested hydrogen effects from the literature on dislocation mobility, elastic interaction between defects and the cohesive strength of the crack tip were emulated at different hydrogen charging conditions. In the current testing conditions emulated, the hydrogen effect on the cohesive strength replicated the experimental findings the best. However, the results were found to be system-specific. Which hydrogen effect is critical and causes the enhanced crack growth rate depends on the conditions of the tests. The project has had a close collaboration with international partners in Japan, Germany, France, Iran and Belgium. This project has created new knowledge in the understanding of the hydrogen effect on the fatigue life of steels used in offshore O&G applications.

HyFlex has developed a new testing rig able to measure the fatigue life of samples under hydrogen charging. Hydrogen was found to enhance the crack growth rate. These results were validated with the Discrete Dislocation Dynamics (DDD) model. The hydrogen effect on the cohesive strength replicated the experimental findings however, the results were system-specific. This project has made possible the post-analysis of the dislocation evolution along the fracture surface more agile and easier by developing a technique able to capture dislocations in the scanning electron microscope (SEM). The new experimental and theoretical methodologies Hyflex have generated will have an impact on the hydrogen research community since they open new ways for testing and understanding the detrimental effect hydrogen has on structural materials. With the new era of green hydrogen just taking-off, Hyflex will represent a very important background, setting the Norwegian activities in a very good position.

It is well known that the presence of atomic H in ferrous materials under cyclic stresses affects the fatigue behaviour. Platforms, umbilicals, risers, flowlines and subsea pipelines are continuously subjected to oscillatory environmental loads and H, from cathodic protection or H2S containing fluids. Degradation by H under such conditions would manifest as reduced resistance to fatigue crack growth. The HyF-Lex project aims to investigate, understand and measure the influence of hydrogen on fatigue crack growth. To achieve this, in situ fatigue testing at the macro-scale under hydrogen-charging conditions, combined with advanced nano- and micro- characterization of the cracks and plastic zones will be used. These results will give input to a hierarchical model framework to address the combined degradation effect from fatigue and hydrogen. Comparative investigations of H uptake under CP and H2S conditions will provide information regarding the charging conditions for the fatigue testing. The project team consists of a balanced team of young and experienced scientists at NTNU and SINTEF that has a well-established network both nationally and internationally. International cooperation is ensured by the participation of three European research institutes who are at the forefront of the research field of fatigue and hydrogen degradation. Industrial relevance of the project is assured by an industrial advisory group containing five companies that are strong stakeholders in the Norwegian oil and gas sector.