Wind and salty water: corrosion and fatigue. Not so fast!

Steel does not react that optimal to salty water

Monopiles are the most commonly used foundation for offshore wind turbines, and they are made of steel. But the environment is not a kind one, consisting of salt water and air with waves and wind loading. As Richard Pijpers underlines, steel does not “react that optimal” to these conditions, causing degradation. But, as he further explains, “it’s not only the fact that it corrodes - so that there’s a rust layer - but there's also a fluctuating loading. That causes at critical locations the issue of fatigue in combination with corrosion.”

To design an optimal monopile

Though it might seem as if everything about steel, corrosion, and resulting degradation mechanisms is known already, the combination of corrosion and fatigue in offshore environments is not fully understood yet. Richard Pijpers explains: “Yes, we all know of rust, but we would like to find out how fast degradation mechanisms occur, and we try to optimise the structures in a way that we can use as much material as needed, but not too much, to have an effective solution.”

Namely, previously, a lot of fatigue tests have been done with specimens in a salt water environment, based on which design curves were generated. However, these designs are usually either too conservative or not conservative enough. “So, we would like to pin-point in a better way how the structure reacts to the environment and what's the specific mechanism, so that we can act upon it,” Richard Pijpers summarises.

To save time, effort and costs

In addition to uniform corrosion, or the general material loss making the monopile walls thinner, a small pit in can take shape. When corrosion occurs, in that notch, the material is extremely vulnerable. As a result, sharp cracks want to initiate and propagate. Richard Pijpers says “What we specifically would like to address in our model is when that happens.” Research has already shown that corrosion doesn’t need much time to threaten the structure of the monopile design. With the use of small-scale samples, this project can track the growth of pits and notches, seeing exactly when and where the degradation is going to happen, to help identify the moment at which these sharp fatigue cracks occur. With this knowledge, more adequate mitigation measures can be chosen in the design.

Richard Pijpers admits that there are multiple mitigation measures possible already, such as increasing the thickness of the steel, applying protective coatings, or using sacrificial material. However, these measures are costly or can be activated only at a later stage, when corrosion may have already occurred.

Thus, Richard Pijpers explains, the new C-FLO (Corrosion - Fatigue Life Optimisation) model achieves to “present to designers ways of choosing the optimal mitigation measures.” Additionally, it would allow the industry to make better use of inspection data to extend inspection intervals. And that would already save an incredible amount of time, effort, and costs.

Validation with field data is needed

With the extensiveness and novelty of the project, a whole new world is revealed, requiring the build-up of new corrosion expertise. As can be expected of GROW, this means that – as Richard Pijpers explains, “the only way of doing that is to make alliances with partners, to team up, to determine exactly what the main questions are.” Joke Luyten elaborates: “Everyone benefits from each other. And it’s good to understand what the others are doing because then you can have a better intuition on how things will go. This variety of partners from all over the world collaborates not on a playground for scientists, but for a model that will be very important for the industry.”

With new research comes, of course, a variety of surprises. Richard Pijpers illustrates that “we are in our lab environment doing a lot of dedicated tests for generating data, but it’s also so vital to have data from the field.” Though significant information has been obtained so far, he stresses that “the real validation in the field is a very important follow-up”.

This project itself, already, aims to design a monopile in a more optimised way, and also to prevent surprises when the monopiles are already built. Joke Luyten states “you don’t want a crack that you did not expect. So, I hope to make sure that the involved companies are proud of the results, and that they can use them.

After all, with a better understanding of the corrosion processes, the industry can optimise the design and maintenance of the monopiles and thereby reduce the costs of offshore wind energy.