The number of offshore wind turbines is set to rise in the coming years. The steel foundation structures, known as monopiles, must be able to withstand the salt water and the force of the wind and waves. However, too little is currently known about the effect of these loads on the working life of the foundations. TNO is developing models to predict and increase the lifespan of these monopiles.
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To find out more about the TNO research into ‘corrosion-fatigue’, please contact Richard Pijpers.
When designing foundations for wind turbines for a fast-growing offshore market, you need to take account of the combined effects of the varying wave and wind loads (fatigue) and the salt water (corrosion).
‘At present, our knowledge of these factors is lacking,’ explains Richard Pijpers of the Netherlands Organization for Applied Scientific Research) (TNO). ‘As a result, the designs are conservative; they use relatively thick-walled constructions and unnecessarily expensive coatings. The effectiveness of these precautionary measures is uncertain, which means that in practice, problems with defective parts can still arise and lead to costly repairs.’
Lifespan of wind turbine foundations
Estimating the lifespan of wind turbine foundations is a crucial step towards optimising the use of materials and reducing the cost of manufacture and maintenance. ‘We only have a limited amount of information about the degradation mechanisms that affect the foundations of offshore wind turbines,’ Pijpers says.
‘Our research group has a lot of knowledge about making models for steel constructions, such as bridges. We focus on the impact of fatigue on the lifespan, which is also a contributing factor in offshore constructions. The varying wind and wave loads can cause tiny cracks to form. In the case of offshore wind turbine foundations, it is particularly important to be able to analyse the rate at which these cracks grow in salt water. This is why we are developing models to estimate the lifespan of the critical components in these constructions, taking the effects of both fatigue and corrosion into consideration.’
“We focus on the impact of fatigue on the lifespan, which is also a contributing factor in offshore constructions”
‘We started this research into ‘corrosion-fatigue’ within TNO. The next step was to involve a larger group of market parties via the GROW consortium,’ Pijpers explains. ‘This resulted in the C-FLO project [Corrosion Fatigue Life Optimisation], in which we also work with non-GROW partners.
Our aim with the 14 partners now involved (Deltares, DNV-GL, Eneco, innogy, Ørsted, Parkwind, Posco, PPG Coatings, Rijkswaterstaat, Shell, Sif, TU Delft, Van Oord and Vattenvall/Nuon) is to increase our understanding of the mechanisms that affect the lifespan. Eventually, we hope to transpose our project onto the practical situation, so that we can optimise the design, manufacture and maintenance of the foundations used for offshore wind turbines.’ The project is funded with a ‘Top Sector Energy Grant’ from the Ministry for Economic Affairs.
“In this way, we can use simpler, less conservative designs to combat fatigue and corrosion”
Simpler designs to combat fatigue and corrosion
The three-year project started in mid-2019, and preparations for the first experiments are now in full swing. ‘We are initiator and project manager of the C-FLO project on behalf of TNO, and we also provide substantive knowledge, particularly regarding the constructional side,’ Pijpers says.
‘The aim is to use models to estimate the residual lifespan, and to compile strength curves. You can then use the models to calculate various design and maintenance scenarios. In addition, it will be easier to modify design guidelines in line with specific situations, enabling us to use simpler, less conservative designs to combat fatigue and corrosion.’ In other words, he is keen to stress, ‘from fundamental knowledge to practical application’.
Knowledge useful for other applications
A positive side-effect of C-FLO is that in future, the project can be used as a basis for understanding mechanisms relating to other types of constructions, including dam walls and engineering structures on waterways such as locks and maritime structures.