MAXWind

Introduction

• MAXWind – MAintenance, Inspection and EXploitation Optimization of Offshore Wind Farms subjected to Corrosion-Fatigue
• Number of partners: 4
• Coordinator: University of Ghent
• Project type: Fundamental research
• Start date: 01/03/2020
• Duration: 4 years
• Budget: 2.222.915 €
• Funded by the Energy Transition Fund (ETF)

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Partners

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Project Context

In April 2018, it was publicly announced that as part of the exit strategy from nuclear power, Belgium will double the area of its North Sea waters made available to offshore wind parks after 2020. After 2020, a new, 221-squarekilometre area near French waters is planned. This strategy clearly emphasizes the support of the Belgian government for wind energy as an important energy supplier for the future. In the near future there will be parks with different lifetime and this requires a sound lifetime assessment procedure to ensure the good functioning of the wind turbines and secure the energy supply. The steel support structures (jackets, monopiles) are subjected to both fatigue and corrosion, impacting their lifetime. There is a need for accurate lifetime assessment tools that can help offshore wind farm owners and operators to optimize the wind farm in the future lowering the Levelized Cost of Energy (LCoE) below 60€/MWh. This is of importance for the farms that are currently in operation and need an optimized maintenance strategy and possible extension of their exploitation, as well as for new projects which can also benefit from lifetime assessment tools in the design process.

Project description

Fatigue and corrosion are critical failure causes of offshore wind support structures. The low accessibility and high inspection/repair costs of large structures in corrosive environments is motivating remote monitoring and optimized inspection and maintenance plans, based on continuous assessment of the structure’s reliability.

Due to the time-variant uncertainties associated with corrosion, applied loads, stress prediction and modelling of the deterioration mechanisms, a more advanced lifecycle reliability assessment is necessary to assess the structural safety and to support decision making. The project aims to reduce the uncertainties by combining inspection and monitoring data (about cracks and corrosion), load measurements and estimates (stresses, wind, wave) and improved material property models.

This information can be employed to regularly update the failure probabilities and thus structural reliability. This also necessitates to have reliable models that can simulate the real behavior of the structure to predict qualitatively and quantitatively the degradations due to corrosion and fatigue. These numerical models will be calibrated with available and new field data (unique long‐term load measurements on offshore wind turbines).

The focus of MAXWind will be on monopile foundations, as these are currently the most used type of foundation in Belgian waters and there is a large amount of data available on these type of foundations. However, the established methodology can be extended to include other foundation types like jacket structures and floating foundations. 

Project objectives

The global goal is to get a much better estimation of the remaining life of in-service wind turbines and to derive optimized inspection and maintenance plans for a group of similar structures. This will make it possible not only to define the optimal inspection plan, leading to reduced maintenance costs while ensuring functionality and safety, but also to calibrate the safety factors of design standards attempting to enhance the fatigue analyses in the design stage.

The following (fundamental) knowledge gaps are identified and will be tackled in this proposal:

• What are the interactive effects of corrosion and fatigue load on crack initiation and propagation at welded joints?
• Can we develop a validated method to update the failure probabilities of structural details where fatigue occurs (welded joint) considering the collected environmental data and those obtained from inspection/maintenance (corrosion, cracks)?
• Can the methodology for a single welded joint be extended to the other non-monitored joints and to the entire structure?
• Can we extrapolate the knowledge learned from a few monitored/inspected OWT structuresto update the probability of failure of similar structures in the entire wind farm?
• How much cost reduction is expected once the optimal maintenance plan is defined?

The following project goals have been set:

• In-field measured data
• Numerical routines for analysis and filtering of load data
• Validated model for non-linear damage accumulation
• Integrated model for combination of corrosion and fatigue
• Methodology for integration of corrosion and crack size information in smart S-N curve
• Optimized virtual sensing methodology for one structure and extension of Fleet leader concept for structural differences
• Structural Reliability and Risk-based Maintenance Methodology

Impact in Belgium

To meet the objectives related to renewable energy, wind energy plays an important role. If wind energy has a large share in the total energy production, wind turbine parks will need to provide reliable energy and this independent of their lifetime. Currently, inspection, maintenance and repair actions induce large costs and it is to date unclear how this will evolve with higher lifetime. Costs directly related to corrosion are hard to assess, as damage results from the interplay of fatigue and corrosion. The NeSSIE project (North Sea Sollutions for Innovations for Corrosion in Energy), in which OWI-Lab was involved through Sirris, studied the economic impact of corrosion on marine structures, including off shore energy structures. From this study, it can be extrapolated that the cost of the corrosion repairs in splash zone for the Belgium wind farms is around 2.75m€ every year. Other indirect costs are even much higher, but very hard to assess. These are all included in O&M costs. Possible extension of exploitation might offer great benefits and cost reductions, on the basis that safety and energy supply can be guaranteed. The figure below illustrates the different stages of the lifecycle of offshore wind parks and indicates the focus points of the MAXWind project.

Dissemination of results

Open-access toolbox and dataset

• Toolbox for analysis and filtering of load data with fatigue life estimation in mind: https://github.com/OWI-Lab/py_fatigue
• Fatigue block loading experiment database: https://osf.io/6y5sd

Report

• Offshore corrosion monitoring campaign: download report here

Manuscripts in international journals

• M. Henkel, W. Weijtjens, and C. Devriendt, ‘Fatigue Stress Estimation for Submerged and Sub-Soil Welds of Offshore Wind Turbines on Monopiles Using Modal Expansion’. Energies, 14(22), 7576, 2021. https://doi.org/10.3390/en14227576 
  • Describes a numerical method for the reconstruction of strain and stress at different heights along the circumference of a monopile based on some discrete strain measurements at the monopile-tower interface. The approach has been validated based on monitoring data from a Belgian wind farm.
• Hectors K., De Waele W., An X-FEM based framework for 3D fatigue crack growth using a B-spline crack geometry description, Engineering Fracture Mechanics, Vol 261, 108238, 2022. https://doi.org/10.1016/j.engfracmech.2022.108238 
  • Numerical algorithms were developed in Python that allow the growth of a crack (weld defect) in a structure subjected to complex loading conditions to be integrated into finite element software.
• Sadeghi, N., D'Antuono, P., Noppe, N., Robbelein, K., Weijtjens, W. & Devriendt, C., Quantifying the effect of low-frequency fatigue dynamics on offshore wind turbine foundations: a comparative study, Wind Energy Science. 8, 12, p. 1839-1852, 2023. https://doi.org/10.5194/wes-8-1839-2023 
  • In this article, the impact of low-frequency dynamic events on the service life of operational wind turbines was quantified and their possible causes were identified.
• F de Nolasco Santos, P D'Antuono, K Robbelein, N Noppe, W Weijtjens, C Devriendt, Long-term fatigue estimation on offshore wind turbines interface loads through loss function physics-guided learning of neural networks, Renewable Energy, 205, 461-474, 2023. https://doi.org/10.1016/j.renene.2023.01.093 
  • Estimation of long-term fatigue damage evolution by training neural networks with embedded physical knowledge. The training was done with monitoring data (SCADA and accelerometers) obtained over 9 months in a Belgian wind farm.
• F. Mehri Sofiani, S. A. Elahi, S. Chaudhuri, K. Hectors, W. De Waele, Quantitative analysis of the correlation between geometric parameters of pits and stress concentration factors for a plate subject to uniaxial tensile stress, Theoretical and Applied Fracture Mechanics, Vol 127, 104081, 2023. https://doi.org/10.1016/j.tafmec.2023.104081 
  • Analytical equations were developed for the stress concentration factor of corrosion pits based on their normalized dimensions. For the first time, the critical regions in a corrosion pit (i.e., where a crack can develop) have also been identified.
• Hectors K, Vanspeybrouck D, Plets J, Bouckaert Q, De Waele W, Open-Access Experiment Dataset for Fatigue Damage Accumulation and Life Prediction Models. Metals, Vol13(3), 2023. https://doi.org/10.3390/met13030621 
  • A unique database was developed with results from hundreds of fatigue experiments with varying load amplitude, and a method was proposed to objectively assess models for the calculation of damage accumulation.
• Mishael J, Morato PG, and Rigo P, Numerical fatigue modeling and simulation of interacting surface cracks in offshore wind structural connections, Marine Structures, Vol 92, 1-17, 2023. https://doi.org/10.1016/j.marstruc.2023.103472 
  • Describes a system model for the reliability of an offshore structure subjected to fatigue, incorporating both the interaction and propagation of cracks.
• De Nolasco Santos, F., Noppe, N., Weijtjens, W. & Devriendt, C., Farm-wide interface fatigue loads estimation: A data-driven approach based on accelerometers, Wind Energy. 27, 4, p. 321-340, 2024. https://doi.org/10.1002/we.2888 
  • Investigated the use of fleetwide IoT accelerometers to estimate stress cycles across the entire fleet. Also investigated the added value of population-based training, in which a machine learning model is trained with data from a diverse and representative group of turbines.
• N Hlaing, P Morato, F de Nolasco Santos, W Weijtjens, C Devriendt, P Rigo, Farm-wide virtual load monitoring for offshore wind structures via Bayesian neural networks, Structural Health Monitoring, Vol. 23(3), pp. 1641-1663, 2024. https://doi.org/10.1177/14759217231186048  .
  • Bayesian neural networks have been developed and trained on the basis of extensively instrumented wind turbines (fleet leader). The models make it possible to predict the lifespan of other wind turbines in the park for which only SCADA data is available.
• F. Mehri Sofiani, J. Tacq, S.A. Elahi, S. Chaudhuri, W. De Waele, A hybrid probabilistic-deterministic framework for prediction of characteristic size of corrosion pits in low-carbon steel following long-term seawater exposure, Corrosion Science, Vol. 232, 112039, 2024. https://doi.org/10.1016/j.corsci.2024.112039 
  • A model has been developed to simulate the growth of corrosion pits by combining analytical descriptions of the corrosion process and probabilistic datasets of corrosion activity measured at the Blue Accelerator. The results have been validated using 3D scans of corroded coupons from monopiles of a Belgian wind farm.
• H. Saeed, R. Vancoillie, F. Mehri Sofiani, W. De Waele, Mode I stress intensity factor solutions for cracks emanating from a semi-ellipsoidal pit, Materials, Vol. 17(19), 4777. https://doi.org/10.3390/ma17194777 
  • Development of analytical formulas and a surrogate model for the stress intensity factor at the tip of physically short cracks in a corrosion pit. Based on this, crack growth and lifespan can be determined.

 

Conferences, workshops, symposia (presentation and manuscript)

• Giro F, Mishael J, Morato PG, and Rigo P, Inspection and Maintenance Planning for Offshore Wind Support Structures: Modelling Reliability and Inspection Costs at the System Level. Proceedings of the ASME 2022 41st Int. Conf. on Ocean, Offshore and Arctic Engineering (Hamburg, Germany), Vol 2, 2022. https://doi.org/10.1115/OMAE2022-78269
  • Comparison of system-based inspection and maintenance strategies versus component-based. The system-based ones are more effective at managing the risks of structural failure.
• N. Sadeghi, K. Robbelein, P. D’Antuono, N. Noppe, W. Weijtjens and C. Devriendt, ‘Fatigue damage calculation of offshore wind turbines' long-term data considering the low-frequency fatigue dynamics’, presented at TORQUE 2022 (Delft, Netherlands), published in Journal of Physics: Conference Series, Vol. 2265, 032063, 2022.  https://doi.org/10.1088/1742-6596/2265/3/032063
  • Description of the method for quantifying fatigue damage due to very low frequency stress reversals. The method has been validated using a dataset obtained after 3 years of monitoring a wind turbine in the North Sea.
• D’Antuono P., Weijtjens W., Devriendt C., On the Minimum Required Sampling Frequency for Reliable Fatigue Lifetime Estimation in Structural Health Monitoring. How Much is Enough?. Presented at European Workshop on Structural Health Monitoring. EWSHM 2022. Published in Lecture Notes in Civil Engineering, vol 253, 2023. https://doi.org/10.1007/978-3-031-07254-3_14
  • Quantification of the influence of measurement frequency when monitoring dynamic strain on lifetime calculations. It is shown that the ratio of the maximum frequency in the measuring signal to the measuring frequency must be greater than 10 to avoid an underestimation of the lifetime.
• F. Mehri Sofiani, S. A. Elahi, S. Chaudhuri, K. Hectors, W. De Waele, A numerical study on tensile stress concentration in ellipsoidal corrosion pits, presented at the International Conference on Structural Integrity and Durability ICSID 2022 (Dubrovnik, Croatia), published in Procedia Structural Integrity, Vol 50, pp. 51-56, 2023. doi 10.1016/j.prostr.2023.10.066 https://doi.org/10.1016/j.prostr.2023.10.066
  • Quantification of the increase in stress at corrosion pits in a steel component and identification of the most critical zones in the corrosion pits from which cracks can develop.
• S.A. Elahi, F. Mehri Sofiani, S. Chaudhuri, J.A. Balbín, N.O. Larrosa, W. De Waele, ‘Investigation of the effect of pitting corrosion on the fatigue strength degradation of structural steel using a short crack model’, presented at the International Conference on Structural Integrity and Durability ICSID 2022 (Dubrovnik, Croatia), published in Procedia Structural Integrity, Vol 51, pp. 30-36, 2023.  https://doi.org/10.1016/j.prostr.2023.10.063
  • Quantification of the reduction of the fatigue limit (permissible stress level to obtain a service life of 10 million cycles) of carbon steel exposed to an offshore environment.
• Mishael J, Morato PG, and Rigo P, Propagation of Interacting Cracks in Offshore Wind Welded Structures through Numerical Analysis, Proceedings of the 34th International Ocean and Polar Engineering Conference (Rhodes, Greece), 2024.  https://onepetro.org/ISOPEIOPEC/proceedings-pdf/ISOPE24/All-ISOPE24/ISOPE-I-24-442/3417264/isope-i-24-442.pdfhttps://onepetro.org/ISOPEIOPEC/proceedings-pdf/ISOPE24/All-ISOPE24/ISOPE-I-24-442/3417264/isope-i-24-442.pdf
  • Expansion of the structural reliability model of an offshore wind turbine on a monopile. The interaction of welding defects leads to accelerated crack growth and possible coalescence of individual cracks.

Conferences, workshops, symposia (without manuscript)

• S.A. Elahi, F. Mehri Sofiani, S. Chaudhuri, W. De Waele, Numerical study on the effect of pitting corrosion on the fatigue strength degradation of offshore wind turbine sub-structures using a short crack model, 18th EAWE PhD Seminar on Wind Energy (Bruges, Belgium), 2022.
• F. Mehri Sofiani, S. A. Elahi, S. Chaudhuri, W. De Waele, A numerical study on corrosion-fatigue in offshore wind turbine sub-structures, 18th EAWE PhD Seminar on Wind Energy (Bruges, Belgium), 2022.
• Mishael J., Giro F., Morato P.G., and Rigo P, System Structural Reliability Modelling for Offshore Wind Welded Connections, 18th EAWE PhD Seminar on Wind Energy (Bruges, Belgium), 2022.
• F. Mehri Sofiani, S. A. Elahi, S. Chaudhuri, W. De Waele, Pitting corrosion and its transition to crack in offshore wind turbine supporting structures, poster presentation at Faculty of Engineering and Architecture Research Symposium (FEARS 2022, Ghent, Belgium), 2022.
• S.A. Elahi, F. Mehri Sofiani, S. Chaudhuri, W. De Waele, Fatigue strength degradation of structural steel in sea environment due to pitting corrosion, poster presentation at Faculty of Engineering and Architecture Research Symposium (FEARS 2022, Ghent, Belgium), 2022.