ESR 8

The first larger offshore wind farms approach the end of their design lifetime, which is typically 20 years. For these offshore wind turbines, the decision on lifetime extension will soon become significant, as it is an appealing option to increase return on investment. Lifetime extension describes the continued operation of the asset beyond the end of their design lifetime. Lifetime extension is possible if the system holds a remaining useful lifetime beyond the original lifetime anticipated during design. A major challenge is lacking industrial experience regarding prediction of lifetime performance under various operation and maintenance scenarios. Therefore, industry and academics give increasingly more attention to assessments of remaining useful lifetimes and decision support concerning end-of-life scenarios for offshore wind turbines.

Monopile support structures for offshore wind turbines are the focus of this project. The main research objectives are:

• Implementation and verification of a low cost remaining useful lifetime assessment of offshore wind turbine support structures. Information from design, monitoring, and inspections are combined to set up and update predictions of remaining useful lifetime.
• Development of a decision model for lifetime extension of offshore wind turbines based on remaining useful lifetime estimates. The decision model is updated with information from inspections and monitoring.

Approaches to structural reassessment are analysed for their suitability regarding lifetime extension. In detail, (I) analytical assessment through renewed lifetime calculation using aero-hydro-elastic simulation tools, (II) vibration-based load monitoring, and (III) practical assessment through inspections and fatigue crack growth analysis are compared.

Academic I

Norwegian University of Science and Technology (NTNU)
Performed: January – July 2016

Academic II

Cranfield University
Performed: March – July 2017

• Ziegler, L, Smolka, U, Cosack, N and Muskulus, M. (2017). Brief communication: Structural monitoring for lifetime extension of offshore wind monopiles: can strain measurements at one level tell us everything?. Wind Energy. Sci., 2, 469-476, https://doi.org/10.5194/wes-2-469-2017, 2017.

• Ziegler, L, Gonzalez, E, Rubert, T, Smolka, U and Melero, JJ (2018). Lifetime extension of onshore wind turbines: A review covering Germany, Spain, Denmark, and the UK. Renewable and Sustainable Energy Reviews, 82(1), 1261-1271, https://doi.org/10.1016/j.rser.2017.09.100.

• Ziegler, L, Voormeeren, S, Schafhirt, S and Muskulus, M (2016). Design clustering of offshore wind turbines using probabilistic fatigue load estimation. Renewable Energy, 91, 425-433. DOI: http://dx.doi.org/10.1016/j.renene.2016.01.033 Link: http://www.sciencedirect.com/science/article/pii/S0960148116300337

• Ziegler, L and Muskulus, M (2016). Fatigue reassessment for lifetime extension of offshore wind monopile substructures, Journal of Physics: Conference Series, 753 (092010). IOP Publishing. DOI: 10.1088/1742-6596/753/9/092010 Full text: http://iopscience.iop.org/article/10.1088/1742-6596/753/9/092010

• Ziegler, L, Schafhirt, S, Scheu, M, & Muskulus, M (2016). Effect of Load Sequence and Weather Seasonality on Fatigue Crack Growth for Monopile-based Offshore Wind Turbines. Energy Procedia, 94, 115-123. DOI: http://dx.doi.org/10.1016/j.egypro.2016.09.204 Link: http://www.sciencedirect.com/science/article/pii/S1876610216308736

• Ziegler, L, Voormeeren, S, Schafhirt, S and Muskulus, M (2015). Sensitivity of wave fatigue loads on offshore wind turbines under varying site conditions. Energy Procedia, 80, 193-200, https://doi.org/10.1016/j.egypro.2015.11.422.

• Ziegler L and Muskulus M (2016). Lifetime extension of offshore wind monopiles: Assessment process and relevance of fatigue crack inspection. 12th EAWE PhD Seminar on Wind Energy in Europe, 25-27 May 2016, DTU Lyngby, Denmark: Download full text

• Ziegler, L and Muskulus, M (2016). Comparing a fracture mechanics model to the SN-curve approach for jacket-supported offshore wind turbines: Challenges and opportunities for lifetime prediction. Proceedings of the ASME International Conference on Offshore Mechanics and Arctic Engineering, Volume 6: Ocean Space Utilization; Ocean Renewable Energy. https://doi.org/10.1115/OMAE2016-54915.

• Ziegler, L (2016). Lifetime extension of offshore wind monopiles. INORE 10th European Symposium, Nantes, France. Download Full Text

• Ziegler, L, Smolka, U and Muskulus, M (2017). Strategies for low-cost assessment of lifetime extension of offshore wind monopiles. 13th EAWE PhD Seminar on Wind Energy in Europe, 20-22 September 2017, Cranfield University, UK Download Full Text

• Ziegler, L, Lange, J, Smolka, U, & Muskulus, M (2016). Repowering and life extension: when does it make sense to switch? WindEurope Summit 2016, Hamburg, Germany. https://windeurope.org/summit2016/conference/proceedings/index2.php