Validation of virtual sensing for the fatigue assessment of subsoil and submerged components of offshore wind turbines

Offshore wind energy is a young industry which continuously grows in importance for the success of the energy revolution. Currently the first generation of offshore wind turbines (OWT) reaches design life raising questions about structural reserves of the turbines for potential life time extensions. Designs of OWTs are driven by the fatigue damage accumulation on critical locations of the turbine substructure. As fatigue is influenced by the combined loading of wind, wave and the first dynamic modes, actual fatigue progress will deviate from assumptions in design. Monitoring strategies enable to learn about the actual fatigue damage accumulation on the dry part of the turbine structure by sensor measurements. However,to draw conclusions from fatigue progress to the remaining useful lifetime critical locations need to be assessed. As critical locations are usually found in welds of the submerged or even sub-soil part of the structure, methods are required to estimate fatigue instead of measuring it directly i.e. virtual sensing. This research starts with an introduction of different virtual sensing techniques before choosing Modal Decomposition and Expansion (MDE) for fatigue estimation on the most common offshore wind substructure types. First, MDE obtains modal amplitudes from acceleration and strain measurements. In the subsequent expansion step these amplitudes are expanded to virtual measurements at arbitrary locations. The algorithm uses a reduced order model that can be obtained from either a FE model or measurements. In this work, operational modal analysis is applied to obtain the required stress and deflection shapes for optimal validation of the method. Furthermore, the measurements that are used as input for the algorithm are constrained to measurements from the dry part of the substructure. Within a proof of concept based on numerical simulations, MDE is applied to the submerged welds of the OC4 jacket design. While limitations of the method are found regarding brace welds due to local dynamics, MDE is successfully applied to the leg welds of the jacket. The most common foundation type in offshore wind is the monopile. This research validates MDE on subsoil welds of a monopile foundation. After reconstructing strain histories in arbitrary locations on the substructure, fatigue assessment over various environmental and operational conditions is carried out. The technique is found capable of estimating fatigue with high precision for locations above and below seabed. In contrast to the application to jackets, analysis on monopiles is based on a dedicated measurement campaign at Nobelwind farm in the Belgian North Sea. Therefore, also challenges and solutions arising from working with OWT measurement data are described.