The increasing demand for renewable energy sources means more offshore wind turbines. Currently, steel monopiles are the most preferred foundation type for supporting wind turbines and comprise about 70% of the newly commissioned wind turbines on European shores. Monopiles are generally large-scale structures that require efficient strategies to achieve the required penetration depth in the seabed. During a typical monopile installation, hydraulic impact hammers are commonly used to achieve the penetration depth by administering thousands of blows. Each blow generates a comprehensive stress pulse that penetrates downward to provide enough energy to overcome soil resistance. Considering the potential detrimental effects of the stress and accelerations generated during monopile installation, it is imperative to monitor the installation processes and associated impacts. For instance, the measured data can be used to estimate the bearing capacity and consumed fatigue life of the support structure.
Notably, various monitoring methods like contact sensors have several limitations that make them unsuitable for monitoring pile installation. Currently, the research on reliable non-contact measurement techniques has intensified. However, the existing classes of non-contact methods, such as optical techniques, are not suitable for application in systems subjected to dynamic loads and extreme marine environments. It is well known that steel structures are weakly magnetized, and their magnetization is sensitive to plastic and elastic deformation. As the generated magnetic field pervades the surrounding areas, magnetic field measurement provides a non-contact approach for determining the deformation of the structure. This principle is a promising approach for developing high-performance non-contact methods based on magenetomechanical effects. Unfortunately, most of the existing in-situ experimental research on the magnetomechanical effect are based on quasi-static loading conditions that are incomparable to dynamic loadings that characterize pile installation.
To overcome the above shortcomings, Peter Meijers (Postdoctoral researcher), assistant Professor Apostolos Tsouvalas and Professor Andrei Metrikine from the Delft University of Technology developed a different non-contact method for in-situ measurements of the magnetomechanical response of steel monopile during impact pile driving. This new method takes advantage of the resulting magnetomechanical effects owing to the ferromagnetic nature of the steel structures. The impact-induced changes in the magnetic stray field of the ferromagnetic structure were compared to the measured strain to analyze the effect under dynamic loading conditions. The authors demonstrated how the measured magnetomechanical response under dynamic loadings could be used to develop a feasible and reliable non-contact technique for deducing the impact-induced deformations. Their work is currently published in the research journal Engineering Structures.
The research team showed that the measured stray field strongly correlates with the simultaneously measured strains in terms of the amplitude ratio and frequency content for the hydraulic hammers blows. Attaching the magnetometers used to collect the stray field data to the hammer’s sleeve maintained the relative position of the sensors throughout the installation process. Moreover, the maximum deviation from the stray field and the peak strain exhibited a linear relationship. Furthermore, the proposed non-contact method produced inferred strain signals that satisfactorily corresponded to the measured strains, suggesting its feasibility and practicability.
In summary, the authors conducted a full-scale in-situ measurement campaign during an onshore monopile installation. They provided a unique data set containing the magnetomechanical response of a steel structure subjected to high impact dynamic loading. Based on the collected data, the ability of the newly proposed non-contact method to elucidate the impact-induced changes was successfully validated. Thus, the present method can be used as a promising alternative to overcome the limitation of conventional measurement devices. In a statement to Advances in Engineering, first author, Peter Meijers explained their study findings will enable continuous monitoring of impact-induced strains during the installation of large-scale steel monopiles.
Meijers, P.C., Tsouvalas, A., & Metrikine, A.V. (2021). Magnetomechanical response of a steel monopile during impact pile driving. Engineering Structures, 240, 112340.