Increasing demand for energy from renewable sources has resulted in ambitious plans to construct a large number of offshore wind farms in the coming years. In relatively shallow water depths, the preferred support structure for wind turbines is the steel monopile, which is a thin-walled cylindrical structure. To decrease the cost of the generated electricity, larger wind power generators are commissioned, which has led to a significant increase of the size of the foundation piles. Currently, monopiles are most frequently driven into the seabed by means of hydraulic impact hammering. Aided by the compressive stress wave generated by each hammer blow, the pile gradually progresses to the desired penetration depth. The stress generated by each hammer blow can inflict plastic deformations at the pile head, which can jeopardise the delicate alignment required for the bolted connection between the superstructure and the monopile. Furthermore, the repeated elastic deformation of the pile leads to material fatigue, which reduces the service life of the structure. Hence, monitoring the deformation and stress resulting from the hammer blows is vital to assess the structural health. Offshore, however, dedicated sensors are seldom employed, due to time constraints and the harsh marine environment. In addition, contact sensors can easily be damaged by hammer-induced high-amplitude strains. To this end, this thesis develops several alternative methods to monitor the deformation in a monopile during installation. These methods are non-collocated (i.e. a quantity is measured at certain location to infer the structural quantity of interest at another position), and, preferably, non-contact. By considering the propagation of elasto-plastic waves, a non-collocated method to quantify the amount of plastic deformation inflicted by a hammer blow is first proposed. As a part of the energy contained in the stress wave excited by the hammer blow is used to permanently deform the structure, the stress wave becomes distorted. At a certain distance below the pile head, the energy flux is determined that is carried out by the stress wave through a cross-section of the pile. The difference between the measured value and the expected energy flux from a linear-elastic simulation with the same hammer forcing provides an upper bound for the amount plastic deformation inflicted by a hammer blow. The main benefit of this proposed method is that the sensors are employed outside the region where the highest strains occurs, reducing the risk of damaging the sensors. However, data is collected with sensors which are attached to the pile, leaving the aforementioned restrictions to the sensor deployment in place. To enable the widespread monitoring of steel structures subjected to dynamics loads, non-contact methods are needed. For the development of a non-contact method to infer the hammer-induced deformations, the magnetic stray field of the steel structure is analysed, which permeates the space around it. As the structure's magnetisation depends on elastic and plastic strains through the magnetomechanical effect, it is expected that the magnetic stray field, which is generated by the magnetisation, conveys the information about the present strain state of the structure to the sensor. Contrary to experiments on the magnetomechanical response of structural steel reported in literature so far, a steel cylinder has a significant demagnetising field due to its geometry, creating a non-uniform spatial distribution of the magnetisation. Additionally, magnetomechanical data under dynamic loads are scarce. Hence, a unique laboratory-scale experiment was designed, in which a steel cylinder was repeatedly impacted by a free-falling concrete mass, providing the first insights into the magnetomechanical effect in dynamically-loaded structures with a substantial demagnetising field. In between impacts, the magnetic stray field was mapped to analyse the evolution of the remanent stray field, i.e. the stray field when the structure is unloaded. Due to repeated impacts which only generate elastic strains in the structure, the remanent stray field evolves towards a metastable magnetic equilibrium. When a new peak strain is introduced, the stray field converges towards a new equilibrium, displaying a tendency towards a global magnetic equilibrium. However, as soon as plastic deformation forms, the evolution of the remanent field deviates from this trend as a result of the increased dislocation density, which, in turn, reduces the material's ability to remain magnetised. This behaviour serves as a basis for a non-contact method to detect and localise regions of plastic deformation in a steel structure subjected to repeated impact loads. This novel method is the first non-contact technique to infer structural deformation proposed in this dissertation. In the lab-scale experiment, strain gauges and a magnetometer registered the transient magnetomechanical response during each impact. When the magnetisation is at a magnetic equilibrium, a strong correlation is found between the axial strain and the magnetic field variation around the remanent state. The amplitude and direction of the transient magnetic stray field varies with the circumferential position of the magnetometer, indicating that the response is partly determined by the magnetisation in the vicinity of the sensor. To simulate the measured response, an isotropic magnetomechanical model was developed in this thesis that, for the first time, accounts for the demagnetising field of the structure. The capability of this model to reproduce the measurement results are limited though. It is envisaged that the model may be improved by accounting for anisotropy and by including the remanent magnetisation. To date, limited data have been published on the in-situ magnetomechanical response of large-scale steel structures in a weak ambient magnetic field. Consequently, an in-situ measurement campaign was performed to measure the magnetomechanical response of a monopile installed onshore with a hydraulic impact hammer. During the campaign, several magnetometers were employed using different sensor configurations. Similar to the lab-scale experiment, the position of the magnetometer relative to the pile determines the amplitude and direction of the transient magnetic field. Next to a good correspondence between the strain and magnetic signals, a polynomial relation was found between the peak strain and the maximum deviation from the remanent field expressed along the major principle axis. Using the inverse of this relation and a magnetometer which retains its position with respect to the pile, a novel method to infer the elastic strain from the transient stray field is proposed, which shows a promising correspondence between the inferred and measured strain signals. Additionally, the working principles for a new alternative technique to monitor the pile penetration using non-contact sensors are proposed. For each of the four non-collocated methods introduced in this work, directions for improvements and steps to generalise the techniques are discussed. The main benefit of the non-contacts methods in particular is that they eliminate the onerous process of attaching the sensors, enabling swift deployment and providing the opportunity to reuse the sensors. Although the new methods in this dissertation have mainly been applied to the installation of monopiles, the potential application of these non-collocated methods is much wider. Ultimately, they could be used to monitor any large-scale steel structure subjected to dynamic loads. @en

Current offshore wind turbine installation and positioning methods require mechanical equipment attached on the lifted components and human intervention. The present paper studies the development of a contactless motion compensation technique by investigating a magnetically controlled pendulum. The technique involves the interaction of a magnetic pendulum with an electromagnetic actuator. Two control modes are considered: the imposition of a desired motion to the mass and the motion attenuation of a prescribed pivot excitation. The numerical model is validated and calibrated against experiments and demonstrates excellent predictive capabilities. The control exerted is effective for a broad range of excitation frequencies and amplitudes. Important parameters associated with the performance of the technique such as the separation distance of the magnets and the saturation of the controller are identified. The controllability regions for effective control depending on the characteristics of the excitation are derived. The force amplitude of the contactless actuator is comparable to currently-used active tugger line control systems, but with the additional advantage of both attractive and repulsive forces. The findings of this paper illuminate the path for the further development of a non-contact control technique which has the potential to increase the efficiency of offshore wind installations.@en

This paper reports on a measurement campaign in which the magnetomechanical response of a steel monopile is recorded during installation with a hydraulic impact hammer. By comparing impact-induced changes in the magnetic stray field of the structure to the measured strain, this effect is analysed for the first time under dynamic loading conditions on such a large scale. It is shown that the measured stray field displays an excellent correspondence with the strain in terms of frequency content and amplitude ratio for hammer blows that induce compressive strain pulses of different magnitude. Using the data, a non-contact method is developed and validated to infer the hammer-induced strains using the dynamic magnetic stray field. The proposed method can be applied during pile installations when the use of conventional strain measurement devices is challenging, e.g. in the offshore environment.@en

The use of bolted connections between the tower and a support structure of an offshore wind turbine has created the need for a method to detect whether a monopile foundation plastically deforms during the installation procedure. Small permanent deformations are undesirable, not only because they can accelerate fatigue of the structure; but also because they can lead to misalignment between the tower and the foundation. Since direct measurements at the pile head are difficult to perform, a method based on non-collocated strain measurements is highly desirable. This paper proposes such a method. First, a physically non-linear one-dimensional model is proposed, which accounts for wave dispersion, effects that are relevant for large-diameter piles currently used by the industry. The proposed model, combined with an energy balance principle, gives an upper bound for the amount of plastic deformation caused by an impact load based on simple strain measurements. This is verified by a lab-scale experiment with a uni-axial stress state. Second, measurement data collected during pile driving of a large-diameter pile show that the proposed one-dimensional model, while able to predict the peak stresses, fails to accurately predict the full time history of the measured stress state. In contrast, an advanced model based on shell membrane theory is able to do that, showing that a bi-axial stress state is needed for these type of structures. This requires an extension of the theory for plasticity quantification presented in this paper.@en

To study the irreversible changes in the magnetic stray field surrounding a steel structure caused by impact-induced elastic and plastic deformations, a steel cylinder was repeatedly subjected to axial impacts of various magnitudes. Due to impacts that induce elastic deformation, the measured magnetic stray field of the structure converges to a global magnetic equilibrium. However, as soon as plastic deformation develops, a deviation from this trend is observed. From the spatial distribution of the stray field, the location of the plastic deformation is determined. Subsequently, the underlying processes of the measured evolution of the stray field are discussed and successfully incorporated into an elementary model of the structure's magnetisation to simulate the results from the experiment. It is expected that the reported observation is useful for a class of engineering applications in which non-contact and non-collocated measurements can be utilised to identify structural damage under dynamic loading.@en

Current methods to infer the penetration of a steel monopile during an offshore installation are rather inaccurate. Since a large number of foundation piles will be installed offshore in the coming years, a reliable technique to infer the penetration depth is vital. This paper proposes a non-contact method to monitor the pile progression into the seabed based on measurements of the magnetic stray field that permeates the air surrounding the structure, eliminating the necessity of a predefined pattern on the pile’s surface. A simple magnetisation model for the monopile is proposed from which the relative motion between the moving pile and a stationary magnetic field sensor can be extracted. Comparison between the measured and simulated stray field data show a promising correlation, providing the basis for the new non-contact monitoring technique that is applicable offshore. @en

Due to the increasing need for energy from renewable resources, a large number of offshore wind farms are planned to be constructed in the near future. Despite the plethora of available foundation concepts for offshore wind turbines, the monopile foundation is the most widely adopted concept in practice. To predict the installation process for a monopile a so-called drivability study is performed. Such a study allows one to decide on a number of key parameters for the installation process, such as, the appropriate size of the hydraulic hammer, the number of hammer blows and energy input needed to reach the final penetration depth, and the induced stresses in the system. The latter is important for the prediction of the fatigue life of the pile. Currently, drivability studies are based on one-dimensional wave equation models as first proposed by Smith in the 1950s. These models are valid as long as the diameter of the pile is small compared to the excited wavelengths in the structure due to the hammer impact. For large-diameter monopiles that are currently being used in the offshore wind industry, the latter condition is not met and the effect of stress wave dispersion can no longer be neglected. In this paper the classical wave equation model is amended by an extra term which accounts for the lateral inertia of the cross-section, resulting in the so-called Rayleigh-Love rod theory. With this new model, a parametric study is performed in which the effect of stress wave dispersion on the induced stresses and the number of hammer blows needed to reach the final penetration depth are assessed. A comparison with the results obtained from the classical model is also included in order to define the applicability range of the models. It is shown that the effect of stress wave dispersion can not be neglected for a drivability study of large-diameter monopiles. @en

The present study introduces a modified version of PD Control for the case of a magnetically controlled pendulum. The response was observed in both experimental and numerical simulations taking into consideration the non-linearity posed by the system. The modified PD controller was compared to the simple counterpart for further concrete justification of its superiority. The results attained highlight the benefits of the modified PD control in all facets of control performance, namely the efficiency, the accuracy of the representation of the interaction, the sensitivity on alterations of the control gains as well as the prediction of the experimental response by the numerical simulation. Thus, the control method proposed can serve as a promising foundation for the further development of a non-contact position control technique for offshore wind turbine installation purposes. @en

An experiment has been performed on a magnetic pendulum, which interacts with an electromagnet. The free non-linear vibrations of the pendulum-magnet system are studied to identify and analyse the system’s characteristics. Due to the presence of the electromagnet, a modulation of the pendulum’s natural frequency is observed. A mathematical model is formulated that is able to reproduce the experimental results.@en

Recent developments in the construction of offshore wind turbines have created the need for a method to detect whether a monopile foundation is plastically deformed during the installation procedure. Since measurements at the pile head are difficult to perform, a method based on measurements at a certain distance below the pile head is proposed in this work for quantification of the amount of plasticity. By considering a onedimensional rod model with an elastic-perfectly plastic constitutive relation, it is shown that the occurrence of plastic deformation caused by an impact load can be detected from these measurements. Furthermore, this plastic deformation can be quantified by the same measurement with the help of an energy balance. The effectiveness of the proposed method is demonstrated via a numerical example.@en