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.

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The increase of demand for offshore wind energy resulted in an increase of larger wind turbines. These turbines are often placed on top of a monopile (MP) substructure. The conventional method for installing these MPs uses a jack-up vessel. However, due to the local bathymetry and soil conditions. This method has its drawbacks, such as unavailability due to soil conditions, impracticality in excessively deep waters, or high costs. Heerema Marine Contractors (HMC) aims for the novelty of installing these MPs by using semi-submersible crane vessels (SSCV). Installing the MPs with a floating vessel is an attractive alternative due to its accessibility and speed. To counteract the extra motions of the floating vessel a Motion Compensated Gripper Frame (MCGF) is utilized. This gripper frame uses eight rollerboxes (RBs) equipped with polyurethane (PU) rollers to keep the MP in position, being able to install MPs from 7 up to 12.5 meters in diameter.

During pile driving, vibrations will propagate through the MP, inducing vibrations within the RB. Existing pile driving models fall short in accurately describing the behavior of large diameter MPs. In large diameter MPs the effect of the so called ‘breathing’ of the MP is discarded as the radial coupling is neglected. In addition, the material properties of the PU rollers are undefined or uncertain.

This thesis investigates the coupling between the MP and RB vibrations by using a numerical model derived with the Finite Difference Method (FDM). Three models were proposed: (1) the MP model, (2) the RB model and (3) the coupled model. Where the latter is coupled by incorporating a spring-dashpot system to simulate the behavior of the linearized PU rollers. The MP is considered behave axisymmetrically, simplifying the 3D wave equations to a 2D system of equations. The resulting motions from these models are directly compared to the motions obtained from a Finite Element Method (FEM) simulation in Abaqus. The MP model, in particular, exhibits good agreement with the Abaqus model. The RB model and coupled model predict higher accelerations than their FEM counterparts, aligning with expectations as the FDM model is computed by the 1D Euler-Lagrange beam equation, directing all stresses and forces directly into bending of the beam.

Analysis of the dynamic stiffness in both the MP and RB reveals that the RB is vulnerable to excessive vibration when the roller stiffness aligns in such a way that the eigenfrequency of the RB intersects with the ring frequency of the MP. This susceptibility is particularly notable in the case of small-diameter MPs (7-7.7m), where the roller stiffness that causes excessive vibrations, is close to the assumed base value of 200 kN/mm. In the category of large diameter MPs, a roller stiffness exceeding 1650 kN/mm could produce a similar effect. As the roller stiffness is a critical parameter for the response of the RB, it is important to know its value during the design phase. To prevent or mitigate excessive vibrations, it is essential to choose a roller stiffness that avoids eigenfrequency intersections between the RB and the MP. This proactive step is vital for optimizing the performance and stability of the MCGF during pile driving operations.

It is crucial to acknowledge that this thesis employs a simplified hammer input force for a 12.5m diameter MP, showcasing predominantly low frequencies. In reality, higher frequencies occur, which could result in more energy density, a significant consideration given that smaller diameter MPs result in higher ring frequencies. For future analyses, determining specific hammer forces corresponding to different diameters is therefore recommended.

Furthermore, the comparison between the industry practice for calculating resulting stresses in the RB and those derived from the numerical model indicates higher stresses in the industry approach. However, it is noteworthy that the predominant contribution to maximum stresses originates from the prestress on the MP, constituting 93% of the total stress. As a consequence, the additional stresses attributed to accelerations are relatively negligible in comparison.

The increase in demand for renewable energy has resulted in higher demand for wind energy. To meet this requirement, the wind turbine sizes are increasing rapidly, and this results in increasing load on the connections between MP-TP and the segments of the tower.
This research focuses on traditional L-flange connection and novel C1 wedge connection, former the most widely used in the offshore industry and other being new to the offshore industry. The objective of this thesis is to evaluate the limits of the L-flange and C1 wedge connection. The design of the L-flange connection carried out on the basis of Petersen’s theory[1]. It is designed to have higher ULS resistance with steel mass as low as possible. This analytical designed is then compared to finite element analysis (FEA). Previous study of Cheng[2] is used to validate the setup and methodology for FEM in Ansys.
For the same overturning moment, C1 wedge connection is designed using the design tool provided by C1 Connections. The design check for flanges is carried out. The design is then compared to finite element analysis (FEA). Fatigue limit state was verified for both the connections.
Based on the study, the following conclusions are made. Firstly, L-flange connection has around 30% higher ULS resistance compared to design overturning moment of 609MNm whereas C1 wedge connection has 55% higher ULS resistance. Secondly, L-flange connection has higher meridional deformation at the same elevation in the shell as compared to C1 wedge connection. The gap opening at the interface of the flanges is studied and it is observed that C1 wedge connection opens after the loss of contact force at the interface which is generated more efficiently through the pretension of stud whereas L-flange connection being an eccentric connection starts opening without full loss of contact force at the interface and the secondary path of load transfer is activated. This results in the lower meridional rigidity provided by the L-flange connection as compared to the C1 wedge connection. Lastly, C1 wedge connection provided an opportunity for several optimizations to have the same ULS resistance as that of L-flange connection. The mass of steel reduced for these optimizations. The L-flange connection weighed 53.72 tonnes for both ULS and FLS criterion whereas C1 wedge connection weights 20.8 tonnes for ULS criterion and 28.8 tonnes for FLS criterion.

To overcome the problem of wave-induced vessel motions on the crane tip, technologies have been developed to compensate for these motions. Most vessels nowadays, are equipped with dynamic positioning systems, these systems reduce the yaw, surge and sway movement of the vessel. This leaves, the motions roll, pitch and heave uncompensated. To reduce these motions, two main elements of motion compensation can be identified, consisting of heave compensation and anti load swing compensation. Existing technologies, like active and passive heave compensation tackle one of these elements. Other methods like the Stewart platform can compensate for both the heave motion and the load swing of the crane by stabilizing a platform on which the crane is mounted. In existing literature, not much work has been done to incorporate both the heave compensation and anti-swing control procedures in a combined control method. It would be much simpler if the crane could compensate the heave and payload swing by using only the crane's actuators.

This thesis, studies the mechanical feasibility of a motion compensated knuckle boom crane, compensating for the wave-induced vessel motions by controlling the available actuators in such a manner, that the crane tip stays stationary. A typical knuckle boom crane has three actuated degrees of freedom, consisting of the slewing, boom luffing and jib luffing angle. To study such a system, a numerical model is developed that combines the multi body dynamics of the knuckle boom crane with models for the hydraulic actuation of the crane and incorporates the full six degree of freedom vessel motions. The actuated degrees of freedom are controlled using three separate proportional integral controllers. The desired trajectories for the controllers are calculated using the geometric relations of the crane, based on the assumption that the desired crane tip coordinate in the global axis system is known.

Using the numerical model, insight is gained in the natural frequencies, forces/moments on the system, the necessary hydraulic properties and motion compensation performance. From the simulation results, it can be concluded that the cylinder forces and slewing torques increase when the motion compensation is turned on, compared to an uncompensated crane. Comparing the simulation results, to the Barge Master T40 crane, it can be concluded that cylinder forces, cylinder sizes, stroke velocities and power requirements are in line with requirements that can be achieved with existing technology. The main limitation, is expected to be the rated torque of the slewing motor, when the crane is positioned parallel to the vessel. In this position the main contributor to the motion compensation is the slewing motor. Using the proposed concept, motion compensation performance comparable to the existing Barge Master T40 is achieved. From the presented simulation scenarios, results indicate that the operational parameters and motion compensation performance of the system are feasible and competitive compared with existing technology.

The need for sustainable solutions in the energy sector has led to the rapid increase of offshore windfarms around the globe. The areas with ideal soil conditions are gradually being occupied, pushing the offshore wind industry to search for methods of installing wind turbines in hard terrain. So far, the installation of monopiles in weak rock, if successful, required the use of additional casings and grouting. In this thesis project, an alternative installation method is investigated, in which the monopile itself is used as casing, avoiding the waste of extra material.

The ‘drill-drive’ installation method under investigation consists of an initial drill-out of the rock from the inside of the pile and its subsequent driving into the seabed with a hydraulic hammer. No casing is used and a cutting toe is equipped in the bottom circumference of the pile, being responsible for breaking through the rock.

Through finite element analysis, the structural integrity of both the monopile and the cutting toe during the drill-drive process is assessed. A driveability analysis is conducted for a seabed of different types of rock and the first conclusions on the feasibility of this novel installation method are drawn.

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

Pre-piling templates are often used in the offshore industry to ensure the vertical installation of foundation piles for jacket structures. Boskalis is currently preparing a project that uses these templates in the Taiwan Strait, which is a region known for its relatively soft soils and seismic activity. Consequently, the piles are very long in order to enable sufficient bearing capacity. In earlier work, an Orcaflex model was developed to predict the dynamic behavior of 90 meter long piles stabbed in the template with 5 meter penetration into the seabed, which resembles the situation prior to pile driving. The obtained results indicated excessive loads on the template due to large swaying motions of the piles. This problem is caused by resonance: the natural swaying frequency of the long piles interferes with significant wave energy. In this thesis, this model is further developed and improved by identifying and addressing the following three shortcomings.

First, the model does not account for wave diffraction effects at the piles. A simplified model is developed to study the impact of wave diffraction on the pile swaying amplitude in the frequency domain using a theory by MacCamy and Fuchs. In order to model wave diffraction effects in Orcaflex, diffraction-corrected added mass coefficients are assigned to each pile node based on spectral density analyses of the acceleration of the waves relative to the pile. Second, in the adapted model, the effect of flow oscillation is not considered in the drag coefficient definition, which leads to inaccurate modelling of the drag loads. This deficiency is corrected by modelling the influence of the Keulegan-Carpenter number on the drag coefficient. To do so, the oscillation period parameter of the Keulegan-Carpenter number at each pile node is assumed based on spectral density analyses of the velocity of the waves relative to the pile. Drag coefficients are calculated from the Keulegan-Carpenter numbers and related to Reynolds numbers. This relation is used in Orcaflex. Third, the pile clamp modelling is simplified in the adapted model. A realistic clamp design is assumed to model the clamp more accurately. Based on the results of time-domain simulations, the maximum loads on the template decrease in short-wave sea states when diffraction is accounted for. Further, while model results with the revised drag coefficient show an increase in pile swaying velocities, the maximum loads on the template remain roughly identical. Yet, with the revised pile clamp, the model predicts significantly higher loads on the template. Overall, the results show a violation of the template design limits in several load cases. This implies the need for either redesigning the template or finding a work method solution to mitigate the pile swaying. The latter is pursued by modelling vessel wave shielding.

By turning the vessel in a certain heading relative to the dominant wave direction, one can shield the installation site from waves and reduce the loads on the piles. To model this, a hydrodynamic diffraction analysis is performed in Ansys Aqwa. Velocity potential data is obtained to describe the influence of the vessel on the waves near the piles, which is subsequently incorporated into the Orcaflex model. Now, the predicted loads on the template no longer exceed the design limits, which demonstrates that shielding is an effective strategy to ensure workability, provided that the vessel roll motion does not become excessive. Since roll can cause large crane tip motions that would complicate the operation, it is recommended to further study the vessel motions during shielding.

Pile driving can cause plastic deformation to the head of the driven pile, which in turn can lead to connectivity issues when the superstructure is installed. Moreover, plastic deformation reduces the estimated service life of the structure. Therefore, it is necessary to monitor the strain levels during the installation in order to detect if the yield limit is exceeded. Conventional methods to detect this, like strain gauges, can be hard to install and are prone to damage. Hence, alternative methods to monitor the onset of plastic deformation in the structure are needed.

In this thesis, a promising alternative is developed through a lab-scale experiment. The experiment consists of a scaled version of steel pile that is impacted axially by a falling concrete mass. With the measurements, a prototype for the monitoring method has been developed and successfully validated.