The offshore wind industry is rapidly expanding, featuring larger turbines in deeper waters and new geographical locations, leading to increased uncertainties. These developments pose significant design challenges for maintaining the simple and structural robust monopile structures, which are currently the most popular foundations for offshore wind turbines.
This study aims to investigate major contributors to fatigue on XXL monopiles supporting 15 MW and 22 MW turbines based on metocean conditions across different geographical locations. Furthermore, the impact of guyed monopiles has been analysed based on numerous water depths and soil parameters. These aspects have been largely unexplored in the existing literature.
The research uses a frequency domain monopile fatigue estimation method that integrates aerodynamic effects with hydrodynamic excitations. The method assumes a uniform wind profile and white noise wave spectrum to compute the stress response spectrum. By applying a linear correlation between the stress response spectrum and hydrodynamic excitation, the stress is determined over a wave scatter diagram, considering the joint probability of wind-wave conditions. The approach uses time series loads, computed by the aero-hydro-servo-elastic load analysis tool OpenFAST. Additionally, a dimension scaling reduction is used to reduce the mass of the monopile when incorporating the guyed lines.
The findings reveal that fatigue is dominated by scenarios lacking aerodynamic damping, such as wind-wave misalignment and idling, where directional spreading of metocean conditions has lower influence. Furthermore, fatigue damage is significantly affected by the positioning of the system’s natural frequency relative to the peak wave period. A noted limitation to the model is the exclusion of turbulent wind effects.
Regarding the guyed monopile analysis, the dimension reduction strategy shows a significant mass reduction in deeper waters. The stiffness of the system is determined by the tendon parameters, where the envelope of the natural frequency is larger in clay conditions than for sand conditions, and it increases for increasing water depth. Using a feasible tendon set-up shows higher fatigue damages at the critical location when compared with the conventional monopile fatigue damage. However, lower fatigue damages are found at other locations along the monopile length. Additionally, it is concluded that using stiff tendons results in a high risk of snap loads especially when creep of the tendon lines is considered. The results show potential for guyed monopile systems especially in deeper waters, reducing the mass, whilst maintaining similar fatigue damages as conventional monopiles. These results encourage the need for extra research on the topic of guyed monopile systems.

This research contributes to a novel non-contact installation method for offshore wind turbines, utilizing electromagnetism for positional control. Floating wind installation requires efficient control methods for payload positioning. Current methods use tuggerlines that are physically attached and work through tensile attractive forces. In contrast, electromagnetic control offers a non-contact solution and allows for control through both attractive and repulsive forces. This research explores the applicability of multiple data-driven control algorithms on a magnetically controlled pendulum system. These algorithms are evaluated for both positional control and motion attenuation under pivot-point excitation, using simulations of a numerical model of the electromagnetic pendulum. The model-free control algorithms considered in this study assumed that changes in the system’s state are solely due to the control output. These methods aimed to optimize the control output based on either the state error or estimates of the state gradient with respect to the control output. However, this assumption was found to be valid only for systems dominated by the control response with limited dynamic behaviour over time, which was not the case for the electromagnetic pendulum system.

The study culminates in the development of a controller based on the online estimation of the magnetic interaction force. This controller, named Proportional-Derivative Force estimating neural network controller (PD-FeNN), is a neural-network based state-dependent PD-controller. The neural network is trained during an operational learning phase on estimations of the magnetic interaction force, which are derived using the model of a linear undamped pendulum. By incorporating the neural network, this approach eliminates the requirement of the previous state-of-the-art controller to manual model the magnetic interaction force based on experimental data. The PD-FeNN controller is tested on both numerical and physical models of the electromagnetically controlled pendulum. The results demonstrate efficient control across a wide range of excitation frequencies and amplitudes for both motion attenuation and positional control. As a successor to the modified PD controller, the PD-FeNN controller improves upon its predecessor by enabling positional control without requiring a model of the magnetic interaction. This advancement enhances the applicability of non-contact motion control for payloads in the offshore wind industry.

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.

Wind turbine installation vessels (WTIVs) are ships that are specifically designed to install offshore wind turbines. These WTIVs have four or six large truss-like legs that are lowered towards the seabed by means of jacking systems. These jacking systems are regulated by control systems to ensure the leg is lowered with a constant velocity regardless of external disturbances. The bottom of these legs are outfitted with spudcans, which have a conical shape to allow for penetration into the seabed. When these spudcans have settled in the seabed, the hull is lifted above the waves. Consequently, wave-exciting forces on the hull are prevented and the motions of the hull are near-zero so that the on-board crane can perform the installation operations with minimised disturbances.

The objective of this thesis is to develop and analyse a model that is able to describe and simulate the dynamics of these jacking systems in great detail in response to external loads and the dynamics of the WTIV. The control systems of the jacking systems are included in this model to simulate and evaluate the interaction between the control system and the dynamics of the WTIV. Two conventional control systems are considered: the Volts-per-Hertz (V/Hz) and the direct torque control (DTC) method. In the process of lowering the legs and subsequent platform lifting, a transient phase can be identified during which the spudcans are penetrating the seabed. Due to the periodical motions of the ships, multiple impacts with the seabed are expected. Additionally, the jacking systems and the leg undergo a change of load direction as initially the leg is in tension and the jacking systems are generating power, and afterwards the leg is in compression and the jacking systems are consuming power. This thesis is focused on this seabed penetration phase as this phase introduces complicated dynamics. In literature, no model is available that has the abilities to simulate the WTIVs and its jacking systems with control systems in such level of detail.

This research gap is addressed by developing such a simulation model. This model is written in Python and developed using finite element (FE) techniques and solved using numerical time integration. Seabed characteristics are derived using a detailed coupled Eulerian-Lagrangian (CEL) FE models. Multiple control strategies are simulated and evaluated, each differentiating how the velocity and torque setpoints of the jacking systems are calculated. From the simulation model, it is found that in order to achieve load sharing between jacking systems, torque and velocity require to be independently controlled which only the DTC method has the ability to. Furthermore, each of the jacking systems should be provided with its own power supply. Best performance and stability was achieved when each chord of the leg is given a common torque and velocity setpoint, which is equivalent to a common torque and setpoint per leg in reality. Moreover, load sharing can be improved without a control system by increasing the relative stiffness ratio between the chord and the mechanical contact between rack and pinion.

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.

HeeremaMarine Contractors (HMC) owns and operates several semi-submersible crane vessels (SSCV) used for offshore heavy lift operations. Throughout the engineering phase of a (dual crane) heavy lift, dynamic lift models are generated by combining bodies with their hydrodynamic properties, inertia and spring-dashpot elements. The models represent the mass-spring system of a heavy lift over the different lift stages during a topside installation. These stages characterize the free floating, load transfer, free hanging and set down
phases.

Hook load fluctuations are governed by the relative vertical motion of the load and the crane boom tip. At the load transfer phase this motion is governed by both the motion of the vessel and the barge, whereas for the free hanging phase it is mostly effected by the motion of the vessel. To achieve safe and successful projects, an accurate prediction of the load and motion responses is essential while advising offshore personnel about the lift to perform. At the moment an inconsistency exists between predicted load fluctuations and offshore
crane measurements. This is the main reason for this research.

Until now a simplified spring-damper system is used to incorporate the hoist wire reeving system of a crane.This simplification is not fully justified and three goals are set up to model this in a more detailed manner. Firstly, the driving parameters of the load-crane-vessel system are assessed. Secondly the dynamic behavior of the wire reeving system is captured in a numerical model in Simulink. Finally, the dynamic load fluctuations of the model are compared with offshore measurements to both validate the model and analyze the results.

The dynamical model of the load-crane-vessel system is solved in the time domain and it consists of two parts. The first part considers the sheave and wire system of the crane. An equation of motion is derived for each sheave where dry friction is taken into account and leads to a stick-slip effect. This friction originates from the sheave bearings and from the bending friction of the steel wire rope that runs over it. When a load is raised, the stress in each rope part increases from the winch to the dead end. With a lowering operation the effect is the opposite. Due to stick-slip this force difference remains in the crane wires after the operation. The friction factors of the sheaves in the numerical model are tuned with steps observed in offshore load measurements during crane operations.

The second part of the model imposes the measured vessel motions to calculate the motion of both crane boom tips and the topside. This is performed by applying their influences as external forces on the free bodies. With the relativemotion the response of the hoist wire forces is determined. A coupling of these two parts can be made by removing the element of the hoist wire in the imposed motion model and replacing it by a pair of two nonlinear forces determined fromthe sheave wire model. These forces have an opposite sign and
are equal in absolutemagnitude and phase.

The effect of friction on dynamic hook load fluctuations is also consideredwith inputs of different amplitudes and frequencies. The hook load fluctuations at the measuring sheave are lower than applied fluctuations in the model when friction is taken into account. The simulated force at the measuring sheave is better represented with higher load fluctuations as the stick condition is exceeded earlier.