With the increasing water depths of the new offshore wind farms, the challenging soil conditions, the availability of assets, and other factors, jack-up installation vessels may no longer be suitable to complete the installation scope of work for the new wind farms. Therefore, installation methods and techniques using floating vessels must be further developed to allow safe and efficient installation of wind turbines.

Due to the response of floating installation vessels, excessive motions can be transmitted to the lifted object, making the installation operation very weather sensitive. To increase workability, Heerema Marine Contractors (HMC) has developed the RNA method for wind turbine installation using a semi-submersible crane vessel. This method uses a temporary support structure on the vessel deck (installation tower) where the RNA is fully assembled with the assistance of the GREP (Guided Root End Positioning) tool to constrain the blade root to the top of the installation tower and consequently the hub. Finally, the RNA is installed in a single lift on the WTG permanent support structure. However, the GREP tool is compatible only with a specific range of wind turbine blade dimensions; therefore, for a different-size wind turbine blade a new GREP tool must be designed, fabricated, and mobilised.

This project proposes an improvement to the HMC's RNA method to eliminate the necessity of the GREP tool. That is, a motion-compensated Stewart platform attached to the crane boom, where the blade is fixed to be installed in the hub on top of the installation tower. The project is developed by investigating the initial assumptions; those are crane boom stiffness, blade deflection, installation tower motions and their influence on the vessel's response, and aerodynamic loads acting on the blade during installation. The kinematics (inverse and forward) and dynamics of a Stewart platform are formulated, as well as the mechanical concept of the proposed system and the blade installation process using a Stewart platform attached to the crane boom.

Furthermore, to eliminate the requirement of the GREP tool, a control system is developed to compensate for the blade root motions relative to the hub. The motion control system uses sensors to measure the hub's motions and generate the Stewart platform actuators' set points. Different possible sensor set-ups are evaluated, and a filter is designed to reduce the influence of the sensors' noise. The control system is developed on the basis of feedback PI (Proportional and Integral) and adaptive feedforward control to the actuators (hydraulic cylinders).

It is concluded that it is technically feasible to use a motion- compensated Stewart platform for blade installation in the RNA method. However, the economic aspects of the proposed solution must be investigated. ... Read more

Floating offshore wind turbines offer a means to access wind resources in waters that are too deep for bottom-fixed offshore wind turbines. To operate a floating wind farm, it must be possible to perform maintenance on the turbines. A critical maintenance aspect is the exchange of a major component such as a blade. It involves a lift of a large component to and from the turbine, which is subject to dynamic excitations from environmental loads. Jack-up vessels are used for major component exchanges on bottom-fixed offshore wind turbines, but due to the increased water depth in floating wind farms these types of vessels cannot extend their legs to the seabed and operate. As an alternative, an onsite blade exchange using a turbine mounted crane is considered in this thesis. The lifting dynamics during the operation are studied, as a knowledge gap is identified in this area.

A 15 MW floating wind turbine on a tension-leg platform (TLP) is considered as a case study. This type of floater is considered the most difficult to disconnect from its mooring and tow to a port for maintenance using a shore-based crane. The Offshore Self-Climbing Crane (OSCC) from Huisman Equipment is considered as maintenance equipment, which consists of a base docked on the TLP and a lattice structure coupled to the turbine tower, on which a crane is mounted. Due to the mass of the OSCC, the tension in the tendons of the TLP is reduced, which lowers its natural frequencies in surge, sway and yaw and shifts them closer to wind excitation frequencies. The roll and pitch natural frequencies of the TLP are lowered due to the top mass of the crane and the coupling between the bending modes of the lattice and the tower, shifting them closer to wave excitation frequencies. Snap loads in the tendons occur for sea states with wave peak periods near the roll and pitch natural periods at significant wave heights of 2.5 meters and above.

Steps of the blade exchange operation are studied. The operability of the installation of the OSCC is found to be limited by vessel motions, while the TLP remains relatively still. Installation of the OSCC is found to be a bottleneck in the blade exchange due to low operability and a limited number of suitable installation vessels.

Free-hanging blade installation is not deemed possible, due to low installation operability and the unconstrained yaw mode. The stiffness in yaw resulting from a line-up tool attached to the blade root or yoke is assessed. The low stiffness of the line-up tool at the yoke results in a large response due to crane tip displacements caused by wind-induced motions of the TLP. Placing a line-up tool at the root of the blade results in the highest operability of the blade lifts. Yoke motions during its attachment to the old blade become limiting instead. The design choices of the top crane mass, lattice stiffness and the type of line-up tool are related to the operability of the blade exchange.
... Read more

With the increase in need for offshore wind exploitation a research is set up with aims to develop a preliminary design tool. The introduction begins with background information on what role floating wind will play in the offshore wind industry and why this is a relevant topic. A general overview of floaters is presented with a comparison of the advantages of each type of floater. As well a chronological review of approaches to preliminary design. After which a numerical load and response model is developed. First theory is discussed regarding load and response modelling as well as the fatigue damage calculations. After the theory is covered, a test case is set up using the IEA 15MW reference turbine. The turbine is subjected to aerodynamic and hydrodynamic loading. This loading is calculated using Morison’s equations and a simplified thrust coefficient. In the test case the IEA15MW reference turbine is put atop a large floating substructure. For the environmental condition a site off the coast of Norway is used with an adjusted depth to allow for the size substructure. The response calculation is then, where the equations of motion are set up in matrix form, and terms in all of the matrices are then derived for the mass, added mass, stiffness and damping matrix. The stiffness matrix considers hydrodynamic stiffness and mooring stiffness. The terms in the damping matrix are derived from aerodynamic and hydrodynamic damping. Hydrodynamic damping only considers viscous damping terms. The model is then tested for a set of regular and irregular wave and wind conditions to analyse the system’s behaviour. For the fatigue calculation the substructure is divided into welded areas that will be investigated for lifetime global fatigue. This response model is validated by comparing natural frequencies of a smaller well documented 5MW floating spar. Optimization theory is covered, where multiple gradient and non-gradient based approaches are discussed. The simulated annealing algorithm is developed and tested on a set of test functions: Ackley & Booth. The algorithm shows quick convergence for areas that have small changes. The optimization problem for this research is formulated. The design space is visualised and compared to the most similar test function. After which, the constraints used by the optimiser are presented. The first constraint set consists of logical design constraints determined by the spar geometry. After which, the constraint set also includes limitations on the extreme response in time domain simulation. In the results chapter, it is found that this slows down the process so much that it is not feasible to include fatigue damage in the constraint function. Furthermore, it is found that global fatigue damage wouldn’t be a governing constraint. The results chapter discusses the importance of mooring stiffness in this optimization. Present the results from constraining the optimiser with the time domain response and compare the fatigue lifetime of the optimal designs. It is found that global fatigue is not a governing design consideration. It is found that both geometrical and response constraints are relevant for this type of optimization. However using global fatigue as a constraint is both irrelevant and computationally infeasible for the optimization of a floating substructure. Finally, a discussion is presented where the work is critiqued, and recommendations are made for further work. ... Read more

With an increasing shift to renewable energies, solutions are required to overcome that variability and unpredictability of power production that comes with it. Hydrogen is one solution that can overcome this issue since it can be produced using renewable energy through electrolysis, it can be transported to shore efficiently by pipelines and it can be stored, like natural gas. Not only can hydrogen be used as a way to store energy, but it can also be used directly as a basis for steel production and for long-distance transport. Using wind energy, specifically offshore wind farms, as the source for hydrogen is an idea that is gaining attention. Currently, wind turbines are electrically connected, bringing their electricity to shore by high-voltage cables, allowing hydrogen to be produced onshore. Hydrogen could however also be produced at a central location in the offshore wind farm or at each individual offshore wind turbine, removing the need for high-voltage cables. The purpose of this research is to explore different methods of hydrogen production and validate a model that can simulate each method. The goal is to be able to compare the different methods and quantify the losses of producing hydrogen offshore as opposed to onshore. A time-based simulation model has been developed to represent an offshore wind farm, which would either produce electricity or hydrogen, which eventually arrives onshore. High-resolution wind data are used to correctly include short-term fluctuations in wind output. The N.O. Jensen wake model is implemented to capture the wake effects. Since wake is typically modelled steady-state, specific attention has been given to include the moving of the wake through the wind park. The power outputs for each turbine in the model could be accurately determined based on the incoming wind data. It is found that the spatial effect reduces wind power variations at the wind farm level to about 22\% of first turbine in the sting, as expected. The output of the simulation model compares the power losses of producing hydrogen through four scenarios. This model uses a total of eleven wind turbine strings of seven 10 MW wind turbines each. The power output calculated would then be used to determine the amount of hydrogen produced. By applying electrical power loss equations and calculating the pressure drops for hydrogen traveling in a pipeline by assuming steady-state and isothermal, the losses are calculated for each scenario. A sensitivity study is performed to see the effects of changing model parameters on each of the different scenarios. The model developed in this thesis allows for comparison of electricity production and hydrogen production by offshore wind turbines. It is found that producing hydrogen at the wind farm level and then transporting hydrogen to the shore (Scenario 2) has lower overall energy losses (kg of hydrogen available on shore) than using electrical cables. The same goes for individual offshore hydrogen production (Scenario 3). A high-level exploration of the economics indicates that both scenarios 2 and 3 are more economically feasible when only looking at transportation costs. It is recommended that further research is done to be able to determine which method of hydrogen production would be the most effective by taking the transportation losses and economics into account.

... Read more

With the Paris climate accords signed in 2016, most countries have committed themselves to ambitious climate targets during the next decades. One of these targets is a dramatic increase in the overall energy portfolio's market share of renewable energies. This increase in renewable market share will, for a large part, consist of newly built offshore wind farms. In turn, this rise in offshore wind energy projects is expected to be especially dramatic in northern regions, where high and constant wind speeds prevail. However, as offshore wind farm projects move further north, additional challenges need to be faced. One of these is the technical challenge to design offshore wind farms for possible encounters with drifting sea ice. To tackle this challenge, a proper understanding of the mechanics associated with encounters of drifting sea ice with offshore wind turbines is essential.
Such encounters are currently primarily understood phenomenologically, and the associated models simulating these encounters – or ice-structure interactions – therefore are phenomenological as well. Moreover, most ice-structure interaction models are fundamentally one-dimensional, whereas ice-structure interactions are generally not one-dimensional. This mismatch holds especially for ice-structure interactions with offshore wind turbines, where wind loads are generally misaligned with ice loads causing highly two-dimensional ice-structure interaction problems. Therefore, the first half of this work sets out to extend one of the industry-leading phenomenological one-dimensional models – the Hendrikse (2017) model – to a two-dimensional environment. The ultimately developed Zero-friction contact Area variation Model By Omnidirectional Numerical Ice (ZAMBONI) attempts to do so by introducing practical extensions rather than introducing new assumptions. Nevertheless, one extension does entail a shift from current one-dimensional ice-structure interaction models. Namely, the assumption that ice experiences neither friction at the ice-structure interface nor internal shear forces. Consequently, much of the correctness of this model hinges on this extension. To assert the correctness of ZAMBONI. A comprehensive verification campaign is performed as well as a simple order-of-magnitude validation campaign. Although both confirm the extensions' correctness, further validation is required, especially concerning the zero-friction principle. Upon developing and discussing this two-dimensional ice-structure interaction model, the second half of this work couples ZAMBONI to an offshore wind turbine model to gain further insight into ice-structure interactions. These dynamically coupled two-dimensional simulations serve two purposes. Firstly, to compare one- and two-dimensionally simulated load cases of aligned ice and wind. Secondly, to perform newly simulable load cases of misaligned ice and wind. Four primary findings are discussed. Firstly, as hypothesized, introducing a disturbing wind load lowers the ice-structure contact area, causing smaller loads and displacements due to ice loads. This effect is especially well observable for misaligned wind loads and low far-field ice velocities. Secondly, a new ice-structure interaction regime is observed where ice and structure synchronize in the structure's first bending mode. This synchronization occurs most dominantly for two-dimensional ice. Thirdly, frequency lock-in occurs solely in the second bending mode and is terminated at lower ice indentation speeds for two-dimensional than for one-dimensional ice. Finally, small ice-wind misalignments, which are most common, appear highly similar to load cases of fully aligned ice and wind. ... Read more

Power optimization through wake steering and axial induction control is a well investigated topic in wind energy, which is generally proven to work. The influence of control manoeuvres on the fatigue of static components is generally discussed, but drivetrain fatigue due to wake steering and axial induction control is rarely discussed, while it is known that the drivetrain is a highly vulnerable part of the wind turbine and its downtime can result in a significant increase in cost. Having a better understanding of turbine wake interaction and wind farm power optimization and its influence on drivetrain dynamic behaviour serves as a reference for future wind farm cost optimization and predictive maintenance. The main research question answered in the thesis is as follows: To what extent does wind farm power optimization increase profit when wind farm power production and drivetrain bearing fatigue damage is considered? Multiple test cases for wake steering and axial induction control are considered, where different yaw angles, γ, and the blade pitch angles, β, are chosen for the upwind turbine. For each test case, power production and bearing damage is studied. A cost estimation is made and for a range of energy prices, the most profitable test case is found. For verification, a two and four wind turbine case in an uniform wind field is considered. Power production results for this low turbulent case are studied and compared to literature. Turbulent wind field results show that both wake steering and induction control result in a limited power production increase of 0.78% for γ = 7° and 0.17% for β = 1°, shown in the Figure below. The power production increase for the two and four wind turbine case in the uniform wind field for wake steering and induction control are 4.78% for γ = 15°, 16.6% for γ = 20°, 0.19% for β = 1° and 5.04% for β = 3° respectively. Overall absolute bearing damage of WT1 and WT2 increases with increased yaw angles for WT1 and the overall bearing damage of WT1 and WT2 decreases with increased blade pitch angles for WT1. INP-A and PLC-B bearing damage significantly increased for the downwind turbine. In the high turbulent wind field (TI = 0.2), when considering two wind turbines, wake steering can result in an increase of profit ranging from -€3,70 to €4,-, while axial induction control can result in an increase in profit ranging from €3,- to €40,-. In the low turbulent wind field (TI = 0), when considering four wind turbines, the power production increase for wake steering can result in a profit increase ranging from €30,- to €130,-, while axial induction control can result in a profit increase ranging from €15,- to €60,-. Both wake steering and induction control can result in increased profit. The desired control manoeuvre is highly dependent on the ambient wind, wake overlap of the downwind turbine and the wind farm arrangement. ... Read more

Climate change, as a result from global warming, requires an energy transition: the reduction of greenhouse gas emissions from fossil fuels and a radical innovation of the global energy system to proceed apace. Offshore wind is an important source of clean, renewable energy, and it plays a key role in the transition. 80% of the worldwide offshore wind is to be produced on locations in deep waters; here floating foundations are required, that to date are far more expensive than their bottom-fixed counterparts. To reduce costs of floating wind energy, reliable, detailed predictions of the system’s loads and motion response are crucial. Floating offshore wind turbine structures are designed using ’aero-hydro-servo-elastic’ software codes that simulate the dynamic response of a floating offshore wind turbine system to the offshore environment. Predictive accuracy can be improved by comparing simulation results from a model of a known system against measurements taken from the real-world system, a so-called model validation. One promising state-of-the-art aero-hydro-servo-elastic software code is BHawC/OrcaFlex, developed by Siemens Gamesa Renewable Energy (SGRE). Due to its novelty, however, validation of the code has only been carried out to a limited extend, giving rise to uncertainty about the interpretation of simulation results. The purpose of this MSc. thesis project is to validate the performance of BHawC/OrcaFlex by comparing its simulated load and motion results to measurements on a real-world floating turbine from the Hywind Scotland floating offshore wind farm (Hywind). Measurement data and a description of the ’as-built’ system were made available by the wind farm owner Equinor ASA. In order to establish an achievable level of modelling accuracy and predictive value of BHawC/OrcaFlex, the code was verified against another aero-hydro-servo-elastic software code: OrcaFlex, by setting up a similar model of the Hywind system in both codes. Limited information is available on the performance of OrcaFlex in floating wind load and motion predictions. Therefore, it was in turn verified against a wide range of industry-standard aero-servo-hydro-elastic software codes, using a modeled system that closely resembled the Hywind turbine and load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. OrcaFlex predictions matched very well across all load cases. The main differences were attributed to differently modeled additional linear hydrodynamic damping, as the official damping prescription resulted in prediction errors. In the BHawC/OrcaFlex verification against OrcaFlex, both models were subjected to multiple load cases that step-by-step increased in complexity, to further isolate causes of discrepancies between the models. Simulation results from running both models appeared to be nearly identical, though some discrepancy was observed from due to the simplified aero-servo-elastic OrcaFlex code. The final validation of BHawC/OrcaFlex to full-scale Hywind measurements is performed at below-rated, rated and above cut-out wind speeds with multi-directional wave and current components. In general, BHawC/OrcaFlex motion frequency domain predictions appeared to correspond well to the actual Hywind measurements. Most phenomena in the low-frequency, wave-frequency and high-frequency region were captured by the simulations. However, large errors were observed in the mean surge, sway and bridle line tensions predictions. Most discrepancies were found originating from errors in the model set-up, e.g. lack of hydrodynamic damping, simplifications in the wave model or errors in the mooring system set-up. Tuning of the mooring system showed improvement of the results, but further improvements could be made. Several sensitivity studies were added on parameters, such as hydrodynamic drag, tower damping and mooring drag. This showed overprediction of the surge/sway and roll/pitch frequency responses can be mitigated by both additional linear and viscous hydrodynamic damping. The main recommendations for further research are to further analyse errors identified in the model set-up. In addition, some yet unexplained phenomena that are not captured by BHawC/OrcaFlex in the current model, are to be addressed. Finally, a the development of a standardized approach to relate model validation studies in the field of floating wind to cost improvements could further quantify the value of future comparison studies. ... Read more

Offshore wind energy is one of the solutions to meet the growing demand for renewable energy. The offshore wind turbines producing this energy keep increasing in size and, as a result, the monopile foundations are becoming larger and heavier. The traditional jack-up installation vessels have limited crane capacity and many of these vessels are unable to install the XXL monopiles. Therefore, the offshore industry is currently investigating a new installation method using a motion-compensated gripper frame on floating vessel with a dynamic positioning system. The gripper frame is attached to the vessel and encloses the monopile with a ring to keep it vertical during the installation. In addition, the gripper frame compensates for the vessel motions such that the vessel motions do not influence the monopile motions. The purpose of this thesis is to investigate the feasibility of such a motion-compensated gripper frame and to determine what control settings minimise the monopile inclination and the force exerted on the monopile. The system is composed of three main bodies: the vessel, the gripper frame and the monopile. The monopile and PID controller, which controls the amount of force exerted on the monopile to keep it vertical, have been modelled in the frequency domain to gain insight in the effect of changing the control parameters. To model the dynamics of the coupled system an OrcaFlex model has been set-up. The system has been tested for various values of proportional and derivative gain, kP and kD respectively, in various wave conditions. First the perfect control system is tested, where the force to keep the monopile vertical is applied instantly and the vessel motions are fully compensated. However, as the real world is never perfect, the system tested for sensor lag and imperfect motion compensation as well. The results are judged based on three criteria regarding the maximum monopile inclination, actuator force and actuator stroke. Resonance is observed in case a value of kP is selected such that the natural frequency of the monopile and controller matches the wave forcing frequency. Adding derivative gain kD limits the monopile motions and force exerted in this case. To limit the monopile motion the proportional gain should be selected such that resonance is avoided. Two different control settings are investigated and it has been found that a relatively high value of kP of 10,000 kN/m in combination with a kD of 11,000 kNs/m is a suitable setting based on the three criteria. Furthermore, bow quartering waves is the favourable wave direction compared to head waves for the system considered in this thesis, as the force on the monopile is more evenly distributed over the actuator in x- and y-direction. Introducing a sensor lag into the system results in higher monopile motions and forces on the monopile. If the sensor lag exceeds 0.3 s it leads to instability of the monopile for both settings. The effect of not fully compensating the vessel motions is found to be limited due to the fact that these motions are slowly varying. The results of this work contribute to a better understanding of the dynamics of the system in various wave conditions. Furthermore, it provides insight in the effect of sensor lag and imperfect motions compensation, contributing to the design of a motion-compensated gripper frame for the installation of XXL monopiles. ... Read more