As the shift towards renewable energy continues, larger monopiles are being installed to accommodate for the increasing size of wind turbines. To allow for safe and economical installation of these monopiles, reliable and efficient models to predict hydrodynamic forces during installation are needed. This thesis investigates the hydrodynamic forces and inner water surface behavior acting on a very large (15 meter) diameter monopile during a vertical lowering operation.

Extreme wave impacts can damage ships and pose a risk to those on board. An extreme wave impact can be green water: a wave impact on a ship's deck or superstructure, or slamming: a ship's underside slamming on a wave. To prevent serious accidents these green water and slamming impacts need to be minimized. Predicting the probability and impact pressures can make minimizing impacts possible. Extreme wave impacts are challenging to predict as they are multiphase, nonlinear, turbulent and rare. The rarity, complexity and variety of impacts have resulted in limited studies into the statistics of extreme wave impacts, causing questions about the probability, distributions and ranges in which impacts occur. To predict extreme wave impacts answers to these questions would be helpful.

The goal of this thesis is to study the statistics of extreme wave impacts. To fulfill this goal a large data set of extreme wave impacts is collected. A new testing facility is created to collect data by adding a wave maker to an existing recirculating tank. In the test facility water and waves flow past the model, allowing for long testing times. Three large experimental data sets with a ship with forward velocity in head waves are collected. The collected data is in total 246 hours of experimental data over 23 test cases, representing over 2766 hours of continuous sailing at full scale.

From the first experimental data set the probability of occurrence of green water and the expected maximum pressures during green water events are identified. The data set contains green water events in different sea states, forward speeds and drafts. Two proposed methods to estimate the probability of green water occurrence are compared. One method is based on the probability of water exceeding the deck and one on a ship’s freeboard and the significant wave height, the former being in better agreement with the data, the latter being more practical for designers. The maximum pressures caused by green water are distributed according to the Fréchet distribution, also called extreme value distribution II. With the newly identified distribution, an equation to calculate the probability of a pressure limit being exceeded for a ship in operation is formulated. This first data set shows that the distribution of the time between green water occurrences is exponential, indicating that when green water occurs is independent of the time since the last occurrence.

The second set of experiments is aimed at identifying the influence of surge on green water and slamming. Long-running experiments with forward velocity and irregular waves are repeated with and without surge. Surge is found to increase the probability of green water events, but the impact pressures on deck and the probability of a green water event reaching the deck box decreases when the ship is free to surge. In this second data set green water and slamming events turn out to not occur independently as both event types cluster. The clusters occur for large probabilities of occurrence, which is why the first data set did not show these dependent clusters. Clusters are caused by large pitch motions. Larger pressures on deck are found for clustered events.

The conditions under which green water occurs and the relation between water exceeding the deck and green water are investigated. The relation is not direct and a difference between green water and exceedance that does not develop into a flow on deck is identified. A proposed prediction method follows from the difference between green water and exceedance. Pitch is identified as an important indicator for green water as green water events consistently occurred with large forward pitch motion, while exceedance also occurred with neutral pitch.
A prediction method of probability is proposed that implements separate limits for the motions and wave elevation that occur simultaneously, thus including the phase difference between the motions and wave elevation.

Design variations with different drafts and freeboards at the bow are tested in the third set of tests. A large set of 3263 green water events in irregular waves with forward velocity is experimentally obtained for six different bow designs. The data demonstrates that both freeboard and draft at the bow affect the probability of green water. Increasing the draft at the bow increases the swell-up, reducing the effective freeboard and in turn, increasing the probability of green water.

Increasing the freeboard results in a decrease in the probability of green water, as expected. However, the probability is not reduced equally for different green water impact pressures. The joint probability of green water occurrence and pressures shows that increasing the freeboard only decreases the probability of low-pressure events. Increasing the freeboard increases the probability of high-pressure events. These results highlight the importance of statistics when designing for green water.

The large experimental data sets have been combined with a machine learning method: SINDy. The models are trained to predict the acceleration of heave and pitch with the parameters heave, pitch, velocity of heave and pitch and the wave elevations along the hull. As a first step, a model is trained on fictitious data. The data is based on empirical response amplitude operators. The resulting model represents a damped mass-spring system with external forcing. The identification of the damped mass-spring system with external forcing is sensitive to random noise in the input data. Models have also been trained on the experimental data available. The models trained on experimental data did not result in the expected damped mass-spring system with external forcing model. The likely cause is noise in the experimental data.@en

Greenwater events, characterized by the overtopping of water onto offshore structures during extreme weather conditions, pose significant challenges due to their complex nature and the severe consequences they can have on both structural integrity and human safety. Despite the substantial forces involved, the failure mechanisms induced by greenwater loads remain underexplored, particularly concerning local structural failures that can lead to long-term degradation such as micro-cracking and corrosion. This thesis aims to fill this gap by investigating the local failure mechanisms induced by greenwater loading on offshore structures.

Through a combination of small-scale experimental setups and numerical Computational Fluid Dynamics (CFD) simulations, this study seeks to identify the dominant mechanisms that lead to material failure during greenwater events. The research involves simulating greenwater impact under controlled laboratory conditions and comparing these results with those generated by numerical models. A key aspect of the study is the parametric analysis, which correlates specific sea-state characteristics to the intensity of greenwater events providing a more detailed understanding of the conditions under which these failures occur.

The findings from this research aim to curve a path which later studies can follow, resulting into the knowledge acquired in the specific field to ultimately lead to more resilient offshore structures, reducing maintenance costs, prolonging the lifespan of marine vessels, and enhancing safety protocols.

A Landing Platform Dock (LPD) facilitates amphibious operations by allowing landing vessels such as the Landing Craft Vehicle Personnel (LCVP) and Landing Craft Utility (LCU) to embark and disembark from its internal flooded well-dock. The smooth transition of these landing vessels is critical, but can be hindered by wave motions (i.e. sloshing) within the well-dock, making safe embarking and disembarking more difficult.

Existing literature mainly describes wave motions within a well-dock, but does not address how to minimise these motions. This thesis aims to develop a preliminary design to reduce wave energy entering the dock by implementing an optimised wave attenuation solution.

ComFLOW is used to observe that wave motions inside the well-dock are highly non-linear and correspond to the LPD’s oscillation period. The wave motions within the dock are caused by a drop in water level near the well-dock entrance. This causes waves to roll into the dock. In shallow water conditions like in the well-dock, the propagating waves hereby show amplitude dispersion and wave breaking.

Using a system engineering approach, a bottom-hinged pitching flap was found to be ideal for wave attenuation in shallow water conditions. The flap’s attenuation performance was examined using ComFLOW by adjusting design parameters such as: mass, center of gravity, length and mechanical damping.

It was found that high-period waves with high wave heights fully couple dynamically with the flap, resulting in several outcomes. First, the flap follows the wave frequency, resulting in sub-optimal energy dissipation with use of radiation damping. Second, tuning the flap’s natural period to match the incoming wave period, increases pitching angles but reduces angular velocity, leading to decreased wave attenuation. Third, lowering the centre of gravity increases the restoring moment and angular velocity. This results in enhanced energy dissipation of higher-order wave components. Fourth, increasing the flap’s length improves wave attenuation due to enhanced reflection and radiation damping. Fifth, adding mechanical damping decreases attenuation performance, likely because the reduction in angular velocity worsens the energy dissipation effect for higher-order wave components. This research revealed that the best performing solution, with low a centre of gravity, attenuates 28.7% of wave energy entering the well-dock. Furthermore, optimising the flaps natural period to the incoming wave period was less effective, attenuating only 19.7% of the wave energy.

Offshore mussel cultivation is gaining popularity, with the longline technique emerging as a popular method for cultivation. The longline technique entails multiple backbones from which mussel lines that loop around it are suspended and which are kept afloat by buoys. Overdulve Offshore Services designed the Cees Leenaars, a semi-submersible mussel farm that employs the longline method for mussel cultivation. However, the configuration of the longlines and the offshore location of the farm introduce several challenges. The mussel will be exposed to high loads due to wind, current, and waves. Therefore, the question remains if the flow through the mussel farm will be sufficient to provide nutrients to the mussels. Hence, it is of importance to model the behaviour of the mussel droppers in waves.

The model will be constructed by first reviewing the motion of a single 3D pendulum, followed by the motion of a double 3D pendulum. For both situations the Lagrangian method is used to construct the equations of motion. Semi-Implicit Euler is used as integration scheme for the discretization of time. The results provide insights into the chaotic and complex characteristics of pendulums.

Thereafter, a model for a string will be constructed and this model is eventually expanded to a model for a mussel dropper by adding constraints to the last pendulum of the string. The method used for these models was different than for the model for a single and double pendulum due to instability issues. For the string Newton's second law was used to determine the equations of motion, since using this method made it easier to apply external forces and constraints to the system. Instead of a semi-implicit integration scheme an implicit one was used, resulting in an improved stability of the system. Due to the switch from semi to a fully implicit integration scheme, a nonlinear solver was build to solve the set of nonlinear equations at every time step. Later on, it was noticed that the implicit scheme accounted for too much numerical damping. In order to reduce the numerical damping, the Crank-Nicolson method combined with the theta-method was applied.

After constructing the model, an experiment was conducted for validating the model and for determining the damping coefficient of the system, which was still unknown. The experiment consisted of a physical model of the dropper and a camera which recorded the motion of the physical model. A code was written to determine the position of the pendulums in each frame and these positions were calculated relative to the angle each pendulum made with the horizontal axis. The experimental setup was reconstructed by the numerical model and compared in order to determine a value for the linear and quadratic damping coefficients. The model overdamped the amplitude of the motion right after the dropper was released from its equilibrium position. This could be due to numerical damping, due to errors in the experiment or due to a faulty translation of the experiment to the model. This provides the numerical model with not enough damping to let the motion die out, but it better represents the behaviour of the dropper for larger motions. These coefficients were found to be most suitable because from an engineering perspective it is safer to underestimate the damping in the system and assume higher amplitudes of the motion.

Lastly, the 3D model is used to simulate a mussel dropper in waves, as was the aim of this research. The dimensions of the mussel dropper are chosen to represent a real-life mussel dropper. The sea state to which the mussel dropper is subject will be one occurring every once in 25 year in the North Sea. The mussel dropper is simulated using seventeen pendulums. One pendulum is reviewed to determine the response of the mussel dropper. The time signal of this pendulum and the response spectrum of the pendulum show how the pendulum reacts to the incoming waves. A phase space plot is constructed to visualize the chaotic and complex behavior of the mussel dropper in waves.

This report aims to investigate the dynamic simulation of an IMOCA 60 sailing yacht in big wave condi-tions. These yachts are equipped with hydrofoils, which significantly increases their speed. However, this increase in speed introduces a challenge: when encountering large waves, the yacht can experi-ence a ”crashing” behavior, where rapid acceleration leads to a collision with the wave ahead. These crashes can be severe enough to cause injuries to the crew onboard, making it essential to understand and mitigate these occurrences.
A Dynamic Velocity Prediction Program, DVPP, was developed to explore the yacht’s behavior in waves. This DVPP systematically models all the forces acting on the yacht, allowing them to be solved in the time domain. Particular attention was given to hydrodynamic forces, with a nonlinear Froude-Krylov force calculation to accurately represent the effect of waves on the yacht’s hull. Next to this, a correc-tion has been applied to the diffraction and radiation forces to take the effect of foiling into account. Furthermore, a correction has been applied to the aerodynamic forces to account for the flapping of sails due to changes in apparent wind angle.
To validate the DVPP’s accuracy regarding hydrodynamic and static forces, a heave decay test and RAOs of a Wigley hull were calculated. Based on these results, the DVPP agrees with the refer-ence data, which gives confidence in the DVPP. A qualitative validation was conducted to evaluate the DVPP’s ability to simulate an IMOCA 60 in wave conditions. These simulations demonstrated that the DVPP with the implemented corrections could accurately simulate an IMOCA 60 yacht in waves, as the results corresponded with those from a DVPP developed for an ocean-racing trimaran.
Further investigation was performed on the effect of the foils on the yacht. A parametric study revealed a clear correlation between the yacht’s behavior and sea state: higher sea states lead to more severe crashes. Further investigations into foil chord length and rake angle were also conducted. The analysis showed that a longer chord length tends to result in less influence of waves on the yachts speed, likely due to the increased drag associated with a longer chord, which limits the yacht’s speed.
Additionally, it was found that a lower rake angle leads to more severe slowdowns. This is attributed to the influence of wave orbital motion on the foils; at a lower rake angle, the increased angle of attack generated by the orbital movement increases lift as the wave approaches the stern of the yacht, leading to higher speeds and more significant impacts with the wave ahead. Furthermore, recovery from these crashes is slower with a lower rake angle, as the hydrofoil produces less lift overall. Based on this parametric study, it can be concluded that a larger chord length and a higher rake angle are preferred to minimize accelerations during slowdowns. However, further investigation is needed to understand how the yacht’s overall design influences its behavior in waves.
Lastly, a longer simulation, with challenging environmental conditions, was performed to investigate whether the DVPP could be used to simulate crashes in waves of an IMOCA 60. The results showed several slowdowns where the G force was above the threshold for a crash. This indicates that the DVPP can simulate these extreme events. Upon further analysis, it was concluded that the first part of the slowdown occurs due to the foil submergences, and a second slowdown occurs when the hull enters the water. Based on the results of the parametric study, the recommendations of a larger foil chord length and higher rake angle were applied to the simulation case; with these changes, the slowdowns were much lower, and the occurrence of crashes was reduced.
Furthermore, it is recommended that future research focus on enhancing the accuracy of the DVPP, particularly in the modeling of nonlinear hydrodynamic forces, radiation, and diffraction effects. Since an engineering solution was implemented, incorporating unsteady sail forces into the simulation to account for the effects of sail trimming on the yacht’s performance is also crucial for stable results in big waves. Further research is needed to present a method that is backed by further physics.

This study investigates the effect of dynamic properties of a structure on the magnitude of impacts loads of breaking waves. A literature review in the field of violent wave impact on flexible structures is done. From this review, it is concluded that the hydro flexibility of structures have great impact on the fluid-structure interaction. In earlier studies, it is found that models of constructions in breaking waves are not accurate yet, using knowledge that is available up until now. There are studies that show suggestions of influence on the structures own properties. However, no extensive research had been done in this field. That is why the main objective of this study is to research if the natural frequency of a construction is of influence on the impact force of violent waves.

To answer the research question, a model is made and an experiment is carried out. The model consists of two parts, first a pendulum is used to represent a simplified structure to interact with a breaking wave. Using the model, a simulation of pendulums with different natural frequencies is done. The pendulums interact with the same wave each time. The second part of the model consists of a beam that is impacted by a breaking wave. Simulations are carried out where beams with different natural frequencies are impacted by the same wave.

To verify the model and further elaborate on the research question, an experiment is done. This experiment is done in the Sloshing rig at the faculty of Mechanical Engineering at the TU Delft. During the experiment, four different plates with different natural frequencies are used. They are exposed to the same wave impact each time. The model and the experiment results are compared and used to form a conclusion.

It is concluded that wave impacts are of higher magnitude on structures with a higher natural frequency. The difference between the measured and predicted forces indicates that the fluid- structure interaction of these plates lead to this difference. This can be explained by the different fluid- structure interaction for plates with different natural frequencies. If velocities of the plates with higher natural frequencies are higher, the Morison component of the impact force becomes larger. Therefore these impacts can become higher than for other structures. As the natural frequency of the plate increases, the results show relatively higher measured deflections as well.

The Landing Platform Dock vessel is mainly designed to support amphibious operations in which landing operations play an important role. For operating landing craft the vessel has a well dock at the stern, designed to station several landing craft. Landing operations are restricted by the motions of the Landing Platform Dock vessel and by the motions of the water in the flooded well dock. Due to turbulent flow in the dock entrance and nonlinear wave motions inside the dock, the (dis)embarking procedure can be very hazardous for landing craft. Therefore it is necessary to execute this operation as safe as possible, meaning that the water motions in the well dock should be investigated thoroughly.
The primary objective of this thesis is to investigate how various physical mechanisms, when combined with wave characteristics, influence the flow within the well dock. The wave profile within the dock arises from the interaction of two physical mechanisms: radiation, caused by ship motions, and diffraction, which is observed when the ship remains stationary, causing incoming waves to diffract around its hull. Specifically, this study wants to determine which of the two physical mechanisms radiation or diffraction has a more pronounced influence. Also, it investigates how different wavelengths impact these mechanisms and the results combination of independently analysed mechanisms with a model that integrates both mechanisms from the start of the simulation.
The second objective centers on the development of an accurate and efficient model to capture the physical mechanisms of the flow within the well dock of an LPD. The validation for this model derives from model tests conducted at TU Delft[20].
The third objective focuses on simplifying the model in order to facilitate the design process. Upon validating the model from the second research question, wave kinematics are captured using a wave probe located at the dock entrance. These kinematics are then used as inputs for a 2D model. Notably, this model specifically represents the well dock and the ramp region, excluding the complete ship structure. For validation, the same procedure employed in the second objective is followed, referencing experiments conducted at TU Delft[20].
All Computational Fluid Dynamics (CFD) calculations were executed using the ComFLOW program.

The offshore wind industry is increasingly constructing wind turbines farther from the coast, in deeper water, and under more extreme conditions. This requires larger (monopile) foundations and necessitates new installation methods. An important factor affecting workability is the dynamic behavior of the monopile during installation.
The objective of this thesis is to develop a method for determining the hydrodynamic loads caused by internal sloshing in an open-ended monopile (MP) as it transitions from a horizontal to a vertical position in the splash zone.
First, the resonance frequencies of the internal water column are predicted with analytical approximations based on linear theory. Distinction is made between piston mode and sloshing. Two numerical methods, linear potential flow (LPF), and computational fluid dynamics (CFD) are used to verify the resonance frequencies. Since the CFD analysis is done in 2D, a 2D representation of the open-ended monopile is considered. Due to the presence of viscous effects in CFD, the resonance observed with CFD consistently occurs at a lower frequency than for the analytical and LPF methods. Also it is found that an decrease in inclination angle of the monopile with respect to the horizontal, while maintaining the same submerged length, results in lower resonance frequency for both piston mode and sloshing in both LPF and CFD.
To assess the accuracy of LPF in describing the motion of the internal water column, it is compared to the CFD model. Input excitation in the CFD model is low enough to avoid non-linear sloshing modes and other non-linear behaviour of the free-surface. The ComFLOW 2D CFD model has been validated against various works from the literature for the accurate representation of gap resonance frequencies.
For both the piston mode and sloshing resonance, discrepancies between the two numerical methods are found, which can be attributed to viscous effects. At resonance viscous effects are nonnegligible, therefore the LPF method over-predicts the severity of the piston mode and sloshing. The influence of both the submergence of the monopile and its inclination angle with respect to the horizontal is considered.
The hydrodynamic coefficients for added mass and damping are found with forced oscillation for both upright and inclined geometries. While good agreement is found between the LPF and CFD results away from resonance, the CFD results in the vicinity of the resonance frequency are used to tune the LPF model, by way of additional linear damping, to achieve more accurate results.
It can be concluded from the present work that the resonance of the internal water column near the first sloshing mode significantly affects the overall hydrodynamic force and must be taken into account. At the peak hydrodynamic force observed during the first sloshing mode, the sloshing induces forces 5.59 times higher (submergence of 5 meters), 3.62 times higher (submergence of 10 meters), and 2.33 times higher (submergence of 15 meters) compared to cases where sloshing is not considered.
Looking forward, it is strongly advised to conduct forced oscillation tests with larger amplitudes, as this explores the effect of non-linear chaotic sloshing. Additionally, expanding the CFD analyses to 3D, where more non-linear sloshing effects are expected, such as swirling, is recommended. Furthermore, given the differences in results between LPF and CFD, it is valuable to validate the findings through model experiments.

When waves break, air and water mix and small air bubbles are formed in the water, creating aeration. The mixture of air and water alters the behavior of the fluid. Traditionally, it has been assumed that aeration has a damping effect on impact, as exemplified by the use of air bubbles in the swimming pool below the Olympic 10m platform and during synchronized 10m platform jumps to reduce the force exerted on the swimmer's body.

However, in recent years, it has been found that this assumption does not hold true in all situations involving aeration. When treating aerated water as a non-compressible fluid, it does exhibit a damping effect on impact. However, when treating aerated water as a compressible fluid, the speed of sound of the mixture can actually change with implications for the propagation of density waves. This means that an aerated water impact can feature pressure oscillations that increase the force instead of reducing it. Consequently, hydraulic structures that have been or will be constructed with the assumption that aeration only has a damping effect, may be affected.

Understanding the effect of wave-induced aeration on a horizontal platform is crucial for the design of new hydraulic structures in a more efficient and cost-effective manner. This research aims to determine the maximum force experienced by a horizontal surface during an aerated wave impact. The investigation involves using a numerical method, followed by a series of self-conducted small-scale sloshing experiments. The simulations contribute in two ways: they aid in understanding the problem and the magnitude of forces acting on the experimental setup, and they assist in verifying the experimental results once the experiments are completed. For the experiments, a new tank layout has been designed to facilitate the creation of aerated water inside the tank. The tank is used for conducting experiments where aerated wave impacts are compared to non-aerated wave impacts for different wave impacts. The experimental results confirm both the effect of pressure reduction due to aeration when the aeration level is near to 4%, as well the effect that the maximum pressure can increase for aeration levels between 1 and 2%.

Over the past few decades, there has been a significant increase in global energy consumption. To address the risks associated with climate change, there is an increasing urgency to transition towards renewable energy sources. With this growing demand, the interest in offshore wind energy has increased significantly, leading to the construction of larger bottom-founded offshore wind farms. In order to fulfill this rising demand, The International Renewable Energy Agency has estimated that a total of 2000 GW of installed offshore wind power is required to achieve net-zero emissions by 2050. However, the current installed capacity stands at approximately 35 GW, indicating a substantial gap that needs to be addressed. The offshore industry faces a major challenge in meeting this demand and making a substantial contribution to the supply of renewable energy sources.

As the size of wind turbines continues to increase, their monopile foundations also grow in weight and dimensions. Conventionally, monopiles are installed by upending them on the vessel, lifting them into the air and lowering them onto the seabed. However, this installation procedure is not feasible for extra-large (XXL) monopiles. This is because these XXL monopiles exceed the crane capacity of the, relatively new, installation vessels. To prevent the vessels from becoming outdated, an alternative approach to upending has been devised. This method involves utilizing the buoyancy of the monopile itself to compensate for the insufficient crane capacity on board of the vessel. This innovative upending technique is referred to as the trapped air method. This research explores this method as the influence of imposed buoyancy on the system's behavior in such operations has not been addressed in earlier research.

The main objective of this thesis is to examine the dynamic behavior and determine the natural frequencies of a monopile experiencing an upward facing buoyancy force. Moreover, it is desired to quantify the workable limits of the concept. First, a literature study is conducted, followed by a study that investigates the mechanics of the system. The monopile suspended from a crane, can be simplified using double pendulum models. Through the analytical approach, the influence of each force, mass and inertia component in the complex system can be examined individually. This assessment results in the equations of motion that serve as the foundation for the numerical model.

To gain initial insights into the behavior of the monopile, exploratory tests are conducted in a purpose-built experimental setup. It is observed that as the buoyant force increased, the monopile searches for an equilibrium to stabilize the system. Hydrodynamic effects induce a noticeable shift of the center of gravity of the monopile towards the waterline. Additionally, when a current is applied to the partially submerged monopile, motions in the sway direction are observed and suggests the presence of vortex induced motions.

Building upon the exploratory tests, decay tests are conducted at TU Delft to investigate the behavior and loads experienced at the cranetip. The results revealed that the presence of buoyancy significantly reduces the side-lead load to approximately X\%-Y\% of the maximum allowable load in the horizontal direction, which nearly half of the load observed without induced buoyancy. Additionally, the vertical loads reached approximately X\%-Y\% of the maximum allowed vertical loads. The buoyancy in the system effectively reduces both the vertical and horizontal loads in the cranetip. Furthermore, the presence of buoyancy leads to a significant decrease in the natural frequency of the system.

The numerical model is based on the equations of motion derived through the analytical approach. Initially, validation of the model is performed by using the scaled model dimensions. The frequency alignment indicates that the numerical model accurately models the natural frequencies on model scale. The model is then used to simulate a full-scale scenario. It can be concluded that numerical approximations of the frequencies closely align with those derived through the analytical approach and those observed during the model experiments. The close agreement among three different approaches provides strong validation of the accuracy and reliability of the numerical model in predicting the natural frequencies in full-scale scenarios.

In head seas, the offshore construction vessel Pioneering Spirit experiences wave amplification in the slot between its bows. Wave run-up results in a water jet at the closed end. To avoid potential damage of structures in the slot and improve operational safety, more insight into this phenomenon is needed. The aim of this research is to investigate under what circumstances water jets occur within the slot of a floating structure in waves and find suitable means for attenuation of this effect. The solution should not interfere with pipelay equipment in the slot and be detachable for when the slot is required for other purposes.

Literature research on wave damping solutions in moonpools, floating breakwaters, and fixed free surface breakwaters indicates that a plate type fixed free surface breakwater offers an elegant option for wave attenuation. However, limited published research exists on the optimal design of such plates and their application for preventing wave run up and water jets on adjacent structures. Therefore, this study evaluates simple designs to gain new insights.

A linear Boundary Element solver is used to find a simplified model of the slot in which large waves occur, in order to save computational time when using a nonlinear solver.
It follows that the assumption that the vessel does not radiate waves can be made, seen as the effect of radiation on the wave elevations in the slot is small. A simplified model of the hull that is found to experience similar linear wave effects has a box-like shape with a slope underneath the hull and oblique planes to recreate the wave amplifying effect of the slot.


The Computational Fluid Dynamics (CFD) solver ComFLOW is used to simulate two different sea states in which water jets occur in the slot of the simplified model. Seen as three-dimensional (3D) simulations require a significant amount of computational time and power and water jets also occur in a two-dimensional (2D) setting, the simulations are performed in 2D.
The wave attenuation mechanisms in terms of reflected, lost and transmitted wave energy are evaluated for each breakwater design as well as the mean net force on the breakwater.
The results show that a plate under an angle of 60 degrees has small wave transmission and reflection, induces large wave energy loss and experiences a small mean net force. Furthermore, this model prevents the most water jets in both sea states compared to other models.
The results also indicate a linear correlation between the transmission coefficients of breakwaters and their average reduction of the net force on the hull corresponding to the water jets. There is no direct link between the prevention of water jets and the transmission past breakwaters, seen as the occurrence of water jets depends on the phase and shape of the wave as well as the incident wave height.

Further optimisation of the geometry of the 60 degree inclined plate and addition of porosity and appendages is recommended to increase energy dissipation and reduce forces on the breakwater. Additionally, future research should explore the relationship between transmission coefficients and the occurrence of water jets and also consider other factors beyond wave height that contribute to jet formation such as phase and the shape of the wave.

Surrogate modelling techniques such as Kriging are a popular means for cheaply emulating the response of expensive Computational Fluid Dynamics simulations. These surrogate models are often used for exploring a parameterised design space and identifying optimal designs. Multi-fidelity Kriging extends the methodology to incorporate data of variable accuracy and costs to create a more effective surrogate. This work recognises that the grid convergence property of CFD solvers is currently an unused source of information and presents a novel method that, by leveraging the data structure implied by grid convergence, could further improve the performance of the surrogate model and the corresponding optimisation process. Grid convergence states that the simulation solution converges to the true simulation solution as the grid is refined. The proposed method is tested with several problems involving both synthetic and realistic multi-fidelity data acquired with Computational Fluid Dynamics simulations. The synthetic results indicate that our method surpasses the most well-known multi-fidelity Kriging model in terms of both accuracy and cost given the proper conditions. The performances of both surrogates in the optimisation cases using Computational Fluid Dynamics simulations were similar. Data-supported conditions for the proposed method to be effective, and a procedure to cheaply recognise them, are provided. To further improve and validate the method, recommendations are given at the end.

The development of floating islands is an attractive option to provide living/working spaces at sea, as it would be a more economical solution for many locations at sea where the water depth is high. However, large second-order wave forces cause the floating island mooring system to become expensive as the wave height increases. So, in many locations with an intermediate water depth, land reclamation is still a more economical solution. A breakwater connected to the wave-ward side of the floating island can reduce the mean wave drift force of the entire structure by attenuating wave energy and thereby lowering the mooring costs. In this thesis, the optimal geometry of this breakwater is designed.

An optimisation is performed to find the ideal geometry of this breakwater such that it attenuates the maximum amount of wave energy while experiencing a low mean wave drift force. A parametric design of a breakwater is made, with six varying factors/parameters that define its shape, such that the geometry could range from a flat plate just below the water line, a large box-type breakwater at the water surface, a deeply submerged wedge-type breakwater, or anything in between. ComFLOW (a simulation method for free-surface flow) is used to determine the mean wave drift force and wave attenuation performance of all the breakwaters in the design space for two different regular wave conditions: Wave Condition 1 with a wave height of 3.0 metres and a peak period of 6.0 seconds, and Wave Condition 2 with a wave height of 9.0 metres and a peak period of 10.4 seconds. After all ComFLOW results are gathered, an optimisation method Design of Experiments is used to map the dependency of the six input factors that define the geometry, on the performance of the breakwaters, to come up with the geometry of the optimal breakwater.

To obtain the most accurate representation of reality in the numerical environment of ComFLOW, a validation of the programme is performed in two phases. Firstly, ComFLOW results are compared to analytical formulas, derived from linear wave theory. And secondly, ComFLOW dissipation characteristics are compared with the results of a physical experiment involving a regular wave plunging over a barred beach profile. From this followed, a minimum of twenty cells is required at the height of the structure to give reliable results of the mean wave drift forces on the structure. ComFLOW results for the wave transmission, wave reflection and forces on the structure followed the same trend as linear wave theory, with a converging offset. The wave energy dissipation characteristics of ComFLOW of all different grid sizes used in comparison with the experiment in which waves were plunging over a breaker bar, showed results that were in compliance with the experiment.

In Design Iteration 1, 94 different geometries were simulated in ComFLOW, of which 46 were completely submerged. Around 80\% of all submerged breakwaters experienced a mean wave drift force in the opposite direction of wave propagation. An explanation for this negative mean wave drift force is sought by the set-up and set-down of the water level, which occur around breaking waves. This results in a difference in hydrostatic pressure, and if the breakwater is positioned in between, it will experience a mean wave drift force in the opposite direction of wave propagation. The length of the sloping beach of the breakwater needs to be shorter than the wave length; otherwise, the set-up will be above the beach, and this results in a contribution to the mean wave drift force in the same direction of the wave propagation. The optima found by \acrshort{doe} based on minimising the mean wave drift force were either a shallow box-type structure relatively far below the water surface or a large structure closer to the waterline with a sloping beach on its wave-ward side, which induces the waves to break and thus dissipate wave energy. For maximal wave attenuation performance, the height and length of the breakwater must be maximised.

A cost function is developed that computes the reduction in captical expenditures (\acrshort{capex}) the breakwater provides to the floating island. Therefore, the construction costs of the breakwaters and the reduction in the mooring costs of the floating island are calculated. The difference between the two is the total cost reduction that the breakwater provides. An optimisation is done on maximising this total cost reduction for Wave Condition 1, which resulted in an optimal design of a breakwater with a length of 10.0 metres, a depth of 9.6 metres, a draught of 6.2 metres and its wave-ward side is a sloping beach that induces the incoming waves to break. It can provide a cost reduction of 24.4 k\texteuro per unit width. Compared with a floating island designed by the 3-year Space@Sea project for a location in the North Sea, this breakwater would deliver a 51% mooring cost reduction and a 7.7% reduction in total costs.

In this report a new enriched finite volume method is introduced. This method will be particularly useful for fluid-structure interaction problems. In engineering many problems are in the domain of fluidstructure interaction. Examples of these are water in sea locks, blood flowing in vessels and sailing ships. Fluid problems are usually solved with the finite volume method (FVM), and structures in this fluid flow are handled by discretizing the fluid flow around this structure. The mesh therefore needs to be conforming around the structure, and remeshing is needed when the the structure is moving over time. Remeshing is a computationally expensive process, therefore in FEM a method called enriched FEM is developed, which removes the need for remeshing. Enriched FEM uses so-called enrichment functions; these enrichment functions decouple the structure from the mesh. This concept is implemented in the finite volume method to get an enriched FVM. In order to come up with this method; FVM and enriched FEM are compared to see if the concepts of enriched FEM can be implemented in FVM. The FE method that is used is the discontinuous Galerkin (DG) method, which is a method that uses components of FEM and FVM. It discretizes equations as in FEM, but the elements are connected using flux functions as in FVM. DG is used to develop a high-order FVM and to introduce enrichments in FVM. The result of this thesis is a method which can implement enrichment functions in FVM, such that remeshing is no longer needed. The method is compared with conforming FVM and enriched DG, and it uses less computational power than both methods. The method also recovers the optimal convergence rate for a non-conforming FVM discretization. The enrichment functions are decoupled from the FVM discretization, therefore the method can be added to existing FVM solvers without changing the non-enriched part. The method is tested on a problem where waves are generated in a box.

When sailing in heavy seas, it happens that the water washes over the bow. This shipping of water is generally divided into two types. The term white water is used when the water is more of a spray and is generally harmless. Green water is more violent and can pose a danger to the crew and structures on board.

Research on green water has mainly focused on the arrival of green water on deck and the different types. Impacts are generally considered as result of green water wave and tested by a dam-break problem. The impact of green water has not been studied as solely the impact of water on a structure. Drop test experiments are conducted in different ways for different shapes in the past. However, there have not been tests before with green water using drop tests. Drop tests are deemed suitable to test shapes with different deadrise angles and create aerated water.

The aim of this work is to investigate the effects of green water on deck structures by studying the maximum impact pressure when considering the deadrise angle and the aeration.

In this thesis the influence of shape and aeration on the maximum pressure at impact is investigated. To investigate the effect of aeration and shape, the impact tests are conducted using a drop tower experiment. For this experiment different increasing deadrise angles are chosen from 0± or a flat plate up to 30±. In addition test with aerated water are done up to 4% which is in range of real green water waves. The aeration and the shapes are chosen in such a way that non-compressible, linear compressible and non-linear compressible effects take place. It can be seen that with a flat plate, the impact increases almost instantaneously and produces a high peak. Adding aeration reduces this peak considerably. Fromexamining the impact of the wedges, it can be concluded that the pressure rises more slowly at a higher deadrise angle. The almost instantaneous drop in pressure seen with a flat plate changes to a smoother rise at a high deadrise angle.

In addition to the experiment, numerical simulations are carried out to verify the solution. The numerical simulations also give a better insight into the pressure across the width of the object. Upon examining the numerical simulations, it is concluded that the results for deadrise angles of 15± and 30± are equivalent to the results of the results of the experiment. However, the simulations for a flat plate result in an high pressure which is probably due to a low number of data points which capture the peak.

It can be concluded that the shape of the structure and aeration have a significant influence on the maximum pressure. However, the decrease due to a higher deadrise angle is more significant than the effect of additional aeration. With higher aeration, the maximum pressures are closer together and the difference in deadrise angle has less effect. When examining the impact pressures the effect of both deadrise angle and aeration can be seen. The maximum impact for a non aerated flat plate impact measured is 9.93[bar ] and is decreased by 8.8[bar ] for a wedge impact with a deadrise angle of 30± which drops in water that is 4% aerated. The influence of both shape and aeration can be reviewed independently which leads to an reduction of impact pressure for a flat plate of 59 percent when 4% air is added to the system. When only changing the deadrise angle to 30± a reduction of 86% is found.

To facilitate the installation of larger monopiles multiple players in the offshore wind market are investigating the floating transport of monopiles to installation locations. This brings the challenge that when these floating monopiles are to be installed by heavy lift vessels, they must safely be handed over to the installation vessel and hooked onto the main crane of the installation vessel for installation. Current literature mainly focusses on two topics. Some studies exist on the behaviour of a floating monopile near a heavy lift vessel. Studies also exist that study the behaviour of a monopile hanging on the crane of a heavy lift vessel. The step between these two moments in time, where the floating monopile is handed over from the tug towing the monopile to installation vessel, is the focus of this study. This thesis presents a method for the hooking of a floating monopile to the crane of heavy lift vessel Oleg Strashnov and a model for the hooking operation. This model is used to determine a maximum allowable sea state for this operation. Two methods for hooking a floating monopile to the crane of a heavy lift vessel were defined. In the first option, the monopile would be moored alongside the vessel and kept in position by a supporting tug while the rigging is handed over to the heavy lift vessel and the rigging can be hooked onto the vessel crane. The second option entails positioning the monopile perpendicular to the heavy lift vessel, with fenders placed between the monopile and vessel...

This thesis study is intended to research the effects on peak pressure during impact of a deformable structure with a liquid. To find an answer to this question, a novel wedge was designed to perform free fall wedge experiments at the TU Delft. This report describes the progress that lead to answering the research question:

What is the effect of dynamic deformation of a structure on the maximum pressure at the wedge surface during impact?

To create an overview of the problem, the various systems and knowledge required to answer the research question are discussed. Among others, a decision on wedge deformation type is made, the importance of multiple impact velocities and the sensors required to measure data of the impact are discussed. The proposed concept to perform experiments is with a deformable wedge that is allowed to fold around the keel upon impact with the free surface. Hypothesis is that the deformation on impact could lead to initial lower impact pressure but dynamic response could lead to higher peak pressures at later stage of impact.
In the next phase, estimations of impact pressures, forces and accelerations are used to finalize the design of the wedge. Strength calculations were performed on the critical parts of the wedge. It is concluded that the analysed parts are strong enough to withstand repeated loading of the structure. Also during this phase, an extra experiment into dynamic calibration of the accelerometer was performed. By curve fitting the expected acceleration to the experiment data, the error of the accelerometer is determined. The accelerometer used in this experiment has an error of 0.48% which is within the limits of the manufacturers requirement of ≤1%. Before the start of the experiments the limitations of the setup are discussed and an uncertainty analysis is performed on the sensors.
The experiment consists of a series of free falls with various wedge setups, either rigid or deformable. A rigid setup of the wedge is tested at impact velocities of 2, 4 and 6m/s and deadrise angles of 10 and 20 degrees to create a base line for maximum impact pressures. The results of the rigid experiment are compared against previously tested rigid wedges. It is concluded that the novel wedge showed similar peak pressures compared to previously tested rigid wedges. The deformable wedge is dropped at all impact velocities and deadrise angles with three different springs, varying in stiffness.
Deformation of the wedge during impact resulted in decrease in peak pressure. Decreasing spring stiffness and increasing impact velocity resulted in increasing deformation of the wedge. With these results the research question set in the first chapter of the report is answered.
The answer to the research question is that impact induced deformation of the structure leads to lower peak pressures. Dynamic effects were observed but did not lead to higher pressure than the rigid experiments. While further research into the matter is required, this result may already lead to improved ship design which could lead to lower manufacturing costs and lower emissions.

During specific operations, offshore marine contractor Heerema Marine Contractors reduces the draft of their Semi-Submersible Crane Vessel (SSCV) such that the floaters are submerged in close proximity to the free surface. This draft is also known as inconvenient draft. At this draft the motions of the vessel do not fully comply anymore with the computed prediction using linear diffraction theory. A proper motion prediction is required to ensure a safe execution of the offshore operation. The key issue is the submerged part of the floater, since discrepancies occur as soon as there is only a small water column on top of the floaters. Motion RAOs are computed via the hydrodynamic coefficients and wave loads. The reason why this leads to discrepancies in the motion RAO is not yet known. In addition, qualitative hydrodynamic data should be gathered for an object submerged in close proximity of the free surface.
Therefore, in this study the cause of the discrepancies in motion RAO is studied on an experimental basis for an object submerged in close proximity to the free surface. The scope is narrowed down to a two-dimensional cross-section of a SSCV-floater. Numerical simulations using linear diffraction theory are performed in WAMIT. The results are compared to experimental data obtained via model tests performed at the towing tank facilities at the faculty 3mE at Delft University of Technology.

Based on experimental data is concluded that linear diffraction theory does not predict physical phenomena that satisfy the boundary conditions and that underlie principles of the theory. Despite the fact that the experimental data does satisfy the boundary conditions of the numerical simulation, discrepancies occur at the hydrodynamic coefficients and wave load. As a result, it can be concluded that linear diffraction theory malfunctions at the inconvenient draft region, and therefore is not the correct theory to determine RAOs for an object on inconvenient draft.
Discrepancies in motion RAO are found to be dominated by the discrepancies the wave load, except for the frequency at which the added mass equals negative inertia of the body. Discrepancies for added mass are less significant than for the damping coefficient and wave load. At higher frequencies inertia becomes dominant over damping and damping deviations affect the RAO less.
The pitch motion about the center of the cross-section does not represent a rotational motion about same the degree of freedom for an SSCV. However, it allowed to experimentally investigate the global numerical extremes. In addition, it led to the finding that it seems that the rotational data is more affected than the heave data.

There is a strong suspicion that poles in the complex plane are the cause of the discrepancies in the hydrodynamic data, which result of the used Green's function by Wehausen and Laitone. This suspicion is based on characteristics of the numerical data in combination with findings in literature and the experimental data, accumulate.

Currently, an approach that makes use of free surface damping is applied to predict the motion of an SSCV at inconvenient draft. Based on experimental data can be concluded that the use of free surface damping without further modifications does not result in an accurate motion prediction.

Lastly, the effect of nonlinearities when increasing the oscillation or wave amplitude. The wave load is found to be more prone to nonlinear effects than the hydrodynamic coefficients.The force signal remained dominantly harmonic, but higher harmonic forces did show up. Furthermore, with the exception of most tests at the largest tested submergence, higher harmonic waves were measured and visually observed during the tests. These higher harmonic waves arose at the transition from the shallow to deep water regime and vice versa. These forces were found to have hardly any effect on the force signal.

Stimulated by the energy efficiency indices introduced by the International Maritime Organization, the increased attention for the environment as well as economical pressures emphasize the relevance of accurate resistance estimation for ships. This leads to a particular interest for added resistance in waves, a resistance component that is currently hard to predict. One of the uncertainties lies within the phenomenon of bow wave breaking, which is expected to have a nonlinear, reducing impact on added resistance. A literature review reveals a lack of research done on this aspect. However, a better understanding of the phenomenon is key to improve the accuracy of added resistance estimates. If current methods are proved to overestimate added resistance, present estimations of required engine capacity would suffer the same bias. As a result, less powerful engines could then be installed which in turn would lead to more efficient ship designs.
Within this context, this thesis aims to gain new insights into added resistance by studying how it is affected by bow wave breaking. To reach this objective, experiments combining two fields, namely experimental hydrodynamics and optical measurements through stereo vision, are performed at the Delft University of Technology. The ship model no. 523 of the Delft Systematic Deadrise Series, a hard chine planing hull, is towed through calm water and regular head sea conditions at a constant speed. The speed and wave conditions are selected such that they gradually cause the bow wave to break. The ship model's speed is varied between a Froude number of F_r = 0.15 and F_r = 0.30. The incoming wave steepness is varied between a wave height to length ratio of H/λ = 1/60 and  H/λ = 1/30 for short, intermediate, and long wave conditions, corresponding to a wavelength to ship length ratio of λ/L_pp = 0.5,  λ/L_pp = 1.1 and λ/L_pp = 2.0, respectively.
During these experimental runs, the breaking of the bow wave is evaluated through visual observations, the added resistance is measured using resistance tests, and the relative wave elevation is assessed using a newly developed waterline detection method. Using the fact that the hull is semi-transparent, this latter method employs stereo imaging through cameras that are placed inside the ship hull. The waterline can then be traced using a Canny edge detection algorithm which is based on the intensity gradient at the free surface. By solving the stereo correspondence problem, the hull can be reconstructed and the detected waterline projected onto 3D coordinates.
The analysis of the experimental results shows that the linear approach, in which added resistance is proportional to incoming wave amplitude squared, does not hold. With increasing steepness, the added resistance coefficient decreases. Contrary to expectations, no clear correlation between the added resistance coefficient curve and the onset of bow wave breaking is revealed. However, the decrease in added resistance coefficient is strongest for conditions where the bow wave breaks most violently, which indicates an effect of bow wave breaking. The intensity of breaking could not be quantified by these experiments and therefore it is suggested to extend this study by applying CFD methods.
The relation between the added resistance and the incoming wave amplitude squared does not consider nonlinear phenomena (e.g. induced by green water and wave breaking). It would thus not take into account a possible decrease in relative wave elevation due to bow wave breaking and this is hypothesized to be the source of added resistance overestimation. This hypothesis is studied by reconstructing the relative wave elevation from optical measurements. Bow wave breaking seems to specifically affect the maximum relative free surface elevation with respect to the undisturbed waterline while its minimum elevation is characterized by the disappearance of the stationary bow wave for intermediate wave conditions, F_r ≥ 0.2. Considering the complexity of added resistance which is influenced by different factors, bow wave breaking effects are difficult to isolate. Nonetheless, the analysis of the experimental results led to the introduction of an alternative added resistance coefficient which nondimensionalizes the added resistance by the relative wave amplitude squared. This coefficient takes into account different factors affecting added resistance, e.g. the combined effect of ship speed, incoming waves, and bow wave breaking, and shows a constant trend compared with the common added resistance coefficient. These findings highlight the importance of an accurate relative wave height estimation.