Accurate simulation of non-linear free-surface wave interaction with structures as if in the open sea requires truncation of domains for computational feasibility. In absence of special boundary measures, truncation introduces artificial boundaries with spurious wave reflection back into the computational domain. The common way to prevent reflection is by means of dissipation zones, which are often larger than the domain of interest in order to achieve sufficient accuracy. Local absorbing boundary conditions pose a potentially more efficient alternative, but are limited in accuracy due to corners on Cartesian domains (advantageous for future implementations of Volume-of-Fluid methods). This work proposes a methodology that preserves a Cartesian structure of the interior grid, while mapping to a circular domain boundary through the use of curvilinear coordinates. On the circular boundary, higher-order absorbing conditions can be applied without corner limitations. The linearized shallow water equations are discretised using a staggered finite volume discretisation. Two second-order Engquist-Majda boundary conditions, extended with generating wave capabilities, are expressed in the coordinate system, implemented and benchmarked using a first-order Sommerfeld boundary condition, using a centred and off-centre source configuration. Results demonstrate that the methodology allows consistent implementation of higher-order absorbing boundary conditions with low spurious reflection. ... Read more

Maritime vessel slamming into aerated water is a significant area of study for high-speed and lightweight craft. Upon impact with water, safety risks can lead to structural failures, endangering human life and resulting in economic losses. The design must be optimised for material usage with appropriate safety factors based on accurate knowledge of the loading and response under several impact conditions. Structural flexibility makes hydroelastic effects unavoidable, with the magnitude governed by impact type, velocity, and structure geometry. Entrained air from free-surface breaking renders the water weakly compressible, further altering the slamming dynamics. This work presents a physical foundation and experimental dataset for validating numerical models of more realistic impact conditions.

In this work, a novel experiment was set up for a combined evaluation of hydroelastic slamming in aerated water. A wedge with a 15 degree deadrise angle was designed and constructed to represent ship bow slamming. The wedge bottom consisted of interchangeable steel plates of varying stiffness. Contact and optical measurement techniques were used to capture and visualise the loading and response over the plate width. Experimental test conditions include plate thicknesses between 1 and 3 mm, impact velocities between 2 and 5 m/s and an air fraction in water between 0 and 2 %.

The results show that structural stiffness and impact velocity, captured by the hydroelasticity factor RF jointly govern the slamming response. The air fraction in water redistributes impulsive loads over time and modifies the wetted natural frequency through the influence of water density. For high-velocity impacts with significant plate deformation, as well as for impacts approaching the quasi-static regime, a lower peak strain was found for impacts in aerated water. Furthermore, for the condition where the wetting time approaches the first wetted natural period of the structure, response amplification due to hydroelastic effects was observed, with a further increase in load for the aerated water condition. This study demonstrates that aeration can amplify structural response, making it essential to take into account the physical aeration effects in the design of ship plating for slamming conditions. The structural response was found to capture the underlying physical behaviour more reliably than the pressure measurements alone. The effects of the small-scale experimental setup on the results were also identified and documented in detail. ... Read more

This study investigates the possibilities of stern slamming and the effects on a heavy lift vessel that is on dynamic positioning during offshore operations. When such a vessel encounters following waves the stern becomes susceptible to high-pressure impacts. These impacts could lead to whipping effects throughout the hull. These possible vibrations can lead to discomfort for the crew and for important mechanical failures of critical DP systems. This poses a risk during the offshore operations and could lead to an abortion of the operation until the slamming stops. This study focuses on identifying how slamming occurs in conditions in which a vessel can perform offshore operations and what parameters have an effect on the pressures generated by the impacts. Also, a relation between the slamming and responses is drawn to find out how certain components could fail due to the whipping effects.

The research consists of two different steps. The first is a two-dimensional incompressible Volume-Of-Fluid model in ComFLOW is used to simulate the hydrodynamic wave impacts on the stern of a heavy lift vessel. Simulations cover two irregular sea states representative of Beaufort scales 4 and 5 and a range of different drafts for the vessel from a minimum draft of 6 meters to the design draft of 8 meters. For this 2D simulation are made to ensure relative quick computation and fundamental results. Because of the 2D simulations and the vessel being stationary, only heave and pitch motions of the vessel are modelled. Also, simulations with a series of different peak wave periods are set up with and without the vessel motions to see the effect of the incoming waves and vessel motions on the impacts on the stern. Mesh refinement and grid convergence analyses are conducted to test the accuracy of the CFD model.
The second approach uses the output of the CFD model to calculate the vibrational responses due to the wave impacts on the stern. The hull of the vessel is idealised as an Euler–Bernoulli beam and decomposed into its first four bending modes. Pressure time histories extracted from the CFD simulations serve as asymmetric loading inputs to two independent vibration response solvers, a Duhamel integral formulation and the Cummins impulse response equation. Both solvers compute time-dependent modal amplitudes and reconstruct spatial velocity fields along the beam to evaluate Root Mean Square velocities at critical locations.

The results from the CFD simulation show that in both irregular sea states significant slamming impacts occur in all loading conditions of the vessel considering the selected sea states. However, the closer the vessel gets to the design draft the lower the average recorded impact pressures are. For a sea state with a Hs of 1.1 meters and a Tp of 4.6 seconds, the average impact pressure reduced from 390 kPa at a draft of 6 meters to 80 kPa for a draft of 8 meters. For a sea state with a Hs of 1.65 meters and a Tp of 5.1 seconds, the average impact pressure reduced from around 4200 kPa at a draft of 6 meters to around 250 kPa for a draft of 8 meters. However, in both sea states impacts with pressures well above the 1000 kPa were recorded on the stern. Also, lower and higher periods seem to increase the average impact pressures, likely due to steeper waves. The vessel motions in all cases reduced the average impact pressures between the 25.7% and 46.1%.
Structural response analyses show close agreement between the Duhamel and Cummins methods, with discrepancies under 2.5% arising from different treatments of memory effects. Predicted RMS velocities at the stern exceed typical comfort thresholds between 4 and 6 mm/s for impact loads around 200 to 300 kPa and can approach equipment safety limits of 18 mm/s even in moderate sea states for the higher observed load of 750 kPa or higher. This indicates that slamming induced vibrations may pose fatigue and operational risks. ... Read more

This Master's thesis explores the modelling of dynamic power cables (DPCs) used in floating offshore wind turbines (FOWTs), focusing on how lower- and higher-order models capture the cables' dynamic behaviour.

The study is motivated by the increasing use of floating offshore wind systems in deeper waters, where dynamic cables are required to transmit the power. To manage tension and reduce fatigue failure, a lazy wave configuration (LWC) is commonly employed as configuration for a dynamic power cable.

Two modelling techniques are compared: a custom-developed linear finite element model based on the Modal Superposition Method (MSM) and a more computationally intensive, non-linear model using OrcaFlex. The MSM approach is based on the linearisation of the system’s dynamic behaviour using mode shapes, which significantly reduces computational cost compared to high-fidelity methods like OrcaFlex. In fact, the MSM model runs over twelve times faster than OrcaFlex, making it particularly suitable for early-stage analysis. Results from both models are analysed in the time domain, with focus on displacements, bending moments and axial tension to identify fatigue-prone areas.

The findings show that the lower-order MSM model, while limited by linear assumptions, accurately captures displacement in tension-dominated regions and effectively identifies fatigue-prone locations. Compared to the OrcaFlex model, MSM computes conservative fatigue life estimates, as demonstrated through simulations across multiple surge spectra, imposed at the top-end. In contrast, the higher-order OrcaFlex model, which accounts for geometric non-linearities, offers more accurate predictions under larger wave loads and is better suited for detailed fatigue analysis of selected sea states.
... Read more

This document explores the wave run-up loads on offshore monopile structures, particularly those experienced by the main access platforms during severe storms. Such incidents have been observed to cause significant damage, necessitating a deeper understanding and accurate prediction of these loads.
The main objective of this research is to compare various vertical wave run-up load models, focusing on their prediction accuracy and computational efficiency.
First the wave run-up heights and wave run-up loads of analytical models are compared to each other. These results are calculated with the maximum wave heights and water depths on the Dogger Bank. It can be seen that the wave run-up heights and therefore the wave run-up loads are unrealistically high. The main reason for this is that the analytical models are used outside the boundaries where they are validated for. This leads to less reliable results.
Theoretical and numerical limitations are in this thesis to increase the reliability of the results. ... Read more

Improved modeling of the response of offshore wind turbines (OWTs) is key to optimizing design and reducing maintenance costs and downtime. A major challenge is the modeling of hydroelasticity, which is not yet well understood for non-uniform structures such as OWTs. Previous research on hydroelasticity during impact events focused on uniform systems, and the effect of non-uniformities on hydroelasticity has not been properly studied yet. Two characteristic non-uniformities of OWTs are the end mass (from the rotor-nacelle assembly) and the added mass around the base of the structure resulting from its submersion depth. The main research question in this thesis was: What is the effect of end mass and added mass on the hydroelasticity of non-uniform systems, such as offshore wind turbines, under breaking wave impact?

This question was answered through an experimental campaign. Experiments were conducted using a two-dimensional OWT model with varying end masses and submersion depths, which was impacted by a focused breaking wave in the wave tank at the Ship Hydromechanics lab of TU Delft. The force and structural response were measured and compared against a non-hydroelastic rigid model. Computational fluid dynamics simulations were performed using ComFlow, and a Finite Element Method model was created to get a quasi-static estimate of the model response used for comparison.

By increasing the end mass and added mass, the first natural period of the models approached the loading duration of the impact. This resulted in a peak force reduction of up to 30% compared to rigid models, indicating that the role of hydroelasticity increases as this ratio approaches 1.0. However, an effect on peak force reduction due to submersion depth for structures with similar period ratios was also seen, indicating the complexity of hydroelasticity for non-uniform structures. Comparing the structural response in the experiments against the quasi-static estimate showed larger errors for models with a period ratio close to 1.0, underestimating the response by up to 27% for the maximum deflection and 75% for the maximum acceleration. The results in this thesis show that the characteristic non-uniformities of offshore wind turbines significantly influence the hydroelastic behavior of such structures during breaking wave impacts.
... Read more

Green water events—where large masses of water wash onto a ship’s deck—pose a significant threat to breakwaters and other deck structures. The impacted structures are often treated as if they were rigid, overlooking potential hydroelastic responses. This thesis investigates the role of hydroelasticity in green water impacts on forward-tilted, flexible breakwaters through a combined experimental and numerical study. The research question of this thesis is ”What is the importance of hydroelastic effects during a green water impact on a steel breakwater?”, which was divided into four sub-questions:

RQ1: What is the relation between the variables of the impact (plate angle θ, fluid wedge angle α, impact velocity v) and the resulting load and pressure during impact of a rigid plate?
RQ2: For what combination of θ, α, v and plate stiffness does hydroelasticity occur during impact of a flexible plate?
RQ3: In what manner does hydroelasticity affect the load on the breakwater and what is the magnitude of these effects?
RQ4: What is the magnitude, shape and frequency of the deformation that occurs during a hydroelastic impact and what is the relation between the variables of the impact (θ, α, v, plate stiffness) and the resulting deformation?

Experiments were conducted using a sloshing tank that generates wave impacts that are similar to dam-break type green water events. A range of plate angles and thicknesses was tested to capture rigid and flexible responses. Load cells, pressure sensors and laser displacement sensors measured force, pressure and structural deformation. A high-speed camera was used to study the shape and velocity of the incoming wave.

Numerical simulations performed with a VOF type numerical method based on the Navier-Stokes equations provided initial estimates of force magnitudes, rise times, and impact velocities. Comparison with experimental data showed similarities between the numerical model and the experimental data, but differences were also found. In part, these differences could be attributed to the numerical method being a one-phase model which means that the effect of air entrainment was ignored by the solver.

In the experiments, a large hydroelastic effect on the maximum impact force and the force over time was found. Rise times were decreased and force maxima were increased by up to 31% or decreased by up to 49% relative to the maximum force on the rigid breakwater. Maximum deformation was reduced by up to 83% relative to deformation that was predicted by a quasi-steady analysis. It was concluded that the interaction between deformation and wave loading should always be considered, because this interaction can significantly affect the applied loads and the deflection of the structure. ... Read more

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. ... Read more

This report explores recent developments in research on breaking wave impacts on offshore structures. Through a literature review, research questions are formulated to form the basis of the thesis, along with the planned approach for addressing these questions. The report begins with an overview of breaking waves, their chaotic nature, the influence of fluid-structure interaction and effects of scaling. The literature review identifies a knowledge gap regarding the influence of time-varying mass on structural response and whether wave impact characteristics can be determined through analysis in the spatio-temporal domain. The main research question is: How do hydroelastic effects influence the structure response during specific breaking wave type inter actions? To investigate this question, a two-dimensional scale experiment is carried out using a sloshing tank to generate waves and examine their impact on a plate. The behaviour is compared to a CFD model capable of computing rigid wave impacts.
The results show that for a cantilevered plate, the relative angle between the incoming wave and the plate strongly influences the magnitude of peak forces and pressures. Different wave geometries excite distinct structural responses, and frequency analysis of the plate deformation reveals a link to the wetted interaction between the wave and the plate. Plate deformation is governed by the combined effects of flexibility, aeration, and wave alignment, which interact non-linearly to determine the overall response, highlighting the complex dynamics of hydroelastic wave–structure interaction. ... Read more

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. ... Read more

Driven by more stable wind conditions and abundant space, the offshore wind market has grown significantly, leading to larger turbines being installed farther offshore in deeper waters. Floating crane-vessels, with their large capacity cranes, can take the next generation of wind farms to new depths. However, the mitigation of wind- and wave-induced motions still remains a challenge.

This thesis explores the use of frequency domain models, full-scale data and Control Moment Gyroscopes to enhance the floating installation of wind turbine towers. The resulting contributions highlight the challenges and relevance of offshore motion compensation, advancing the state-of-the-art and contributing to the future of the wind sector. ... Read more

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. ... Read more

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.
... Read more

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.
... Read more

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. ... Read more

Accurate modelling of ultimate or maximum hydrodynamic loads is crucial for the design of offshore wind turbines and ensure the survivability for its design life time. High fidelity direct Fluid-Structure Interaction models can provide accurate results but are too computationally expensive to deploy for the many environmental loading conditions that need to be evaluated during the design phase. Indirect numerical methods that separately obtain the wave kinematics and subsequently use those to calculate the hydrodynamic loads are widely used in the industry. Often extended linear wave theory is deployed to obtain the kinematics. However, for storm wave conditions and breaking wave events, this theory breaks down and requires engineering solutions such as separate slamming models to provide conservative force estimations. Fully nonlinear wave kinematics might directly represent steep and breaking waves, omitting the requirement of slamming models.
This study evaluates the performance of the non-hydrostatic wave model SWASH in simulating fully nonlinear wave kinematics that are subsequently used to obtain the hydrodynamic loads with the Morison equation. The project focuses on the extreme events of a typical 50 year return period storm in the North Sea. Deterministic comparison of the time series of the hydrodynamic loads of the nonlinear model with available experimental data and a linear model, that served as a benchmark representing the industry method, showed mixed results. Several large overshoots were observed in the nonlinear results for non extreme events, which were not present in the experimental data. Load estimates for extreme events were of mixed accuracy, both over and under estimations of the hydrodynamic load magnitudes were observed. The study concludes that while SWASH offers valuable insights into nonlinear wave dynamics, further refinement is needed to improve its reliability in load predictions.
Future research should initially focus on refining the implementation of SWASH, tackling the large overshoots by including a wave breaking turbulence model. ... Read more

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. ... Read more

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. ... Read more

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.
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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%. ... Read more