This study investigates the hydroelastic response of very flexible, free-floating membranes in Faraday waves, with a focus on the influence of sheet thickness. The motivation for this work arises from the need to understand wave-structure interactions involving very flexible floating structures (VFFS), which are relevant for applications such as (offshore) floating photovoltaic panels ((O)FPV).

Laboratory-scale experiments were conducted using vertically oscillated membranes of varying thicknesses (20–200 micrometers) floating on a water surface. To ensure a reliable comparison and establish baseline measurements, free-surface reference experiments were first performed using silicone oil, which provided controlled conditions with minimal contamination effects. Additional experiments on deionized water allowed for direct comparison between hydroelastic and purely fluid cases. The experimental setup combined imaging, digital image correlation (DIC), and synthetic Schlieren methods to capture the coupled wave–membrane dynamics. These techniques provided quantitative measurements of both membrane deformation and underlying wave fields, including amplitudes and wavelengths, across a range of excitation frequencies and acceleration amplitudes. This enabled precise determination of the onset of Faraday-wave instabilities and a detailed characterization of the spatial deformation patterns of the floating membranes.

The results demonstrate a strong dependence of hydroelastic behavior on sheet thickness. Increasing thickness enhances the bending stiffness and inertia of the membrane, resulting in longer dominant wavelengths, higher critical accelerations, and modified wave amplitudes compared to very thin membranes. For the thinnest membranes, classified as VFFS, localized wrinkles were observed at low excitation frequencies. Their presence indicates dynamic stress variations and local in-plane tensions induced by wave–membrane interactions, phenomena not captured by standard continuum models. Furthermore, the onset of instabilities and wave amplitude behavior for thicker membranes revealed the combined effects of increased mass and bending stiffness, highlighting the transition from highly compliant to more rigid floating regimes.

Taken together, these findings provide experimental evidence for the critical role of sheet thickness in governing hydroelastic response. The results clarify how very flexible floating structures interact with surface waves and how this interaction evolves as thickness increases. Beyond fundamental fluid–structure physics, this work offers practical insights for the design and modeling of VFFS in engineering applications, such as optimizing the stability of floating photovoltaic modules and controlling wave-induced motion of thin maritime membranes. ... Read more

There is a lack of knowledge on the hydrodynamic behaviour of an AdBm Noise Mitigation System (NMS) designed to reduce sound during monopile piling, which is relevant for improving design and safety. The NMS contains panels designed to mitigate underwater noise. Model tests were performed on both an actual AdBm panel and geometry-based perforated panels, and hydrodynamic coefficients in the form of added mass (C_a), drag (C_d), and linear damping (C_b) were determined for the heave and surge directions, improving the understanding of hydrodynamic loads on the NMS.

Experiments were conducted in the Towing Tank No. 2 at TU Delft, where both forced oscillation and wave tests were performed with test conditions based on regular environmental waves. The hydrodynamic coefficients in heave and surge have been determined and show that the nondimensional Keulegan-Carpenter number (KC) is the most dominant parameter, which leads to the coefficients being expressed as functions of KC. Tests were performed with a single panel and three panels in series to study interaction effects. The main findings showed a significant decrease in C_d and C_b up to 70% within the tested KC range using panels in series, resulting in lower hydrodynamic loads compared to using a single panel. Furthermore, the results of the forced oscillation and wave tests were compared. The hydrodynamic coefficients were found to be similar for low KC values, but the forced oscillation results increasingly overestimated the hydrodynamic coefficient values as KC increased. However, the range of comparison was constrained due to limitations in the wave maker capabilities. The findings contribute to a better understanding of the hydrodynamic loads on the perforated models that reduce the knowledge gap of the hydrodynamic behaviour of the NMS, providing a basis for improving the design parameters for the deployment system.
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The increased consciousness of the negative impact of human-induced climate change has resulted in rapid developments within the offshore wind industry. With Wind Turbine Generators (WTGs) becoming increasingly larger and available shallow water sites becoming scarcer, new offshore wind solutions must be developed to accomplish the goals set by today’s policymakers. The new deep water sites available for offshore wind are not applicable for the current benchmark of monopiles and jack-up vessels. New state-of-the-art applications, such as Floating Offshore Wind (FOW), come into play in the global offshore wind market. To meet this demand, Heerema is developing installation methodologies for floating to floating installation applications. One of these methodologies is the RNA+ method (Rotor Nacelle Assembly + Tower), lifting a fully assembled WTG module in one lift from the vessel deck on top of its foundation to limit the number of critical lifts. Operability studies are required to examine whether such operation is feasible. As the FOW industry is the new kid on the block, little is known about FOW’s configuration and other influences. Therefore, this research aims to determine the effect of design parameters of the three key components of an offshore floating to floating installation on its operability. These key components are the WTG, floater, and installation vessel. For this research, the IEA 15 MW reference turbine with an altered tower, a Tension Leg Platform (TLP) provided by Intecsea, and Heerema’s Semi-Submersible Crane Vessel (SSCV) Sleipnir were used as WTG, floater, and installation vessel respectively.

A numerical base case model containing these key parameters was built to address the influence of the design parameters. This base case models the free-hanging stage of a complete WTG module suspended in the SSCV’s crane 3 m above the TLP in a water depth of 150 m. The numerical model was analyzed in the Frequency Domain (FD), considering only first-order effects. With respect to this base case, all parameter variations were compared. A mean JONSWAP spectrum was used as wave spectrum.

The results show that this installation method is sensitive to long wave periods ( > 8 s). The clearance between
the nacelle and crane-boom is deemed the most governing limiting criterion. The relative vertical Z-tip motion between the tower bottom and TLP top and the side-lead angle of the crane hoist wire are the secondary governing limits. Design parameters that influence the static clearance between the nacelle and crane-boom have the most impact on the total operability. With the current design parameters world’s largest SSCV has a limited operability for installing the modified version of the IEA 15 MW reference turbine with a single crane lift. Alterations to increase its crane boom reach and clearance are needed to perform this single lift installation. The hub height and nacelle casing size of the WTG limit the operability significantly. Furthermore, due to its relatively small size, stability and stiffness in heave direction, the TLP hardly affected the operability.
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Previous research into floating offshore wind turbines consistently shows some manner of discrepancy between numerical and experimental simulations when it comes to low frequency forces and motions. A part of solving this issue is the generation of accurate data for the calibration and validation of the numerical models, needed because of the coupled analysis introduced due to the relatively large wind force. The goal of this research thus became to generate accurate experimental data focussed on the low natural frequency of the mooring system in surge direction. This data had to be accompanied by an uncertainty assessment and should encompass multiple mooring systems for a good comparison.
In order to achieve these goals experimental model tests were set up using bichromatic wave sets in a long towing tank. The bichromatic wave sets allowed for the creation of beating patterns, targeting the specific low frequencies on and around the surge natural frequency via the difference frequencies. The long towing tank reduced the amount of reflections and clutter in the tank. Together these factors ensured accurate excitation of model at the desired frequencies.
The model used was a 1:96 semi-submersible model with three buoyancy columns supporting a central tower column. The depth in the tank being 1.25 m at model scale ensured a relatively deep testing environment. Two different mooring systems were tested, each with a different surge natural frequency but with the same semi-taut fibre rope-chain lay-out. A comprehensive measurement system consisting of force transducers in the mooring lines and wind force device and a camera tracking system enabled accurate measurement of both the force and motion of the model. Experiments revolved around examining the influence of the difference frequency in the wave excitation on the low frequency behaviour of the mooring system and floater.
From the lead mooring line force time series, it could be observed that during the surge natural frequency test the mooring line had triple the force compared to the wave tests with the monochromatic components, showing a clear effect of the bichromatic beating pattern. From the tests it also became clear that mooring system 1 had rear lines which were slack during wind and wave excitation. These slack lines had a number of effects on the behaviour of the model during the tests, such as increasing the mooring line force and surge response.
The tests thus show that the low frequency response comes from the difference frequency in the bichromatic wave sets, as was the goal. The heightening of the response around the natural frequency also shows that getting close to this frequency amplifies the effect. The comparison between the test results and the results from the OC5 project show that this is likely also the case there. At the same time the surge response does not show this heightened response. Which is likely due to the non-linear stiffness of the mooring system.
Furthermore, it was seen that the slack lines have a very large impact on the force in the mooring lines, especially on the lower frequencies and should thus be avoided at all times. The damping ratio as a parameter for the low frequency response is both physically and experimentally (excluding the slacked lined system) consistent and shows potential for development.
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Coastal communities around the world are facing significant challenges such as erosion, flooding, and storm surges. These issues highlight the need for nature-based coastal management solutions that can help mitigate the negative impacts of these events. Coastbusters Consortium developed such a solution in the North Sea. This solution is a system that utilizes a bivalve long line approach, where blue mussels are allowed to grow and eventually attach to the seabed, forming a reef structure that can help induce natural accretion of sand, attenuate storm waves, and reinforce the foreshore against coastal erosion. This reef structure can help to enhance coastal protection and reduce the impacts of these significant challenges on coastal communities.

To enhance the current long line system, it is imperative to develop numerical models that can accurately predict the forces acting on the slender cylinders in current and waves. One approach to this is to use the Morison equation, which accounts for both drag and inertia force. However, there is a lack of understanding regarding the drag and inertia coefficients in the literature that are used in the Morison equation. To address this issue, the present research conducts three experiments on a 3D-printed model of a dropper line, at a one-to-one scale, in a towing and wave tank. To treat the dropper line as a cylinder, a characteristic diameter is used. The 3D model is developed based on a thorough evaluation of the existing dropper lines at De Panne. These experiments aim to determine the drag and inertia coefficients for the dropper line in different steady and oscillatory flows, which can aid in the design of more efficient and effective bivalve aquaculture systems and integration into numerical models.

To determine the drag coefficient of the current towing experiments were conducted and forced oscillations and waves experiments were conducted to determine the drag and inertia coefficient in waves. The parameters used were based on the current and wave regimes at De Panne, and were expressed in terms of Reynolds and Keulegan-Carpenter numbers. The results showed that for continuous current, the drag coefficient of a dropper line containing blue mussels was determined to be $C_D$ = 1.2 for Reynolds numbers between $3.0*10^4$ and $1.0 *10^5$. In oscillatory flow, the drag coefficient varied between $C_D$ = 2.3 - 3.5, and the inertia coefficient varied between $C_M$ = 1 - 2.5 for Keulegan-Carpenter numbers between KC = 5 - 28.

The experiments conducted in this study included evaluations of the characteristic diameter and shape of the dropper line.
Results also showed that a difference existed between the coefficients obtained from the forced oscillation and wave experiments. Possible explanations for this difference were investigated, including free surface effects and flow differences. The results obtained from this study can be applied to the design of bivalve aquaculture systems and their integration into numerical models. These findings contribute to improving the efficiency and effectiveness of nature-based coastal management strategies for mitigating the effects of erosion, flooding, and storm surges on coastal communities. Further research is needed to fully understand the complex dynamics of the bivalve long line system and its interactions with the coastal environment.
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Floating offshore wind turbines offer opportunities to harvest wind energy at deep-water locations, where the construction of fixed-base turbines is infeasible. The dynamic power cables, which interconnect turbines and transport the generated electricity, are under large dynamic stresses due to the environmental loads and the motion of the floating platform. Limited knowledge about the structural behaviour of these cables is available, which is why there is need for new analysis and design methods. This MSc thesis presents a method for the preliminary design optimization of the dynamic power cable configuration. A parametric model of a dynamic power cable is built in the commercial software package OrcaFlex, from which motions, loads and fatigue on the cable can be calculated. Following that, a radial basis function model-based optimization algorithm is applied to find the optimal cable configuration for an arbitrary environmental scenario. The key performance indicator here is fatigue damage on the copper conductor, which is expected to be critical due to the cyclic loading on the cable and the poor mechanical properties of copper. Experiments are carried out to test the optimization model’s validity in terms of convergence, robustness and efficiency. The final method is capable of consistently finding a near-optimal dynamic power cable configuration design within reasonable time. Additionally, findings are presented about the fatigue behaviour of the DPC, what causes the fatigue damage and how to mitigate the effects. ... Read more

The penetration of the renewable energy sources in the global market has been constantly growing over the last years. This trend is expected to accelerate even more in the future due to the improving technologies, the economies of scale, the competitive supply chains and the improving developer experience. The main challenge for a faster and wider implementation of these resources is their intermittent character and the limitations that this implies. A concept that successfully deals with this variable energy output is the Hybrid Power Plant (HPP). HPPs combine at least two different sources of energy with the goal of delivering stable power with reduced fluctuations throughout the year. In addition, HPPs present various other synergies such as in operational costs or in the developing processes, that could lead to a reduced Levelized Cost of Energy (LCOE). The goal of this thesis is to assess the potential synergy of a combined wind and solar park for the case of the Energy Storage Lake (ESL) of the project Delta21. The Delta21 project has a twofold character: firstly, the usage of the ESL as a large battery integrated with green energy production and secondly the protection of the inland against floods due to high sea level or superfluous river discharge. The main challenge for this specific case is the large water level fluctuations within the lake in a daily basis due to the operation of the lake. The feasibility of a hybrid wind and solar park can be examined by various different perspectives and disciplines. Among these relevant components, the most frequently studied ones are the optimization of the energy resources, the use of common electrical infrastructure and the effect of the intermittent wind turbine shadows on the solar panels. Due to the special conditions, with the large water level fluctuations met at the lake, two more components become critical. These are the type of foundation of the wind turbines and the mooring configuration for the floating solar units. This project is evaluating a new potential synergy that regards the use of the wind turbine towers as anchor- ing points for the floating solar units. Therefore, the research is focused on the estimation of the forces acting on a floating solar unit. These forces are a combination of wind loads acting on the solar panels and the freeboard of the floaters, and wave forces acting on the submerged part of the floaters. In particular, the technology of the floating solar boat as introduced by GroenLeven and the small-scale wind turbines as designed by Dutch Wind are adopted for this project. Analytical formulations are used for the calculation of the wind loads. As for the wave forces, the linear potential theory is used and the calculations of the hydrodynamic coefficients and wave exciting forces are performed through the Boundary Element Method (BEM) software NEMOH. After the computation of first order wave forces and the corresponding responses of the floaters, the far-field approach is used for the estimation of the Quadratic Transfer Function (QTF) of the drift force. The methodology developed in this project and the accuracy of the model are validated with the use of reference data that regard the interaction of connected floating units placed in a close proximity and the corresponding second order wave forces as predicted by the far-field approach. ... Read more

Operational optimization of vessels is valuable for the planning and execution of maritime operations. Accurate and efficient models to predict vessel motions are needed to make reliable operational decisions. The wave-induced vessel response can be modelled in terms of a Response Amplitude Operator (RAO) and a wave spectrum. Uncertainties related to the parameters that govern the RAO can significantly influence the reliability of the vessel motion prediction. To decrease these uncertainties, the maritime sector has realized the potential of using vessel motion measurements. As a result, it is envisioned that a vessel response model might include an identification module that searches for model parameters using measurements of responses to make a reliable prediction.

This study presents an identification procedure to handle the inherent uncertainties of vessel model parameters, aiming to improve vessel motion prediction. The identification procedure identifies the vessel's RAO by the measured response spectrum and nowcast wave spectrum, with the goal of finding the heave and roll natural frequencies. The natural frequencies provide information on the vessel’s parameters. This is used to identify the parameters related to the mass distribution and damping of the vessel. These were found by minimizing a cost function, that quantified the difference between the measured and predicted response spectrum, using an optimization method. Identifiability analyses of the parameters were performed on two case studies.

For the first case study, a synthetic data set is created with the vessel response model to simulate the vessel motions. Tests were conducted with five different wave spectra and several vessel headings, constituting diversified scenarios. The RAO was identified by the measured response, the wave spectrum, and a sinusoidal function to describe the directional dependency of the RAO. Using the synthetic data set, the identification algorithm successfully identified the parameters with good agreement to their actual values. The second case study involved the examination of parameter identification on real onboard vessel motion measurements. In most of the cases, the RAO could be identified from the measurements and the natural heave and roll frequency was found. The identified parameters resulted from the identification procedure and improved the vessel motion prediction compared to the initial prediction, but still, deviations remained. The identified parameters are verified against a different measured data set. The results show that the identified response spectra approach the measured responses, indicating that the identified parameters are reusable.

In summary, it was found that the parameters have a great influence on the output of the vessel response model. Therefore, it is essential to have a thorough understanding of the correct operational parameters for accurate motion prediction. The established identification procedure shows to be a good addition to existing vessel motion models to identify input parameters at relatively low computational cost. ... Read more

Climate change is triggering an ever-growing demand for renewable energy. The U.S. is still far behind Europe when it comes to offshore wind energy. They have made ambitious plans to reach 30 GW in offshore wind energy by 2030 while currently 42 MW is installed. One of the main challenges in the U.S. is the installation method since a legislation called the Jones act prevents the usage of European installation vessels for shuttling (the conventional method). Building a Jones act. compliant installation vessel is a large investment which comes with risks and long lead times. Feedering is an alternative strategy, but barely any research on it is available. Here, a feeder vessel sails back and forth from the storage port to the (non-Jones act. compliant) installation vessel to supply Wind Turbine Generator (WTG) components. These components need to be lifted from the floating feeder in order to be installed. In the literature, this step is deemed to be the riskiest. However, barely any technical research is available with regards to the lift-off.

In the first thesis of this double degree program, a lift/installation sequence called the direct installation method is deemed to be highly interesting with respect to the logistics and costs. However, this research misses a technical study in order to understand if it is technically reachable to directly install these components. In this offshore engineering thesis, a barge is used as a feeder vessel and tower segments of a 20 MW WTG are chosen as the to-be lifted components. This research focuses on the pre-tension phase before the lift-off. This contains the steps where the crane of the installation vessel is already attached to the tower, pre-tension is building up and the release of the sea-fastening. Here, pre-tension is a percentage of the load that is taken in the crane before the lift-off. This research aims to increase the understanding of whether a tower can be released safely on the floating barge and what can be done in order to realise the idea of a direct installation method.

Frequency, as well as time-domain simulations, are used to investigate the problem. The results show that snap loads occur for pre-tensions up to 10\%. From 30\% and higher, the tower will start toppling. Toppling is initiated due to the inertia of the large tower segment when it is released from its sea-fastening. Toppling the tower is not allowed since this could damage the tower itself, the sea-fastening and/or other components on deck of the feeder. Increasing the limiting wave height is a must in order to make the direct installation method more practicable. This can firstly be done by using more tower segments. Therefore, reducing the size of each segment. Another option is to implement a motion compensation tool that decouples the motions of the feeder and the tower. The third option is to design a seafastening system that reduces the moment after the release, a temporary counteracting toppling system. All in all, can be stated that safely releasing a 20 MW tower segment on a floating barge is highly challenging and more research is required to solve the issues that are found in this research. This is necessary to allow the direct feeder method to be used for future offshore wind installation projects in the U.S. ... Read more

The electricity demand increases globally and requires a shift toward renewable sources to prevent the exhaustion of the planet. The shipping industry is responsible for 2-3 % of the global Greenhouse Gas emissions and Heerema Marine Contractors (HMC) identified floating solar as a promising solution to reduce the emissions, of their crane vessel Sleipnir, during operations. This study is the starting point of a technical feasibility study as a temporary energy supply for Sleipnir. The design is strongly focused on the temporary deployment and limited occupied deckspace during transit. Current floating solar systems are commonly designed for permanent deployment as supporting structure for rigid glass photovoltaic (PV) panels. Furthermore, modular constructions are used to limit the transportation costs however they are not designed for temporary deployment. A new PV innovation is flexible lightweight films which allow a more flexible supporting structure. A flexible thin sheet can be spooled on a drum to make temporary deployment possible. The influence of the wave loading on the coupled hydrodynamic behaviour is evaluated since wind and current loading are predictable based on previous research.

The structural design parameters of the thin sheet and drum are designed to mimic the excitation motion since wave structure interaction has been minimized to reduce the mooring force. Therefore, the draft must be low and the characteristic length related to the bending stiffness of the sheet should be smaller than the excitation wavelength. The draft of the drum should be low to have a natural heave frequency higher than the excitation frequency.

The coupled hydrodynamic response for head loading is evaluated with model tests in a towing tank. The concept is scaled according to Froude to ensure the surface waves, which are gravity-driven, are properly scaled. Regular waves are chosen based on the workability wave spectrum of Sleipnir. The roll and heave response over the frequency domain is indicated by analyzing the stable response at certain frequencies. The motions of the drum are obtained with the use of object tracking based on video recordings. The force within the connection of the system was measured with a force transducer whereas the mooring force was measured with a newly developed 3D-sensor.

It turns out that the heave motion of the system mimics the excitation motion over the wavelengths resulting in small drift forces. Significant rotations of the drum were observed for the longer wavelengths leading to water pumping over the sheet. The overturning moment is driven by the dynamic pressure over the drum diameter and the measured force in the connection generates a counteracting moment. The connection force is proportional to the buoyancy required to submerge the sheet and the acceleration of the free-floating sheet.

The feasibility of an OFPV concept for Sleipnir is demonstrated but the rotations have to be reduced by lowering the natural roll frequency. The drum dominates the coupled hydrodynamic behaviour compared to the sheet. Either the dimensions of the drum should be lowered or the thickness of the sheet must be increased. Decreasing the drum diameter is favourable over a thicker sheet since that would increase the characteristic length. Another option is to adjust the geometry of the drum to a shape where increased water displacement is required for the roll motion.
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With the climate goals of the Paris Agreement and the European Green Deal, countries need to reduce their carbon footprint and increase their renewable energy production. For countries with high population density, land-based photovoltaics (solar) takes up valuable space. Therefore, solar application at sea is considered.

One of the proposed designs is by the Joint Industry Project (JIP) Solar@Sea II. The JIP structure is flexible, inflatable, and considerably smaller than Very Large Floating Structures (VLFS). The goal is to mitigate the installation and transportation disadvantages associated with VLFS. Combining multiple of these singular structures in an array allows for the same operational solar footprint as would be the case for a single VLFS.

To keep the structure at the intended location, a mooring system needs to be designed. For the mooring line analysis, the motions of the attachment points of the mooring lines to the floating structure must be deter-mined. These motions are dependent on the interaction between the fluid, structure, and mooring line system. The development of a numerical method that is able to determine the flexible motions of the structure is required. This method should be suitable for the initial design stages of the structure and mooring system.

A numerical method for the deep-water regime in frequency domain was developed. The structural deformations are determined by means of the Finite Element Method (FEM) in ANSYS. The fluid interactions with the structure are determined by means of a lower order Boundary Element Method (BEM). The combined effect of both structural motions and fluid behavior is captured in an equation of motion. The motions of the flexible structure can then be determined by solving the equation of motion. The method is written in Python and the interaction with ANSYS is achieved by means of an ANSYS APDL interface.

The numerical method was successfully verified by comparison of results with analytical solutions. The nu-merical method is validated by comparing the numerical result with experimentally determined responses of a 1:1 scale JIP structure to incident regular deep-water waves as measured by MARIN. Suitable deep-water test cases were determined from the MARIN data. The undisturbed numerical wave was compared with the undisturbed deep wave measured by MARIN. Finally, three test cases were selected for the validation of the interaction of the structure and the wave. These consisted of a wave longer than the structure, a waveslightly shorter than the structure, and a significantly shorter wave than the structure.

The numerical method was able to accurately predict the structure motions for waves longer than the structure. For waves shorter than the structure, the error between the numerical method and the experimental data increased as the wavelength decreased. The errors found can likely be attributed to the nonlinear interaction of the ballast bags, which are not considered in the current numerical method. This cannot be confirmed based on the available experimental data.

The presented work is part of a larger intended method, which is not yet finished. Satisfactory results were found for the components considered in the verification. The validation provided points for improvement for the current numerical method. The response to waves longer than the structure can be determined accurately. The method is less accurate for waves shorter than the structure. The recommendation is therefore to research the effect of the ballast bags on the structure response in shorter waves. This could result in a better approximation for structural responses in shorter waves. ... Read more

The idea of shared mooring systems has been introduced in the offshore wind farm industry as an effort to lower the overall installation costs for floating offshore wind farms. The reason for building such plants is that using floating turbines allows them to move to deeper waters, reducing visual and environmental impact and allowing footprint reduction, with the downside being an increase in the total installation costs.

Because shared mooring lines create couplings between neighbouring turbines, the use of shared mooring lines adds complexity to the design and modelling of offshore floating wind farms. So far, these systems have been investigated and analysed using either quasi-static or fully dynamic models. With the goal of using a method that can be more accurate in terms of dynamic behaviour of the mooring lines than the quasi-static one and less time-consuming than the fully dynamic one by using a system with less degrees of freedom, this project seeks to further explore the idea of shared mooring lines within a quasi-dynamic model.

A quasi dynamic system is not as complex as the dynamic model but more rigorous than the quasi-static one because it includes terms related to the drag and inertia of the mooring lines. Inertial loads on mooring lines can alter the tension of the mooring line at the fairlead, while drag forces on mooring lines will dampen platform motions, especially for slowly varying motions. These are the two reasons why it is critical to include these effects in the model. In this case inertial terms have not been considered, focusing mainly on including drag terms for both the anchor and shared lines together with the geometric and elastic stiffnesses.

To begin with, the quasi-dynamic model has been applied to a two-turbine system, constrained to move only horizontally in surge as a model assumption. The model has been implemented in Matlab as a two-degree-of-freedom system.

The ultimate goal of this research is to see if this approach can be used instead of conducting a thorough dynamic analysis. As a result, in addition to the Matlab model, an equivalent system in SIMA has been built to perform a full dynamic analysis and allow comparison of the two models' findings.

Based on the model results, it appears possible to obtain good accuracy for both motion amplitudes and tension values in a shorter amount of time than a full-dynamic analysis. Indeed, some correlation has been observed between the results obtained with the quasi-dynamic model and the full dynamic one, especially at lower forcing frequencies. ... Read more

Floating structures have developed significantly in recent years. As land is becoming scarce, the use the water surface will contribute to ease this scarcity. Therefore in recent years, floating structures covering large areas have been developed, also called flexible floating structures.
In this project the focus is set on the mooring system very flexible floating structures (VFFS). At the TU Delft, two towing tanks can be used to investigate the mooring system of VFFS, however first a reliable measuring system is required that is able to examine a specific part of the mooring system. Conventional setups that measure the mooring forces consist of large instruments, as these instruments only have a small effect on their investigated structure (vessels). The response of VFFS is dominated by elastic deformations and differs from conventional rigid structures. For VFFS, these type of instruments will have a large effect on the structure motions and thus these conventional setups cannot be used. Therefore, a new measuring system is required to conduct small scale experiments with VFFS, and the following objective is formulated: Develop an instrumented mooring system for VFFS at model scale for the towing tank at the TU Delft and determine its accuracy.
A new concept is developed in this project. This concept resulted from an extensive concept development where all functions of the system were analyzed. With the use of a Morphological Chart and a Multi Criteria Analysis the best concept was selected. For this concept, it was determined that the focus should be on the sensor configuration and calibration procedure.
First, the optimal sensor configuration of the concept was specified by analysing the working principle of the concept. Second, the calibration procedure was further analyzed. From this analysis, three calibration procedures were developed: the single sensor calibration matrix, the full fixed calibration matrix and the full rotated calibration matrix. From literature and theory, it was not possible to determine in advance what calibration procedure should be selected, and therefore the performance of the procedures were verified with experiments. All calibration procedures were executed, whereafter the performance of the different procedures were compared. The two main considerations for the comparison were the accuracy and the usability of the procedures. After performing the comparison, the main conclusion was that the full upright calibration procedure is the optimal procedure.
To verify the concept under realistic conditions, an example application was performed in the towing tank No.1 at the TU Delft. By doing this, the concept has proven to be suitable to measure the mooring force and transform them into usable data.
In this project a new concept was developed into a working system. This system forms an excellent base for extensive research into the mooring system of VFFS, and is a good addition to the measurement instruments for the towing tank at the TU Delft. It is concluded that the system is able to measure the mooring forces and the direction. The accuracy of the system still has to be improved, and with additional research the working concept can be further developed. ... Read more

Offshore floating photovoltaic (OFPV) gain increasing attention in research and from project developers, due to the need for more renewable energy production. OFPV must be proven to be both technically and economically feasible to be a viable option in the offshore environment. Several incidents of floating PV systems have been observed for mainly inland applications of floating PV. These incidents, which occurred due to malfunction of the system, may induce several risks. Hence, the risks associated with OFPV technology should be assessed, as these risks may affect the feasibility and overall support of the developments. However, the current early stage of industrial development present a gap in the standards and guidelines, as well as the availability of data on all levels, to perform a risk assessment. The aim of this research is to outline the process of assessing technological risks resulting from loss of function (failure) of operating offshore floating photovoltaic systems. The general steps of risk assessment procedure are performed on an OFPV system in this thesis. In the initial step, a generalized OFPV system was modeled and analyzed, which includes the functional decomposition of the system to element-level and a quasi-static load analysis. Then, a preliminary hazard analysis (PHA) is performed to identify a broad scale of hazards and failure mechanisms of the OFPV system in its environment, followed by a qualitative failure mode and effect analysis (FMEA), to relate the identified hazards to the components of the OFPV system. Based on the FMEA and observed incidents, it was found that failures of the mooring lines are a major threat to floating PV systems. To determine the quantitative likelihood (probability) of OFPV system failure in relation to its components, a reliability analysis was performed with a fault tree analysis (FTA). The fault tree analysis includes sensitivity analysis with Birnbaum and Fussell-Vesely importance measures, to allocate the most important elements that contribute to failure of the system. Due to lack of probabilistic data for OFPV, element failure rates were estimated based on substitute data sources, which made the results are prone to data uncertainty. Therefore, structural reliability analysis is included, since this systematically includes uncertainties and correlations, by providing a probabilistic method to determine the failure probability of an element of the FTA. As mooring failures were identified as a major threat to OFPV, this element was analyzed in more detail. Initially, a limit state function was defined to describe the condition under which a mooring chain failure occurs, in terms of random load and resistance variables. The first order reliability method (FORM), including the Rackwitz-Fiessler transformation and the Rosenblatt transformation, was used to iteratively determine the probability of failure of the failure event. Metrics to assess the contribution of the random variables to the probability of failure are included with sensitivity factors. To determine the consequences of mooring failure, hypothetical accident scenarios were modeled with an event tree analysis (ETA). Finally, the results obtained from the previous steps of the risk assessment process are summarized and evaluated. The risk matrix and ALARP principle were introduced, to evaluate the acceptability of the risks. In addition, the contributors to uncertainty of the results are qualitatively assessed per step. This research presented the tasks and potential difficulties that an analyst may encounter when performing a risk assessment for an OFPV system. Guidelines and standards for risk assessment of OFPV are desirable to evaluate the technical and economic feasibility of several concepts, towards further industrial development of the technology. Moreover, this research did not intend to obtain actual quantitative results for a specific OFPV design or concept, mainly due to the lack of descriptive and probabilistic data. With further development of OFPV technology, and as more data becomes available, this research can serve as a foundation for future risk assessments of offshore floating photovoltaic systems. ... Read more

This study is focused on the decommissioning of a jacket with a heavy lift vessel. During the transport the crane suspended jacket hanging in air can start swinging due to swell waves. This can result in undesired risks such as a collision between the jacket and the vessel. To prevent this undesired risks from occurring a solution must be sought. A possible contingency scenario to prevent this risks from occurring is to (partly) submerge the jacket. The aim of this study is to investigate in which way the jacket motion response changes if it would be in the water. A research methodology will be derived to investigate this. This will be done by investigating the a jacket transport with the Thialf. As a starting point, and for later comparison, the behaviour of the a jacket in air, while it is free hanging from the cranes of the Thialf, will be investigated in a frequency domain solver Liftdyn. The resulting modes and response between the model and measurements indicate a good starting point. Thereafter the jacket will be lowered in to the water and the resulting forces will be determined. The forces on the jacket will be determined using the linearised Morison equation. For the damping two approaches are investigated, i.e. the absolute velocity approach and the relative velocity approach. Subsequently different submerged depths are investigated and the effect of adding tugger winches is investigated. The operability for the different scenarios is derived and compared. It is observed that by submerging the jacket the pitch mode is limiting the operability for wind waves. This mode can be damped by adding tugger winches. By submerging the jacket deeper the operability improves. For swell waves the operability is improved when the jacket is submerged. It is concluded that submerging the jacket can be used as a contingency scenario for a swell train, but it is crucial to know the behaviour of the jacket. Because submerging it at the wrong depth could lead to large resonant responses. And by understanding the behaviour of the jacket tugger winches can be used to damp out a specific modes. ... Read more

This thesis presents the potential of solar energy to answer the increasing demand for sustainable energy. Multiple floating solar parks are installed on inland water bodies. This trend is a result of the lack of surface for PV systems and a result of legislation on national and international governmental levels, for the reduction of greenhouse gasses. This inland technology is currently well established. Still, in densely populated areas such as the Netherlands supplementary surface could be beneficial to unlock the full potential of solar energy. Ocean and sea surfaces are a possible, but challenging solution to this problem. These offshore water surfaces could be used for electricity or synthetic fuel production. Offshore locations are attractive because of the abundant availability of surface. Still, an OFPV-system needs to overcome multiple technical and non-technical challenges and requirements. This challenge demands technical solution within the constraints of a suitable business case. Furthermore, the impact on people and ecology also needs to stay within ethical borders. An OFPV can be subdivided into three physical subsystems: an energy converting system, a position monitoring or mooring system and a floating support structure. Multiple concepts were proposed in literature or by the industry. All proposed concepts are variations of support systems that provide a surface for conventional PV-technology. It also was concluded that none of the proposed concepts is fully developed. Currently, only one system is in the testing phase. The current state of the technology, and the high potential of an OFPV are the drivers behind the search for a new concept design. In this thesis, the requirements for an OFPV support structure were formulated, and a brainstorm resulted in the buoy and beam concept for further research. The buoy and beam structure is a system in which submerged beams form a triangle grid are held together by floating buoys. The buoys support triangle platforms, by carrying the corners of the platforms. All connections between the buoys, beams and platforms are fully hinged. Because of the hinges, the structure can move with the waves and therefore mitigates the loads it experiences. In the second part of this thesis, the feasibility of the buoy and beam structure was assessed. It was concluded that the heave response is the most critical response to the feasibility of the concept. When the platform would heave too much, the waves will slam into the platforms and the solar panels, leading to damage and excessive loads on the structure. Consequently, the heave response and the relative wave height seen from the moving triangle platform are researches and modelled. The structure was identified as a hydrodynamic transparent structure. Consequently, the relevant hydrodynamic loads are obtained with the Morison equation and Airy wave theory. All damping terms are linearised. A differential equation incorporated the constrains for both, the construction and the hydromechanic loads. The mechanical equations can be linearised by assuming small angular displacements of the beams. This differential equation is implemented in a calculation tool in MATLAB. The code is verified over multiple steps. The tool made it possible to obtain the response based on the topology and the main dimensions of the buoys and the beams. With this calculation tool relations between the dimensions of the buoys and beams on one side and the frequency response, on the other hand, are identified by simulating different variations of a simplified starting design. The following conclusions were made on the eigenfrequencies. Firstly, it was noted that the eigenfrequencies of the system are in proximity to each other. Secondly, it was shown that the first and second eigenfrequency could be calculated in a simplified manner. Thirdly, it was seen that the buoy diameter to mass ratio has a positive linear correlation with respect to the eigenfrequency, i.e. the stiffness to mass ratio of the system severely influences the eigenfrequency. Furthermore, it was seen that an increasing beam length has a slight linear correlation with the eigenfrequency. Also, it was concluded that a large hexagon system (for example, nineteen buoys) could be beneficial because a hexagon structure creates a large surface with respect to the number of buoys. Moreover, in reality, the response should be lower than the model prediction, as a result of the high number of beams that cause damping. Additionally, a hexagon system should have a minimal change in response for different incoming wave angle. To obtain a feasible system, the response peaks of the structure should not overlap with the wave spectrum. Based on the additional simulation, it was concluded that it should be possible to design a system with a low eigenfrequency resulting in a system that will only slightly move with the waves. Additional relations between the dimensions of the buoys and beams and the response peaks were found. The requirements to obtain a low eigenfrequency and a modest response match, which positively influences the feasibility. Some further steps on the research of the heave response and the relative wave height are needed. The calculation tool can still be improved. A preciser estimation of the response peaks is of significant importance. Some further iterations must be executed to determine the full potential and feasibility of the concept design. Furthermore, in a scientific point of view, it will be highly interesting to validate the model on a model scale. In the development of the buoy and beam structure, additional critical responses need to be researched. The forces in the hinges and the response in horizontal direction need to be known for further assessment of the feasibility. Next to the technical aspects, the human, ecological and economic implications of the buoy and beam structure need to be researched. ... Read more

Wave impact tests are performed in the LNG industry to design the structure of the cargo containment system (CCS) of the vessels and to study the physics of breaking wave impacts. The state-of-the-art methodology divides the wave impact flow into two parts: the global flow and the local flow. The global flow considered as the solution of the incompressible Euler equations for the liquid and gas in the tank; and the local flow as a perturbation of the global flow. However, researchers have found it to be a challenge to generate repeatable global flows of focused breaking waves and, therefore, to draw definitive conclusion about the contribution of the global flow variability in the impact pressure variability. In this context, a new facility was designed and built at MARIN in the framework of the SLING project to further investigate the physics of sloshing impacts: the Multiphase Wave Lab (MWL), a wave flume of 10 m in length, 0.6 m in width and water depth of 400 mm.

This thesis aimed to identify and evaluate the sources of variability of the global flow of focused waves, to define the repeatability criteria and to determine the theoretical conditions that would lead to global flow repeatability in the MWL. To achieve the objectives, both theoretical and experimental work have been required.

Three main sources of global flow variability were identified: (1) water depth variation, (2) long bounded waves (seiching) and (3) currents induced by seiching. These drivers of variability were modeled in a wave generation and propagation algorithm from which the sensitivity of the global flow to the sources of variability has been addressed. The results showed that the water depth is the most critical driver of variability and that repeatability would be achieved if its difference between experiments is below 0.5 mm.

Based on the sensitivity study and the characteristics of the wave maker, a criterion to experimentally quantify global flow repeatability was derived from the Sobolev norm of the Fourier space of free surface elevations at a distance from the focal point. To validate the theoretical value, impact waves were generated using a wave focusing technique. Linear wave and wave making theories were used to compute the paddle motion generating the wave impact of interest. The breaking wave was designed using a two-parameter Ricker amplitude spectrum formulation, which defines the contribution of each frequency to the total breaking wave energy and therefore changing its characteristics (crest thickness, crest stability, gas pocket size…). Image processing techniques were used to measure wave maker motion and free surface elevations from video recordings.

While more repetitions are required to confidently conclude about the validity of the criterion, the experimental results showed that when the ‘dissimilarity’ value was below the theoretical threshold, exceptional repeatability of the global flow was obtained. ... Read more

The use of renewable energy sources like solar and wind energy for the generation of electricity are expected to increase in the coming decades. However, these sources are intermittent. Therefore Electrical Energy Storage (EES) systems are needed in the near future to assure a reliable electricity supply. These systems should also be able to deliver power in longer periods of time with low generation from wind and solar energy sources. An example of such an EES system is the GreenBattery, developed by AquaBattery. This is a flow battery that stores the electricity by splitting salt water in an acid and a base. Because the main component of the GreenBattery is salt water, this battery is safe in use, as it cannot catch fire or explode. Furthermore, this makes the battery environmentally benign and cost competitive with other EES systems. The main focus of this thesis is to make a concept design of a GreenBattery that can be placed on a water like a lake. This concept is evaluated in a case study regarding the energy storage lake of the Delta21 project. This battery could provide a reliable backup electricity source for e.g. the Port of Rotterdam or stabilise the electricity output of renewable sources. Therefore, the main research question of this thesis is: ‘How can AquaBattery’s GreenBattery be realised on the energy storage lake of the Delta21 project, providing long term storage?’ To answer this question, first the design basis is explored. Based on this, concepts are generated using a morphological chart method and the most feasible concept is chosen using a multi-criteria analysis. This chosen concept is worked out in more detail, after which its technical and economic feasibility is investigated. The most feasible concept turned out to be a floating concept. This concept uses floating rigid tanks to store the different liquids present in the GreenBattery. The power unit, where the electricity conversion takes place, is placed on top of these tanks. Furthermore, the feasibility of integrating a solar photovoltaic system on the floating tanks is investigated, because of the large area available for multiuse on such an island. The resulting dimensions of the floating tanks is 98 by 98 by 3 m (length, width, draft). Four of these tanks can be coupled together to form one floating island. Such an island can store about 560 - 800 MWh of energy, depending on the required storage duration and about 3.2 MWp of solar panels can be placed on top of the tanks. In total, about 300 of such islands can be placed on the energy storage lake of the Delta21 project. Such an island is technically feasible, because the natural periods are larger than the expected wave periods. Therefore, the motions and forces of the island will be minimal. Furthermore, in terms of costs, the floating GreenBattery is cost competitive with other EES systems and therefore will be economically feasible as well. ... Read more

Floating breakwaters are applicable in several offshore applications to protect downstream structures from excessive wave loads and to reduce their motion response. This thesis investigates the impact of the leading platform, functionally a breakwater, on the RAOs and response of the platforms behind it in frequency domain. The structures are only hydrodynamically coupled. All models were analysed for head waves only. Variables in model design are investigated using a combination of diffraction software and solving equations of motion in six degrees of freedom for each body. The variables are the gap between the breakwater and the first platform, the width, i.e. side perpendicular to wave direction, of the breakwater and the gap between platforms. Base case dimensions for the platforms and breakwater are chosen based on the natural frequencies and wave transmission coefficients respectively. These choices are made to function well for the wave spectrum at the chosen site. Each case analysed included a breakwater and 10 platforms downstream of it. It was found that he imperfect efficiency of a breakwater means that the first few platforms behind it act as breakwaters too; albeit for much lower wave energies. Increasing the gap between the breakwater and the first platform behind it results in a decrease in RAO of the platforms due to increasing hydrodynamic coupling. The impact on pitch RAOs is greater than the impact on the heave RAOs. Increasing the width of the breakwater results in minimal reduction in RAOs of the platforms behind it at large gap sizes. At small gap sizes, there is an adverse relationship. Hydrodynamic coupling between the platforms can lead to shared natural frequencies within the design frequency range, leading to a large motion response. This can be prevented by changing the gap size and thereby the hydrodynamic coupling and moving the natural frequency outside of the design range. A breakwater and multiple platforms downstream of dimension L=100 m, B=100 m, T=5 m, with a gap of 80 m between the breakwater and platforms and 100 m between the platforms themselves was shown to effectively reduce the motions of the downstream platforms. The first few platforms exhibit higher heave than the other platforms, but similar pitch to the other platforms. Therefore, their use cases in a floating city must be chosen accordingly. The potential negative impacts of hydrodynamic coupling between the platforms means that the platforms must be further apart from each other, resulting in floating cities with a much larger footprint than previously expected. Further research into the motion response for 2D structure layout and different wave directions would be interesting follow-ups to this thesis. ... Read more

Jumbo Maritime is a shipping company active in the heavy lifting market. Jumbo uses stabilisers to enlarge the stability of the ship during lifting. By using stabilisers Jumbo can perform heavy lifts with a relative small ship, this is one of the major advantages. Downside of using stabilisers is that the installation and de-installation is a time-consuming process. Besides that the usage also causes safety issues, more and more port authorities no longer approve the usage of stabilisers. Research is done in finding alternatives for the use of stabilisers, focusing on alternatives for new ships that need to be build. Looking at Jumbo’s position in the market shows the following unique selling points, for the J-type: • Limited draft compared to its competitors • Length < 150 meters On the one hand the usage of smart stabiliser to speed up the installation and de-installation of stabilisers is looked at. Because smart stabilisers are still stabilisers, also alternatives are analysed that do not require a stabiliser at all. By widening the ship, the ship can be stable enough to perform lifts without extra support from stabilisers. A model is created to be able to determine the required width for each concept. This model is generated based on the input and requirements set by literature, market analysis and Jumbo’s specifications. Some of these requirements are: limited dimensions, minimum required stability, anti heeling and deadweight. The four concepts that are created as input for the model are: • Base concept, concept 1..., Concept 2... Concept 3...Concept 4...: A case study is performed to be able to compare the concepts. Three cases are created, which are actual relevant cases for JumboMaritime: For each concept the costs are determined to sail the certain case. Because the heavy lift market consists of somany single jobs that differ a lot on weight, complexity and distance it is difficult to map the revenues. The costs are determined per job, to be able to compare the different concepts. Costs consists of the capital costs, fuel costs and operational related costs. Besides cost per job, also cost per ton/mile is calculated to determine the economical speed. For case 1 the economical speed is 14 knots, which is similar to the required design speed. This means that the ships sails most cost efficient at this speed. The time that is saved by eliminating the stabiliser is used to reduce the sailing speed. In this way the same amount of jobs can done in the same time. This research shows that the saved fuel consumption as a result of slower sailing can compensate the longer sailing time and increased resistance for a wider hull concept. Comparing the concepts in the different cases shows that concepts 3 and 4 are more beneficial in case the stabiliser usages goes up. It can be concluded that awider hull shape can operate at 2-5%lower costs. The fuel consumption of the V-shape decreases more significantly at lower drafts. It is an advantage at ballast sailing or during sailing light cargoes. Besides the lower costs, the increase of deck space for the wider concepts is added value in the heavy lift market. An other important advantage of the wider ship concepts is the elimination of safety issues because the usage of stabilisers is not needed any more to lift heavy cargoes safely. ... Read more