Offshore wind energy has become a crucial element of the global energy transition, with the North Sea being a major hub for offshore wind farms. As first-generation farms approach the end of their operational life, decommissioning offshore wind export cables has emerged as a significant technical challenge. This thesis focuses on the feasibility and limitations of using the cable pullout method for decommissioning offshore wind export cables, particularly examining the influence of burial depth and soil conditions.

This research is structured in two main phases. The first phase involves a comprehensive literature review to identify critical knowledge gaps and establish an overview and understanding of existing decommissioning practices. The second phase employs simulations using OrcaFlex software to model and analyze various cable pullout scenarios. These simulations focus on determining the limits and constraints imposed by burial depth and soil relative density for sandy soils commonly found in the North Sea.

Key findings from the literature study underscore the complexities of decommissioning, which encompass legal, environmental, economic, and technical considerations. One of the main conclusions is the importance of adopting a 'design for decommissioning' approach during the initial cable installation. This involves designing cables and selecting burial depths that facilitate easier future removal, thus promoting sustainability and cost-effectiveness. Additionally, this approach can help mitigate potential environmental impacts and regulatory challenges associated with cable removal.

Soil modelling plays a crucial role in understanding how burial depth and soil conditions influence resistance forces during cable pullout. Factors such as shear strength and burial depth are analyzed to determine the forces that oppose cable recovery. The model includes scenarios for fully drained, fully undrained, and partially drained uplift resistance, implemented in a Python script to simulate real-time resistance during pullout operations. To address buried cables, an external soil model is integrated into OrcaFlex, allowing dynamic simulations of soil resistance during cable pullout.

The simulation results with a 525 kV HVDC export cable reveal how soil resistance significantly increases with greater burial depth and higher soil density. These findings highlight the critical importance of burial and soil conditions in planning decommissioning operations and suggest that additional deburial techniques may be necessary when cable pullout is not feasible.

This thesis provides valuable insights and recommendations for future research and practical applications, aiming to support the offshore wind industry's evolving needs and enhance the sustainability of decommissioning processes. These findings are crucial as the industry anticipates a substantial increase in decommissioning activities, with offshore wind capacity expected to continue to grow.

The offshore wind industry is rapidly advancing, expected to play a crucial role in renewable energy production, with a growing emphasis on addressing associated environmental concerns, particularly the impact of underwater noise generated during the installation of wind turbines. In response, bubble curtain technology (BCT) has emerged as a potential solution. While the BCT operational concept has been demonstrated in practice, a detailed investigation related to fluid dynamics, geometric characteristics, and operational parameters has yet to be conducted. It's important to note that this need for investigation is specifically applicable to large BCT systems. This research aims to enhance the understanding of unsteady compressible flow inside BCT hoses and examine the impact of operational parameters on the pneumatic aspects of the bubble curtain system along the hose length.

This thesis systematically defined its fundamental objectives, beginning with the development of a 1D unsteady model for compressible flow in pipeline systems. This model, which served as the basis for subsequent studies, was verified against the existing literature, revealing a 2-4% difference in flow rates and pressure responses. Subsequently, the scope was broadened to include nozzle configurations, which were gradually included in the pipeline model. The model evolved through configurations with one, three, and five nozzles, where larger diameters increased dampening on transient pressure fluctuations. As the research progressed, the final model, incorporating five nozzles, was used as the basis for the scaled Bubble Curtain Technology (BCT) model.

A sensitivity analysis for this study was carried out utilizing parameters from existing research. The sensitivity study specifically emphasized the influence of geometric (hose and nozzle diameters, hose length, nozzle spacing) and operational factors (discharge coefficient, water depth, air flow rate) on the flow dynamic of Bubble Curtain Technology (BCT). The main findings from this research included reducing backflow with smaller diameters and lowering reverse flow with greater discharge coefficients and airflow rates. Changing the hose length and nozzle spacing proved effective for adjusting the required flow rates. The investigation also found that nozzle diameter and discharge coefficient had a considerable impact on nozzle flow rates, with a 2-3% increase over reference values at the maximum value range. Other geometric and operational parameters in the tested ranges had a relatively lower influence on the nozzle flow rates or generated pressure variations.

The scaled BCT unsteady compressible flow dynamics model presented in this thesis is still in its early stages of development, but it can serve as a basis for the development of full-scale pneumatic models that can enhance BCT and lessen the environmental impact of offshore wind farm operations.

This thesis investigates the erosion process of cohesive soil due to a submerged, moving, inclined water jet. This study provides a visualization and description of the failure mechanics of cohesive soil due to a submerged inclined water jet and develops a new equation for estimating the erosion process of cohesive soil while including the angle of the water jet as a parameter. Figures providing support to this new equation are also provided in Chapter 7. The primary variables tested are jetting angle, stand-off distance, forward velocity, and jetting velocity.

The study is guided by two primary research objectives. First, to visually and descriptively understand the failure mechanics of cohesive soil subjected to a submerged inclined water jet. Second, to develop a method of predicting the erosion process of cohesive soil, considering variable jet angles, stand-off distances, forward velocity, and jet velocity of a submerged inclined water jet.

Experimental testing was conducted in the "Dredging Lab" of Delft University of Technology. The experiment involved eroding cohesive soil blocks using half of a circular nozzle placed along the wall of a flume. This nozzle configuration allowed for the nozzle to be visible during testing. I designed several half nozzles that were tested in a flume provided by TU Delft. Nozzles with a variety of nozzle diameters were designed to have either a 25, 45, 65, 90, 115, 135, or 155 degree jetting angle. Seventy-two clay blocks with a known undrained shear strength were used as the cohesive test soil, allowing for immediate replacement after each test. Detailed experimental procedures are outlined in Section 5.3, offering insights into the design and execution of the tests.

The data analysis provides evidence that the jetting angle has a notable impact on the erosion process of cohesive soil. This includes a “deflecting jet” failure mode, the formation and prediction of the sediment plume, and the estimation of the erosion cavity depth. The failure mode of the soil can be seen in Section 5.5 and data analysis figures are provided in Chapter 6. Section 7.1.2 provides a new estimation for the erosion depth of cohesive soil using an inclined water jet (Equation 7.17). This study contributes to our understanding of cohesive soil erosion and methods of estimating erosion processes. The findings emphasize the crucial role of the jetting angle and provide a foundation for future research aimed at refining erosion prediction models and exploring additional parameters influencing the process. Practical applications may include improved design considerations for projects involving water jet erosion such as deep-sea mining, water injection dredging, trailing suction hopper dredgers, as well as other dredging processes involving water jetting.

The current supply of critical materials falls short of keeping up with the required pace to comply with the commitment governments worldwide made towards a decarbonised energy mix by 2050. To avoid dependency on large suppliers, lower the pressure on land mining expansion and still meet the metal demand of the rapidly growing green economy, more companies are turning their focus towards the deep-sea, where these critical metals are present in the form of nodules. However, sediment plumes from nodule collection can negatively impact marine ecosystems and hinder mining operations. This research investigates how slope angles and directions affect the near-field dispersion of turbidity currents discharged from a moving source during deep-sea mining (DSM). The study focuses on the Clarion-Clipperton Zone (CCZ), rich in the so-called polymetallic nodules.
Experiments were conducted at TU Delft's Offshore and Dredging Laboratory using a modular flume tank with an experimental setup scaled at 1:20. The configuration included a moving cart and diffuser system on a sloping seabed, with precise control over slope angles and velocity ratio. A homogeneous suspension of glass beads (used instead of CCZ sediment) with water was prepared in a mixing tank and discharged into the flume to simulate a turbidity current. Measurement techniques involved the acoustic Doppler velocimeter (ADV) and ultrasonic velocity profiler (UVP) sensors for mixture concentration and velocity data, complemented by multi-angle camera footage for visual analysis. Various slope angles (-5° to 5°) and velocity ratios (source velocity over discharge velocity, ζ = 1.00 and 1.25) were tested to observe their effects on the behaviour of a turbidity current, with device calibration and controlled experimental parameters ensuring accurate and comparable results.
A notable phenomenon was observed during the experiments, namely the formation of a bulge directed towards the diffuser. A higher velocity ratio and steeper slopes led to more significant bulges, with the bulge length and impingement angle increasing with slope steepness. Regarding velocity ratios, higher ratios resulted in larger dispersion angles and larger turbidity current heights in downhill driving experiments, while uphill conditions showed an inverse trend. The slope angles influence the dispersion by reducing the dispersion angle for steeper slopes in downhill driving experiments. Data analysis from the UVP, camera footage, and concentration measurements support these findings.
Future research should focus on influencing factors on bulge formation, assessing the influence of the concentration on the dispersion of a turbidity current, identifying parameter ranges for floating plumes, conducting experiments with reduced source velocities, scaling down experiments and using real CCZ sediment in saltwater.

The increasing demand for renewable energy and the need to achieve global decarbonization goals have led the energy sector to explore remote areas where Floating Offshore Wind Turbines (FOWTs) are expected to play a significant role. These far from shore located turbines harness strong and consistent winds, but their deployment in deeper waters poses challenges. In addition, the intermittent nature of wind energy, grid congestion, and transmission losses further complicate meeting the energy demands. Combining FOWTs with offshore hydrogen production offers a promising solution by reducing reliance on electrical infrastructure. Green hydrogen, produced from renewable sources, provides a versatile and proven option for decarbonizing hard-to-abate sectors. The integration of hydrogen production directly from a FOWT presents a compelling alternative, but the transportation of hydrogen from Hang-Off Point (HOP) at the FOWT to the Termination Point (TP) at the seabed through flexible pipes requires ensuring the structural integrity under static and environmental loading.
This research evaluates the impact of various parameters in lazy wave configurations on the tension and curvature behavior of flexible pipes used for hydrogen transport in offshore wind-to-hydrogen systems, utilizing simulations performed in OrcaFlex (OF). The research shows that failure modes, tension, overbending, and compression, are only exceeded in non-optimal wave configurations, with the Minimum Bend Radius (MBR) being a more critical constraint than the Minimum Breaking Load (MBL). However, both limits are breached when compression, looping at the Touch Down Point (TDP), or collapse has already occurred, suggesting that the pipe may be overdesigned or that the lazy wave configuration represents a conservative design approach. Furthermore, research also finds that environmental factors have minimal impact on tension and curvature, with static effects contributing for 86\% of total tension and 78\% of total curvature. This indicates that static analysis can provide an initial configuration without the need for computationally demanding dynamic simulations, as the design limits of MBL and MBR are exceeded in similar static and dynamic scenarios. Lastly, the design of a lazy wave configuration can be simplified to three configurational parameters: a buoyant section parameter (the outer diameter of the Buoyancy Modules (BM), pitch between BMs, or the buoyant section length), the first section catenary length, and a total configuration parameter (total length or horizontal distance between the HOP and TP), since the parameters within each group exhibit similar behavior in terms of tension and curvature responses when varied individually.

The demand of offshore wind energy has increased enormously in the last decade, and continues to do so in the foreseeable future. The monopile will remain the most important foundation structure. There is however not yet a durable solution for its main disadvantage; the under water sound radiation during installation. In this report a new, silent installation method is therefore explored: screwing monopile into the ground. In this thesis the interaction between the monopile and soil is explored, along with the addition of a screw thread. Based on this the driving requirements are determined. Then a possible driving mechanism is shown, considering the driving power, connection between the ship and the monopile and driving equipment. Lastly, the implications of the logistics is explored.

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

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

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

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

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

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

This thesis aims to investigate the effectiveness of the contained dredging system (CDS) in avoiding turbidity during dredging activities. Turbidity is a common issue that arises during dredging activities and causes the sediment to be suspended in the water column, which can have negative impacts on the environment. The CDS is a novel way of dredging that focuses on avoiding turbidity, and it has been developed by Hydrex, an Antwerp-based firm. The research aims to determine whether the CDS provides a solution to the environmental problems associated with turbidity at contaminated sites. The research methodology involves conducting a literature study, theoretical analysis, and real-life testing of the CDS. The main research question is ”To what extend does the CDS create turbidity?”, and a set of sub-questions is formulated to answer the main research question. Based on the analysis and findings presented in this thesis, it can be concluded that dredging activities can have a negative impact on the environment through the generation of turbidity. Traditional dredging techniques such as the clamshell, backhoe, trailing suction hopper dredger (TSHD), and cutter suction dredger (CSD) all create turbidity to varying degrees. However, methods such as silt curtains, sedimentation ponds, polymer flocculants, pH adjustment, vegetation buffers, and bio-engineering can be used to minimize the spread of sediment and reduce turbidity levels. Furthermore, new dredging techniques have been developed to minimize turbidity. The contained dredging system (CDS) was developed to minimize the negative impact of dredging on the environment by focusing on avoiding turbidity by dredging within an air chamber. However, the theoretical analysis of the CDS showed that it creates turbidity when the critical height of the dredged wall is exceeded. The CDS was designed to create no turbidity, but the theory and calculations presented in this thesis suggest that it does create turbidity. In conclusion, the CDS, although developed with good intentions, does not fulfill its requirement of not creating turbidity. The device creates turbidity due to the low critical height of the vertical wall, which will collapse after dredging. However, the CDS could still be used in non-sensitive areas or in very polluted areas where accurate dredging is necessary.

Despite the technological improvements in the dredging industry in the last decades, there are components that have not been investigated accordingly. One of them is the bow coupling bend pipe which can be found on trailing suction hopper dredgers. In contrast to most pipelines, the specific bend pipe is not only a means of transport for the dredged material but has a structural role, as well. Currently, it provides support to the subsequent pipes, which have no other connection to the vessel than the bow coupling bend pipe. Moreover, the abrasive nature of the slurry causes severe wear in the dredging pipes, with even more damage in bend pipes. These two factors lead to the relatively frequent replacement of a large amount of material, which also translates to an uneconomical procedure.

The objective of this thesis is to explore the potential of designing a new bow coupling bend pipe, which focuses on the reduction of the replaced material and the extension of its lifetime. This can be achieved by analysing the two aforementioned factors, in order to improve the management of the original material, resulting in an economical solution.

This study does not only analyse the design of bow coupling but also contributes to the understanding of slurry wear in bend pipes. Precisely, the report starts with a thorough explanation of slurry transport, describing also the three mechanisms of slurry wear and the parameters that influence the wear rates in a bend pipe. Furthermore, a literature review on the wear profile of bend pipes takes place in order to determine the expected wear pattern in the bow-coupling bend pipe. A total of five different wear zones are proposed to
describe the wear rates in specific areas in the bend. The lack of research on slurry wear in large-scale bend pipes necessitated the use of assumptions to adapt the literature outcome on the bow coupling bend pipe. The information obtained from that research is used for the development of various concepts, considering also other aspects that contribute to the main goal of the thesis. Several concepts are investigated and compared based on the priorities of the project. The final design is further discussed, proceeding to modifications that improve its performance. The final geometry and materials of the system are defined in the finite element analysis. To do that, company guidelines and related standards were used to define the loads on the system as well as required realistic scenarios in which the design had to prove its structural feasibility. Finally, before providing recommendations for future work, the proposed design is compared with the current one, highlighting improvements in the three aspects of the thesis objective.

In particular, using almost 60% of the material required for the current design, the new solution extends the bend pipe’s lifetime at least three times. That proves the remarkable effectiveness of the new design in the management of the pipe material, which is also directly linked with economic benefit. It is estimated that the capital expenditure of the new system will be twice as high as the current case, while the replacement cost is reduced by about half.

Implementation of an Explicit Algebraic Reynolds Stress Model in Isogeometric Analysis for seperating flows using a two-equation k-omega model. The model is verified for simple cases such as homogeneous isotropic decaying turbulence, and the turbulent backward facing step

Wave overtopping, in which a wave washes over the crest of a dike and damages its inner slope, is one of the many mechanisms that is found to be responsible for dike failures. As these dikes are generally covered with grass, insight into the erosion resistance of these grass covers against wave overtopping is desired. To assess the performance of dikes against wave overtopping, various concepts exist, such as the widely used cumulative overload method. However, these concepts lack a thorough understanding of the physical mechanisms at play during grass cover failure as a result of wave overtopping. Furthermore, the methods are generally not time-efficient and are labor intensive, leading to high costs. Therefore, a time-efficient and predictive method is desired that is based on the physical mechanisms of grass cover failure.

The grass pull device, which is used extensively in this thesis, may serve as an alternative for the existing assessment methods. The device, which is reminiscent of a tensile test used in mechanical sciences, is able to exert various load mechanisms on the grass cover. In this thesis, the grass pull device is used to study various aspects of grass cover failure. Special attention is given to the influence of cyclic loading on the grass cover. Additionally, material properties have been derived that may serve as input for numerical grass erosion models. Furthermore, the influence of grass roots, subsoil type and pore saturation on the failure mode of grass covers was investigated.

The results of this study showed a continuous growth of deformation and a decrease in stiffness when the grass cover is loaded cyclically. The behavior of grass during cyclic loading was found to be comparable to other composite materials, such as fiber-reinforced plastics. The material properties Young's and shear modulus were derived. The Young's moduli were found to be slightly overestimated, while the shear moduli were found to be comparable to what may be expected from literature. Differences in grass cover properties on different subsoils were identified, showing that grass covers on clay are generally better at resisting deformation, while having a brittle failure mode. For grass covers on sand, a large spread was observed and the material was found to deform easily, while still providing resistance at large deformations.

Based on the findings of this study, recommendations were made to improve the grass pull device. It was found that the grass pull device was successful in providing insight into various physical processes. Whether the grass pull device will be able to capture all relevant erosion mechanisms remains questionable, but it has proven to be a successful addition to existing assessment methods.

Reducing dredging activities in the harbors brings benefits both economically and environmentally. However, this means that ships may have to sail very close to or even through the mud layer on the seabed, with possible effects on ships’ resistance and manoeuvrability. This study has made a step in understanding these effects on ships’ resistance. In order to study the effects of the different mud characteristics on the passing vessels, non-Newtonian models have been implemented in ReFRESCO, the CFD code developed by the Maritime Research Institute Netherlands (MARIN) in collaboration with several non-profit organizations around the world. The objective of this study was to test the ability of ReFRESCO to simulate and assess the influence of a fluid mud layer on the resistance of a ship. RANS simulations are performed for the flow around a KVLCC2 benchmark vessel at model scale with h/T=1.2 and a Froude number of Fn=0.064. Calculations were carried out varying systematically the mud layer thickness (h_mud/T=0.1-0.3), the mud density ratio (ρ_mud/ρ_water=1.05-1.2) and the mud yield stress (τ_y=0-1 Pa). The mud is modelled with the regularized Bingham model. The results show that the resistance of the ship increases in the presence of a mud layer, mainly in the pressure component. The pressure resistance shows a significant peak when the internal Froude number 〖Fn〗_i approaches one. This suggests that the increase in resistance is mainly due to the dead-water resistance, which is highly dependent on the (combination of) ship’s velocity, mud layer thickness and density. The influence on the resistance due to the yield stress appears rather limited, and it depends on the density ratio between water and mud. Results also suggest that yield stress has mainly an effect on the viscous pressure component.

Marine structures can sustain damage due to violent wave impacts that are characterized by complex dynamics involving both water and air. Methods that predict impacts loads for structural design often assume the water as incompressible. These predictions can be inaccurate during impacts where air is entrained in water, as that greatly increases its compressibility. Initial impact forces may decrease due to a cushioning effect. However, the total severity of the impact may increase due to temporal spreading and oscillations of impact pressure. Previous research on aerated water impacts covers breaking waves against fixed structures, flat water slamming or green water events, but not the interactive motion of rigid marine structures in free surface waves. This work aims to evaluate the effect of aerated water impacts on the dynamics of floating rigid bodies in irregular waves. As a part of this work, a state-of-the-art numerical simulation method for aerated water impacts is extended with a monolithic coupling for rigid body motion. Additionally, boundary conditions for wave generation were implemented for a validation of the method during breaking wave impact. Results of the verification of both extensions compare well to other literature. It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications. It can be concluded from this thesis that the compressibility effects of aeration on penetration depth after cylinder slamming can be neglected for entry Mach numbers below 0.1. For further research, it is recommended that this numerical method is used for a range of experiments with various geometries and higher impact velocities. This could provide more insight into situations where aerated water could have important design implications.

Technical developments of the last decades have led to an increase in design space for non-nuclear submarines, especially regarding the increasing amount of feasible power plant configurations. To be able to select the most suitable power plant configuration, it is important to know the effect on the complete submarine design and all systems on board. In the last years, Nevesbu has developed power plant optimization and sizing models to be able to select the best power plant configuration for a certain design. However, these models do not take auxiliary systems into account. The common way used of estimating the main parameters of auxiliary systems in an early design stage, is scaling based on existing submarine designs as reference. When new technologies are applied and alternative power plant configurations are selected, such as totally lithium-ion battery powered designs, reference designs cannot be used any more to make an estimation of the required auxiliary systems. Therefore, new methods have to be developed.
The objective of this thesis is to gain knowledge about, and quantify the direct and indirect consequences for the auxiliary systems when lithium-ion batteries are implemented on a large scale in an entirely battery powered submarine. Design support tools will be developed to support the designer during the design of the battery and auxiliary systems. The associated main research question is:
How can a designer be supported, in an early design stage of an entirely lithium-ion battery powered submarine, when trying to determine the size, weight and energy consumption of systems that, among others, support the safe implementation of the new batteries?
For this thesis, the safe implementation of lithium-ion batteries in an entirely battery powered submarine is studied. This type of battery has the risk of thermal runaway, which is a cascading process in which the battery releases all its stored energy, generating a lot heat and toxic gases, causing an increased risk on an explosion. Research has shown that a foam injection system could be used to mitigate the risk on thermal runaway and such system is also capable of extinguishing other types of fires inside the submarine, creating a safer working environment for the crew members. A foam injection system requires pressurized air, but the amount does not have a significant impact on the pressurized air storage.
The increased endurance of the entirely lithium-ion battery powered submarine requires a reevaluation of the environmental control systems. The amount of chalkholders as CO2 absorbers and oxygen candles to generate oxygen increases due to the increased endurance. The additional weight and volume of these consumables are undesirable. A regenerative CO2 absorption system is introduced which is smaller and lighter, but requires a significant amount of the installed battery capacity, causing a reduction in endurance.
To support a designer in an early design stage, several design support tools are developed: for the lithium-ion battery system, the foam injection system and the pressurized air system. These tools estimate main parameters such as weight, volume and heat load based on system designs and input parameters such as size and weight of components and the pressure hull radius and length. The tools show the designer the consequences of design decisions and provides important information about knock-on effects on other systems. Furthermore, the tools can be used to optimize the weight and volume of the battery system and foam injection system.
A case study, based on a concept design of an entirely battery powered submarine showed the usability of the tools, quantified the effect of these systems on the overall concept design and the importance of the results in an early design stage. The number of crew members had to be reduced with seven to implement the regenerative CO2 absorber and LOX tank. The required pressurized air storage did not change due to the implementation of the foam injection system, but the overall submarine became safer. After changing the concept design, the weight and stability still satisfied all requirements. Taking the output of the tools into account during an early design stage can prevent large design modifications.

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.

With the transition towards renewable energy, offshore wind power farms are being constructed or planned in many places involving placing wind turbines in the sea. The monopiles for these wind turbines are vertical cylinders which are situated in a flow with high Reynolds numbers and low to intermediate Keulegan- Carpenter numbers. The Reynolds and Keulegan-Carpenter number are dimensionless numbers for the quantification of respectively currents and waves. Previous research has not covered the Reynolds and Keulegan-Carpenter numbers in which energy generating devices are situated. The current research is focused on filling the gap and investigating the drag forces on these cylinders via means of experiments. Additional simulations focussing on the free surface and end effects were also conducted. The forces at play and the Morison equation to model these forces have been evaluated. It was discovered that in the investigated range, the drag and inertia coefficients depend on both Reynolds and Keulegan-Carpenter numbers. By analyzing the results of the experiments in the time and frequency domain, indications of vortex shedding and second-order harmonic wave forces, as well as forces from wave-wave interaction were found. The Morison equation itself is analyzed to find that it either underestimates or neglects these forces. By means of simulations, the effects of the free surface and the aspect ratio of the cylinder on the drag and inertia coefficients are found. Lastly, it is proposed to use a rewritten Morison equation to find the drag on a cylinder in a flow with irregular waves. This equation is found to describe the drag forces more accurately.

Dam reservoirs form a crucial part for human society storing water, controlling floods, providing hydropower, water for irrigation and drinking. Annually 1% of the worldwide dam reservoirs storage capacity is lost, caused by sedimentation. The inflow of sediment and reduction of flow velocity and turbulence in the reservoir pro- vides favourable conditions for settling. Several sediment transport mechanisms are responsible for this, one of these is the turbid density current. The turbid density currents settle as the reservoir becomes wider, and it is affected by forces along the top and the bottom of the current. Recently, focus on reservoir engineering has shifted from primarily structural dam design towards complete sediment management strategies. In order to improve the sustainability of dam reservoirs, many management techniques are developed that inhibit or mitigate sedimentation. However, the effectiveness of these techniques is not yet known. This thesis provides an additional concept for sediment management in dam reservoirs consisting of channelling of turbid den- sity currents in dam reservoirs. The channel provides controllable parameters. The aim is to study the effects of channelling turbid density currents.
The study starts with a literature review, to describe sedimentation, sediment transport, and turbid den- sity currents in dam reservoirs, including their analytical and numerical descriptions.
Two computational models study the concept: a steady-state model and a numerical model. The steady- state solution and is based upon an equation for open channel flows modified for turbid density currents. This model is used to investigate the effects of hydraulic radii and slope of the channel on the turbid density current — secondly, the dynamic numerical solution. An analytical description is provided using the one- dimensional shallow water equations, consisting of the continuity, momentum and particle conservation equations. The solution includes four sources: deposition, erosion, gravity and friction. It omits water en- trainment and bed deformation. The model is discretised using the Generalised Lax Friedrichs method. First validation and investigation of the quality of the source terms are done. Subsequently, the model, including the four source terms, is used to study the effect of slope, hydraulic radii, concentration and sediment size in the channel. Expanding the numerical study by a Water Injection Dredging case in which local velocity, concentration and height are increased along a certain length to study possible effects.
To conclude, channelling turbid density currents is a viable solution to improve sediment transport. The slope and depth of the channel have the most significant effects. The generalised Lax Friedrichs method provides a valid and straightforward discretisation method for the numerical model. Furthermore, the model provides an easy, quick and simple to use tool to make first estimations of the effects of channel dimensions.

Once a semi-submersible is operating at an inconvenient draft (A shallow draft with limited water column above the pontoons), the passing waves over the pontoons can not keep their linear motion and energy will be transferred to higher harmonic wave frequencies. This means the hydrodynamic response will also contain energy at these higher frequencies. Conventional linear diffraction solvers are not able to solve the combined response of the wave frequency and its higher harmonics, which then result in a non-physical solution for the wave loads, motions and internal loads. This research aims to obtain a better insight into the hydrodynamic response at the inconvenient draft and ultimately on the internal loads of the semi-submersible. In the first part of this research, model test solutions, linear potential solutions obtained with WAMIT and CFD solutions obtained with ComFLOW are compared. the comparison shows that ComFLOW is able to provide more accurate wave loads compared to the linear potential solver. The higher harmon- ics observed in the measured wave loads during model tests are correctly predicted by ComFLOW, although maximum deviations of 30% are still observed between measured and predicted wave-load amplitudes.
However, ComFLOW is not able to solve the motions of a free-floating semi-submersible correctly. Due to pressure peaks in the wave-exciting forces, solving the equation of motion results in an incorrect motion response and ultimately results in incorrect internal loads. Although a significant effort was made - in close collaboration with the ComFLOW developer - to improve this functionality, the results remained unsatisfactory. Even so, in order to obtain an insight into the higher harmonic response contributions to the internal loads, a parameter study has been conducted, using tests in which the semi-submersible was held captive. This parameter study was conducted by systematically varying wave amplitudes and draft, and resulted in situational limits at which the higher harmonic response contribution becomes significant and the linear relation between the incoming wave and the hydrodynamic response is lost. This limit is shown to be dependent on the Ursell number. Furthermore, it is demonstrated that the significance of the higher harmonic response contribution increases from 10% to 40% throughout the inconvenient draft, while the most severe situations resulted in a higher harmonic response contribution of 50% of the total response amplitude. In the final part of this study, an attempt is made to couple the wave loads from ComFLOW to internal loads. A quantitative analysis is made on the effect of the higher harmonic responses of the wave loads on the internal loads. The time domain simulator aNySim is used combined with the wave- exciting forces on a captive semi-submersible calculated with ComFLOW. A multi-body analysis is used to obtain the internal loads on the aft and front part of the semi-submersible. This did not provide the correct answers because the stiffness of the spring damping between the two sections affects the higher harmonic response contribution. This method overestimated the higher harmonic response contribution. A better understanding of the joint and the joint stiffness/damping of a dual-body simulation may solve the encountered problems.

Our world is rapidly changing, leading to various technological achievements in diverse areas of expertise. Among the most evolving are autonomous robots, machines and vehicles, enormously extending human capabilities. While this trend became quite familiar for land-based vehicles, innovations in the underwater domain lag behind, mainly due to the harsh environmental conditions encountered. This in large contrast with worldwide growing needs for autonomous underwater solutions, like dredging. Nevertheless, for underwater vehicles becoming autonomously, various technological challenges must be overcome. The most essential one is finding a proper answer to the question; where in the underwater world am I? Autonomous robots are usually be able to operate in complex environments using external reference systems such as Global Positioning System (GPS) to locate themselves inside their environment. However, these are not accessible in underwater applications, since water strongly attenuates electromagnetic signals, notably GPS. Hence, various alternative methods for Simultaneous Localization And Mapping (SLAM) for underwater applications are employed, among which the method of anchoring inertial measurements on environmental landmarks, perceived with an exteroceptive Sonar. The ultimate goal of this procedure is to correct for errors in the inertial measurements, using the environmental perception available in the Sonar scans. Nevertheless, due to practical underwater issues, it still is not readily available and even barely investigated in open scientific research. This thesis aims at the development of an underwater localization algorithm, using onboard inertial sensors and Sonar. The overall system design consists of several individual software procedures, executing certain assignments, together ultimately solving for the localization task. The developed system was tested using ground truth in form of a real-world dataset obtained several years ago in an abandoned marina. This showed that most individual procedures were able to provide good results, while having a low computational complexity in relation to online operations. However, it is quite a challenge to obtain results accurate enough to properly correct for positional errors, by using the designed system. Although showing potential, it still is not entirely decent in terms of accuracy and computational burden. This is likely due to the sparsity of the extracted Sonar measurements. Data is lacking to further verify the system, hence several questions remain open regarding its universal applicability in case of different scenarios, dynamic environments and long trajectories. In this context, more in-depth research is needed into the algorithms that are influenced by the Sonar measurements, to increase the accuracy of the overall system, while gaining more insight in its computational behaviour. Additionally, also more real-world experiments are essential, to extensively verify the designed system. In conclusion, the results showed that it is possible to correct for inertial errors, using the system developed. Furthermore, it demonstrates that most procedures individually produce good results in real-time. Hence, this study can be seen as a positive step in the right direction, forming a basis for future research in solutions that are generically applicable in online real underwater world operations.