In this study, the excavation process of a low pressure vertical impinging jet in cohesive soil has been investigated. Mass flow excavation is a hydraulic, low pressure, subsea excavation method whereby a large volume flow is applied to the seabed through which the seabed is eroded, and the soil is transported. The erosion process of a cohesive soil by impinging jets depends on many variables (e.g., jet flow velocity, standoff distance, grain size, undrained shear strength). The erosion processes, especially for dynamic pressures of lower than two times the undrained shear strength, are not fully understood. It is still unknown what the influence of different soil and jet parameters are. As a result, it is still unknown what the expected scour rate will be during a mass flow excavation process of cohesive soils. The goal of the research is to quantify and be able to predict the production of a mass flow jet on cohesive soils. For this purpose, the relevant parameters of cohesive soil erosion are investigated, and fluid velocity profiles of the jet are related to clay bed failure mechanisms. Special attention is put on the effect of the consolidation coefficient on the erosion process. Based on literature and experimental research, different soil failure mechanisms have been investigated. The main result of this study is a relation between the main jet and soil parameters, and the erosion velocity development.

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.

Subsea rock installation is widely applied in the offshore industry and utilized for a wide range of purposes including but not limited to: pipeline protection, scour protection, insulation of pipelines, upheaval buckling prevention and seabed preparation. Tideway Offshore Solutions is specialized in subsea rock installation and currently operates three state-of-the-art fallpipe vessels. Their vessel 'Flintstone' makes use of an innovative closed fallpipe system to provide high accuracy subsea rock installation. The presence of rocks in the water column of the fallpipe increase the density of the mixture in the fallpipe. The density difference between the mixture in the fallpipe and the density of the surrounding sea-water results in a water level drop in the fallpipe. To keep this water level drop within acceptable limits extra water is added to the fallpipe system which accelerates the fallpipe flow. The accelerated fallpipe flow can result in high outflow velocities of the rock mixture at the fallpipe exit. High outflow velocities of the rock mixture can eventually result in increased impact velocities of the rock particles on the seabed. Increased impact velocities of the rock particles on the seabed can lead to unsatisfactory rock berm shapes resulting in the need for remedials. To have their fallpipe system perform as efficient as possible Tideway Offshore Solutions was interested in possible measures to reduce the outflow velocity of the fallpipe which resulted in this thesis. In the first part of this thesis different concepts, that could potentially reduce the outflow velocity of the fallpipe, are generated and conceptually analyzed. The information acquired from this analysis is used as input for a multi criteria analysis that resulted in the selection of the most promising concept, the use of a deflector. The deflector will act as a flow deflector at the fallpipe exit thereby decreasing the impact velocity of rock particles on the seabed. In the second part of this thesis a complete three-dimensional computational fluid dynamics (CFD) analysis is performed on the fallpipe outflow with and without deflector. The CFD program used to simulate these situations is ANSYS Fluent. The simulations for both cases are performed for two different turbulence models, the k – ε and k – ω SST turbulence models, the distance from the fallpipe exit to the seabed is varied as well and a range of deflector angles and dimensions are simulated. The fluid flow velocities obtained from the CFD analysis are used as input in a MATLAB model to compute the rock particle trajectories in a two-dimensional plane. Combining the rock particle trajectories and their velocity components it is possible to compute the impact velocities of the rock particles on the seabed. The results of the trajectory model set up in MATLAB showed that a substantial decrease in impact velocity of the rock particles on the seabed can be achieved by using a deflector at the fallpipe exit.

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.

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.

Green water events on ships in extreme waves pose a considerable risk to personal, operational, and structural safety. Considering these issues, novel experiments were done to investigate the interaction between focused breaking waves and a ship at forward speed. The investigated interactions are global motion, global loads, and forces on a deck structure. For the experiments focused, breaking waves were created, which served as loads of the green water events. First, wave focusing with prescribed characteristics was investigated in numerical settings. It was shown that focusing the waves with iterative methods does not lead to a converging solution. Three breaking waves were created in a wave basin. The characteristics of the waves were retrieved from 2D numerical simulations. The parameters of the spectrum of each wave were calculated from the simulation results. The experiments were conducted systematically, where the position of the longitudinal center of gravity (LCG) of the ship was varied with respect to the focus point of the breaking wave at the focusing time. Using the same wave input resulted in green water events and loads on deck structure with different characteristics caused by different breaking stages of the focused wave. The effects of different ship velocities and different focused breaking waves were also explored throughout the experiments. It was shown that using the same focused wave, i.e., the same energy input, the breaking stage of the wave during the impact has a large influence on both the magnitude and time development of the loads on the deck structure. Force and pressure peaks were the largest and had an initial spike-like development when the wave during impact was in an unbroken stage. Impacts with the wave in its broken stage resulted in significantly lower loads and longer-duration load development. Such changes in the loads show that the ship's position with respect to the breaking wave is essential to consider even for the same input energy, posing additional challenges for establishing the maximum load in a sea state.

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.

When a ship reaches the shore, the water depth reduces and thus the under-keel clearance also reduces. The ship keel clearance influences the maneuverability of the sailing ship. In this thesis project, the link between the fluid mud rheology and its density is first investigated. The exponential relation reformulated between the density and yield stress necessitates the direct measurement of rheology for navigability. It was confirmed true that the yield strength grows exponentially with the density of the fluid mud. However, at higher densities of mud, more thixotropy is observed in the flow curves. Besides, the Power-law relations were found for the relation with volumetric concentration, for yield stress in agreement with fractal dimension theory. Second, quantification of the thixotropy effect of fluid mud is conducted to justify the importance of the time-dependency effect in the rheological modeling. We found that an unremolded fluid mud has high thixotropy even at high shear rates. However, diluted and remolded fluid mud at high shear rates found a negligible thixotropy effect. We also observed that Houska's modeling curve coincides perfectly with the flow curve of fluid mud at high shear rates, and also it integrates the thixotropy into the modeling. Third, the total resistance of a plate moving through fluid mud is measured and compared to the frictional forces calculated using the available analytical formulas of a plate moving in Bingham fluids and Power-law fluids. The moving plate is an abstraction of a vessel's keel sailing through the fluid mud. We found that the total resistance of the plate moved in fluid mud at low velocity agrees with the frictional force of a plate moved in Bingham fluids. As the velocity increases, the stagnation pressure becomes significant, and the deviation of the total resistance of plate and frictional force of a plate increases. Thus, to estimate the total resistance of a plate moved in mud, normal forces (stagnation pressure) and vortices at edges should be investigated because these need to be subtracted. The experimental data in this thesis are made resourceful to help researchers validate their Computational fluid dynamics(CFD) models in the application of sailing through the mud.

Scarcity of (green) energy resources drives mankind further offshore and towards deeper waters. Protection of subsea infrastructure is of eminent importance in these environments. The key parameter to have a good control over the installation of rock is the falling time. For larger water depths up to 1200 meters, the accuracy of the falling time prediction becomes more important because a small deviation can result in a large difference between prediction and reality.

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.

Due to an increased demand for transport, ships become larger, needing a larger navigable depth. For these reasons a waterway needs to be dredged and a Cutter Suction Dredger is a vessel suitable for this operation. A Cutter Suction Dredger is a floating vessel which removes sand, clay or soft rock from sea or river beds. It has a cutter head with pickpoints attached to it. By rotating and swinging, the pickpoints are pushed into the soil, disintegrating it. The soil enters the cutter head where it is mixed with water. From inside the cutter head it is hydraulically transported to the vessel via the suction mouth and pipe. The rotational speed of the cutter head can be varied by the vessel operator. When increasing the rotational velocity and swing speed, more production can be obtained. However, this leads to an outflow of water and dredged material near the ring, spilling the soil. When the Cutter Suction Dredger is employed for cutting sand, the sand particles are easily kept in suspension due to the rotating motion before it is sucked up. A cutter suction dredger is also used for cutting rock, leading to large pieces, which are more influenced by gravity and the centrifugal force. Due to these forces, the pieces are thrown out of the cutter head more easily than smaller sand particles. The pieces of rock which are thrown out of the cutter are considered spilled. This spillage is unfavourable since this material has to be dredged a second time or is left on the sea floor. When the material is left on the sea floor, a larger layer of soil needs to be dredged for creating the same navigable depth. To reduce spillage, the processes contributing to spillage should be quantified in order to design a better cutter head or working method. This dissertation contributes to this goal by presenting a validated model for simulating the spillage of rock particles inside a rotating cutter head. Such a model can be used to quantify different processes and test new cutter head designs… @en

The increasing size of today's ships is a major concern for navigation in confined waters. In order to ensure safe manoeuvres, port authorities prescribe, among others, a minimum under-keel clearance that must be maintained by the ships during navigation. However, the seabed of ports situated at the estuaries or along rivers is often covered by mud as a result of sedimentation. Hence, while the position of a solid bottom is clearly defined and can be easily detected by sonar techniques, the presence of deposited sediments makes the definition of "bottom" and "depth" less clear. This also poses some questions on the optimal dredging strategy to adopt to minimise maintenance costs while ensuring the required safety. For practical reasons, port authorities define the (nautical) bottom as the level where the mud reaches either a critical density or a critical yield stress (i.e. the shear stress below which the fluid behaves as a solid-like material). However, an optimal choice that minimises dredging activities while preserving the required safety shall also take into account the behaviour of ships. As the understanding of the link between mud rheology and ships' controllability and manoeuvrability with muddy seabeds is rather limited, this research project was started. With the rapidly increasing power of today's computers, Computational Fluid Dynamics (CFD) has become a viable option to study this problem. The CFD code selected for this research is a multi-phase viscous-flow solver developed, verified and validated exclusively for maritime applications. As such, it was originally developed for Newtonian fluids only. Since mud exhibits a non-Newtonian rheology, the `step zero' of this research was to implement the Herschel-Bulkley model, which allows to numerically simulate two important flow features of mud, i.e. its shear-thinning and viscoplastic behaviour. Other rheological characteristics, such as thixotropy, were not considered in this study as they are deemed of minor importance at this stage. The next step was concerned with ensuring that the modification of the flow solver to account for the non-Newtonian rheology of mud was correct. This was done by using the Method of Manufactured Solutions (MMS), which allows to rigorously verify the code against user-defined exact solutions. The verification exercises showed that the code performs as intended for both single- and two-phase flows of Herschel--Bulkley fluids. The illustrated procedure can be readily adapted to verify the correct implementation of other rheological models that may be implemented in the future. In this case, it is recommended to examine, in addition to the grid convergence of velocity and pressure, also the grid convergence of the apparent viscosity as the latter is particularly sensitive to coding mistakes related to the implementation of the new rheological model. While code verification ensured that the Herschel--Bulkley model was correctly implemented, obtaining fully-converged solutions for realistic non-Newtonian problems may still be difficult. The non-Newtonian solver has thus been tested on the laminar flow of Herschel-Bulkley fluids around a sphere, as the latter is the simplest three-dimensional flow exhibiting features that are typical of the flow around ships, such as boundary layer development and flow separation. Although obtaining a fully-converged solutions was indeed challenging, it was possible to replicate data from the literature with good accuracy. This provided confidence to employ the CFD code to simulate ships sailing through fluid mud. The verification of the CFD code was followed by validation of the mathematical model. The problem of a ship sailing through fluid mud was simplified into a simpler one, i.e. a plate moving through homogeneous mud as to mimic a portion of the hull penetrating the mud layer. The objective was to investigate the accuracy of the (regularised) Bingham model (which is a special case of Herschel-Bulkley) to predict the frictional forces on a plate moving through mud. The comparison between experimental and numerical data showed that the ideal Bingham model well captures the relative increase in the resistance due to the increase in the mud concentration but, at low speed, it tends to over-predict the resistance. On the other hand, choosing a lower regularisation parameters seem more favourable, both from the numerical and physical perspective. In fact, this research showed that better predictions at low speed were achieved by using lower regularisation parameters that were determined from the first points in the mud flow curves. It should be noted, however, that the thixotropy of mud and possible deflections of the plate during the experiments may prevent drawing definitive conclusions. Finally, one question arising when simulating a ship sailing through a non-Newtonian fluid is how accurate are standard Reynolds-Averaged Navier-Stokes (RANS) models, which are developed for Newtonian fluids, when applied to non-Newtonian flows. In the last step of this dissertation, the accuracy of three RANS models was assessed against published Direct Numerical Simulations (DNS) data for pipe flows. From this study it was concluded that, among the three tested Newtonian RANS models, the SST model produced the best predictions and it is reasonably accurate for weakly non-Newtonian fluids and for high Reynolds numbers. In addition, a new RANS model, labelled SST-HB, has been developed. The new model showed good agreement with DNS of pipe flows in the mean velocity, average viscosity, mean shear stress budget and friction factors. However, the new RANS model was calibrated and tested for pipe flows only, a relatively simple internal-flow problem. Hence, the applicability of the new model to complex external flows, such as the flow around a ship, still requires further investigations. Furthermore, RANS simulations with some realistic mud conditions predicted laminar flow in the mud layer. In this case, the use of the standard SST model is recommended. The developed and tested CFD code, together with other insights provided by this research, can be used in the future to both numerically investigate the effect of mud on ships and to obtain the hydrodynamic coefficients for manoeuvring models. These models could then be used in real- and fast-time simulators for research and commercial purposes, but also for pilots training. @en

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.

Jet trenching operations in sandy seabeds have been extensively studied and are well understood, offering a preferred method for offshore cable installation due to its relatively low environmental impact. However, the transition to clay seabeds presents significant challenges, demanding a deeper comprehension of jet trenching dynamics in cohesive soils. Recognizing this need, DEME NL Offshore, a Dutch company specializing in offshore operations, seeks to develop a predictive model tailored for jet trenching in cohesive soil environments. This thesis addresses the complexity of jet trenching in clay seabeds through a comprehensive computational fluid dynamics (CFD) modelling approach. Beginning with an extensive literature review, various methods for modelling jet trenching processes are explored, highlighting the critical parameters influencing trenching outcomes. While existing models primarily focused on jet penetration depth, this study identifies the need to integrate fluid dynamics principles and particle behavior to achieve a better understanding of jet trenching dynamics. A CFD model is developed to cover the fluid dynamics in the trench behind the jetting sword. Subsequently, a langrangian model is applied to determine the trajectory of cohesive particles of different diameter in the trench. Key Findings: • For the scenarios considered in this thesis, jet trenching in clay soils is not feasible by the accumulation of particles at the trench bottom before the cable touchdown point. • Particle diameter significantly influences trenching outcomes, underscoring the importance of further investigation. • Existing models inadequately account for clay block formation and behavior, indicating a gap in understanding. • Empirical validation and field testing are essential to enhance model accuracy and reliability. • Integration of field observations and experimental data can refine the model for real-world scenarios. In response to these findings, recommendations for future research endeavors are proposed, emphasizing the importance of empirical validation and field testing to enhance model accuracy and reliability. By integrating field observations and experimental data, trenching models can be refined to better predict real-world trenching scenarios, thereby optimizing offshore cable installation processes. In conclusion, this thesis contributes valuable insights into the challenges of jet trenching operations in cohesive seabeds and lays the groundwork for future research in the field. By addressing the recommendations outlined herein, future endeavors can advance trenching methodologies, ultimately improving the efficiency, reliability, and sustainability of offshore cable installation.