Submarine power cables are commonly buried below the seabed to protect them from external hazards and to ensure long-term operational reliability. In morphologically active areas, however, trenching is complicated by sand waves and megaripples. These bedforms introduce local slopes and crest regions where the cable must adapt to rapidly changing seabed geometry. In practice, this can lead to insufficient burial and the formation of standing bights, where a section of cable remains locally elevated instead of settling into the trench.

This thesis investigates the mechanical response of submarine power cables during trenching over sand-wave-dominated seabeds, with particular focus on the conditions that may lead to standing-bight formation. A back-analysis of DEME project data is first used to identify practical trends in burial performance. The field data show that rougher seabeds, steeper local gradients, and cable properties such as submerged weight and bending stiffness influence the achieved depth of lowering. However, the available operational data do not allow the individual mechanisms to be isolated with certainty.

To study the cable mechanics separately, a simplified static model is developed. The cable is represented as a free-hanging, inextensible slender rod subjected to submerged self-weight and internal force components. The governing non-linear elastica equation is solved numerically using a shooting-based procedure. For each cable type and prescribed uphill geometry, admissible equilibrium configurations are identified and evaluated using end tension, horizontal distance, exit angle, and overall cable shape.

The results show that standing-bight susceptibility cannot be assessed from residual tension alone. The same end tension can correspond to different cable geometries, depending on the horizontal and vertical internal force components. For a given uphill geometry, a practical tension window exists. If the tension is too low, the cable sags excessively and reaches the prescribed height too early, creating a standing-bight-prone geometry. If the tension is too high, the cable remains too straight and may require too much horizontal distance to reach the target depth, leading to loss of depth of lowering. The favourable window depends strongly on cable properties, prescribed height, and available trench length.

Within the scope of this thesis, standing-bight formation is therefore interpreted as a loss of geometric mechanical compatibility between cable properties, residual tension, and seabed geometry, rather than as the result of a single dominant parameter. The results provided indicate mechanical tension windows that can support trenching assessment and motivate further dynamic and soil-interaction modelling. ... Read more

Subsea rock installation plays an important role in the protection and stabilization of offshore infrastructure, such as pipelines and cables. Fallpipe vessels enable the accurate placement of rocks on the seafloor at large water depths. The fallpipe is used as a large transport system between the vessel and the project depth. The internal dynamics during these operations remain poorly understood. Mixture densities, particle velocities, waterdrop, and air entrainment are important properties of the process that cannot be monitored during operation due to practical limitations. Existing numerical studies largely omit the effects of water inlets and entrained air. This thesis addresses these knowledge gaps through an experimental and numerical investigation of the flow dynamics inside a vertical fallpipe.

A Froude-scaled physical model (scale factor of 11.7) was designed, constructed, and tested at the Boskalis Hydrodynamics lab. A transparent PMMA tube of 4.4 meters in length was used to represent the fallpipe. The translucent material allowed direct visualization of the processes inside the fallpipe. Properties such as density, rock concentration, and production were further recorded using a loadcell and pressure sensors. Full-scale productions ranging from 370 to 1360 tonne/hr were scaled down and tested. Experiments combined production rates between 0.22 and 0.84 kg/s with four water‑inlet configurations (0, 4, 8, and 16 inlets). Each inlet had a diameter of 10 mm and was located at a depth of 0.265 m. The combined measurements from the loadcell and pressure sensors were combined with visual analysis to obtain key parameters, including mixture density, concentration, waterdrop, particle velocity, and air fraction.

Results show that the addition of water inlets substantially lowers the mixture density and rock concentration while increasing the velocity of both the water and rock fraction velocities within the fallpipe. Configurations without inlets experienced full blockage from production above 0.49 kg/s, which in full size would mean at productions above 820 tonne/hr. A configuration with 8 or 16 inlets open showed oscillatory plug forming around the inlets. The cause for this behavior can be attributed to the jet streams entering through the inlets. Rock particles are decelerated once they arrive at the inlets. Air entrainment was observed in all configurations with water inlets and increased with production rate, reducing the mixture density measured between sensors. The waterdrop inside the fallpipe can be reliably estimated from pressure measurements and closely matches visually tracked values.

A one-dimensional drift-flux was created and compared to the test results. The model uses mixture momentum, hindered settling effects, wall effects, and the addition to create simulations of the inside of the fallpipe. Results obtained from model simulation compare well to the results of the conducted experiments. A consistent deviation within the simulation results can be attributed to the absence of a modeled air fraction within the model. The trend that was not captured by the model is the influence of the aforementioned inlet dynamics, as this effect is not included in the model.

This thesis provides the first combined experimental–numerical study of water addition and air entrainment inside a fallpipe. The results provide new physical insights and establish a validated modeling framework that can support and accurately predict efficient subsea rock installation operations. Recommendations are provided, including the incorporation of air fractions and detailed inlet dynamics, for future experimental research. ... Read more

Main objective of this thesis is:
"Verify & Improve an existing, or develop a new, formulation to estimate the solids effect on the centrifugal pump for the transported slurries. Furthermore, find knowledge or data gaps and perform experiments to fill these gaps."

In the slurry transport industry, such as the dredging industry, the clear water pump curves are not applicable for transport calculations. This is due to the decrease in pump performance, expressed in meter mixture head, during the hydraulic transport of the mixture. This decrease occurs due to the presence of solids in the mixture. In the past 60 years, lots of researchers have investigated the effect of solids in centrifugal pumps.

Each researcher formed a formulation which described a specific data set of particles with certain particle size and density with a specific pump size.

The purpose of this thesis is to determine which formulation is the best formulation for the dredging industry. This industry is characterised by pumping quartz particle size ranging from d50 = 0.1 - 2 mm, with a density around 2650 kg/m3, using pump impellers ranging from Dimpeller = 0.4 - 2.5 m. These specific characteristics are evaluated with scaled experiments in this thesis.

An extensive literature study is conducted, where all known formulations are evaluated and checked against each other. After analysing this comparison, the most relevant formulations are selected to be evaluated deeper by conducting experiments at the dredging lab of the Delft University of Technology.

The experiments took place from August 2018 till February 2019 and were performed at the dredging lab. During the experiments three narrow graded quartz materials are tested with d50 values of: 0.139 mm, 0.285 mm and 0.859 mm. The test set up uses a centrifugal pump with Dimpeller = 0.4 m. Each material is tested over the range from 0 - 40% concentration by volume for three different flow rates at constant RPM. With the help of a butterfly valve, the flow rate is tested over the range from 60%to 140%Q/QBEP . Q/QBEP is the ratio of the actual flow rate to the best efficiency flow rate of the pump, for the selected RPM setting. The range 60% to 140% is the usual operating windows for the dredging industry. ... Read more

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

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

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

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

It was found that high-period waves with high wave heights fully couple dynamically with the flap, resulting in several outcomes. First, the flap follows the wave frequency, resulting in sub-optimal energy dissipation with use of radiation damping. Second, tuning the flap’s natural period to match the incoming wave period, increases pitching angles but reduces angular velocity, leading to decreased wave attenuation. Third, lowering the centre of gravity increases the restoring moment and angular velocity. This results in enhanced energy dissipation of higher-order wave components. Fourth, increasing the flap’s length improves wave attenuation due to enhanced reflection and radiation damping. Fifth, adding mechanical damping decreases attenuation performance, likely because the reduction in angular velocity worsens the energy dissipation effect for higher-order wave components. This research revealed that the best performing solution, with low a centre of gravity, attenuates 28.7% of wave energy entering the well-dock. Furthermore, optimising the flaps natural period to the incoming wave period was less effective, attenuating only 19.7% of the wave energy.
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With the shift to renewable energy becoming more important than ever before and offshore wind farms being one of the sustainable energy resources available, many offshore wind farms are currently being built.
One of the difficulties of these wind farms built with monopiles is that the monopiles and electricity cables must be protected from scouring and falling objects. The protection is usually performed by placing a layer of rocks around and over these objects. For deep water up to 400 meters, vertical fall pipes are used to place the rocks directly under the rock placement vessel. This is impossible when placing rocks around monopiles, as the monopile stands in the way of where the vessel would have to lie during the rock placement procedure.
The inclined fall pipe (IFP) was introduced recently to combat this issue. The IFP, which hangs diagonally next to the vessel, can place rocks a couple of meters to the vessel's side, leaving a safe distance between the vessel and the monopile. Rocks tend to form clusters inside the pipe during placement with an IFP. This means the rocks exit the pipe in bursts of clusters, resulting in uneven rock layers if the rocks are placed during the movement of the vessel. Another result is less accurate placement of the rocks as the rocks at the outer layer of a cluster, during the free fall, are pushed further out, away from the intended placement location.

This thesis investigates the processes of rocks moving inside an inclined fall pipe and simulates this process numerically. This is achieved with a Finite-Volume solver for the fluid coupled with a Discrete Element model for the particles. The results are compared to existing experimental data.

The simulation results show the same rock behavior as in the lab tests for angles up to 60 degrees. The clusters start forming from the small volume concentration differences within the rock layer. The higher densities concentrate and become clusters, while the lower densities become gaps. In the simulations, this process is caused by the differences in the flow velocity of the fluid and the resulting drag differences of the particles. This knowledge is a first step towards finding solutions to lower the formation of clusters and thus, the more accurate placement of the rocks on the sea floor.

Furthermore, multiple points of improvement for the solver have been found. For angles of 75 degrees and steeper, the rock movements of the simulations miss a certain chaoticness, which may be attributed to missing turbulent drag forces in the solver. The velocities predicted by the solver are too high; multiple possible causes are speculated. The simulation results follow the same trends as the lab results, meaning they could be used for qualitative comparisons. Various additions to enhance the solver are proposed.
The influence of a system parameter, the distance factor was noticed to be stronger in shear flows than in homogeneous flows during a system parameter variation. This distance factor relates the fluid-particle interaction radius to the particle diameter. Finding limits for this parameter in shear flow could enhance the solver's prediction capability.

With the shift to renewable energy, more monopiles will be placed. The simulations show which forces cause the cluster formations and can be used to investigate ways of reducing them. This can reduce the spillage of rocks during operation, making the entire rock placement more efficient and will reduce costs.
The solver with the enhancements for better prediction capabilities could be used to further optimize the rock placement accuracy and efficiency. This would reduce wind farm costs and help the shift to renewable energy. ... Read more

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. ... Read more

Gravel is a material with multiple purposes in construction and infrastructure. Its usage ranges from construc-tion of roads, building navigational channels to land reclaimation. Gravel can be obtained through multiple methods, such as dredging with ships. One of such dredging ship is the trailing suction hopper dredger (TSHD). However, in the seabed the gravel is thoroughly mixed with sand, thus during the collection process the unwanted sand is pumped up with the gravel. It is not uncommon to receive up to 80% sand and only 20% gravel. In order to use the storage of the ship more efficiently, the gravel is separated from the sand through an on board sieve installation. The sand is released back into the water and the gravel is collected in the hopper. This has several disadvantages, such as wasting energy by pumping unwanted sand which is returned back to the seabed. The turbidity in the seabed that occurs by releasing the fine sand which settles slowly to the seabed. And the requirement of a large sieve installation to separate the gravel from the sand, which reduces the already limited space of a ship. In order to limit the mentioned disadvantages, it would be favourable to bring the separation process at or near the seafloor. Through a literature study the best underwater separa-tor has been selected. From this it is concluded that the usage of a jet that pushes fine sediment out is the best for underwater separation. The fine sediment is pushed to the upper part of the pipe, whilst the coarse particles resist the jet and remain at the lower part of the pipe. The fine particles at the top are sucked in by a retour pipe, which guides them down to the seabed where it is released with a lowered environmental impact. Further research on this concept is done through the use of computer simulations which models the conditions using a drift-flux solver. The concept has been tested with different configurations to determine the influence of several input variables. The chosen variables are the inlet velocity, jet velocity, sediment size and sediment concentration. For these configurations the separation efficiency of the system is determined by analysing the retained gravel and the filtered sand. The containment of gravel is generally over 90% and highly satisfactory. The maximum separation of sand is 50%. These results indicate the viability of the system and thus research on improvements are recommended to further increase the systems effectiveness and efficiency. ... Read more

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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The electric pulse fishing technique is no longer an option for fishermen. Therefore, they have to fall back on beam trawl fishing. This method has several disadvantages such as high fuel consumption and much soil disturbance. This system therefore deserves an investigation into improvement.
There is various literature on interaction of offshore structures with the soil. Processes in which a structure is also towed through the seabed are ploughing and cutting. Experiments with tickler chains are done and also numerical models are created.
Experiments were performed to acquire the required information that could not be retrieved from literature. In these experiments a chain element is towed through a sand layer in various configurations in a controlled way. Parameters that are varied are: velocity, chain link diameter, angle of attack, density, grain size.
A decrease of the angle of attack leads to a decrease of the tow force. The force per unit length is defined by dividing the tow force by the projected length. The projected length perpendicular to the towing direction, decreases for a decrease of angle of attack. However, the force per unit length also decreases for a decrease of the angle of attack. That means that the projected length is not the only explanation for the decrease in force.
The experiments indicate that the forces for higher penetration depths and higher chain link diameters are very high. An increase of density leads to a significant increase of the internal friction angle for low confining pressures. The increase of the internal friction angle leads to a higher passive soil pressure coefficient. The increase of this coefficient can certify the high forces.
Two dimensionless parameters are defined: the dimensionless force and the Froude number. An estimate for the increase of the passive pressure coefficient is integrated in the dimensionless force to clarify the high values for dense soil. After plotting the experimental results expressed in these dimensionless parameters, a linear trend line representing the data with reasonable accuracy of R2 = 0.611.
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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.
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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…
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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. ... Read more

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.
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To meet energy demand towards a low-carbon future, the global market demand is growing for metals such as cobalt and nickel which are major elements in batteries. Polymetallic nodules, which are formed on abyssal plains at depths ranging from 4 to 6 kilometres and are distributed in high abundance on the top of the seabed, contains several times more cobalt and nickel than the entire global terrestrial reserves. This has raised the interest to exploit these resources from the deep ocean. The seafloor mining tool (SMT) can move along the soft sea bottom and can collect polymetallic nodules. While doing so, it will also entrain sediments and water. The excess of water and sediment entrained is discharged at the back of the SMT, forming a sediment plume. The sediment plume dispersion has strong adverse impacts on deep-sea environment.Thus, it is essential to study the sediment plume behavior in order to limit plume dispersion and thus to reduce its environmental impact.
Experimental research is a powerful technique to study the plume behaviour. However, experiments sometimes take a long time due to complex set-up. In comparision, numerical analysis can save time and costs when solving complex problems. Furthermore, numerical modelling can provide deeper understanding and flexibility for boundary conditions and sediment types, which is applicable on both model and prototype scale. Previous numerical studies have noted the significant role of flocculation in limiting plume dispersion, but flocculation process has not been modelled explicitly. This study aims to establish a numerical model to study flocculation process and its effect on sediment transport.
Previous flocculation-fluid dynamics modelling has applied a Euler-Euler method with additional population balance equations. The disadvantage is that many equations need to be solved. To avoid excessive computational costs, the sediment transport is described by a multiphase drift-flux model in this study. The flocculation process is modelled by a discretized form of population balance equations. The author has found that, by multiplying the particle volume, the population balance can be efficiently incorporated in the phase continuity equations in the drift-flux model. The population dynamics of particle aggregation and breakup can thus be characterized by the phase transition terms in the phase continuity equations. Hence, no additional equation needs to be introduced and solved.
Verification is carried out to check conservation relationships and iterative convergence of numerical results. Then, an initial numerical investigation has shown the results can qualitatively show the three settling stages (i.e., flocculent settling, hindered settling and compression settling) found in the experimental studies. Afterwards, the collision efficiency is calibrated using the settling column tests conducted by Enthoven (2021). The results of calibration show a good fit to the experimental data. Another advantage is that numerical simulations can provide the particle size distribution over time, which is not measured in the experiments.
The major novelty of this study is the coupling of the drift-flux model and the population balance equations, which inherits both the characteristics of population balance and the merits of drift-flux model in reducing computational costs. The flocculation modeling technique as proposed in this study can be incorporated as a module into an extended drift-flux model to predict the dispersion of deep-sea mining plumes. ... Read more

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. ... Read more

The research focuses on sedimentation in reservoirs. Subjects include settling distribution in a reservoir, dredging and flushing of sediment in reservoirs, impact of sedimentation on reservoir performance and financial effects of long term sedimentation in reservoirs. ... Read more

Wave overtopping is responsible for many dike failures, dike breaches and severe flooding. The erosion resistance of dikes to overtopping waves is determined by the strength of the grass cover. Therefore, a thorough understanding of the erosion resistance of grass covers is desired to enable a good and effective dike management. In the existing approach to assess the erosion resistance of grass covers, the erosion resistance is empirically quantified in terms of local hydraulic loads. However, despite the cyclical nature of overtopping waves, no insights into the dependence of the erosion resistance on successive loads are available. To improve the understanding of the resistance of grass covers to wave overtopping induced loads, understanding of the development of the mechanical behaviour of grass sods is essential. In this study the mechanical behaviour of grass covers under tensile loads have been examined by means of direct pull tests executed in the field. Especially the influence of repetitive wave loading and the conditions during testing were of interest. The results of this study show that the deformation of the grass cover due to tensile load is time dependent. The failure mode of the grass cover is influenced by the degree of saturation of the grass cover. The maximum strength and mechanical behaviour of the grass cover is influenced by accumulation of damage. The findings presented in this study provide a better understanding of the processes that influence the mechanical behaviour of the grass cover. Moreover, insights in the development of the process of damage accumulation are revealed. Based on the findings of this study, a method is proposed to determine the resistance of grass covers against repetitive loads by means of direct pull tests. In this method the resistance of grass covers against repetitive loads is described by an estimation of the fatigue limit. Relating the fatigue limit of a grass cover to hydraulic loads, enables an assessment of the erosion resistance of a grass cover at a specific location. ... Read more

In dredging operation, the high-pressure water jet is widely used for the excavation of soil. To study the jetting process and optimize the dredging devices, the moving vertical water jet penetrating cohesive soil experiments were carried out by Nobel (2013). However, in terms of the design optimization for the dredging devices, it is not easy to change the jet scale and soil properties during the experiment due to time and economic constraints. Some detailed physics during the jetting process, e.g. pressure on the soil surface and shear plane inside the soil during jetting, were also not monitored during the experiment. Therefore, numerical simulation is chosen to optimize the design of dredging devices and study the physics of the jetting process. A CFD (computational fluid dynamics) numerical model is built to simulate the moving jet penetrating cohesive soil. The soil is modeled as a Bingham plastic. The sediment transport is modeled by using drift-flux model. The moving jet modeling is achieved by using dynamic mesh algorithms AMI (arbitrary mesh interface) and A/R (cell layer addition removal). The CFD numerical model has been validated with the experiment of Nobel. After the validation, an analysis of the jetting process based on this CFD model is accomplished proving that the CFD model can reveal the details of the soil failure process during jetting. This thesis work reveals that it is possible to describe the hydraulic excavation of cohesive soil with reasonable accuracy using CFD numerical model. The CFD model can also reveal the details of the soil failure process that could not be retrieved from the experiment. Since the model is generic, it can be applied for a jet bar with multiple nozzles. This is helpful to improve the design of dredging equipment, optimize the operational settings and estimate the production. ... Read more