The increasing demand for rare minerals, such as lithium, cobalt, and copper, driven by the growth of the world population and the transition towards sustainable energy technologies, has become a pressing concern. These minerals are crucial for electrifying the transportation sector through electric vehicle production and are in high demand for the thriving high technology industry. However, their scarcity and high prices due to supply shortages necessitate alternative sources to meet these demands. In recent years, deep-sea mining has emerged as a promising solution to address the growing need for rare minerals. The vast potential reserves in the ocean floor offer an enticing opportunity for exploration and extraction. However, deep-sea mining comes with its engineering and environmental complexities that require thorough investigation and understanding. This research delves into the experimental study of a Coandă-Effect-Based Collector, aiming to understand its behaviour regarding water entrainment and cohesive sediment erosion. The focus is on understanding the collection mechanism to minimize clay pickup and maintain low clay concentration in the discharged mixture. This is vital in mitigating the impact of deep-sea mining activities on the marine environment. This research provides valuable insights into water entrainment and cohesive sediment erosion in the context of the Coandă-Effect-Based Collector. The findings emphasize the significance of operating parameters and shed light on the complexities of the collection mechanism. Future studies should explore additional data collection to understand the influence of the secondary jet duct better and employ reliable methods for measuring clay concentrations in the discharged water. Overall, these findings have important implications for optimizing collector design and mitigating the environmental impact of nodules mining activities.

Cutter suction dredgers (CSD) are highly vulnerable to large wave loading due to their stiff, spud-based mooring. Many mitigations targeted at reducing the resulting stresses on the spud and other critical operating components have been proposed in recent years, most notably by adding a level of flexibility to the connection between the pontoon and the spud for rotational motions. Investigating the merits of the implementation of a flexible spud keeper however are hindered by the lack of accurate, CSD-specific models that are able to simulate real waves in the working area, as well as the multibody system of a CSD and its complex interactions with the sea floor. In this thesis, an ANSYS AQWA model is developed to conduct an operability analysis of a CSD in operation in offshore, coastal conditions. Three modelling objectives were established to provide a complete analysis of the effect of increased flexibility in a spud keeper: adding non-linear wave effects, improvement of the soil boundary conditions, and building a dynamic multibody system of a CSD in ANSYS AQWA. A wave model is developed that expands upon standard Linear Wave Theory (LWT) through the use of Stokes 2nd Order Wave Theory (S2). S2 was found to be valid for a larger range of water depths and wave parameters than other wave theories, mainly in shallow, coastal conditions, which is the main operating area of a CSD. These non-linearities cannot be implemented into a frequency domain (FD) model, therefore a time domain (TD) model is required. A combination of S2 waves in a JONSWAP spectrum is used to simulate the motion and force response in TD, as well as second-order forces from the interaction of wave groups with the CSD. The model output is then used as a means of verification of these assumptions and to show the effect of the wave shapes on the spud bending stress. An altered spud model is proposed for shallow, wide spuds, using a rotational spring system instead of the commonly used clamped model. For the use-case of a CSD it shows a spud's tendency for rigid motion below the mudline, while also providing an upper operational limit for soil failure around the spud. The model output showed this upper limit to likely not be a important operational limit, but it can be used to investigate spud rotation below the mudline. Mooring effects from the cutter are absent in older models, but are added in the form of spring elements to better approach reality. Finally, a number of CSDs with different levels of flexibility are tested for four operational limits. The model shows a clear shift in motion resonance towards lower frequencies when the stiffness is decreased, while operability analysis in TD presents a significant increase in operability when a CSD is free to move in pitch. This completely flexible connection provides a major reduction in spud bending stress, but is governed by the acceleration limit of the pontoon. Other reduced stiffness designs are shown to only provide limited benefits to the operability.

Context: The shift to green energy offers a need for offshore wind farms. The transition from a relatively cheap energy production from carbohydrates, to a young offshore wind energy industry needs innovations to lower the cost. Increasing the efficiency of wind turbines and lowering the installation cost of a wind farm, will eventually make wind energy cheaper then energy produced by coal. A part of the installation process is the burial of the submerged power cables. The fragile power cables have to be buried a couple meters below the seabed, to be protected from anchors, fallen objects, and fishing activity. Problem definition: To submerge the power cable into the seabed, a trench have to be made. The soil of the seabed can consist of sand, silt, clay, rock or any combination of them. This research focuses on the trenching though clay. Clay has a very low permeability compared to sand, and clay has cohesive strength. For trenching in clay, a narrow plough is mostly used. These ploughs have a small frontal area and are build to cope with the high ploughing forces. A prediction of ploughing forces and velocities are made by models in preparation of cable burial projects. With accurate predictions, the best equipment can be chosen and a planning of the trenching operation can be made. The prediction models take into account the geometry of the plough and the soil characteristics. A reduction in pulling force results in an increase in ploughing velocity and therefore lowering the time and cost of cable installation. Approach: A large part of the pulling force in clay is caused by the adhesive force on the sides of the plough sliding through the clay. The adhesive force is therefore the main focus of this research. There are two main goals: the first goal is to investigate the adhesion force in relation to the strength of the clay; the second goal is to investigate ways to reduce the force caused by the adhesion. The adhesion factor is a parameter that is included in the prediction models. In literature of construction and agricultural engineering, predictions of adhesion factors and ways to reduce the adhesive force can be found. However, the circumstances during subsea ploughing are vastly different then in the other fields of engineering. Therefore, the approach of this study is to use small scale ploughing experiments with different strengths of clay to investigate the two goals under subsea ploughing conditions. For the experiments, a test setup is used. In this setup, a block of clay of a meter long can be hold into place in a water tank and be submerged. On top of the water tank, an electric motor can pull a cart over rails. A small scale plough can be bolted underneath the cart. During an experiment, the plough is pulled through the clay and the velocity and pulling forces are measured with sensors. Results and conclusions: The adhesion factors of three types of clay are found. The softest clay with a undrained shear strength of 25 kPa has a adhesion factor of 0.43. Literature shows that the adhesion force is about 1.0 at 25 kPa. This low adhesion factor could be the result of the frontal cutting that disrupt the clay. The residual shear strength is lower then the undisturbed clay. For the medium (80 kPa) and hard (131 kPa) clay the adhesion factor is respectively 0.68 and 0.53. These values are on the higher side of what literature reports. This could have to do with the relative high velocity during ploughing. The downward trend of adhesion factor with increasing clay strength does correspond to literature. To research the possibility of reducing the adhesion force, three small scale ploughs with modified adhesion surfaces are tested. The Alpha plough, which uses vertical gaps, reduced the adhesion force by 52 percent in both soft and hard clay. The Bravo plough, which uses convex shapes, reduced the adhesion force 39, 72 and 54 percent in respectively soft, medium and hard clay. The Charlie plough, which uses water nozzles at the adhesion surface, reduced the adhesion force by 70 percent in soft clay and 63 percent in hard clay. This experimental study obtained valuable knowledge of the adhesion factor and possibilities to reduce the adhesion force.

Density waves inhibit the stability of (long) pipelines, which cannot be predicted using current day design tools. Density waves are formed due to an inverse relationship between sand eroding and settling in a pipeline. By designing and building a dedicated flow loop, an experimental study on how the erosion and sedimentation of sand in a pipe is influenced by particle size and concentration is conducted. Moreover, it is determined whether the erosion and sedimentation process can be modeled numerically using existing analytical pick-up functions. This could be applied for predicting density waves in horizontal pipelines. From the experimental study it is found that at higher concentrations, the caterpillar-like movements of the bed, associated with the sedimentation and erosion unbalance at high concentrations, were observed more and more frequently. The influence of the mean grain size diameter can be characterised as the ability of the mixture to trigger amplifying density waves in the horizontal measurement section. Only the coarser sands used in the experiments were found to experience the sliding-stopping behaviour. Implementing different analytical pick-up functions to simulate the experimental study resulted in sufficiently accurate results. Depending on the pick-up function on how much calibration was required the measurements could be simulated adequately.

This study is the first to experimentally show two wave mechanisms regarding density wave amplification with long horizontal slurry transport. There is bed-driven and suspended-driven density wave amplification, in which the grain size determines which mechanism is dominant. Density wave amplification in hydraulic pipeline transport causes significant risk during operation with the consequences of blockage. Current design methodology for pipeline transport considers mixture velocity and density constant over space and time. However, these conditions are only possible in laboratory circuits where conditions can be controlled carefully. In real-world conditions, concentration varies significantly over time due to the natural dredging process in which a dredging vessel takes slurry from the seabed. Density wave amplification can be differentiated into two different flow categories. Both long horizontal transport and a combination of vertical and horizontal transport. With the first category, there are two main theories that explain the amplification of density waves: 'erosion and sedimentation imbalance' and 'the unstable slip point of the bed'. Here, density wave amplification only occurs in the presence of a bed.  In the second category, there is one theory called the: 'transient accumulation theory' which is applicable to a combination of horizontal and vertical transport. With this density wave, amplification can occur far above the deposit limit velocity. Mixture velocities change when density waves travel from horizontal to vertical orientation and vice versa. When mixture velocity changes density will change. The influence of grain size, concentration and the centrifugal pump on density wave amplification has not been researched yet. A test loop has been built with an inner diameter of 46mm and a length of 46 meters. The goal of this laboratory circuit is to investigate the mechanisms that result in the amplification of density waves. Two types of density waves were measured: bed-driven density waves occurring with coarse sediments (Dorsilit 7: d50=1040 μm and Dorsilit 8: d50=619 μm) and suspended-driven density waves occurring with fine sediments (Dorsilit 9: d50=316 μm and Zilverzand: d50=240 μm). With bed-driven density waves, there is fast amplification and multiple sharp waves which can result in areas where no concentration is left. With suspended-driven density waves, there is one smooth wave, and amplification takes multiple loop lengths.

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.

Numerous offshore wind farms have been constructed recently in the southern part of the North Sea. Their infield and export cables are buried for protection against dropped or dragged objects. In sandy soils, it is common to use tracked remotely operated vehicles, equipped with two water jetting swords. These swords fluidise the seabed and generate a backward flow of water-sediment mixture, allowing the cable to sink into the seabed. The southern part of the North Sea has a highly variable seabed topography characterised by sandwaves and megaripples. These seabed features have a significant influence on the trenching process. Existing models do not allow for an accurate estimation of the influence of seabed slopes on the trenching process and are often not based on fundamental physical processes. Two separate numerical models are developed; a jet trenching model describing the cable burial process and a traction model describing the seabed trafficability. The jet trenching model is divided into three parts; an erosion model describing the erosion of soil by the waterjets, a sedimentation model describing the re sedimentation process and resulting trench shape and a cable model describing the cable deflection. The erosion and sedimentation model combined describe the flow of water and sediment in the trench. The erosion model is based on a specific energy approach to determine the maximum allowable trencher velocity, limited by the eroding capacity of the jets. The sedimentation model describes the flow of water-sediment mixture through a rectangular channel, based on the shallow water equations. The channel width is able to evolve due to breaching and the bed elevation is controlled via erosion and sedimentation. The shallow water equations are solved on a staggered grid, following a one-dimensional finite volume scheme. A moving boundary is imposed on one side of the grid to simulate trencher movement. Seabed topography can be imported to model trencher performance on sand dunes. Tractive performance of the vehicle is modelled by considering its driving state. A constant velocity is assumed, hereby balancing thrust and resistance forces. Resistances due to static sinkage, slip sinkage, seabed slopes, current and internal running gear friction are included. The driving thrust force is found by integration of shear stress over track-seabed contact area, including effects of slippage and constant seabed slopes. A sensitivity study has been performed on the jet trenching model, where a strong influence on achieved depth of lowering was found to be caused by grain sizes and depth of the jetting sword below seabed. Influence of trencher velocity on depth of lowering was found to be associated with grain sizes. A higher trencher velocity has a positive effect on the achieved depth of lowering in coarse sand, whereas in fine sand the trencher velocity has a negligible influence on the depth of lowering. Validation of the model with field data shows reasonable agreement regarding average depth of lowering. When including sand dunes, results of the model show a similar depth of lowering trend as observed in field data. However, the amplitude of depth of lowering variation is underestimated by the model. The sensitivity study performed on the traction model showed that resulting slip ratio and power demand have a strong dependency on track-seabed contact area and corresponding normal pressure distribution. Work remains to include the effect of variable seabed slopes, since the current model is based on constant seabed slopes.

CSD spillage is defined as “any soil that is dislodged above the lowest cutter tip trajectory of a single swing, but is not sucked into the suction pipe”. In addition to higher energy consumption and material wear for delivering the targeted depth, spillage can lead to a variety of environmental issues. As of yet, no analytical model exists in literature that can estimate spillage rates for a given set of cutting parameters. This thesis presents the Sand-Rock Cutting Spillage Model (SRCSM), an engineering model that is particle size-agnostic and makes use of cutting parameters that are all available to the dredge operator. Prediction accuracy of 5 percentage point is achieved with a two-disc potential flow model complemented with empirical closing relationships. A triad of forces governs flow in the cutter head for typical cutting conditions: a centrifugal, suction and gravitational force are considered. For the centrifugal pump effect, and centrifugal pump effect only, the flow inside the cutter is considered steady, non-gravitational, inviscid and non-axial. This allows for the derivation of a pressure-discharge affinity law from the Navier-Stokes. The axial pump effect is governed by the mixture velocity at the suction mouth. It is hypothesized that the pressure difference over the discs drives an inflow at the disc closest to the nose. Centrifugal advection and rapid redeposition spillage are considered the two most significant spillage types out of the six classified. Centrifugal advection can be determined by identifying the onset of radial outflow at the disc near the cutter ring. The magnitude of rapid redeposition flow and its concentration depend on mixing effects that are proportional to the ratio particle settling velocity and the mixture velocity squared. The model is calibrated with three coefficients. User input parameters are the cutter geometry, cut-type factor fd,type (-1 for under-cut), bank slope angle ξ, cutter inclination angle λ, bank height h, step size lstep, rotational velocity ω, settling velocity vts, swing velocity vs, mixture velocity vm and material densities. Spillage=f(Dring,Dnose,Dpipe,b,fd,type,ξ,γ,h,lstep,ω,vts,vs,vm,ρq,ρb,ρw ) For calibration, an inverse flow number θ-1 is used that is proportional to the ratio of centrifugal flow over mixture flow. Spillage rates from SRCSM are in high agreement with reference data for sand (Miltenburg, 1983) and rock (Den Burger, 2003) in an under-cut swing. A sensitivity analysis suggests that most cutter head dynamics are adequately incorporated. The model is less reliable for (non-typical) inverse flow numbers of  θ-1= 6 [-] and higher due to a mixture velocity that drops below zero. In addition, the model is calibrated for a relatively high cutter inclination angle of 45 [deg] and bank angle of 45 [deg]. Caution should be observed with the results. It is also suggested that mixing effects related to the swing velocity are incorporated more explicitly in the model. For typical sand cutting conditions, the highest spillage reduction (-4.6%) is achieved by a 1 [%] smaller step size. For rock, the highest spillage reduction (-0.63%) is achieved for a 1 [%] decrease in swing velocity. Spillage appears to follow the theorem of Ellington (1934): it don’t mean a thing if it ain’t got that swing.

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.

Dams are an essential part of the Dutch flood defence works to protect the hinterland from flooding by the sea. Sand closures are the construction of closure dams with sand as the main building material. The rising and falling tide generates a current through the closure gap, which increases in strength as the gap becomes smaller. This current is quite strong and transports a significant amount of sand from the gap to areas where the current is weaker. The sand that is transported away from the closure gap is called the sand loss. The changing tide and constantly moving sand grains result in a dynamic environment. This makes it difficult to estimate the total amount of sand that is necessary to close the dam. It is important to be able to assess the amount of sand loss accurately, because it could make the difference between needing an extra dredging vessel or not. The costs of these dredging vessels are about 1 million euro's a week; therefore, having an unnecessary vessel present due to very conservative sand loss predictions is costly. Laboratory experiments and a numerical model were used to investigate the morphology during bedload transport under a lateral expansion. A streamwise bar formed behind the tip of the lateral expansion for both the laboratory experiments and the numerical simulations. However, it was not possible to replicate the bedforms that occurred during the laboratory experiments with the numerical model. These bedforms are deemed to be important for the morphology during bedload transport. The numerical model was used to expand the research to suspended sediment transport by decreasing the grain size of the sediment. In the obtained morphological patterns, four areas are distinguished where the morphology was found to depend on the flow velocity and the degree of turbulence in the flow. It is expected that a variation of these patterns can be found during a sand closure, although these findings should be validated with a laboratory experiment or in situ measurements. Therefore, it is recommended to set up a laboratory experiment to investigate the morphology under a mixing layer for suspended sediment transport. Based on the results of the laboratory experiments and the numerical model, it is advised to look into the possibility to use the sand deposited as a streamwise bar during a sand closure as an additional borrow pit to aid the closure process, since it would lead to less sailing time for the dredging vessels. For the first stage of a sand closure, it is advised to use a two-dimensional model that incorporates a bedload transport formula to model the sediment transport. It is important to use a parametrization of the bedforms that occur during bedload transport. For the final stage of a sand closure, which is the most critical phase due to high flow velocities, high degree of turbulence and time restriction, it is advised to use a three-dimensional model with a pick-up equation that takes the influence of turbulence on the grains into account, such as the pick-up formula of Okayasu et al.

The Port of Rotterdam experiences large amounts of siltation from the upstream rivers in its basins and canals every year. Furthermore, container vessels increase in size every decade and therefore require a larger navigational depth. These two factors cause the yearly amount of dredged material to increase. Currently, mainly trailing suction hopper dredgers are used to remove the large amounts of sediment, but these vessels come with high costs and operational time. A more time and cost-efficient dredging method is therefore desired. A potential solution to the increasing problems of costs and operational time by dredgers is the technique of water injection dredging. To make this technique as efficient and effective as possible, this thesis has the objective to measure and analyze flow and sediment properties for different parameter settings of water injection dredging and find the optimal parameter settings of this dredging technique on mud from the Port of Rotterdam. Large-scale experiments have been conducted in the water-soil flume of Deltares, where a jetbar was trailed (or run) over a bed 27 meters long and 0.5 meters deep of port mud while injecting it multiple times with water. A positive correlation is observed between the production rate and jet momentum. The production is related linearly with the jet momentum using the Vlasblom equation combined with a non-dimensionless empirical fitting parameter per run per traverse velocity. What is remarkable is that the intrusion depth by the jets increases with ascending runs while the jet parameter settings stay the same. The data shows that the difference between the mass flux by the density current and the mass flux stirred up by the jets when the sediment concentration of an undisturbed bed is assumed, increases between runs when an increased intrusion depth between those runs is observed as well. This indicates that the volume penetrated by the jets contains a smaller amount of sediment than was initially assumed, thus is disturbed by the previous run and is therefore decreased in strength. This decrease in strength results in a larger intrusion depth during the run itself in comparison to the previous run. The influence of the SOD of the jets is analysed by comparing runs with a SOD to runs without a SOD but with similar remaining jetting parameter settings. When a SOD is applied, the jet pressure applied to the bed is outside the flow development region and therefore the jet pressure decreases with distance from the jet nozzle. The mass of sediment stirred up by the jets, for a SOD of 300 mm, is lower in comparison to a SOD of 0 mm. The density current transports relatively more sediment, however, when a SOD of 300 mm is applied. So, if a large amount of sediment needs to be stirred up, and therefore a large intrusion depth is required, no SOD should be applied and when large horizontal transport by the density current is desired, a SOD outside the flow development region of the jets should be used.

Manganese nodules have been lying on the deep sea soil for centuries before they became known. The first ones to discover the nodules were Murray and Renard, during their scientific cruise between 1872 and 1876 (Murray and Renard, 1876). The Clarion-Clipperton Zone, located between Mexico and Hawaii, appears to have a virtually inexhaustible amount of mineral resources. The manganese nodules consist mainly of man- ganese, but because they also contain metals such as copper and cobalt, the nodules become more and more interesting for our modern society. Another name for the manganese nodules is polymetallic nodules, be- cause of the content of many other metals. By means of a vertical hydraulic transport system the manganese nodules will be transported upwards from the seabed. Given that the nodules are located at a depth of about 5000 meter, it is plausible that they come into contact with the pipe wall during transport. When a nodule interacts with the wall, abrasive wear can occur. Abrasive wear, or sliding wear, causes the nodule to degrade, or decrease in mass, through the sliding contact with the pipe wall. The main research question of this Master Thesis is formed as follows: What is the contribution of abrasive wear to the degradation of polymetallic nodules, in a vertical transport system? The contribution of abrasive wear is tested on the basis of the equation of Archard (1953). In the MTI laboratory in Kinderdijk various manganese nodules are tested in a specially designed experimental setup. The polymetallic nodules available for this Master Thesis originate from the Clarion-Clipperton Zone (CCZ). The first set of experiments is focused on single nodule degradation. The nodules are weighted before and after a test run, hereafter the nodule mass after a test run is divided by the initial nodule mass, resulting in the normalized remaining mass. Each test run consists of a specific sliding length. With the sliding length and normalized remains mass, a prediction can be made of the material specific wear rate factor, noted as k [m2/N]. The experiments resulted into a value of k = 4.16·10−9 [m2/N]. With this k-factor, in the worst case, 97% of the original nodule mass is remaining after 1000 meters of sliding contact. Subsequently, a batch of nodules is tested in the experimental setup, with the expectation that the con- tribution of abrasive wear increases to the total nodule degradation, due to the presence of particle-particle interaction. Recently, Blue Mining has conducted trials in Freiberg, Germany, in an abandoned mine shaft. A vertical pipe of almost 130 meters has been placed inside, and the effect of abrasion on a batch of nodules has been observed. During the batch experiments in Kinderdijk, where the batch of nodules was in a horizontal position, it is seen that the contribution of abrasion is greater, compared to the tests in Freiberg. Using the open-source program OpenFOAM, a vertical turbulent pipe flow is modeled. The pipe has simi- lar dimensions to the pipe of the experimental setup. The simulation was done using a Large Eddy Simulation (LES). By means of a one-way coupling, the data of the flow field is linked to the movement of the simulated particle. Subsequently, point particles were released at various points at the entrance of the pipe. The first series of point particles consists of a diameter of one-third of the pipe diameter, corresponding to the com- mon size of a manganese nodule. The collisions between the point particle and the pipe wall are solved with a Discrete Element Method (DEM), which makes use of a (non-linear) spring-damper system. To test the sensitivity of the model, point particles of one-sixth and one-tenth of the pipe diameter are also released. The number of collisions per total number of simulations are compared with each other. After the sensitivity analysis it turned out that in 15% of the simulations there was contact with the pipe wall, regardless of the diameter of the particle. When this 15% is compared to the experimental results using single nodules, there is a weight loss of around 0.5%. This holds a weight loss of only a single - to a few - gram(s) for the average polymetallic nodule. It can be concluded, according to the experimental trials with polymetallic nodules from the Clarion- Clipperton Zone and by means of a one-way coupled CFD-DEM model, that the contribution of abrasive wear results into a (maximum) remaining mass of 97%. However, it is advised to extend the computer model with a two-way (or multi-way) coupling between the liquid and the solids. It is also recommended to conduct vertical abrasive wear tests with nodules, in order to validate the model and gain more insight in degradation behaviour.

In dredging there are a lot of opportunities to improve the production processes and to make the production cycle more efficient, especially the excavation process of the draghead. The aim of this research is to determine the trailing forces and to estimate the production of the draghead. Besides that, the goal is to get more insight into how the draghead behaves, depending on the trailing velocities. The variations in the trailing velocities, soil characteristics, control settings and draghead and suction pipe geometries that can occur do not make it easy to determine the trailing forces on the draghead and suction pipe. Because of this complexity, it is also hard to define how to estimate the production. Therefore, the scope of this research is limited to just one sand type with specific soil characteristics. An analysis of the draghead and the suction pipe, with a freely suspended visor, showed the physical processes in and around the whole suction pipe system. Because the draghead is fixed to the suction pipe, the influence of suction pipe on the draghead is analysed first. After that, the draghead is divided into two parts, the visor and the visor house. With the use of force and moment balances the trailing forces are determined for every trailing velocity. In addition, the production and its production limits are defined. The calculations show that the increase of the trailing velocity results in higher trailing forces on the suction pipe and draghead. For a velocity of around 2 m/s the draghead, for a Damen SLK600 used in the case study, will lift of from the bed. It should be notified that, among other variables, the dredging depth has an effect on this ‘floating’ point. Moreover, the results showed that the drag forces at common trailing velocities of 1-2 m/s are relatively low compared to the soil excavation forces and therefore have a small share in the total trailing forces. When the suction pipe system is trailed against the current the dragforce becomes more significant. The interaction of the draghead with the bed causes several processes to take place. The resulting relevant trailing forces are mapped and determined. The settlement of the draghead causes a hump of sand to be pushed forwards which result in a sled force and a friction force. Besides that, the flow through the pipes will cause impulse forces in the bends and at the end of the jet pipe out of the nozzle. The jets fluidize the sand which results in the largest production contribution. Furthermore, it can be seen that the penetration depth and cavity width of the jets depend on the trailing velocity and determine the amount of sand that is loosened. The cavities can overlap at low trailing velocities, resulting in a jet production limit. The jets have a significant influence on the behaviour of the visor. The freely suspended visor will drop until a solid bed layer is reached. The cutting force and vacuum force are the dominant forces working on the visor. Application of the equilibrium-moment method showed that the visor is slowly moving upwards when the trailing velocity is increased. However, the cutting layer thickness remains almost constant for an increasing trailing velocity which results in a linear increase of the cutting production. The cutting production contributes, 20-25%, to the total situ production. The total jet and cutting production lead, together with the jet water inserted and ambient water flow, to the total production and mixture density. When the capacity of the dredging pump is insufficient, spillage will occur. This research shows the best possible estimate of the draghead production and corresponding trailing forces. It should be kept in mind that the calculations are based on a simplification of the suction pipe system and the geometries of the suction pipe system of Damen. Nevertheless, a lot of research can be performed into the processes that occur, to improve the results of the calculations. Suggestions for further research would be to determine the magnitude of the vacuum force, the erosion production which is not considered in this research or the on velocity depending cavity width. Validations of the results is another point of consideration. Thus, to determine trailing forces and production, the new approach used in this study contributes to a better understanding of the draghead.

Drag in pipelines is composed almost solely of skin friction drag. The most common technique to achieve drag reduction (DR) is by adding drag-reducing agents. However, in the aerospace industry, various impressive passive drag-reducing techniques have been suggested to reduce skin friction drag in the past decades. Among these techniques, dimpled surfaces form a relatively unexplored terrain. Research into reducing skin friction drag in the turbulent regime by using dimples has been performed in the aviation industry since the '80s. The excessive amount of skin friction drag in pipelines forms an intriguing challenge to break new grounds. Several researchers investigated the potential of DR of turbulent flows using dimpled channels. Even though most of these studies that found DR are disputed, studies published at the National University of Singapore (NUS) obtained positive results time and again. NUS's exact test setup was reconstructed at Delft University of Technology, which has not been done yet as far as the author of this report is aware. Identical pressure measurements were performed, yielding an absolute DR of ≈ 5%, which is slightly less than what was obtained at NUS (>7%). Flat plates return a DR between 8-15%, dimpled test plates returned at DR between 12-20%. The test plates were covered for 99.5% with diamond-shaped dimples of 100 mm long and 50 mm wide. In total, 29 pressure taps were used to determine the change in drag in an 8 m long channel of 20 mm in height. Tests were done at Reynolds numbers, based on half channel width and centerline velocity, between 6,000 and 40,000. Accurate results were perceived up to a Reynolds number of 21,000, likely due to test-setup limitations. Multiple verifications such as two-dimensionality of the ow, comparison of theoretical and experimental skin friction coefficients, and instantaneous pressure tests were used to allow for an objective analysis. The pressure measurements were also supported by 1D hotwire anemometry (HWA) tests and surface oil ow visualizations (SOFV). The majority of the investigated boundary layers were absent of anomalies. It should be mentioned that the viscous sublayer could not be captured, neither a quantitative momentum analysis was performed. However, a shift in the velocity profile, acquired at the same test location, for different test plates was observed. Furthermore, an increased velocity near the wall was observed for dimpled test plates. Surface oil ow measurements did show similar ow patterns to what was recorded at NUS. However, the actual behavior is not investigated through HWA. Hence, it cannot be confirmed nor denied if the change in drag is a consequence of these near-wall ow mechanisms. SOFV did not show irregular ow structures such as ow reversal. Finally, tests were performed with a correction for test volume increase caused by the dimples. After this correction, the drag over dimpled plates increased instead of reduced. Considering other studies on dimpled surfaces that also found an increase in drag, it is strongly believed that the positive effect of dimples in turbulent channel flows does not stem from skin friction drag but rather from the increase in channel volume. Although the precision of the results was relatively high, the accuracy of the wind tunnel was insufficient. Additional research is required to narrow down the 8-15% error that was obtained while testing at plates.

The deployability, regarding mobility, of Boskalis trenchers, is currently determined by practical knowledge. The lack of theoretical knowledge means that risk assessments in the tender phase are less accurate. This reduced accuracy has two negative consequences. First, the trencher may fail. Secondly, the safety margins can be too large. Both consequences result in additional duration and costs. This research considers two ways of failure. The first one is slipping; as a result of this, the track does not have enough traction, which causes the tracks to turn while the trencher does not move forward. The slipping can happen in two ways. The first way is due to soil failure with the grousers fully penetrated. This results in the trencher digging itself in. The other way is that the grousers are not penetrated and the trenchers slips due to the tracksoil interaction. In order to prevent the trencher from slipping, horizontal stability between the soil and tracks should be obtained. The second way of failure is that the trencher sinks into the soil. Vertical stability between the applied pressure and the bearing capacity should be achieved to prevent this kind of failure. In this research a model has been developed that can approximate the operating range of different trenchers. With the help of these operating ranges, the model can make risk assessments in an early phase of the project. There will be little knowledge about the soilspecific parameters and the exact external conditions in this phase. Should there be a risk in the area of mobility along with the project, the model is also able to provide a more indepth analysis. In addition, the model aims to apply to different types and sizes of trenchers. The research answers the following question: ”Which different operational environments and soil conditions have a critical influence on the deployability of a subsea tracked trencher regarding the vertical stability and horizontal stability of the soil¬track interaction system? The model considers the trencher’s mobility in terms of vertical and horizontal stability, where the axes rotate with the slope. Meyerhof’s effective area method approximates the vertical stability. This method is used to take into account the eccentric loads. A traction calculation model approaches horizontal stability. Based on the penetration depth of the individual elements of a track, a shear mode is determined. The available traction force per element can be calculated using these shear modes using the Mohrcoulomb shearing theory. With a case study for the CBT2400, the research question is answered. In this case study, we first look at the different external processes that influence mobility. In addition, the various soil parameters and trencher dimensions are examined. Finally, the results of this study will be compared with two reallife projects. The results show that the mobility of the trencher is better in granular soils than in cohesive soils. In cohesive soils, the trencher’s mobility depends on the external factors that change in the driving direction, like pitch slopes and currents that have a frontal impact. In cohesive soils, the sensitivity is a vital soil parameter besides the undrained shear strength. Since the sensitivity is not included in the soil research, it is recommended to do it in future projects. In granular soils, external factors from all angles must be taken into account, like pitch and roll angles and currents from all directions. With granular soils, especially the relative density and internal friction angle are essential; in addition, the grain size must be taken into account. The grain size influences the permeability of the soil. The permeability, together with the relative density, determine if contractancy and dilatancy will take place in the soil. It could be interesting to change the grouser and track dimensions to improve the trencher’s mobility. However, this depends on the circumstances in which the trencher would have to operate. The model was validated using two cases involving the Borssele and Moray East projects. An important conclusion is that the model can be used to estimate the potential risk areas reasonably. However, it is recommended to test the model in predefined circumstances and soil parameters.

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.

After decades of exploitation of hydrocarbons, the offshore facilities constructed for this purpose are nearing the end of their design life and/or economic operation. According to international law, these offshore structures need to be completely removed. Decommissioning of the substructures often requires for soil to be removed in piles to facilitate pile cutting works below the seabed. One way of achieving this is by deploying a specialized tool, a so-called Soil Plug Removal Tool (SPRT), that operates using hydraulic excavation. This research carried out under supervision of Royal Boskalis Westminster N.V. aims to get a more solid basis for comparison of different SPRT concepts that are available in the market. The tools are designed to handle a wide range of soil types. Removal of cohesive sediment is more challenging, mainly due to the very low water permeability present compared to granular soils. This study therefore focusses on the excavation of cohesive soil types only. In order to verify the performance of several concepts an experimental test program is set up on model scale. The primary goal is to investigate the achievable excavation production in terms of tool progress rate. Therefore, a jetting tool is developed that covers the (complete) spectrum in terms of cohesive soils and performance of the available tools. Two basic SPRT concepts are incorporated in this single tool based on head movement: static or rotating. Jet pressure, clay strength, rotational velocity and set down pressure of the tool are altered during testing on the condition that all other parameters are fixed. This requirement is met for the testing clay by merely varying the shear strength. The testing clay was therefore prepared both with an artificial and natural clay with shear strengths ranging from 20 kPa to 100 kPa. It is found that next to jetting, soil failure can also be attributed to cutting and jet trench failure under influence of the jetting head that rests on top of the clay. For this reason, production values belonging to jetting could not be obtained directly and had to be calculated using jetting theory to distinguish between jetting and jet trench failure. Based on the power that is required to excavate a certain volume of soil (i.e. specific energy), insight is given in the contribution of each failure mechanisms to the production in terms of tool progress rate. During static jetting, the current (nozzle) configuration did not remove enough soil from the jet cavities for the jetting tool to progress downwards. The opposite is true for the rotational tests which comprise the largest part of the test series. An analytical model is proposed to predict the cavity width and depth. This model is only valid for jets with small rotational velocities as encountered in this study. The total production, which was measured, is found to be inversely proportional to shear strength and directly proportional to jet pressure and rotational velocity.