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. ... Read more

Subsea rock installation is an offshore engineering process where rocks are placed on the seabed as protection of cables and pipelines and as scour protection. The inclined fallpipebis a new piece of equipment specifically designed to install rocks close to submerged structures. This thesis investigates the processes of the rock flow in and below the inclined pipe. This is done with a research question in two parts: What is the rock behavior in the inclined fall-pipe, and what is the behavior below the inclined fall-pipe? The research is questions are supported by sub-questions on the influence of pipe angle, production and rock size.

To answer the research questions model tests have been carried out at a scale of 1:15 in the Dredging Lab at TU Delft. The tests were performed with varying pipe angles, production rates and rock sizes. The tests have been analyzed with special focus on velocity, flow behavior, touch down offset from the pipe, and the spread of the rocks. For analysis, video recordings of the tests have been used and the tests have been analyzed with Particle Image Velocimetry software PIVlab.

The results of the tests reveal that the velocity of the rock flow mostly depends on the pipe angle and production rate, and for a lesser part on the rock size. Steep pipe angles increase rock velocity, increasing production leads to a higher average velocity, and smaller rock sizes increase the velocity.

The spread of the rocks and the offset from the pipe are influenced the strongest by the stand-off (SOD) distance between the pipe and the bed. In the tests the SOD was determined by the pipe angle. To compare the tests, they were also analyzed at the same height below the pipe. The results show that the spread of the rocks is only influenced by the height above the floor. The offset is influenced both by the angle and the production. The influence of the production is only visible at lower angles. The increase in production means an increase in velocity and the rocks falling further away. More horizontal pipe ... Read more

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.
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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.
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Due to urbanization, improved living standards and electrification, approximately five times more raw minerals are necessary in 2050 compared to 2018. In deep oceans, the seafloor contains these minerals in the form of polymetallic nodules. Nodules are about the size of golf balls that grow throughout the ocean at depths between 3500 m and 6000 m. They contain a wide variety of metals, such as manganese, copper, nickel, cobalt. Nowadays, for large-scale applications, hydraulic lifting is almost exclusively considered for vertical transportation through the water column. However, there is little research available about using other techniques instead. To tackle this knowledge gap, this thesis studies the feasibility of transporting the nodules using a concept of mechanical lifting. The concept used in this thesis consists of two alternating containers that are lowered and hoisted by lifting and guidance wires. Due to the conditions, such as the large depth, the environmental characteristics and the positioning and heading of the vehicles, there are technical uncertainties regarding mechanical lifting. Risks include the yaw rotation of the container, which might result in rope entanglement and wearing of the ropes. This thesis presents a study into the yawing stability of the concept of mechanical lifting for the vertical transportation of polymetallic nodules, which is a crucial factor to operate reliably.

The research question is answered by performing an experimental test and a CFD analysis. The experimental tests include the dynamics of the system while testing various configurations and is validated by an analytical integration in time and a CFD simulation at model scale. The CFD analysis takes away the uncertainties and unknowns: the drag force, the yawing moment and the fluctuation magnitudes and frequencies. The CFD analysis is performed using the open-source software OpenFOAM and simulates multiple configurations. The results of the simulations are compared to the restoring moment by the guidance wires, by transforming the excitation moments into static and dynamic responses of the system. The CFD model is validated by testing the model with a 2D cylinder and 3D sphere, and by performing a mesh convergence study. The CFD simulations are validated by literature. With the obtained drag forces, the energy consumption is calculated.

From the results, it can be concluded that the system can stably be transported at 2 m/s, as the static and dynamic responses are well within the safety limits. The largest response occurs in the middle of the water column, as the rotational stiffness is the smallest at that location. The dynamic response is smaller compared to the static response, as the high frequent fluctuations (f > 0.075 Hz) are damped. Rope entanglement will not occur during normal operation at 2 m/s. However, critical situations due to incidental events can arise, including a winch failure, friction or a sudden high current. This has not been evaluated in this research and therefore stability cannot be guaranteed. As lowering at 3 m/s with an inclined system and including the current results in a static maximum yawing rotation larger than the safety limit, the stability cannot be guaranteed for operating at 3 m/s. ... Read more

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. ... Read more

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. ... Read more

Due to the increased demand for materials like cobalt and nickel, there is an interest to mine polymetallic nodules from the deep-sea. These nodules are abundantly distributed along the abyssal plains, e.g., the Clarion Clipperton Zone (CCZ) in the North East Pacific. These nodules lay spread on top of a seabed consisting of very fine clayey sediment and will be collected by a seafloor mining tool (SMT). During the operation, the seabed will be disturbed, resulting in a suspended sediment plume discharged by the SMT. This plume can have a significant environmental impact through a blanketing effect on the abyssal fauna and create a disturbance in the water column affecting the entire food web structure. Hence, identifying the critical processes and quantification of sediment plume dispersion is essential to predict the potential environmental impact better and determine what technologies would enable a lower ecological impact. Spearman et al. (2020) investigated turbidity plumes generated by deep-sea mining experiments and discovered faster settling velocity than the theory described. They hypothesize that this is due to flocculation. The settling speed depends on the density, concentration, shape, and cohesive properties of the sediment. Flocculation in the deep-sea can occur in two ways, by salinity or by organic matter. Gillard et al. (2019) showed that the flocculation response of CCZ sediment strongly depends on the concentration and applied shear rate. To analyze to what extent aggregation could influence the plume dispersion, experiments are conducted in which the effect of aggregation can be adjusted selectively. Lock exchange experiments are used to analyze the impact of aggregation by comparing results with illite based on freshwater, saltwater, pre-existing bed, and added flocculant. Additional experiments with artificial CCZ sediment are done to ensure that the illite experiments are not too idealized. To prove that the settling velocity is increasing in saltwater, settling velocity tests have been performed. I performed experiments with illite suspensions in a settling column. An increase in settling velocity is observed due to the rise in salinity up to 75 g/L of illite. Lock exchange experiments are performed to mimic the particle-driven currents. The lock experiments involve a lock release of a fixed volume suspension of sediment from the mixing section into the outflow section. Similar observations were made by doing lock exchange experiments. A decrease in average head velocity is shown for all experiments up to 75 g/L of illite in saltwater. To further induce flocculation and mimic organic matter in the water column, experiments with flocculant were done. Mixing the flocculant, Zetag 4120 or Zetag 8125, in the mixing section will further decrease average head velocity compared to saltwater. Zetag 4120 even ensures very fast settling as the end of the tank will not be reached For low concentrations in saltwater, flocculation is shown as the tail decreases more rapidly than freshwater. Flocculation with higher mass concentrations of illite is shown when flocculant is added to the mixing section. Adding Zetag 4120 to saltwater makes the current even settle quicker. As the SMT will move over a pre-existing bed, experiments were done to see if a pre-existing clay bed has consequences for the behavior of the flow. A clay bed has been made by running a lock exchange experiment with a concentration of 100g/L. This test then had, depending on the next experiment, one day or two days settling time before it was used for the new experiment. One-day bed experiments overall encountered a decrease in average head velocity. Two-day bed experiments overall experienced an increase in average head velocity. To mimic the existing sediment in the CCZ and determine if illite experiments are not too idealized, experiments were done with artificial CCZ sediment. The currents behaved differently compared to illite, as they produced more coherent flows for low concentration. Also, it showed better effects on lowering the ecological impact when looked at mass concentrations of 100g/L of artificial CCZ sediment as gelling occurred. To minimize the environmental impact created by deep-sea mining, the SMT should produce a discharge between 50 and 100 g/L of artificial CCZ sediment. Gelling will occur, which has a positive effect on minimizing the plume dispersion. ... Read more

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. ... 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

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. ... Read more

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 f_d,type (-1 for under-cut), bank slope angle ξ, cutter inclination angle λ, bank height h, step size l_step, rotational velocity ω, settling velocity v_ts, swing velocity v_s, mixture velocity v_m and material densities.
Spillage=f(D_ring,D_nose,D_pipe,b,f_d,type,ξ,γ,h,l_step,ω,v_ts,v_s,v_m,ρ_q,ρ_b,ρ_w )
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.
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In recent years, the demand for minerals and rare-earth elements are escalating due to rapid technological advancements and developments. This condition raises the importance of Deep-Sea Mining (DSM) as an option to fulfill the global demands, for the sake of future ambitious projects. On the other hand, DSM still faces some drawbacks and obstacles in its operations, e.g. environmental impact of its tailings discharge. Thus, the presence of tools for predicting the behavior and environmental impact of DSM tailings becomes crucial for the sake of conducting sustainable DSM operations. Researches, both numerical and laboratory experiments, are then done to achieve this goal. In the study of DSM tailings behavior through numerical simulation, the challenge lies in the ability to implement the complex physics phenomena around DSM plumes to a numerical model. This research is thus aimed to observe one of the parts of the so-called physical phenomena, and numerical constraints on the simulation of DSM plumes: the effects of implementing arbitrary non-orthogonal mesh. Arbitrary non-orthogonal mesh would give users the freedom to refine the mesh based on the required resolution on a certain area in the simulation domain. In this research, an arbitrary non-orthogonal mesh is constructed forming a domain geometry of 3D tank with round pipe as a source of tailings discharge, adapting the laboratory setup of Byishimo (2018). Mesh refinement is done around the pipe discharge area, where the jet mainstream expected to occur, and near the bottom, where settling of solids fraction and high velocity-gradients are expected to occur. Top-hat approximation theory is then used for defining the inlet boundary condition of the simulation domain, and smooth solid wall is used for the bottom boundary. In observing the effect of arbitrary mesh, two parameters are varied: solids settling rate, and differentiation scheme. Following this, six simulation cases are prepared, containing three solids-settling conditions (minimum, realistic, and extreme solids fraction settling) and two differentiation schemes (Gauss Gamma and Gauss Linear). These cases are then simulated using the CFD drift-flux model in OpenFOAM for two incompressible fluids, with ambient fluid and tailings mixture as the two incompressible fluids. Turbulence is modeled using LES method, with Wall-Adapting Local Eddy-viscosity (WALE) LES model for modeling the subgrid-scale of the turbulence. Two simulated field variables are picked to be observed and compared with the laboratory measurement data: flow velocities and local Suspended Solids Concentration (SSC). Simulations show that the constructed domain able to generate stable results using not only Gauss Gamma differencing scheme but also Gauss Linear, which originally expected to give unbounded results. From simulations with various solids settling conditions, it is analyzed that the implementation of solids settling using the mentioned function leads to constant settling rate with inability to re-suspend the settled solids. Thus, simulation of cases with extreme solids settling leads to hyper-concentration of solids fraction in the cells on the bottom. The top-hat profile of the inlet boundary condition is constantly sustained throughout the simulation, resulting in uniform jet vertical velocity profile. The simulations also show that momentum-driven jet-like flow can be observed around the impingement point, while gravity current generated further from the impingement point. The simulation results show that the mesh refinement is enabling simulating and validating the flow with the required resolutions. Moreover, the constructed domain also shows that Gauss Linear can be used for simulating DSM plumes, furthermore using full tank domain. The simulated SSC also turns out not only affected by the simulation domain and mesh, but also the differencing scheme, and the presence of settling and pick-up functions. ... Read more

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. ... Read more

When dredging close to underwater sand slopes, steep slopes might form. In dense sand with low permeability this might lead to the so-called breaching process. The creation of a steep underwater slope marks the beginning of a breaching process.
Pore volumes of densely packed sand tend to increase during shear deformations. This effect is called dilation. As the grain slides over each other, the pore volume increases resulting in increased underpressures. Water, eventually, has to flow in to compensate for this underpressure. The flow rate depends on sand properties. The underpressure keeps the sand body, temporarily, stable. When enough water has flowed in and the sand has dilated enough, sand particles release at the front. This is at the start of the breaching process. This leads to a density current consisting of sand mixed with the surrounding water, which runs down the slope and might cause erosion. The steep front of the slope moves with a certain velocity, which is called the headwall velocity.

A breach can be stable or unstable. A breach is stable if the breaching height decreases in time and unstable if the breaching height increases in time. This work aims to improve the prediction of the stability of a breach. For this purpose I carried out a series of large scale breaching experiments. During these experiments the initial breaching height, slope angles, and sand types and varied.

Literature suggest that the stability of a breach can be predicted using the headwall velocity and the angle at the toe of the breach. The experiments show that this is indeed the case, but that breaches are more stable than literature suggests.

Therefore, to predict the stability of a breach we must know the angle at the toe, and the headwall velocity.
The experiments show that the angle at the toe converges towards the angle that is predicted using equations found in literature.

A formula to predict the wall velocity can be found in literature. Comparison with the experiments show that this formula can predict the wall velocity when no slides are present, but at large breach heights these slides often occur.

The experiments show a clear correlation between the breach height, and the frequency of sliding wedges.
Empirical equations following from the test data predict the percentage of the sliding wedges at different breaching heights are proposed.

A steady state numerical model, to calculate the pore pressure during a breaching process, was programmed in MATLAB. Using these pore pressures, a stability analysis was carried out. This analysis confirms that breaching height is an important factor for the prediction of sliding wedges.
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The first head loss prediction methods for hydraulic transport of solids in pipes, date back to the years '50 of last century. The principles are still applied today. Although it is observed that obtained results may differ from reality, when the circumstances do not represent the situation of the original experiment. To investigate this, the performance of various prediction models is analysed on a large scale laboratory test set-up. In recent publications, several researchers observed disturbances in their expected flow patterns. Described as either 'unsteady flow’, ‘instability’ or 'unexpected mechanism'. Talmon developed the theory that the occurrence of these events could be explained by transient processes in the pipe flow. It is recommended to use a test pipe with great length. To verify, by longitudinal pressure profile measurement, that equilibrium is measured, and that indicated transients are captured by the measuring system. A laboratory test set-up is provided by the CCCC National Engineering Research Center of Dredging Technology and Equipment Co., Ltd. In a joint research program with Delft University of Technology: the pressure drops for water and mixture flow over the pipeline length are analysed. This is done for horizontal hydraulic transport. The focus is on comparing test results with existing theories and find explanations to deviating results. If instabilities are observed, the gathered data can be used for further research on density waves in pipelines. In the liquid flow experiment, three prediction methods derived from the Colebrook-White equation are analysed: Darcy-Weisbach, Swamee & Jain and RangaRadju & Garde. Considering the pipeline to be smooth, the predicted values are similar. The difference between them is almost nil. Furthermore, the result shows good correspondence with the data acquired on the test set-up. In the mixture experiment, four prediction methods are analysed: Durand, Führböter, Jufin & Lopatin and Wilson. They are compared with laboratory data of test conducted in a heterogeneous flow regime. With a velocity ranging from three to six meter per second and concentrations of: 4.4, 8.1, 12.3 and 14.6 percent. In a general approach, considering all transport velocities and concentrations. It is observed that the theoretical principle described by Durand shows the closest resemblance with the experimental data. When only the concentrations are considered, a distinction has to be made. For the two lowest mixture densities, the Führboter method gives the best fit. For the highest two, the best correlation is according to the Durand theory. The principles of Jufin & Lopatin and Wilson underestimate the pressure loss. Where the difference of the former is significantly larger in comparison to the latter. For the lowest velocities of the twelve and fifteen percent slurry experiments, stationary waves over the pipeline length are observed. Due to the length of the test set-up, the transition to equilibrium flow is clearly visible. It demonstrates that longer flow loops give opportunity to conduct further research into the extent of waves. ... Read more

Little is documented in literature regarding hydraulic processes in inclined and large diameter pipelines. The bulk of the previous research dates back 20 to 50 years and was done for small diameter pipelines of up to 150 mm.

This research focuses on gathering data and testing the existing models and ideas with regard to large diameter inclined pipelines and the hydraulic transport within them. The overall goals of the project are to gather knowledge on the less documented and studied principles of pipeline flows and validate ideas and
semi-empirical models from previous researches.

Extensive laboratory experiments were conducted as part of a joint research programme between Delft University of Technology and the National Engineering Research Center for dredging equipment and technology in Shanghai. The experiments were executed with a flow loop with a pipe diameter of 300 mm. It contains a measurement section of over 110 meters, part of which is inclinable. Pipe inclination angles of 17.9, 28.9 and 44 degrees were tested with slurry concentrations up to 15 % at flow velocities between 2 and 7 m/s. The flow velocities, delivered concentrations, total pressures, differential pressures and pump data were recorded. Conducting these experiments on this scale under controlled laboratory conditions is a unique research.

Three semi-empirical models by Worster and Denny, Gibert and Wilson for inclined slurry transport are validated. A comparison is made between ideas from literature regarding deposition limit velocities, delivered concentrations, pipe inclination angles, stratified flow regimes, particle suspension and different flow directions. The observations from previous researches with small pipe diameters are generally in line with the results of the experiments conducted for this thesis. The semi-empirical models prove to deliver accurate predictions of the total pressure gradients in heterogeneous flow regimes. With regard to stratified flows, it is proposed to modify the semi-empirical models by adding a factor to the suspension and solids effect terms. The factor is a function of deposition limit velocity, flow velocity, inclination angle and flow direction. ... Read more

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. ... Read more

Beach nourishments or sand replenishments are applied by the use of pipelines or the rainbow method. Replenished sand is then moved and levelled by bulldozers. This is a passive approach to process the outflow of material. The land based equipment is dependent from tides and water levels, and significant effort is required to install and maintain the onshore discharge pipeline. In remote (off-shore) areas mobilization of site equipment to move the sand may be quite a challenge.
The ideal method considers an active approach regarding processing the outflow of material. Instead of distributing the settled material by site equipment, the pipeline out flow point has to be relocated such that the design could be constructed.
To increase workability the pipeline must have the ability to be relocated in water as on land.
Enabling this approach the pipeline system has to be displaced by some sort of means. The main problem is the rigid behavior of the pipeline. Displacement of the pipeline will result that the entire pipeline length has to be displaced. Assuming that in water relocation of a floating pipeline is not that difficult as floating equipment is able to reach the floating pipeline. When the pipeline is situated on land huge pull or push requirements follows when the pipeline needs displacing.

Concepts both for depositing material as for delivering the material have been generated.
The most promising method for depositing material is to apply a spray pontoon. By adding amphibious propulsion technique to the spray pontoon the pontoon is able to work on the interface between water and land.
The most promising method regarding delivering material to the spray pontoon is by applying a steel pipeline.
During depositing the spray pontoon have to be displaced frequently. Also the spray pontoon has to be able to displace the pipeline system. Properties of the pipeline system dictate the required amount of tractive effort that have to be generated by the spray pontoon. Focus is on maximizing the tractive effort to be generated by the spray pontoon.
To decrease the amount of resistance the pipeline will be mounted on platforms. Focus is on minimizing the required amount effort to displace the pipeline system and by generating flexibility along the pipeline system.
By generating flexibility along the pipeline system the pipeline could swing independently of each other. Production figures will determine the amount of flexibility needed along the pipeline system.

It is technical feasible to apply an amphibious spray pontoon but a uniform concept doesn’t exist because there is a large amount of parameters and aspects involved.
It depends on the type of project, and site conditions which type of platform have to be applied. On project locations were small variations of the water level is to be expected and the soil surface has high bearing capacities values platforms can be applied that have an interaction with the soil surface.
However, on soft soils with low bearing capacity values the soil interaction platforms will experience significant sinkage; the resistance force to displace such platforms may possible not be generated by the spray pontoon. In addition on soft soil the spray pontoon is able to generate a smaller pull force compared to when on sandy soil surface.
On (very) soft soils platforms that don’t have an interaction with the soil surface are advantageous compared to soil interaction platforms.
... Read more

Estimators at the production department of Great Lakes Dredge & Dock Company use a number of models to make production estimates for a job. These models require the slurry concentration in the suction arm of a trailing suction hopper dredge (TSHD) as an input. Currently this concentration is estimated based on results of previous projects. To aid in the estimation of the slurry concentration a model that calculates the in situ production and slurry concentration has to be developed.

During this research a model was developed that calculates in situ production by iteratively finding equilibrium between the actuator moments and the cutting moments around a draghead visor hinge. The slurry concentration is then calculated using the in situ production and a function describing the relationship between slurry concentration and dredge pump flow based on data of an existing TSHD.An extension to existing cutting theory was made to model the effects of the presence of water jets in the cutting teeth of the draghead. In this extension water jets were modeled as a pressure source which provides water to reduce the pore under pressures that cause cutting forces.The production model was evaluated by performing sensitivity analyses. In general the course of the production as a function of the trailing speed that was found could be described as a linear increase followed by a steep drop in production after which production started increasing again at a lower rate. It was found that it was mainly the geometry of the draghead that caused this steep drop. Another finding is that the production at operational trailing speeds (1 to 2 knots) is only of a realistic order of magnitude (between 36 and 125 m^3/min in this case) when still in the linear increase.

The conclusions that were drawn during the development of the model and the sensitivity analyses are that the TSHD draghead production might actually be best described as a linear function of the trailing speed and that the angle of internal friction of the sand is the parameter with the largest influence on the production. Of controllable parameters the pressure in the actuator had the largest influence on the production. That the results of the tooth jet extension correspond sufficiently well with an energy-based type of approach. ... Read more