Deep-sea nodule collection generates suspended sediment plumes that may affect the surrounding benthic environment. Reliable monitoring of suspended sediment concentration (SSC) and particle size near the plume source is therefore important for plume characterization, modelling, and environmental impact assessment. This study evaluates the applicability of multi-frequency acoustic backscatter measurements for determining SSC and particle size estimates under conditions representative of deep-sea nodule collection plumes.

An experimental campaign was conducted in a recirculation tank using two representative deep-sea box-core sediments, referred to as CCZ sediment and Sediment B. Multi-frequency acoustic backscatter measurements were obtained at 1, 2, 4, and 5 MHz frequencies and were evaluated against gravimetric SSC measurements over an experimental suspended sediment concentration range of
0.16 − 32.4 g/L. The recirculation tank was designed to generate a relatively homogeneous suspension of unflocculated deep-sea sediment, thereby simulating conditions within a collector vehicle diffuser. Four acoustic inversion methods were evaluated. Methods 1 and 2 applied commercially available
bin-based inversion approaches to resolve SSC and particle size over range, whereas Methods 3 and 4 applied profile-averaged inversion approaches implemented in a custom Python workflow. The commercially available bin-based inversion methods did not provide reliable results for the investigated fine-grained deep-sea sediments. Method 1 substantially overestimated particle size, by factors of 4.1 and 18.2 for the CCZ sediment and Sediment B, respectively, which corresponded to SSC underestimations of approximately 1.2 and 2.0 orders of magnitude. Method 2 did not yield reliable SSC inversions due to its strong sensitivity to small deviations in the prescribed particle-size parameters (D50
and σ). In contrast, the profile-averaged Methods 3.1 and 4.4 provided quantifiable SSC estimates for both sediments. The inversion performance was governed primarily by the prescribed median particle diameter (D50), while constraining the particle-size distribution width (σ) provided limited additional
benefit. The D50-constrained configurations were therefore selected as the representative cases. Over the common evaluated SSC range of 0.5 − 18.0 g/L, Method 3.1 achieved proportional biases of +36.1 % and +1.2 % for the CCZ sediment and Sediment B, respectively, with corresponding R2 values
of 0.995 and 0.978. Method 4.4 achieved proportional biases of +9.8 % and −15.5 %, with R2 values of 0.944 and 0.867, respectively. For the CCZ sediment, extending the evaluated range to 32.4 g/L resulted in proportional biases of +31.1 % and +19.2 % for Methods 3.1 and 4.4, respectively. Across the selected configurations, the proportional bias remained within approximately ±36 %. Particle-size and distribution width inversions were substantially more sensitive to uncertainties in the acoustic measurements and could not yet be reliably quantified.

For comparison, the optical backscatter sensor (OBS) produced slopes of 0.857 (R2 = 0.983) for Sediment B and 0.546 (R2 = 0.884) for the CCZ sediment. The difference in performance between the sediments demonstrates the dependence of optical measurements on representative sediment-specific calibration. Multi-frequency acoustic backscatter, in contrast, provided range-averaged SSC estimates without requiring sediment-specific calibration, although its effective measurement range significantly decreased with increasing SSC.

Overall, multi-frequency acoustic backscatter shows strong potential for range-averaged SSC monitoring within or close to deep-sea nodule collector diffusers, where particle properties remain relatively stable and theoretical attenuation and scattering models are most applicable. Its ability to provide measurements over a larger representative volume without sediment-specific calibration offers an important advantage over conventional optical point measurements. However, further development is required to improve particle-size inversion and measurement reliability at elevated SSC, followed by validation under representative deep-sea operational conditions. ... Read more

Conventional beam trawling for sole is associated with high fuel consumption and significant seabed disturbance due to the use of tickler chains. With the ban of pulse trawling, waterjet stimulation has been proposed as a potential alternative startle mechanism. However, previous concepts were ineffective and continuous activation was considered economically infeasible because of its high energy demand. Combining waterjets with fish detection technology may offer a solution by activating the system only when marketable fish are present.
The objective of this thesis was to develop and evaluate a trawl concept that integrates waterjet stimulation and fish detection technology into an existing trawl configuration to minimize energy consumption and seabed disturbance while maintaining catch efficiency for sole. An extensive concept development and optimization was performed using knowledge on existing trawl systems, waterjet studies, fish behavior and fish detection technologies. This development and optimization began with the development of functional requirements and generating sub-concepts for different design aspects. These sub-concepts were combined into two main concepts and evaluated based on selected design criteria and stakeholder feedback. Finally, the selected concept was further developed and refined, and the hydraulic water supply system was optimized.
The resulting concept consists of multiple enclosed tunnel-shaped sections separated by side chains and netting. Each section contains its own fish detection gear and selectively activated waterjet housing, supplied by a water supply system positioned inside a wing configuration. The system is integrated into a conventional trawl configuration by placing its accumulator-based hydraulic system inside a conventional wing-with-shoes configuration, and uses a conventional netting and square-shaped net opening. Optimization of this system gave an estimated pump power range from approximately 32 kW at a towing speed of 4.5 knots to around 75 kW at 5.2 knots, which was estimated to be a realistic power limit.
The developed concept appears therefore technically feasible for towing speeds between 4.5 and 5.2 knots, and has the potential to reduce seabed disturbance and improve the practical feasibility of waterjet stimulation compared to continuous activation across the full trawl width. However, the physical response of sole to waterjets, the performance of the detection gear, and the operational behavior of the complete system remain uncertain. Although experimental validation is required before the practical effectiveness, seabed disturbance, catch efficiency, and overall energy performance of the concept can be determined, the developed design shows that selective waterjet stimulation can potentially be an alternative to conventional stimulation. The concept therefore serves as foundation for future experiments and further development in finding a
potentially less invasive and more energy-efficient trawl system. ... Read more

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

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

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

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

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

The expansion of floating offshore wind projects into deeper waters with seabeds composed of hard rock requires alternatives to conventional drill-and-grout anchoring systems. Grouted anchors require offshore mixing of grout, transportation of the grout to the seabed and sufficient curing time before the anchor can become operational. To overcome these limitations, a groutless undercut anchor has been proposed as an alternative. This concept relies on the mechanical interlock between the anchor and surrounding rock, eliminating the need for grout and thereby reducing both installation time and the number of offshore operations. To achieve this interlocking mechanism, a belled geometry must be drilled at the base of the borehole using a technique known as ‘underreaming’. However, the understanding of the rock cutting process for this reaming operation remains limited.

This thesis presents the development of an analytical modelling framework, based on a conceptual reamer design, to simulate and evaluate the rock cutting process during underreaming. A kinematic model is formulated to describe the combined rotational and radial expansion motion of the conceptual reamer. This is coupled with a brittle tensile failure-based cutting model to predict cutting forces and to characterise the rock excavation mechanism. The model simulates different cutting regimes: crushing, relieved cutting, and unrelieved cutting based on tool spacing and the interaction of breakout geometries between adjacent cutting teeth. Two spacing configurations were considered in the simulations: equal and optimal tool spacing. The performance of the reamer is assessed for varying operational parameters on both configurations in (homogeneous) brittle rock. The limitations of the model are the exclusion of shear and ductile (cataclasis) failure mechanisms. Furthermore, the modelled cutting forces were underestimated and approx. 24% lower compared to experimental results. Lastly, the cutting depth is determined from the (prescribed) tool motion and thus not constrained by the operational torque or thrust, which are instead modelled as demands.

The operational parameters investigated were the rotational speed (rpm) and expansion rate ( φ ̇) of the reamer. Analysis showed that the spacing-to-depth ratio of the cutting teeth varies along the cutter arm. Lower values were obtained at the lower part, and higher values for the upper part. Decreasing rpm and increasing φ ̇ decreased this ratio for both configurations. Under identical conditions, the optimal tool spacing configuration showed lower specific energy, despite a small reduction of the production compared to the equal tool spacing configuration. Furthermore, both rpm and φ ̇ were found to influence the cutting production and specific energy. Lower rpm increased the cutting depth per rotation increased, promoting relieved cutting when tooth spacing remained the same. A similar effect was observed with increasing φ ̇. Further load analysis revealed that torque and thrust demand for the optimal tool spacing configuration were consistently lower under varying operational conditions.

Findings of this thesis provide a first estimation of how a conceptual reamer can be analysed by integrating a kinematic framework with established rock cutting theories and experimental observations, thereby offering insight into the cutting process during reaming. ... Read more

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

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

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

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

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

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

The work presented in this thesis investigates the role of flocculation in turbidity flows, with relevance to deep-sea mining but also dredging operations in more shallow waters, as the results are broadly applicable. Deep-sea mining is emerging as an alternative source for critical minerals needed for the transition to clean energy technologies. However, mining activities disturb the seabed, leading to the formation of sediment plumes. These sediment plumes propagate over long distances and impact sensitive marine ecosystems in the area. Similar environmental concerns also arise in the case of dredging projects, where resuspended sediments increase the turbidity in the surrounding waters. These challenges highlight the need for improved understanding of sediment plume dynamics and the processes that control their evolution. Flocculation (the aggregation of small particles into larger ones) could potentially limit turbidity flow propagation. This underpins the work presented in this thesis.


A series of laboratory experiments was designed to study the propagation of turbidity currents under controlled conditions. A lock-exchange flume setup was used to simulate sediment-laden turbidity currents, where detailed investigations of current propagation and floc formation were carried out. The work combines hydrodynamic measurements with advanced particle-sensing techniques to link micro-scale flocculation processes with turbidity current behavior. Experiments were first conducted with clay (illite) in the presence of flocculants (polyacrylamide), and then with a natural clay from the Clarion-Clipperton Fracture Zone (CCZ) that contains organic matter (acting as flocculants). Some experiments were also conducted with non-flocculating quartz as a reference.
The results demonstrate that flocculation can occur rapidly, within tens of seconds, and significantly modifies particle size distributions and settling behavior.

The thesis further explores how seabed characteristics influence turbidity currents. Experiments were performed over beds of different compositions and ages to understand their roles in turbidity flows and floc evolution. The presence of polyacrylamide in the outflow compartment of the lock-exchange flume was found to increase the front velocity when no bed was present, which was attributed to the lubrication effect between the turbidity current and the plexiglass bottom of the flume. It was found that pre-existing beds and their consolidation state affect sediment entrainment and flow propagation. Bed roughness and material type can either enhance or suppress floc formation, thereby altering the mobility of the turbidity current. These findings are directly relevant to operational strategies in dredging and mining, where repeated disturbance of the seabed occurs.

In addition to physical mechanisms, this research also examines the ability of monitoring instruments to detect turbidity current properties, particularly particle size and concentration. Optical Backscatter Sensors (OBS), Acoustic Doppler Velocimeter (ADV), and Laser In-Situ Scattering and Transmissometry (LISST) were used in the lock-exchange setup. Malvern mastersizer and FlocCAM were used to further characterize samples taken at different positions within the lock-exchange flume. The study demonstrates that sensor responses are highly sediment-dependent and that no single instrument can, in situ (lock-exchange) reliably distinguish between primary (unflocculated) particles and aggregates. A combination of lab measurements and in situ sensor techniques is therefore recommended for studying flocculation.

Overall, this thesis provides new experimental insights into the coupling between particle aggregation and turbidity current dynamics. It shows that flocculation, bed interactions, and sensor limitations must all be considered when predicting sediment plume behavior. The outcomes contribute to more reliable assessment and monitoring of environmental impacts associated with offshore engineering activities and offer guidance for future field measurements. The work ultimately strengthens the scientific basis for responsible and sustainable management of deep-sea mining and dredging operations.
... Read more

Pipelines are widely recognized for their efficiency in material transport, and dredging operations rely heavily on them to convey sediment. In pursuit of more sustainable dredging practices, reducing the flow resistance within the dredging pipelines presents a promising approach, as the lower resistance directly decreases the energy consumption of the pump.

A previous study by Yun Peng Yan investigated the effect of the convex pattern on the wear rate in dry bulk material transport. The finding showed that the convex pattern reduced wear by inducing dilation in the bed layer, temporarily creating free space that allowed the particles to roll more easily. This rolling mechanism, observed under dry conditions, is particularly interesting to examine in wet environments. If the same mechanism also occurs inside the pipe in wet conditions, it could influence the flow resistance and potentially improve the efficiency of the dredging pipe. Since no prior research has tested convex patterns under wet conditions, the present study investigates how a convex pattern plate installed at the bottom of a pipe affects flow resistance and wear distribution during the transport of a gravel–water mixture.

In this study, experiments were carried out using gravel with an average particle diameter of d_50=3 mm. The convex pattern plate was evaluated against a plain plate, which served as a reference. The findings revealed that, consistent with previous dry-condition studies, the convex pattern induced particle rolling along the bed within the sliding-bed regime. However, this rolling mechanism did not lead to a reduction in flow resistance. Instead, the convex pattern configuration consistently exhibited higher resistance in both the stationary-bed and sliding-bed regimes. This increase in resistance also affected the deposition limit velocity, which increased from 0.9 m/s for the plain plate to 1 m/s for the convex pattern plate.

In the sliding-bed regime at lower flow velocities, abrasive wear was observed in both configurations. However, the wear behavior changed at higher velocities within the same regime. For the plain plate, the most severe wear was concentrated in the joints between the pipe segments, where the surface irregularities intensified abrasion. In contrast, the convex plate showed reduced wear at the joints and top surfaces, as the stationary particles in the bed provided a protective layer. However, at higher velocities, the upper layers of the bed began to move more rapidly and collided with the crests of the convex structures. This transition shifted the wear mechanism from abrasion to impact wear, causing the most severe damage at the tops of the convex patterns. In both types of plates, the segment directly exposed to the incoming flow experienced the heaviest wear.

In general, the anticipated reduction in flow resistance from the convex pattern was not achieved. The installation of convex structures at the bottom of the dredging pipes does not improve the transport efficiency and, in fact, may increase resistance, making the approach unsuitable as a general wear-reduction strategy. Interpreted through Newitt’s theory and the Darcy–Weisbach relation, the higher resistance observed in the 40 mm pipe suggests that this effect could be amplified in larger pipe diameters. This is because the water resistance decreases with increasing diameter, and then the solid effect increases to compensate for the higher mechanical friction introduced by the presence of convex structures. However, the additional resistance measured in this study was relatively minor and the deposition limit velocity increased by only 0.1 m/s. Since this increase is small, the improvement may still be advantageous in critical pipeline sections where wear protection is required. Therefore, a localized application of convex patterns could remain a practical solution.
... Read more

The urgency of the energy transition, combined with geopolitical tensions over critical raw materials, has increased interest in deep-sea mining of polymetallic nodules. However, there are serious concerns about its environmental impacts. Life Cycle Assessment (LCA) is currently used to support claims that metals from deep-sea mining are environmentally more preferable than those from land-based mining. However, LCA addresses only generic environmental impacts, and previous LCAs comparing deep-sea and land-based mining show considerable variation in outcomes. This study critically examined the applicability of LCA for comparing deep-sea and land-based mining by analysing previous LCAs, developing an aligned LCA, and discussing limitations. Previous LCAs consistently reported lower climate change impacts for deep-sea mining, but the magnitude of this benefit varied due to methodological choices, data inputs, and assumptions, especially regarding energy. The aligned LCA found lower impacts for deep-sea mining in almost all 20 evaluated categories, except transformation of natural land, including the seabed. However, the results depended strongly on LCA design choices: under renewable electricity scenarios, land-based mining outperformed deep-sea mining for climate change and energy resources. Trade-offs emerged, such as high water use from hydropower and ionising radiation from nuclear energy. The study highlighted both strengths and limitations of LCA in this context, and emphasized that it should be complemented by location-specific assessments to capture social, economic, biodiversity, and ecosystem impacts. This study did not conclude whether deep-sea mining is environmentally more preferable than land-based mining, but it identified conditions under which it results in lower generic environmental impacts, and highlighted critical considerations, knowledge gaps, and uncertainties to guide policy making and future research. ... Read more

The global transition to green energy has intensified demand for critical raw materials, increasing interest in deep-sea mineral resources such as cobalt-rich ferromanganese (Fe-Mn) crusts on seamounts like Tropic Seamount in the NE Atlantic. This thesis investigates the relationship between water depth and Fe-Mn crust composition and evaluates whether incorporating this relationship through Co-kriging (CK) improves resource estimation compared to Ordinary Kriging (OK). Geochemical and bathymetric data were analyzed using exploratory data analysis. The estimation workflow included block model generation, isometric log-ratio (ILR) transformation, Landmark-ISOMAP embedding for locally varying anisotropy, variogram modeling, and geostatistical estimation with Ordinary Kriging, Simple Co-Kriging (SCK) and Intrinsic Collocated Co-Kriging (ICCK) as well as Inverse Distance Weighted (IDW) Estimation. Model performance was assessed using Quantitative Kriging Neighborhood Analysis (QKNA) metrics and leave-one-out cross-validation (LOOCV), with a critical evaluation of LOOCV’s limitations.

Results demonstrate a significant relationship between water depth and Fe-Mn crust composition, with a significant improvement in estimation accuracy and confidence when water depth is used as the secondary variable in Co-Kriging, with SCK and ICCK providing more accurate and confident resource estimates than OK. ICCK also performed better than IDW. For the first time, tonnage calculations for metals in Fe-Mn crusts based on a 3D block model and real geochemical data are presented, highlighting Tropic Seamount’s potential as a substantial mineral resource for Europe. The workflow developed in this study, including ILR transformation and L-ISOMAP embedding, proved effective for handling compositional data and spatial anisotropy. The study also identifies methodological limitations, such as the need for improved variography, better sampling distribution, consideration of non-metallic elements, and more advanced cross-validation techniques. The assumption that the top 1 cm of crust represents the entire deposit is noted as a simplification, and future research should address vertical stratigraphy and sampling distribution.

The findings indicate that integrating ILR transformation and Landmark-ISOMAP embedding with Co-Kriging leads to better resource estimation for Fe-Mn crusts, enabling more confident and accurate assessments. This methodology offers significant potential for mineral resource exploration and future research in both marine and terrestrial environments. ... Read more

With the rising global demand for rare earth metals, innovative approaches such as deep-sea polymetallic nodule collection have gained increasing attention. The Clarion-Clipperton Zone (CCZ), one of the most economically promising regions for these resources, is a key focus area. However, before large-scale exploitation can be carried out, it is essential to understand and minimise the environmental impacts thoroughly. One of the primary concerns regarding the environmental impact is the generation of sediment plumes.

This study has aimed to improve the understanding of sediment plumes generated by the tracks of the Seabed Nodule Collector in the NORI-D area. NORI-D is an area within the CCZ. A literature review has been conducted to examine the characteristics of the NORI-D site and the collector’s track design, identifying key mechanisms of plume generation. Building on these insights, a kinematic model has been developed to estimate sediment relocation. A previous study on plume generation during a nodule collector test run in the GSR area has provided a valuable starting point. To pursue the remaining research objectives, a series of laboratory experiments has been performed, focusing on the effects of seabed penetration depth, driving velocity, slip ratio, and sediment type. A scaled model of the collector tracks has been driven over a sediment bed, using an artificial clay developed to mimic the behaviour of deep-sea sediment.

The combined insights from the literature review, kinematic model and experimental work now offer a sound understanding of sediment plume formation and an estimation of the turbidity source term associated with the collector tracks. The results provide an insight into the influence of the tested parameters on plume generation, identifying seabed penetration and driving velocity as the key factors. The knowledge gained from this research could support efforts to minimise the environmental impact of deep-sea mining operations on the seabed. ... Read more

The dredging world is continuously growing, improving and adjusting to new challenges. As the demand for sustainable dredging practices grows and the availability of sand declines, the industry is increasingly exploring alternative materials, including fine-grained soils. This transition presents new challenges, particularly in understanding the behavior of these soils during dredging and reclamation. When stiff cohesive soils are dredged, they often form large lumps or balls, which are subsequently transported and deposited within a reclamation fill alongside a slurry.

The deposited clay lumps create a matrix with significant inter-lump voids, forming a double porosity system, consisting of these inter-lump voids and the voids within the lump, that significantly affects settlement predictability. Current settlement calculation methods typically assume a uniform layer, with soil parameters derived from the soil parameters of dredged material, which are later adjusted based on field feedback. This approach introduces substantial uncertainty, potentially leading to greater risks and increased costs.

This thesis investigates the complex deformation behavior of stiff, lumpy double porosity fills by performing both field data analysis and controlled experimental research. Field observations, including Cone Penetration Tests (CPTs), borehole data, and settlement plate measurements from a real-world reclamation project, are used to evaluate existing settlement models and the limitations of current design assumptions. In parallel, an experimental campaign involving oedometer testing and micro-CT scanning was conducted on clay lumps of varying undrained shear strengths($S_u$) to quantify deformation of macro- and, micro-voids and softening effects.

Findings reveal that traditional 1D-consolidation models, based on homogeneous assumptions, underestimate settlement magnitudes and rates during early loading phases dominated by macro-void collapse. Macro-void closure was observed to occur primarily under vertical effective stresses between 0.5 - 2 $S_u$, after which the fill behaves more homogeneously but remains structurally heterogeneous at the micro-scale. Experimental results highlighted the influence of lump strength and softening processes on overall deformation, with weaker lumps exhibiting void collapse at lower stress levels.

The study concludes that accurate settlement prediction of lumpy fills requires explicit consideration of initial macro-structure collapse, evolving lump strength, self-weight consolidation, and heterogeneous stress transfer. While conventional isotache-based models remain suitable after macro-void closure, early-stage design should incorporate adaptive, probabilistic approaches. Recommendations for future research include refining laboratory testing protocols, improving imaging resolution for micro-structural tracking, and expanding long-term monitoring to better understand the time-dependent evolution of lumpy fills under field conditions. ... Read more

This thesis explores the measurement of low sediment concentrations in pipeline flows, a critical issue in deep-sea mining. The study aims to identify and validate a continuous measurement method capable of accurately detecting suspended sediment concentrations between 10-75 g/L. These measurements are essential for minimizing the environmental impact of sediment plumes generated by mining operations.

The research begins with an analysis of material properties and flow dynamics, emphasizing the role of sediment particle size, and density in influencing suspension and settling behaviours. Highlighting the differences between concentration measurement and density measurement and the additional error involved. A review of flow regimes, turbulence, and their effects on sediment distribution within pipelines sets the foundation for understanding the complexities of representative sampling.

The thesis evaluates a range of measurement methodologies against criteria such as accuracy, range, spatial and temporal resolution, safety, cost, and impact on the flow. Optical, acoustic, conductivity, and radioactive source sensors were unsuitable due to limited range, low accuracy, or safety concerns. Promising alternatives include the U-loop, Coriolis, and vibrating fork sensors. The U-loop offers good accuracy and range, but causes significant pressure drops. Coriolis sensors provide excellent accuracy and broad range, but require careful sampling to ensure reliability. The vibrating fork sensor is simple and has a good range, but suffers from low accuracy and limited spatial resolution.

A key aspect of this research is the experimental setup, which integrates multiple sensor technologies in a controlled flow loop environment. Detailed methodologies for sensor calibration, installation, and data collection ensured the testing conditions. Tests are conducted under varying sediment concentrations, sediment types, grain sizes, and flow velocities to validate the accuracy and reliability of each measurement method.

The results reveal significant differences in sensor performance across test conditions. Test variables such as sediment type, concentration, and flow velocity are shown to influence sensor performance.

In conclusion, suspended sediment concentrations from 10 - 75g/L can be measured accurately using a Coriolis sensor. This sensor directly measures density which can be converted into concentration with minimal calibration or correction, demonstrating superior accuracy and precision across diverse flow conditions. Although it reliably detects finer deep-sea sediment, minor challenges remain when measuring coarser materials, such as nodule fines. Overall, the findings contribute to the development of environmentally responsible deep-sea mining practices, by facilitating real-time and accurate sediment monitoring. ... Read more

This research is conducted in the framework of a Master’s thesis within the Marine Engineering department of the Maritime Transport and Technology branch and in joint collaboration with the Robotics branch of the Mechanical Engineering Faculty of Tu Delft.
The precise control of robotic systems in granular underwater environments is essential for applications such as deep-sea mining, sediment sampling, and seabed infrastructure maintenance. In such environments, the interaction between the robot and deformable substrates like sand and clay plays a crucial role in operational efficiency and system stability. This research investigates how fine-tuning joint stiffness in an impedance controller influences penetration depth accuracy, horizontal force distribution, and force consistency along a trajectory mapped using 3D camera point cloud data. Understanding these relationships is critical for optimizing force control strategies in unstructured and dynamic underwater settings.
Experiments were conducted using a KUKA iiwa 7 robotic arm equipped with an impedance controller, following a mapped trajectory over a real sandbed in both dry and submerged conditions. The point cloud data from a 3D camera provided accurate environmental mapping, ensuring precise trajectory tracking. The results indicate a significant correlation between joint stiffness and penetration accuracy: higher stiffness improved depth accuracy and reduced external disturbances but compromised adaptability in cases where the robot encountered hard obstacles. Conversely, lower stiffness increased compliance, allowing for smoother interactions but at the cost of greater sensitivity to force fluctuations.
Fluid damping in submerged conditions was found to reduce penetration error variability, highlighting the stabilizing influence of water on force interactions. The study also revealed that current robotic systems for deep-sea applications differ significantly from the 7-degree-of-freedom (DOF) KUKA arm used in this research. In practical scenarios, deep-sea mining robots typically feature a single actuated DOF (pitch), with other degrees of freedom facilitated by passive flexibility rather than active control. These structural differences influence force distribution and overall system behavior, emphasizing the need for future studies tailored to real-world deep-sea mining configurations.
Future research should extend beyond sand to softer seabed sediments such as clay, which behaves more like a Bingham fluid and exhibits significantly lower shear strength—potentially by a factor of 5 to 10 compared to sand. Additionally, exploring adaptive stiffness strategies that dynamically adjust control parameters in real time could enhance the efficiency and robustness of underwater robotic systems. These advancements would contribute to optimizing force control for precise, adaptable interactions in unstructured marine environments. ... Read more

Deep-sea mining (DSM) focuses on extracting valuable resources, such as polymetallic nodules, from seabeds at depths of up to 5000 meters. These nodules contain critical metals but pose technical challenges due to extreme conditions like high pressure and long transport distances.

A critical component of deep-sea mining (DSM) is the vertical transport system (VTS), which lifts nodules from the seabed to the surface using a riser system with two-phase (solid-liquid) or three-phase (solid-liquid-gas) flow. The VTS plays a vital role in determining throughput, energy consumption, and the overall stability of the mining operation. Multistage centrifugal pumps are promising for powering the VTS due to their ability to handle high pressures and coarse solids. While the risk of blockages is minimal under normal operating conditions, flow assurance within the pump becomes a significant concern during reflux situations. In such cases, gravity causes the nodule-water mixture to reverse direction, substantially increasing the risk of blockages and clustering. This highlights the importance of analysing reflux scenarios in greater detail to ensure reliable system performance.

To address these challenges during reflux, a test setup is developed to investigate operational and design variables as well as pump geometry. Two model pumps were constructed: one without impeller and diffuser blades, focusing on the effects of internal spacing, and one with blades, aimed at evaluating overall reflux performance. The key vulnerabilities identified in this thesis provide a clear understanding of where issues may arise in the pump design and the underlying mechanisms causing them. This knowledge enables potential users of the multistage centrifugal pump to make informed adjustments, preventing flow assurance challenges in their systems.

Both the pump geometry and mixture variables were thoroughly analysed. For the geometry, the curvature of the impeller blades was found to cause solid accumulation at the entry, posing a significant risk of blockages. This research identified the issue and proposed modifications to the blade edges, which proved highly effective in eliminating accumulation and significantly improving solid flow. Narrowing diffuser vanes and shallow blade inclination were found to promote contact-dominated flow, velocity reductions, and obstructions. At the diffuser-impeller transition, solids entering from multiple directions caused bridging and blockages. Mixture variables also had a notable impact. Solids within the pump's design specifications passed through unobstructed when processed in single-solid batches. However, increased solid concentration at the pump inlet led to accelerating blockage formation. Higher-density solids demonstrated better performance by maintaining higher velocities and reducing the risk of obstructions.

To enhance future testing, it is recommended to use transparent materials, as implemented in this study. This approach provided valuable insights into the flow dynamics and greatly improved the ability to observe and analyse potential blockages. Additionally, optimizing the geometry of the impeller and diffuser blades is critical for improving the pump's overall performance. Adjustments should focus on reducing blockages and promoting smooth solid flow, while carefully balancing throughput efficiency and pump capacity. These design improvements are essential to ensure reliable and efficient operation, particularly in demanding applications as deep-sea mining. ... Read more

Mooring chains of floating offshore wind turbines generate cyclic seabed loading that excavates trenches and may transport sediment out of the trench. This thesis is the first to investigate the physical conditions under which such sediment escape ceases, using a Computational Fluid Dynamics (CFD) model that couples a Boussinesq-type incompressible flow solver with an immersed boundary method and suspended-sediment transport. Forty simulations were conducted across four trench depths (3−9 m), five grain sizes (0.0625−1 mm), and two inflow regimes (0.05-0.36 m/s) representing calm and storm conditions in the North Sea.
A new Escape Potential Index (EPI) was introduced primarily as a simulation prioritisation tool to identify edge cases and minimise the number of non-informative runs. Classical predictors such as the Rouse number were insufficient for this purpose as they do not include confinement or chain-induced uplift. The EPI therefore combines settling behaviour, chain forcing, and trench geometry into a single heuristic ranking. Although not a universal threshold, EPI correctly orders cases by mobility and, when combined with the Rouse number, defines a separation curve that distinguishes escaping from stable configurations and explains why classical suspension theory under predicts escape under cyclic chain forcing.
The results show that trench depth is the dominant control on sediment mobility. Shallow trenches strongly couple chain motion to the flow, producing intense suspension even under weak currents, while deeper trenches reduce uplift through geometric confinement and increased travel distance. A stabilisation threshold emerges: beyond approximately 5−7 m depth, all but the finest sands remain trapped. Grain size dependence follows classical settling behaviour, with fine material escaping across all depths and coarse fractions remaining largely immobile. When the depth–grain size limits are compared with North Sea bathymetry, extensive central and northern deep water areas appear suitable for floating concepts.
The study provides a mechanistic basis for predicting when sediment escape ceases, though limitations such as the rigid seabed, simplified chain geometry, and steady-state inflow imply that results represent upper bound mobility estimates rather than full morphodynamic evolution.
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The global rise in demand for critical metals has renewed interest in deep-sea mining of polymetallic nodules on the abyssal plains. Commercial concepts primarily adopt vehicles with tracks to collect these nodules from the soft cohesive seabeds at depths exceeding 4,500 m. These vehicles need to generate sufficient traction and maintain mobility while minimizing seabed disturbance. Predicting their performance, however, remains difficult because of the complex soil behavior and the limited amount of experimental data available. Most existing mobility models are empirical or assume perfectly plastic soils, which neglect strain-dependent softening and remolding.

This thesis develops a generic analytical framework that connects soil stress–strain behavior to track-scale mobility. The model couples a bearing-capacity-based sinkage formulation with a saturated soft-plastic shear–displacement relation. It allows the use of peak and residual shear strengths, as well as the characteristic displacement to peak strength (Kω), directly as input parameters. The framework handles traction and resistance under quasi-static, undrained conditions and applies to both straight-line and turning motion.

The results show that soil sensitivity (the ratio between peak and residual strength) and Kω control the shape of the traction-slip curve. Larger Kω shifts the peak to higher slip and delays remolding, while higher sensitivity steepens the post-peak drop and lowers available traction once the soil is remolded. Increasing the effective contact length improves traction up to a plateau, after which the additional gain becomes small. In heterogeneous (layered) soils, where strength increases with depth, traction increases with grounding pressure but at a decreasing rate. In homogeneous (uniform) soils, traction decreases with added normal load because increased grounding pressure limits shear mobilization before failure.

During turning, most of the track footprint exceeds the characteristic displacement Kω and the response becomes governed by residual strength. Turning is therefore traction-limited and controlled mainly by soil sensitivity, residual strength and geometry. A clear transition in the minimum turning radius is observed when the required thrust exceeds the residual traction level; beyond this point, small-radius turns become infeasible.

The results indicate that mobility can only be predicted reliably when post-peak soil behavior is included. Turning design should be based on remolded soil conditions, while straight-line operation should remain in the pre-peak region to prevent stalling. Design efforts should focus on optimizing contact length, adjusting grouser height to local seabed conditions, and measuring residual strength and sensitivity for the specific site.
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Interest in deep sea mining (DSM) has increased significantly due to mainly an expected increase in consumption of rare metals and a trend towards a green world. The supply of rare metals is not always reliable, since there is a limited number of major suppliers. An alternative source to the established suppliers on land could be polymetallic nodules found in deposits on the abyssal plains of the oceans.
These nodules contain multiple metals such as nickel, cobalt and rare earth elements and can be extracted by a Seafloor Mining Tool (SMT). As a consequence of the collection of the nodules the SMT erodes the sediment bed as well. The hydraulic collection method of the SMT in this thesis makes use of the Coandă-effect. It is desirable to reduce the amount of sediment picked up from the bed by the collector head of the SMT, as this ultimately reduces the potential disturbance of benthic life by turbidity flows.

Water entrainment of ambient water in front of the collector head and spillage flow behind the collector head caused by the Coandă-effect is also not fully understood, suggesting a deeper understanding of the flow field around the collector head is required.
Operational parameters of the SMT and a secondary jet of the collector head have influence on the flow field and thereby the amount of sediment picked-up from the seabed. The influence of these parameters on bed erosion and sediment collection by the SMT and spillage behind the collector head requires further investigation as well.

To this end Computational Fluid Dynamics (CFD) is used to conduct simulations on the DelftBlue supercomputer at the Delft High-Performance Computing Center. The OpenFOAM solver driftFluxFoam is modified in order to investigate the collector head of a Coandă-Effect-based SMT. In these simulations a small-scale and full-scale collector head is investigated with the focus on the flow field behaviour, sediment collection and spillage. Additionally for the full-scale collector head the influence of 4 collector head parameters and 2 sediment bed parameters on the collected and spilled sediment is investigated.

The results show that the flow field around the collector head of the Coandă-Effect-based SMT is in agreement with previous research. Jet flow dominates near the curved plates due to the Coandă-effect, Water is entrainment in front of the collector head and sediment is eroded and either collected or spilled behind the collector head.
The variation of the 6 parameters have influence on bed erosion, sediment collection and spillage as expected, especially the jet flow through the main and secondary jet. Additionally, the amount of collected and spilled sediment is increased by an increased collection duct flow and decreased by an increased forward velocity. The importance of a balanced combination of the parameters and the influence of the sediment characteristics is also stressed.
The novelty of a secondary jet in the collector head has influence on the sediment collection and spillage as well when the flow rate is varied, suggesting that this can be used in the collector head design to reduce the entrainment of water and thus further reduce the amount of bed mobilization, sediment collected into the SMT and sediment spilled behind the collector head reducing the environmental pressure.
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The growing demand for critical metals such as cobalt, nickel, and copper, essential for renewable energy and green technologies, has intensified interest in deep-sea mining of polymetallic nodules. However, such activities generate sediment plumes that may severely impact mining operation and deep-sea ecosystems. The behavior of these plumes is strongly influenced by flocculation, the process through which fine sediment particles aggregate into larger flocs, altering their settling velocity and transport. While flocculation behavior and settling velocities are well documented for the Clarion–Clipperton Zone (CCZ), no data currently exist for the Cook Islands (CI) region. Differences in sediment characteristics may lead to distinct flocculation and settling behavior, highlighting the importance of region-specific data for reliable plume modeling.

This study characterizes the flocculation and settling behavior of deep-sea clay from the CI region to evaluate the applicability of existing CCZ-based plume models. Laboratory experiments were conducted under controlled conditions using jar tests, laser diffraction, rheometry, and imaging techniques to quantify floc size and settling velocity under varying shear and concentration ranges.

The CI clay exhibited non-Newtonian and thixotropic behavior, with a yield stress above the gelling concentration. Floc growth followed a sigmoidal pattern, well described by a logistic growth model. Higher shear rates and clay concentrations were found to limit the floc size. Importantly, flocs of similar size displayed comparable settling velocities, regardless of their formation history.

When compared with CCZ sediments, CI material showed similar gelling behavior but notably slower settling velocities. In addition, floc sizes observed for flocculation under the same conditions also differ. Consequently, plume dispersion in the CI region is likely to be more extensive, and models calibrated for CCZ sediments are not directly transferable. This study provides the first experimental dataset on CI sediment flocculation and settling behavior, offering critical insights for developing region-specific plume models and designing environmentally responsible deep-sea mining operations. ... Read more

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

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

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

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

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

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

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