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R.L.J. Helmons

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6 records found

A validated co-simulation of grab and moist iron ore cargo

Replicating the cohesive and stress-history dependent behaviour of bulk solids

Journal article - M. Mohajeri, Wilbert de Kluijver, R.L.J. Helmons, C. van Rhee, C. van Rhee, D.L. Schott

The traditional design approach of grabs and other bulk handling equipment consists of manufacturing and testing physical prototypes. A novel design approach is to use a co-simulation of MultiBody Dynamics (MBD) and Discrete Element Method (DEM), in which the virtual prototype of ...

Flocculation effect on turbidity flows generated by deep-sea mining

A numerical study

Journal article - M.F.A.I. Elerian, Ziyang Huang, C. van Rhee, R.L.J. Helmons, R.L.J. Helmons

We have developed and investigated a hydrodynamic model of Deep-Sea Mining (DSM) collector turbidity flows that captures sediment particle aggregation and breakup. Flocculation is expected to have a significant impact on determining the spread patterns of the turbidity flows and ...

Excavation of hard deposits and rocks

On the cutting of saturated rock

Doctoral thesis - R.L.J. Helmons

As a result of the worldwide population and welfare growth, the demand for energy (oil, gas and renewable sources) and raw materials increases. In the last decades, oil and gas are produced from more and more offshore sites and deeper waters. Besides energy, the demand for diverse metals and rare earth elements increases as well. These raw materials are often at the basis of new sustainable technologies e.g. permanent magnets for wind energy and battery packs for electric cars. The availability of these raw materials is essential for a stable development of the world economy. Unfortunately, for some of the crucial raw materials, the availability is sometimes very local and in various cases there is a monopoly forming. To reduce this economic risk, investments are needed to search and extract minerals from new locations. Large, metal-rich fields are found at the bottom of the sea, such as phosphate nodules, manganese nodules, cobalt-rich crusts and vulcanic sulphide deposits (often referred to as Seafloor Massive Sulphide, SMS). These deposits are mainly located in the deep sea, at depths ranging from several hundreds of meters to several kilometers. One of the technical challenges to enable production from these locations is the cutting or excavation process. Experiments have shown that the energy needed to excavate the material increases with water depth. Besides that, it is demonstrated that rock that fails brittle in atmospheric conditions can fail more or less in a plastic fashion when present in a high pressure environment, as would be the case at large water depths. The goal of this research is to identify the physics of the cutting process and to develop this into a model in which the effect of hydrostatic and pore pressures is included. The cutting of rock is initiated by pressing a tool into the rock. As a result, at the tip of the tool a high compressive pressure occurs, which leads to the formation of a crushed zone. Depending on the shape of the tool and the cutting depth, shear failures might emanate from the crushed zone, which will eventually expand as tensile fractures that can reach to the free rock surface. Through this process intact rock will be disintegrated to a granular medium. Additionally, the presence of water in the pores of and surrounding the rock influences the cutting process through drainage effects. The most relevant effects are weakening when compaction and hardening when dilation occurs in shearing and tension. Deformation of the rock causes the pore volume to change, resulting in a under or over pressure. As a result, the pore fluid needs to flow. The magnitude of the potential under pressure is limited through cavitation of the pore fluid, limiting further reduction of the pore pressure. The drainage effects cause the rock cutting process in a submerged environment to show a stronger dependency of both the hydrostatic pressure as well as the deformation rate. The numerical simulations are performed with a 2D DEM (Discrete Element Method). In DEM, the mechanical behavior of a rock is mimicked by gluing loose particles together with brittle bonds. Such a method shows strong resemblance with sedimentary rock. In order to include the effect of an ambient pressure as a result of the water depth and to include the presence of a fluid in the pores of the rock, a pore pressure diffusion equation is added to the model. The discontinuous results obtained with DEM are interpolated to a continuum field through the use of a SP-method (Smoothed Particles). Additionally, SP is used to solve the pore pressure diffusion equation. For that reason, the methodology used in this dissertation is referred to as DEM-SP. Thus far no direct coupling has been found between the input microscopic parameters, that define the properties of and interactions between the particles in DEM, and the resulting bulk properties of the particle assembly. For that reason, a sensitivity analysis is performed in which the effect of the micro-properties on the macroscopic behavior is investigated. Additionally it is proven that the addition of the pore pressure diffusion process to the DEM-SP model corresponds with the effective stress theory. It is also proven that when air is used as a medium in the pores, no significant changes compared to simulations without pore pressure coupling occur. Comparison of the numerical model with a set of tri-axial experiments on shale, in which the deformation rate is varied, shows that the model is well capable to describe both compaction weakening and dilatant hardening. In order to further validate DEM-SP, several experiments from literature are simulated. A comparison of 2D cutting experiments on tiles shows a good match for the chip size, chip shape and the required cutting force. DEM-SP is used to simulated drilling experiments on marble, in which the hydrostatic pressure is varied. These results show that the simulated behavior of the cutting process matches qualitatively with the experiments, i.e. the trend of increasing cutting force with increasing hydrostatic pressure. Furthermore a series of cutting experiments for the purpose of deep sea mining has been simulated. These results match both qualitatively and quantitatively. Additionally, both the experiments and simulations show the existence of a hyperbaric effect. This means that at a hydrostatic pressure which is significantly larger than the tensile strength of the rock the cutting process shear and cataclastic failure are more dominant, while at hydrostatic pressures significantly smaller than the tensile strength the cutting process is dominated by tensile failure and chipforming. Finally, DEM-SP is used to simulate the full cutting motion of a pick point on a rotating cutterhead, in order to investigate the applicability of the method to shallow water depths (<30 m) and to investigate the use of the method for the dredging practice. Even at shallow water depths the effect of an increased hydrostatic pressure shows significant differences. Furthermore, the simulations show a transition from cataclastic towards ductile cutting process based on the cutting depth. Additionally a transition between stick-slip friction of the cut material along the tool is observed, which is an indication for different wear processes. It is proven that DEM-SP is capable of solving drainage related effects in deformation of saturated rock. A range of rock cutting experiments are simulated and the results match well both qualitatively and quantitatively with respect to cutting force and hydrostatic pressure. Further improvement of the model can be achieved by extending the model towards 3D. @en

Assessment of expected production of a deep-sea mining system

An integrated model-based systems engineering and discrete event simulation approach

Journal article - Astrid V. Solheim, Antoine B. Rauzy, Per Olaf Brett, Per Olaf Brett, Steinar Ellefmo, Tonje Hatling, R.L.J. Helmons, R.L.J. Helmons, Bjørn Egil Asbjørnslett

In this paper, model-based systems engineering (MBSE) and discrete event simulation (DES) are combined to assess the performance of an offshore production system at an early stage. Various systems engineering tools are applied to an industrial case concerning the retrieval of dee ...

Bridging the depth

Lessons learned from deep-sea mining for better predicting turbidity plumes

Conference paper - W. Ali, Alex Kirichek, R.L.J. Helmons, C. Chassagne

The insights gained from deep-sea mining (DSM) research regarding sediment dynamics can be utilized to better predict turbidity plumes in shallow marine environments. Small-scale lab experiments can replicate deep-sea conditions effectively, offering an ideal model system to stud ...

Bridging the depth

Lessons learned from deep-sea mining for better predicting turbidity plumes

Conference paper - W. Ali, Alex Kirichek, R.L.J. Helmons, C. Chassagne

Contributed

14 records found

CSD Hydrodynamics

An analysis of a CSD in coastal conditions, and the effect of flexible spud keeper configurations on the hydrodynamic response

Master thesis - J. Dijkstra, C. van Rhee, S.A. Miedema, R.L.J. Helmons, Floris Smeets

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

Design of grabs for coarse material

Enhancing the penetration and cutting depth for limestone

Master thesis - G.J. van Selm, D.L. Schott, M. Mohajeri, Wilbert de Kluijver, R.L.J. Helmons

Due to the constant growth of global maritime shipping, the optimization of grabs used for unloading bulk carriers is more and more relevant. Especially for a coarse material like limestone, the average payload can be improved, by altering the grab design. It is suggested that the penetration and cutting depth of a grab can be improved, which will have a positive effect on the payload. This research is limited to the contact surface of the grab with the bulk material, the knife. Subsequently, the following research question is answered in this research: How to enhance the penetration and cutting depth considering coarse cohesionless material? This is done, by implementing a DEM model using the bulk calibration approach [1], several in situ and laboratory experiments are performed to measure certain macro properties. The experiment is numerically replicated, and micro parameters are changed iteratively until the predicted macro response matches the desired results. A comprehensive study of similar papers considering the penetration resistance using a DEM model is executed, which resulted in selecting the penetration resistance and porosity as target variables. The particle shape of a DEM model has a significant influence on the bulk material behavior. The novelty of this research consists of doing a preliminary analysis before the final calibration process, in which several polyhedral and (multi)-spherical particle shapes are compared. For investigating the penetration resistance, a new test setup for in situ use is developed and constructed. The implementation of this setup proved to be successful, eleven data samples were collected. For the bulk density, an in situ and laboratory setup are compared, of which the laboratory setup proved to be more reliable. Several additional laboratory experiments are executed for the particle density, Particle Size Distribution (PSD) and wall friction. Using the results of the experiments, a DEM model is created, with which a preliminary analysis is done using extreme values of the calibration variables. Four spherical and four polyhedral shapes are compared, which resulted in the two-spherical shape begin the most optimal option. All the polyhedral shapes proved to be not fitting, especially due to their underestimation of the penetration resistance. Due to the high variance of the experimental penetration tests, two parameter sets with identical porosity are calibrated: one with a low resistance target, and one with a high resistance target. Using Design of Experiments (DoE) in combination with surrogate modelling with a linear regression, several optimal parameters sets are created, in which the uncertainty is considered as well. After selecting two optimal parameter sets, the simulations are verified by comparing the experiments and simulations qualitatively. Additionally, a quantitative verification using the stiffness of the force data of each time-step is performed as well. Using the calibrated models, an additional DEM model is created with a bigger particle bed, to investigate the penetration and cutting resistance. By altering the geometry of the knife, a decrease in resistance was achieved, mainly in the cutting phase. It is recommended to consider the flowing behavior of limestone as well, with which a complete material model can be calibrated and validated, and full-scale simulations can be performed.

Seed-sediment Dynamics

An experimental study into the behaviour of seagrass seeds in a sand mixture informing a novel approach for large scale seagrass restoration

Master thesis - A.C.C. Alkemade, Bram van Prooijen, R.L.J. Helmons, Marlies van Miltenburg, C. Chassagne

In response to the sharp decline of seagrass globally, various restoration techniques have been developed and applied; many at limited scale and with limited success. This research focuses on the fluid mechanics and behaviour of seagrass seeds in a sand mixture through physical laboratory experiments. The objective is to establish a technique that incorporates seagrass seeds with sand-capping to develop seagrass restoration methods suitable for upscaling. Due to the limited amount of available seeds, laboratory experiments were conducted to find a suitable proxy for seeds from both intertidal and subtidal Zostera marina populations by tracking and analysing the particle's terminal settling velocity. The average terminal settling velocity of the seeds was 5.97 cm/s, slightly larger than that of sand particles with a diameter of 0.35 mm with a settling velocity of around 5.2 cm/s. Subsequently, the distribution of proxy seeds in the settled sediment of the "sand-capped layer" was determined after dispersal in five sediment-water mixtures with a volumetric concentration ranging from 5 % to 30 %. Mixtures with three-grain sizes grain sizes were released (d_{50} = 0.175, 0.35, and 0.52 mm) in a column of 1m and subsequently in a column of 1.5 m, where only the top 0.5m had a sediment-water mixture, while the area below consisted of a column of still water. Results show a general trend: seeds segregate to the top of the settled layer, further enhanced by larger grain sizes and higher sediment concentrations. Tests using the lowest concentrations (5-10 %) showed the best result, for which the largest number of proxies were located at an optimal depth (1 cm-5.5 cm) for protection and germination when using medium coarse and fine-sized sand grains. The results provide valuable insights into the dynamics of seed segregation in sediment mixtures and the hindered settling effect of a bimodal mixture, showing that seeds move differently to sand in a mixture, experiencing more rotation and drifting within a mixture not observed when following trajectories in a still water column. The larger seed particles segregated to the top of the settled layer due to the difference in behaviour with the sediment mixture. Even though the terminal velocity of the seeds in a still water column was larger.

Sediment dispersion from a moving source

A research into the influence of source speed on sediment dispersion during deep sea mining operations using small scale experiments

Master thesis - K.O. van Pelt, R.L.J. Helmons, S.M.S. Alhaddad, S.A. Wahab

Offshore Pile Drilling From a Floating Vessel

A Dynamic Heave Compensation Analysis

Master thesis - Y.T. Ursem, A.C.M. van der Stap, P. van der Male, R.L.J. Helmons

To fulfil the ever-increasing need for wind energy, European offshore wind farm sites are selected in deeper waters with seabed conditions which can consist of hard consolidated sediments or even rock. The deeper sites require the use of floating wind turbine foundations that are moored off to anchor piles in the seabed. For rock seabed sites, the anchor piles must be drilled. As the water depth of these sites increases, commercially available jack-up vessels are no longer able to operate. Therefore, the anchor pile drilling operation must be performed from the deck of a floating vessel. An extensive techno-economic analysis has led to the finding that a topside-operated drilling rig mounted on a large construction support vessel with heave compensation (HC) added is the most cost-effective configuration. The performed research focuses primarily on the determination of which HC method is most effective at changing water depths of 50 m up to 200 m. Leading to the understanding which site requires the use of active HC, limiting the resources required to construct future floating wind farms. The configurations are tested for relevant wave conditions, determined by assessing potential European floating wind farm sites. Firstly, the research assesses the maximum allowable topside displacements before the drilling column reaches either the operational limits of plastic failure or bottom hole assembly lift-off. Secondly, the operational vessel motions are determined for the relevant environmental conditions. By comparing the results, the need for HC in the drilling configuration is determined. Third and finally, the passive and active HC methods are assessed for a 3-hourly time simulation under the before-mentioned environmental conditions. The assessment is performed using two performance criteria; weight on bit variation and the occurring drill-string stresses. The performed analyses and simulations show that the vertical upward vessel motion is the limiting factor for the operation’s workability. Also, HC is required in every considered environmental condition. Further, the system operating with passive compensation shows a decreased stiffness with respect to the active system, most noticeable at 50 m water depth. This leads to higher frequency vibrations and stress variations being present in the drill-string of the active system. This effect is no longer noticeable for water depths larger than 50 m. For locations with a water depth of 50 m, the active system shows favourable workability results. The active system shows a larger sensitivity to wave conditions with larger wave heights, as the stiffness is larger and more stress variations occur as a response. However, the results remain more favourable in comparison to the passive system as the lift-off percentage is significantly smaller. The passive and active systems show similar results when considering short waves in 50 m water depth, this is best witnessed in the weight on bit and lift-off percentages. For locations with a water depth of 100 m and 200 m, the active and passive HC systems show comparable results for the performance criteria, for all considered wave conditions. The stresses remain within the ultimate limit state, the fatigue damage is negligible in comparison to the time required to perform the drilling operation, and the lift-off percentage for both configurations are in the same order. Therefore, as the workability of the two systems are so comparable for a water depth of 100 m and 200 m the availability, day-rate, and mobilisation complexity of the equipment will determine which HC system is most effective per project.

Guidance, Navigation and Control of Autonomous Vessels

An Implementation using a Control-Based Framework

Master thesis - H.L.J. Taams, S.A. Miedema, A.J.J. van den Boom, M. Bharatheesha, R.L.J. Helmons, P. Mohajerin Esfahani, Peter Rampen

This thesis report proposes a framework to implement Navigation, Guidance and Control (GNC) systems, that enable point-to-point autonomy for displacement vessels. A model-based control approach is chosen as the basis of the GNC systems. The resulting algorithms are implemented for verification in a 1:25 scale model of a Azimuth Stern Drive (ASD) 3111 Damen tug named "Damen Autonomous Ship", aka DASh. First, a compact maneuvering model that captures relevant dynamics of displacement vessel is formulated and identified using system identification. Secondly,the guidance system is automated such that it connects an initial state to a goal state with a collision free path that satisfies all input and differential constraints of the vessel model. To this end, the kinodynamic Rapidly-exploring Random Tree (RRT) algorithm is extended to use a maneuver automaton and optimal motion primitives in its steering function. A learned cost-to-go distance metric for the state space is formulatedto efficiently calculate distance between states, which is used to search for nearest neighbors in the kino- dynamic RRT algorithm. The performance of the planner using the learned cost-to-go distance metric is compared to a minimal curve length distance metric based on Dubins Curves and the commonly used straight-line Euclidean distance metric. It is shown that the learned cost-to-go and the minimal curve length distance metric result in paths of similar performance while the Euclidean metric performs severely worse. Lastly, the navigation and control systems are implemented on DASh. Due to disturbances present in real world environments, the paths must be tracked using feedback control. State estimation for navigation, based on position and heading measurements is performed by implementing an observer using a Extended Kalman Filter (EKF). Non-linear model predictive control (NMPC) in combination with thrust allocation is used to control the vessel during path execution. Due to real time requirements of DASh, the EKF, NMPC and the thrust allocation algorithm are directly implemented inefficient C ++, on the on-board computer of DASh. It is shown that NMPC converges to static reference positions faster and with less control inputs compared to traditional non-linear PD control. It is also shown that time-varying trajectories created by the kinodynamic RRT can be executed successfully. This shows that the identified model is suitable for use in model-based control and that the planned paths indeed satisfy the input and differential constraints of the vessel.

Fatigue assessment of trailing suction hopper dredgers

A study on the contribution of the dredging cycle to the fatigue damage in the bottom door openings

Master thesis - L. Blekman, M.L. Kaminski, J.H. den Besten, Nick Daniels, R.L.J. Helmons

Since the 19th century hopper dredgers have been used for civil operations such as land reclamation and water way maintenance. In the past 25 years a trend of scale enlargement of hopper dredgers driven by economic and productivity reasons is seen. Scaling of the designs has effect on strength and fatigue assessment as well on settling of particles which affect the rate op production. This thesis is focussed on the fatigue assessment of the hopper dredger. Cracked damages in coaming and bottom door areas have been found in relatively large hopper dredgers that are classed by Bureau Veritas. According to Bureau Veritas rules for steel ships fatigue assessment is required if the ship has length greater than 170 m but damages have also been observed in hopper dredgers with a smaller length. This indicates that the hopper dredger design demands for a specific fatigue assessment procedure. The hopper dredger has a unique characteristic load profile consisting of loading and unloading cargo several times a day in combination with small wave heights due to reduced freeboard regulations. Special interest is given on the effect of the loading and unloading cycle in the corners of the bottom discharge openings. It is suspected that the effect of this characteristic cycle may be underestimated and requires an alternative assessment procedure. The main question is what the contribution of this cycle is and how it should be assessed. The relatively low frequency of the cycle raises the suspicion of low-cycle fatigue and this phenomenon is therefore investigated. It is concluded that the loading and unloading cycle has a major contribution to the fatigue damage in the bottom opening corners. In fact 97% of the fatigue damage is induced by the dredging cycle and therefore this cycle could be seen as the sole cause for fatigue cracks in the bottom opening corner structural details. The high stress ranges confirm the suspicion of a low-cycle fatigue phenomenon and therefore further research is done on how to approach this type of fatigue. A method is proposed to take into account bi-axial cyclic plasticity and material hardening based on the use of a maximum principal hot spot stress range. Linear-elastic stresses are corrected to pseudo elastic stresses and effects of SN-curve selection, material selection, bi-axiallity and residual stress are analysed. Based on the found pseudo hot spot stress ranges it is concluded that the linear-elastic stresses in the analysed hopper are not high enough to demand for such a low-cycle fatigue assessment approach. The bi-axial ratio between the two multi-axial plane stress components has no effect on this conclusion.

Preloading of a lumpy clay fill

A study into the closure of the interlump voids of a mechanically dredged stiff clay fill

Master thesis - A.M.L. Dortland, C. Jommi, A.A.M. Dieudonné, H.A. Abels, R.L.J. Helmons

Mechanical dredging of a stiff clay results in clay lumps. Those clay lumps can be reused for the construction of land reclamation projects, and hereby economic and environmental value is generated. Placement of the clay lumps in water result in a matrix of clay lumps and an interlump void space. The collapse of the interlump void space will cause large settlements and needs to be overcome before the site can be used for construction. Preloading is an effective method to close the interlump voids. Stiff clays soften over time due to unloading and swelling. As a result, the strength and stiffness of the clay lumps decrease over time. The presence of discontinuities accelerates the softening process. It was proposed by Leung et al. (2001) that the interlump void space closes under a reduced preload of 25 kPa. The interlump void space closes under a reduced preload because the lumps soften over time. The question rises if closure occurs within a normal construction timespan, i.e., 1 - 2 years, under this reduced preload. In this study, a combined experimental and numerical approach is applied to determine the influence of soil characteristics, softening and the presence of discontinuities on interlump void closure. The influence of softening due to chemical and hydro-mechanical swelling is tested by experimental swell-load tests on stiff overconsolidated Boom clay samples. Additionally, the presence of discontinuities is studied by CT images, and a miniature clay fill test is performed to study the softening time and the rearrangement effect. Furthermore, a numerical study is performed in which the influence of specific soil characteristics on void closure was researched by a sensitivity analysis. By the experimental tests, it is shown that a pore water chemistry change alters the degree of swelling and its compressibility. Furthermore, fissures were identified in the sample material by CT images. The smallest microfissures could not be identified due to the resolution of the images. Consequently, it is impossible to estimate the effect of fissures on the hydraulic conductivity. Therefore, literature data was used to estimate the hydraulic conductivity and this was used as the input in the calculations. In the numerical study, it was shown that MPM could model the softening behaviour over time. The sensitivity analysis showed that the MPM model responds consistently to parameter sensitivity analyses. This leads to the conclusion that MPM can be used as a investigation tool to increase the understanding of the influence of parameter variability on the final interlump void closure problem. The resulting strains of the numerical model comply with the theoretically calculated strains. It was expected that the numerical strains were smaller than the experimentally determined strains, due to the presence of interlump voids in the numerical model. But, the numerical strains are larger than the experimental strains. In the sensitivity analysis, it was found that the final strains and closure time are highly dependant on the soil characteristics. It is likely that the input of the MPM model differs compared to the true sample material and thus different strains result. The time until the interlump void space closes was determined by a simplified geometry in MPM under a preload of 25 kPa. For an unfissured Boom clay, interlump closure takes place after approximately 16 years while for a fissured Boom clay it only takes 2 months. Thus, closure of the highly simplified geometry takes place within a normal construction timespan for a fissured Boom clay. It must be kept in mind that the results are highly dependant on the geometry, lump size and soil characteristics. Therefore, these results can not be generalized into an estimate of the interlump void closure time for any stiff lumpy clay fill. In conclusion, the feasibility of MPM was explored and it turned out to be a promising method to model the interlump void closure problem. Further studies are required to check if the model gives plausible results for more advanced constitutive soil models and geometries. If the results are positive, MPM can be used as a investigation tool to increase the understanding of the interlump void closure time for more refined geometries.

The effect of aeration on the maximum pressure during wave impacts on a horizontal overhang

An experimental and numerical study of the effect of aeration on the maximum impact on a horizontal overhang.

Master thesis - K.P. van Gent, P.R. Wellens, R.L.J. Helmons

When waves break, air and water mix and small air bubbles are formed in the water, creating aeration. The mixture of air and water alters the behavior of the fluid. Traditionally, it has been assumed that aeration has a damping effect on impact, as exemplified by the use of air bubbles in the swimming pool below the Olympic 10m platform and during synchronized 10m platform jumps to reduce the force exerted on the swimmer's body. However, in recent years, it has been found that this assumption does not hold true in all situations involving aeration. When treating aerated water as a non-compressible fluid, it does exhibit a damping effect on impact. However, when treating aerated water as a compressible fluid, the speed of sound of the mixture can actually change with implications for the propagation of density waves. This means that an aerated water impact can feature pressure oscillations that increase the force instead of reducing it. Consequently, hydraulic structures that have been or will be constructed with the assumption that aeration only has a damping effect, may be affected. Understanding the effect of wave-induced aeration on a horizontal platform is crucial for the design of new hydraulic structures in a more efficient and cost-effective manner. This research aims to determine the maximum force experienced by a horizontal surface during an aerated wave impact. The investigation involves using a numerical method, followed by a series of self-conducted small-scale sloshing experiments. The simulations contribute in two ways: they aid in understanding the problem and the magnitude of forces acting on the experimental setup, and they assist in verifying the experimental results once the experiments are completed. For the experiments, a new tank layout has been designed to facilitate the creation of aerated water inside the tank. The tank is used for conducting experiments where aerated wave impacts are compared to non-aerated wave impacts for different wave impacts. The experimental results confirm both the effect of pressure reduction due to aeration when the aeration level is near to 4%, as well the effect that the maximum pressure can increase for aeration levels between 1 and 2%.

Oscillating Disc Technology for Rock Excavation

DEM for non-linear rock cutting tools

Master thesis - B.G. Grashof, R.L.J. Helmons, S.A. Miedema, D.L. Schott

Rock cutting is a challenging process. Heavy equipment, large cutting forces, high wear rates and large amounts of required energy are common challenges when cutting rock. Traditionally, rock cutting is based on a linear motion of the cutting tool. However, a significant improvem ...

The deep sea mining horizontal discharge plume

Small-scale experiments and modelling

Master thesis - B.K. Blankenaar, A.M. Talmon, R.L.J. Helmons, Andrys Posthuma, Frans van Grunsven

The rising demand for minerals and metals and the depletion of land-based resources has created a renewed interest in the extraction of resources from the deep ocean, i.g., deep sea mining. This research focuses on the mining of manganese nodules, these are potato-shaped objects, 5-10cm in size, composed of metals half buried in the seafloor. A Seafloor Mining Tool (SMT) is used to mine the nodules, however, during mining besides the nodules, also the sediment around the nodules is collected. This sediment is separated in the SMT and discharged through a horizontal diffuser located at the rear of the SMT, thereby creating a sediment plume. The sediment plume needs to be reduced for environmental, economic and legal reasons. The sediment plume causes changes in the sediment characteristics, clogging of pores of suspension feeding organisms and burying of the benthic fauna. The plume will spread in all directions, thereby burying still to be collected nodules and reducing the pick-up efficiency, making the process inefficient from an economical perspective. Legal issues will arise when the discharged sediment plume spreads to other exploration areas owned by other parties or to environmentally protected areas. The aim of this research is to investigate the influence of the choice of diffuser height on the reduction of the sediment plume. To achieve this goal a Matlab model was made and small-scale experiments in the laboratory of Dredging Engineering were done. The Matlab model is based on the JETLAG model, described in 'Jets and Plumes' by Lee and Chu (2003), and is a Lagrangian model predicting the centerline trajectory of the plume. The small-scale experiments provide insight in the behaviour of the horizontal discharge plume at different diffuser heights and generate measurements to validate the model. A diffuser designed specifically for these experiments is produced and the height and flow velocity are differed. The experiments consist of visualization experiments, capturing top view and side view images of the buoyant jet, and experiments measuring the velocity and bottom concentration at different locations. The velocity profiles, sediment concentration, deposition pattern and impingement range are compared and analysed. The combined results from the model and the experiments show that the start of the impingement range increases with increasing diffuser height. Furthermore, a lower diffuser height increases the amount of bottom deposition near the diffuser outlet. The measured velocity profiles, near bottom concentration and impingement range are in agreement with this observation. The measured near bottom velocities at 15D from the diffuser outlet are approximately 0.1 and 0.2m/s, depending on the discharge velocity, both have the potential of resuspending settled sediment. This research suggests that the spreading of the plume can be reduced by the choice of diffuser height above the seafloor. Based on the diffuser heights tested in this research a diffuser height of 100mm is recommended. Since a higher diffuser decreases the amount of particles settling close to the source and a lower diffuser increases the potential for erosion. This research has shown that up to a distance of 15D from the diffuser, the near bottom velocities are large enough to cause erosion. Research into the near bottom velocity decay and magnitude further away from the source, inside the turbidity current, is necessary as well to understand the full spreading behaviour and the potential for erosion. Furthermore it is recommended to do research into spreading behaviour of finer particles, especially the behaviour of the actual deep sea sediment in combination with salt water causing flocculation. Moreover, research into the influence of the forward motion of the SMT is recommended since the SMT will create a wake that will influence the spreading behaviour of the plume. Research into the potential creation of a recirculation region caused by the limited entrainment at the rear of the SMT is recommended as well, this can be done by aligning the diffuser outlet with a screen.

Subsea Tracked Trencher Mobility

The development of a model to gain insight of the moility of a subsea tracked trencher in granular and cohesive soils

Master thesis - F.C. Holzhaus, S.A. Miedema, C. van Rhee, R.L.J. Helmons

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.

Large Floating Monopiles

Installation in Heavy Weather Conditions

Master thesis - J. Maliepaard, S.A. Miedema, R.L.J. Helmons, H. Polinder

The offshore wind market experiences large growth over the last decade. There is a clear trend to installation in deeper waters and to larger wind turbines, to maximize the energy output. The majority of offshore windmills use a monopile foundation, which due to the current trends are getting larger. Seaway Heavy Lifting (SHL) is a company that installs these monopile foundations, with their Heavy Lift Vessels (HLV). The largest HLV of Seaway can transport and install three monopiles at a time. However, with larger monopiles it is expected to be reduced to two monopiles or even less. In order to be competitive in the monopile installation market, SHL is looking for a method to transport the monopiles to the HLV, instead of using the HLV for transportation. A previous comparative study showed that floating transport of monopiles to the HLV is the most favorable method. However, SHL is unexperienced with this method and therefore requires a study on developing the best method to install the monopiles within the workability of the HLV. Therefore, the thesis goal is: "Design of a method for installation of large floating monopiles in heavy weather conditions". To design such a method, it will first be determined what steps are necessary and which options can used to fulfill these steps. With a multi criteria analysis will be determined what combination will lead to the best method. The best two methods will be subjected to a sensitivity analysis to determine its feasibility. The method consists of five steps: floatation, towing, mooring, hook-in and lifting. A multicriteria analysis shows that using airbags is the most favorable method of making the monopile float, while using a flange clamp for towing and lifting. In order to reduce motions, the mooring can be either on the side of the vessel in a gripper frame, or by sinking one end of the monopile to the seabed. When the monopile is in moored position the pre-rigged rigging can be taken over by the main hook of the crane. Both methods are modelled in the simulation software package Ansys AQWA. A soil model has been made to assess the influence of the grounding on the motion behaviour of the monopile. Due to the soil properties it is faster and relatively accurate to model the connection between the soil and the monopile as a hinge instead of applying the soil model. The system is subjected to different waveheights and wavelengths, while varying the direction of the environmental forces, the trim of the monopile on the seabed and the orientation of the monopile to the vessel. The installation of a grounded monopile in heavy weather is a feasible option. The simulations have shown that installation is possible up till a significant waveheigth of two meters with a peak period of 8s. The least trimmed position has the best performance over different environmental forces directions. The best option for installation is in a configuration where the vessel and the monopile are facing the environmental conditions head-on, with a 42 degrees monopile trim. However, it has to be noted that other monopile orientations with respect to the HLV are performing good as well, as long as the trim of the monopile is 42 degrees. Using other trim options is possible, but limited to specific orientations. The installation of monopiles with a gripper attached to the vessel seems to be a viable option, but only once the soil model is improved. The simplification of the soil model by replacing it with a hinge gives uncertainties in the real force acting on the interface between the vessel and the grounded monopile. However, when considering the RMS force of 30 minute simulations without taking lateral forces into account, the results are well within the set boundaries. Thus can be concluded that installation of a floating monopile is feasible, when using the grounding method. It is recommended to improve the soil model for both methods, to improve the accuracy of the results. To ensure a safe installation and a safe operation during its lifetime, the structural integrity of the monopile should be investigated. Small deformations can drastically reduce the fatigue life of the structure. An end-plug to lower the monopile to the seabed and reinforce the structure could be an option to ensure the structural integrity. Furthermore, it should be investigated if a gripperframe is a realistic option due to its size. The dimension could turn out impossible for catching and releasing extra large monopiles in heavy weather conditions. Lastly, it turns out that grounding a monopile is more time consuming and difficult than anticipated. Therefore it is recommended to research the installation of a free floating monopile or mooring a floating monopile alongside the HLV.

Horizontal negatively buoyant jets in deep sea mining

The influence of initial concentration of discharged slurries on the spatial spread of turbidity currents

Master thesis - Y.R. Wijmans, R.L.J. Helmons, A.M. Talmon, Alex Kirichek

Recently, deep-sea mining has acquired renewed interest from commercial companies due to a rise in the demand of minerals and metals. Deep-sea mining is focussed on different minerals and rare earth elements found in the oceans that are increasingly used in the manufacturing of goods and products related to renewable energy. In order to collect these minerals, a mining operation at several kilometres depth has to be undertaken. Apart from the technical challenges that mining at several kilometres depth poses, also the environmental consequences receive a lot of attention and criticism from environmental agencies. Deep-sea mining operations are believed to cause damage to the environment by the sediment plume, as well as by producing light and noise which are absent under normal conditions at the deep ocean floor. The sediment plume is generated by the movement of the miner, but more importantly by the discharging of the picked up and unwanted sediment. This happens at the back of the miner using a set of diffusers. In order to acquire a better understanding of the effect that the initial concentration of the slurry has on the spatial spread of the sediment plume and turbidity current caused by the deep sea mining vehicle, experiments have been performed in which the initial concentration has been altered so that the characteristics of the ensuing current can be compared. The main objective of this research is to determine how changing the initial volumetric concentration of the discharge mixture influences the flow parameters after impingement and sediment flux inside the turbidity current for a constant sediment flux and discharge velocity. One of the most important assumptions for investigating the influence of initial concentration of the slurries has to do with the different phases of the discharged flow. During the initial phase, the density difference between the slurry and ambient mixture is thought to be dominant for the settling process, as the mixture behaves as one coherent fluid. After impingement on the bottom, the mixture spreads downstream as a turbidity current. During this phase it is the weight of the solids inside the slurry that causes it to settle. Because there are a lot of solids present in these slurries this process of settling is controlled by both hindered settling and flocculation. Flocculation is the aggregation of cohesive particles, forming a larger, heavier particle. Clay, which is cohesive, is expected to be present at most mining sites, resulting in flocculation to be an important factor in the process. The research has been performed by conducting a total of three different experiments. These experiments consist of pumping a pre-mixed slurry of water and spherical glass particles into a large modular tank, where is it discharged through a diffuser. For each experiment a mixture is prepared with a different concentration, which is discharged into the modular tank with three different flow rates in order to keep the sediment flux constant between each experiment. A total of three diffusers are used, one for each concentration. The diffusers differ in cross section in order to achieve a constant discharge velocity for all experiments. Experiments have been performed using both spherical glass particles and clay. The sediment flux is kept constant between different experiments as it is assumed that the sediment flux of real deep sea mining operations will be mainly determined by the width, cutting depth and velocity of the mainly vehicle. For performing measurements during the experiments, an ADV was used. This device is capable of measuring the velocity in three directions over a profile of 30 mm with 1 mm intervals. The ADV also measures the signal-to-noise ratio, which after performing a calibration can be used for calculating the concentration inside the turbidity current. Together this data can be used for determining the spatial spread of the turbidity current. Based on the findings of this research, it is believed that a higher concentration than the 1% volumetric concentration that is currently being assumed by deep sea mining companies could prove to be beneficial in preventing the spatial spread of the sediment plume and subsequent turbidity current. It is believed that in general, increasing the initial concentration of the discharged slurry, and thereby decreasing the momentum/buoyancy ratio of the buoyant jet, directly leads to both the velocity and concentration of the turbidity current to decrease over a shorter distance after impingement, while the impingement distance itself is also decreased for a higher initial concentration. As such, the total reach of the turbidity current resulting from the slurry discharge from a deep sea mining vehicle can be decreased by using a higher initial concentration of said slurry. It is believed that mainly the higher density difference with the ambient water plays a vital role in decreasing the spatial spread of the sediment plumes produced by deep sea mining operations. The higher density achieved by a high concentration results in the plume to deflect towards to bottom over a shorter distance and a decrease in turbidity current height. This decrease in height is beneficial for the later stage of the turbidity current in which the dying out is dominated by the settling velocity of individual particles.