1,720,984 research outputs found
3D CFD modelling of overflow dredging plumes
No full text3D CFD simulations, lab experiments and field measurements are used to get more understanding of the turbulent near field overflow dredging plume mixing. The focus has been on the most often used dredging vessel: a trailing suction hopper dredger (TSHD). Near field overflow dredging plume mixing is important because it determines the far field dredging plume characteristics and deposition which are essential to know in order to be able to assess the environmental impact of a dredging project. The most important near field mixing processes have been identified in this study. The insights are translated to be usable in every day dredging engineering practise and used in the IMPROVE (IMPact Reducing OVerflow Extension) concept to reduce the environmental impact.Dredging EngineeringMechanical, Maritime and Materials Engineerin
Failure Mechanism of Cutting Submerged Frozen Clay in an Arctic Trenching Process
No full textThis version of the thesis only covers the literature study. The results analysis and conclusion of the thesis are confidential. Pipelines in arctic waters are at risk of being damaged by gouging ice masses. To protect these pipelines, they can be buried in trenches. When trenching in arctic clayey soil, sub sea permafrost can be encountered. The main objective of this research is to study the failure mechanism of frozen clay as encountered in permafrost regions. Knowledge of the cutting mechanism leads to the ability to calculate cutting forces and the specific energy required to excavate material. To find the failure mechanism and answer the research question, a cutting setup was designed and built and cutting experiments were conducted. The setup was designed, based on requirements that result from studied literature on frozen clay and cutting theories. A series of cutting experiments were conducted where a slab of frozen clay was pressed against a transparent wall and the top layer was cut off while filming the process with a high speed camera. The results are measurements of the cutting forces and observations of the failure mechanism.Dredging and Offshore engineeringMarine and Transport TechnologyMechanical, Maritime and Materials Engineerin
Vertical Transport Methods in Deep Sea Mining
No full textA comparison between 4 different transport methods which could be used for transporting offshore mined resources between sea floor and surface: multiple centrifugal pumps, positive displacement (diaphragm) pump, mechanical lifting and airlift.Rivers, Ports, Waterways and Dredging WorksHydraulic EngineeringCivil Engineering and Geoscience
Modeling the cutter head-rock interaction to simulate the dynamic cutting process
No full textMechanical, Maritime and Materials EngineeringOffshore & Dredging Engineerin
Passive suction under mud mats: Model – Testing - Validating
No full textThe subject of this master thesis is related to the passive suction under mud mats. Mud mats are used in the offshore industry to prevent structures from sinking in the soil after installation. If for some reason a structure has to be lifted, on the one side due to an installation error, on the other side due to removal the force needed to lift the structure sometimes exceeds the total submerged weight. This report contains a Literature survey conducted regarding the above subjects. This report also shows the design of a test setup and procedure for lifting a plate from a sand bed. In the end the report follows the modelling of an analytical model which is validated using the test data The focus of this report lies with finding out what basic parameters dominate this force and what the influence of these parameters are during lifting is sand. Experiments will be conducted to test these parameters. In the end these parameters will be used to develop a simple analytical model to predict the order of the breakout force. The model will then be validated with the experiments. The main research question is: How are the breakout force and breakout time influenced by the permeability of the sand and the lifting force? Can an analytical model, that uses these parameters, predicts the lifting force within a certain margin? Literature states that no breakout forces are to be expected due to the large permeability of sand but from cutting theories it is known that under pressures exist around the blade tip, especially for the smaller grain sizes. A test setup was built to study the lifting process and measure the pressure under the plate and his displacement for a given load. The tests were performed in two different sands (Silverbond and Geba Weiss) for a range of different loads with two different plates: A 2-Dimensional setup and a 3-Dimensional setup. An analytical model was created to predict the lifting force for a given permeability and upward velocity of the plate. In the end the model was validated with the test data. The test data showed that for the 2-Dimensional case the pressure profile was of a rectangular shape. A factor 10 in breakout time between the sands can be observed. The 2-Dimensional model gives a good estimation of the lifting load in Silverbond sand when using velocities from the beginning of the lifting process. For the Geba sand, after adjusting the length of the flow paths, the model also gives a good fit. The experiments with the 3-Dimensional plate showed that the pressure profile under the plate is of a rectangular shape with steep slopes towards the edge of the plate. Nothing can be concluded about a relation between the breakout time and the permeability for the same load between the two different sands due to an inertia dominated process. The 3-Dimensional model predict the lifting force accurately for the Silverbond sand using the velocities from the beginning of the lifting process. For the Geba sand the permeability had to be scaled to give a good fit. This is because of model assumptions and using flow paths. Adjusting the tune factor did not give satisfactory results.Mechanical, Maritime and Materials EngineeringMarine & Transport Technolog
Erosion of sand under high flow velocities
No full textIn this thesis the physical process during the erosion of sand at high flow velocities is described. In this context erosion experiments with flow velocities of more than 2 m/s were executed. The experiments were done in the laboratory in the Delft University of Technology, faculty Mechanical, Maritime and Materials Engineering (3mE). With the new collected data the physical process during erosion of sand is analyzed.Offshore & dredging engineeringMechanical, Maritime and Materials Engineerin
Modeling of high speed erosion with a morphological updating routine
No full textErosion is a phenomenon present in several industrial processes. In dredging, the jetting of sand in drag heads erodes the sand bed. In construction of offshore infrastructure such as wind turbines, oil and gas production units, marine pipelines, erosion of material near the foundations can put the stability of structures at stake. Furthermore, rivers or even tsunamis are some of the natural phenomena that can be the cause of erosion. C. van Rhee, 2007 and Bisschop et al., 2015, distinguished two regimes for the erosion of sand dependent on the fluid velocity. On one hand, for low flow velocities, 0.5-1m/s, the erosion process is dependent on the size and the density of the sand grains. On the other, for flow velocities >1.5 m/s, the upper layers of sand are sheared. Densely packed sand has a dilatant behaviour to shearing (see image). This dilatant behaviour leads to a drop of pressure in the interior of the sand-bed, creating a hydraulic gradient and forcing water to flow towards the interior of the sand-bed to fill the voids. The hydraulic gradient caused by the drop in pressure acts against the eroding forces adding resistance to the erosion process. This regime is defined as hindered erosion. The improvements in computing power have led to a spread in the use of numerical modelling for industrial purposes. The aim of this thesis is to develop a numerical solver able to model the behaviour of sand-water mixtures with an emphasis on the erosive process. The numerical model was developed in C++ using the Foam-extend 3.2 framework. The sand is modelled using 2 different approaches. It is modelled as a continuum when in suspension and, through the morphological updating routine when settled in a sand-bed. The fluid motion is modelled by a transient incompressible fluid solver (P.I.S.O) using a collocated arrangement of the unknowns. The momentum exchange between suspended sand grains and the fluid is approached by the Boussinesq approximation of the density. The transport of suspended sand is modelled by an advection-diffusion relation, including the hindered settlement effect. The turbulence model is a standard k-ε model. The erosion process is here modelled using the pick-up flux approach (van Rijn, 1984), with a modified stability criterion (θ_cr). X. Lui, 2008 and N. Jacobsen, 2011, corrected the stability criterion calculated from the sand grain properties (θ_(cr,0)) to include the slope effect (θ_slope). For this work, and following the formulation proposed by van Rhee, 2007, the stability criterion will be corrected to include the resistance due to the dilatant behaviour presented previously in this abstract (θ_vR). θ_cr=θ_(cr,0) (θ_slope+θ_vR ) The solver developed was used in two test cases. First, a settling test, with an initial concentration of sand of c=0.3. For this model, the solver shows a good behavior modeling the settling of sediment, nevertheless, the settling velocity is slightly higher than the one seen in the test. In the high speed erosion test, the velocity above the bed varies from 0-6 m/s. The fitting parameter of this model is the bed roughness; which for this test is 1.05 cm. The bed roughness (ks) was fitted to have the same erosion time. The calculated sand-bed height has values similar to the experimental results. The conservation of sediment presents satisfactory results as the error is lower than 1%, for the settling and the erosion test case. The automatic mesh motion presents certain limitations in this specific application. In the settling case an important shrinking of the mesh will lead to instabilities in the calculations of other fields. In the erosion test, the upper row of cells is greatly deformed sacrificing accuracy near the upper boundary. The mesh deformation should be explored more in depth in further studies.Mechanical, Maritime and Materials EngineeringMarine and Transport TechnologyOffshore and Dredging Engineerin
Improving the efficiency of a flexible fallpipe vessel: An experimental study on the spreading of rock in an impinging plane jet
No full textThis thesis focuses on the spreading of rocks when installed by a flexible fallpipe vessel. The reason that the spreading of rocks is of interest, is because during installation rocks end up outside their designated areas. Theses rocks do not contribute to the desired design and can therefore assigned as lost. Because of the extend of this subject there has been decided to focus only on the spreading during the descend of a rock from the moment that it leaves the fallpipe until it reaches the bed. Possible bed displacements and the build up of the resulting rock berm are thus not incorporated.Dredging EngineeringMarine & Transport TechnologyMechanical, Maritime and Materials Engineerin
Rotational jetting in clay
No full textIn dredging activities submerged jets are widely used for different purposes. Two main applications are considered in this study: 1) a jetting sword on a cable trencher and 2) jets located on a draghead. Both jetting systems are especially designed for jetting sands and achieving large excavation productions in sandy soils. However when cohesive soils have to be jetted, the production decreases drastically. In sandy soils the jet creates wide cavities, while the cavity width produced by a jet in cohesive soils like clay is very narrow. Different researchers found that the cavity depth decreases only a little by an increasing traversing velocity of the nozzle and the cavity width to remain more or less constant, i.e. the jet can process more clay than generally is supplied. This results in the fact that the highest production will be achieved at the highest traversing velocities. However, the traversing velocity of the nozzle is limited by the tool (trencher/draghead). With the use of a rotating jet, more soil is supplied to the jet. The affected area is increased and the combined traversing velocity (normal traversing part + rotating velocity part) is increased as well. In this research the possible increase in excavation production of clay of a rotating jet compared to a conventional non-rotating jet for two purposes is studied; a rotating jet on a trenching machine and a rotating jet on a draghead. This research exists out of two models and two experimental test setups. Bothmodels consist of two modules; a calculation module and a visualization module. The calculation module predicts the excavation production as a function of: 1) Jet pressure, 2) Undrained shear strength, 3) Traversing velocity, 4) Rotational velocity, 5) Nozzle angle, 6) Nozzle diameter. The model is based on the entrainment of water at the back-side of the jet and soil at the front-side of the jet. The more entrainment of soil, the greater the increase in jet mixture density and the higher the decrease in stagnation pressure, resulting in a smaller penetration depth. The visualization module shows the cavity shape based on the cavity parameters modeled in the calculation module. The jet is modeled as a cone based on the geometry calculated in the calculated module. The jet will travel along a trajectory depending on traversing velocity and rotational velocity. The trajectory were the jet has been present, is removed and visualized as excavated volume. Two experimental test setups are developed in order to validate the models. The main parameters varied were the traversing velocity, nozzle angle (45°/60°), rotational velocity and jet pressure. For the trenching setup the increase in excavation production found, lies in a range between 6 and 7 for a traversing velocity of 15 - 215 m/h. For the draghead setup the traversing velocity was varied between 35 and 1800 m/h. The increase in excavation production can be up to a factor 4. The calculation module for the trenching application is able to produce a good estimation of the excavation production for the 45° nozzle. The calculation module overestimates the excavation production of the 60° nozzle compared to the experimental results. For the draghead application the developed model is able to give results close to the corresponding experimental results for traversing velocities lower than 1000 m/h. For larger traversing velocities the model underestimates the excavation production compared to the experimental results. The results of the experiments including the developed models can be used as a base for the design of rotating jets on a jetting sword or draghead.Mechanical, Maritime and Materials EngineeringMarine and Transport TechnologyOffshore and Dredging Engineerin
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