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Item type:Publication, Access status: without files , Numerical investigations of the extraction of submerged foundations by coupled CFD-DEM(American Society of Mechanical Engineers (ASME), 2017-06); ; ; ; Offshore structures are founded on submerged foundations. The excavation of submerged foundations in the sea bed is a difficult task to accomplish when it comes to the decommissioning of these offshore structures. The extraction resistance is a lot higher than the pressure acting on the structure due to hydrostatic pressure, earth pressure and its self-weight. Once the extraction begins, a negative pore water pressure is created until inflowing pore water compensates this negative pore water pressure. This depression is hindering the extraction of the submerged foundation. Additionally, the resistance is dependent on the embedment depth of the structure, the soil properties as well as the extraction velocity, which influences the dimension of the negative pore water pressure. The numerical investigation of this dynamic problem is a limitation for continuum based approaches like the Finite Element Method (FEM) due to the occurring large deformations. These results from the soil bed failing under the movement of the structure and hence starting to flow. Additionally, in order to estimate the created depression, the investigation of the water-soil-interaction is crucial, as the change of the pore water pressure plays a significant role. Therefore, it is necessary to analyze the behavior of the soil particles and the pore water pressure. In order to do this, a coupled Euler-Lagrange approach, namely the combination of Computational Fluid Dynamics (CFD) and the Discrete Element Method (DEM), is used. In these simulations on one hand, the liquid phase, e.g. the water, is considered as a continuum, while on the other hand, for the solid phase, e.g. the soil, a particle representation is chosen. Hence, it is possible to compute the particle-particle-as well as the fluid-particle-interactions. The calculations are carried out with the open source software package CFDEMcoupling®, which combines the discrete element code LIGGGHTS® with CFD solvers based on OpenFOAM®. This paper introduces the coupled CFD-DEM approach to simulate the extraction of a submerged plate in the soil bed. In this work, the soil grains are idealized by spherical particles of different diameters. In order to consider effects of dilatancy and contractancy in the soil bed, different relative densities are investigated. Additionally, a variation of the extraction velocity of the plate is carried out to examine the dependence on the creation of negative pore water pressure. For each case, the extraction resistance is calculated. The flow velocity and the pressure distribution in the vicinity of the structure are analyzed.Publicationtype: Conference Paper91 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Hydrodynamic damping and added mass of modern screw propellersThe operation of ships with "slow-steaming" poses new problems for the torsional vibration analysis of the drive train. It is well known that the propeller determines the essential part of the mass moment of inertia and the system damping. Both values are determined during the initial design phase by semiempirical methods with have originally been developed by Schwanecke and Grim between 1970 and 1980. Since then, propeller designs have changed significantly and it is unclear if modern propeller designs are still covered by these calculation methods. The paper suggests an extension of Grim's and Schwanecke's method for modern screw propellers in homogeneous and unsteady flow.Publicationtype: Conference Paper147 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Static and dynamic analysis methods of position-keeping capability for offshore supply vessels with voith-schneider propellers(American Society of Mechanical Engineers (ASME), 2017-06); ; This paper presents a static and a time-domain method to assess the position-keeping capability of monohull vessels. For the static analysis method, the equlibrium between mean environmental loads and available actuator forces is determined. In case of the dynamic assessment, the motions of the fully actuated ship in all degrees of freedom are simulated in time domain and evaluated by criteria regarding the position and heading of the ship. After first results from validation tests an exemplary application is shown by assessing the positioning capability of a Voith-Schneider propelled offshore supply vessel.Publicationtype: Conference Paper151 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Subsurface ice transport at a transversally towed ship model(American Society of Mechanical Engineers (ASME), 2017-06); ; The subsurface transport of ice along the underwater body of a ship hull or a structure may cause damages to appendages. In order to investigate the conditions under which the ice accumulation occurs, a series of model tests was carried out in the ice basin of Aalto University. The used ship model was towed laterally against the ice with one side breaking level ice. The transport of broken ice floes broken off from the intact ice sheet has been has been monitored with underwater cameras. Both the model drift speed, respectively the ice drift speed, and the ice thickness are found to affect ice accumulation process. The Densimetric Froude number is introduced as measured to determine whether ice floes will accumulate on the upstream of the hull. It is found that ice accumulation is triggered at relatively low Froude number.Publicationtype: Conference Paper92 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Application of a boundary element method for wave-body interaction problems considering the non-linear water surface(American Society of Mechanical Engineers (ASME), 2017-06); ; In this paper an existing time domain panel method, which was originally developed for propeller flow simulations, is extended by implementing the mixed Eulerian-Lagrangian approach for the computation of the non-linear free water surface. The three-dimensional panel method uses a constant source and doublet density distribution on each panel and a Dirichlet boundary condition to solve the velocity potential in every time step. Additionally, a formulation for the acceleration potential is included in order to determine the hydrodynamic forces accurately. The paper gives an overview on the governing equations and introduces the numerical approach. Validation results of the developed method are presented for the wave resistance of a submerged spheroid and a wigley hull. Additionally, the wave diffraction due to a surface piercing cylinder in regular waves is validated regarding the forces and the water surface elevation around the body. Here, the computations are compared with other numerical methods as well as tank test results. Apart from this, the paper deals with an application example showing simulations of an artificial service vessel catamaran in waves. The forces on the hull with and without forward speed are presented. The paper concludes with a discussion of the presented results and a brief outlook on further work.Publicationtype: Conference Paper139 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Calculation of the hydrostatic and structural integrity of docking sequencesThe conditions of competition within ship yards are changing. The current market situation requires a new orientation of the Pella Sietas ship yard with flexible solutions for new ship types. Complex, heavy and ice-going ships show one way for future designs. In view of all the technical difficulties involved in such challenging projects, the first question must be how to handle these heavy constructions with the yards building facilities available. The Pella Sietas yard is using a floating platform for newbuildings. The question arises whether or not this platform is still capable and suited for this kind of ship types. The docking procedure is a complex multi-body interaction that copes with hydrostatic and structural challenges. The docking operation is regulated by the sequence of flooding and emptying ballast water tanks of the dock. At any time of this dynamic operation the hydrostatic stable equilibrium of ship and dock must be ensured. When the ship becomes afloat the keel block system transfers the ships weight on the structure of the dock. It must be ensured that the resulting tensions and deformations do not exceed the maximum permissible values. This paper describes a fast calculation method that determines the mentioned hydrostatic as well as the structural investigations during the docking procedures. The method implies a numerical progressive flooding simulation that calculates the hydrostatics of ship and dock under consideration of their interaction by dock blocks together with the ballasting sequence in the time domain. Furthermore it calculates the block forces distribution by applying the deformation method. In the calculation process ship and dock are modeled as Timoshenko beams and the dock blocks as nonlinear spring elements. Moreover the shear force and bending moment distributions of ship and dock are calculated and the deflection lines are presented. Therefore, the described method enables the ship yard to evaluate quickly the possibility of building new types of ships on the existing building platform and allows evaluating which modifications are useful to enlarge the capacity of the platform even further. It provides a useful tool to minimize local and global stresses and deformations of the interacting bodies during the whole docking procedure by fast optimization of the block system arrangement and the ballasting sequences. As a result the described method could expand the range of flexibility of a given floating dock structure. In addition, the whole hydrostatic and structural integrity of docking sequences can be computed faster and more accurate even at a very early project stage.Publicationtype: Conference Paper85 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Numerical prediction of ship-ice interaction-a project presentation(American Society of Mechanical Engineers (ASME), 2017-06); ; ; ; It is inevitable that commercial shipping and oil and gas resource exploitation activities in the Arctic will increase due to decreasing sea ice extent caused by global climate changes. Significantly more demanding and at the same time less well known environmental conditions create a need for reliable methods to assess icebreaking performance guaranteeing safe performance of the ships operating in this area subjected to various ice conditions. The classic approach of assessing ice-going performance, which combines class rules, experience and model tests, may not be applicable for the Arctic region in full. Furthermore, ship yards experience difficulties due to decreasing time frames and financial restrictions. Therefore this paper seeks to introduce a new development for a realistic and validated direct simulation approach for prediction of the hull load and icebreaking resistance that covers all aspects of the industrial design process and allows a more comprehensive analysis. The breaking model will provide a variable breaking pattern and is able to mimic the influence of the vessel speed and the environment on the ice loading and the predicted breaking length. In order to predict the extreme representative conditions to be simulated, a reverse extreme load prediction methodology is incorporated. An efficient, time dependent dynamic coupling between broken ice fragments, ice features, the 3D flow field and the ship's hull provides resistance values for performance calculations. The computational model will be validated against full-scale data and class rules using deterministic and probabilistic measures. This simulation approach is developed within international research collaboration between Pella Sietas, Rolls Royce Marine, TUHH and NTNU. An overview of the project together with the current status of the ongoing work including first results is presented.Publicationtype: Conference Paper147 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Advanced approaches for coupled deformation-seepage-analyses of suction caisson installation(American Society of Mechanical Engineers (ASME), 2017-06); ; In this paper the installation procedure of suction caissons is investigated by means of coupled seepage large deformation analysis performed with finite element methods. The modelling techniques employed to enable simulations of the penetration of a caisson into the soil under offshore conditions, i. e. several tens of meters below the water level. The numerical model includes a u-p-formulation, which is used to calculate the excess pore pressures and effective stresses from the total stresses. The Coupled-Eulerian-Lagrangian (CEL) approach available in conjunction with the Abaqus/Explicit solver is used. The calculation results are compared to centrifuge tests that were carried out recently at the Centre for Offshore Foundation Systems (COFS). This sheds light on the potential and the limitations of the presented numerical techniques. This paper concludes with a brief discussion of alternative numerical approaches that could be capable of the simulation of caisson installation.Publicationtype: Conference Paper141 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Model ice: a review of its capacity and identification of knowledge gaps(American Society of Mechanical Engineers (ASME), 2017-06); This paper presents a review of the state of the art of modelscale ice with a focus on mechanical behavior, mechanical testing and scaling. The goal of the model-scale ice production is the generation of a material that can represent sea ice in as many aspects as possible. The question therefore is, to what extent model-scale ice complies with this high level requirement and what possible limitations of model-scale ice are encountered. A part of the answer lies in the history of model-scale testing in ice as model ice was originally designed to test ships in ice. The interaction of ships with ice, when breaking level ice, differs from other interaction scenarios in terms of triggered failure processes and consequently in the relevant mechanical properties of the ice. The significance of the forces in particular interaction scenarios is reflected in the applied scaling laws. The standard scaling laws are presented together with published alternatives and their limitation and practicality is evaluated. The relevant interaction forces and ice properties are compiled for level ice breaking, ships in brash ice and offshore structures in slow ice drift. In the production of model-scale ice, the mechanical properties and their measurements play a significant role, as they determine how well the full-scale scenario is scaled. However, latest research in context with earlier published findings indicates that state of the art measurement procedures may not be able to capture the actual ice properties, as the mechanical behavior of model-scale ice might be different than generally presumed. Consequently, this paper presents alternative measurement procedures and highlights existing knowledge gaps which are worthwhile to be addressed in future.Publicationtype: Conference Paper169 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , An experimental method for model propeller-ice interaction in air: concept and first resultsOne of the aims of the German national funded research project ProEis is to develop a methodology for determination of ice loads on model propellers which will then aid in improving and developing software tools for their prediction. For this purpose, a prototype device has been designed at HSVA within this project which is used to guide defined ice floes with one degree of freedom into a model propeller where they are milled and the resulting forces and moments are measured. This paper focuses on the description of the ice feeding device and presents some first results with respect to the physical process as well as measured load characteristics. For now, experiments are conducted in absence of water in order to exclude all hydrodynamic effects. A podded propeller is used which allows measuring of shaft torque and shaft thrust. A high speed video camera is also employed to record the propeller-ice interaction process at 5000 frames per second, allowing to observe the milling process in detail. Two basic impact patterns are observed with respect to their relative load levels. When the greater part of the leading edge of the propeller strikes the ice from the top, i.e. it crushes the granular layer and cuts off a relatively large chunk of ice, the maximum shaft thrust is approximately 1.5 times as high as when the leading edge of the propeller makes contact at the front face of a floe, scraping off a thin layer of ice. Shaft torque is mostly unaffected by the type of impact. The performance of the ice feeding device and the findings of the first tests presented here are reviewed and discussed critically. Recommendations for the planning of such tests and possible improvements of the device are given.Publicationtype: Conference Paper80