Notes for the Students by Marilena Greco:

Transcription

1 Notes for the Students by Marilena Greco: In the following you find topics proposed by MARINTEK, by DNV GL and by SINTEF Fisheries and Aquaculture, for possible Project and Master Theses on hydrodynamic problems. The detailed plans will be finalized in agreement with the interested students. This information is also available on and could be updated in case of additional proposals. So, please check the website if you are interested. In case of any doubt or need of further information about any of the proposed topics, please just contact me. Sincerely yours, marilena.

2 Topics proposed by MARINTEK 1. Experimental study of the local slamming loads due to breaking waves on a vertical column. Figure 1: Overview of experimental setup. Vertical columns are common parts of marine structures like semi-submersibles, spar buoys, tension leg platforms, offshore wind mills etc. These vertical columns experience large slamming loads when exposed to high and steep waves. The highest slamming loads are local in space and are of short duration. Furthermore, they are related to relatively flat impacts, meaning that the angle between the free surface and the structure is small. Under these conditions, air may, or may not be entrapped between the free surface and the structure. If air is entrapped, it causes uncertainty related to scaling of the slamming forces from model to full scale values. A model experiment is suggested in order to investigate the physics of this type of slamming events. This experiment consists of a fixed vertical column, mounted in a wave tank. The model will be exposed to steep waves. A high speed camera is used to investigate the local flow details. Camera angles are indicated in the figure. Other instrumentation like wave gauges and pressure sensors will also be required. The student must plan and carry out the experiment and describe the findings in a scientifically written report. Contact at Marintek: Dr. Bjørn Christian Abrahamsen <BjornChristian.Abrahamsen@marintek.sintef.no>

3 2. Validation and verification studies on the MULDIF solver for body interaction with waves and current. The wave-current interaction code MULDIF has recently been developed at MARINTEK. The program calculates hydrodynamic forces (first order and mean drift), motions, surface elevation for one or multiple bodied in waves with or without current, in either infinite or finite water depth. There exist little data from other numerical codes to verify MULDIF against. It is therefore useful to validate the code by comparing with model tests and full scale measurements. Some comparisons with model test results have been carried out, and these comparisons show results that need to be investigated further, preferably by full scale measurements. CFD analysis could also be a supplement in such an investigation. The results from MULDIF show that there is coupling between the modes of motion (heave to pitch, heave to surge, etc.) due to the wave-current interaction. This is as expected, keeping strip theory codes for forward speed in mind. In offshore application, these couplings have not been much focused on, since they are not present in zero current (for symmetric body geometries). The investigated model test results show good agreement with MULDIF results in low wave heights, but for model tests in large wave heights it seems that the coupling between the modes of motion due to the wave-current interaction disappears. It is believed that this may be due to viscous effects of some sort, flow separation, possibly altering the flow pattern around the structure so that the effect from the wave-current interaction disappears. There may possibly be scale effects on the viscous forces in the model tests, and this raises the question whether or not the coupling between the modes of motions disappears also in full-scale conditions. Task 1: Proposed work to be done is to identify a test case, preferably a semisubmersible, where full scale data of motions as well as wave and current conditions are available. These full scale data need to be investigated to find out if there is coupling between the modes of motion. The MULDIF code should also be applied with the same wave and current conditions and results compared with the full-scale data. If there exist model tests of the same case, this should be included in a comparison with the full scale data and numerical results in order to obtain the best possible understanding of the wave-current interaction and possible viscous scale effects. If full scale data are difficult to obtain, an option is to identify a model test (preferably of a semisubmersible) and use this to validate MULDIF and seek explanations for the different behaviour with and without coupling between the modes of motions that are observed in model tests. It would then be interesting to investigate how small differences in heel/trim angle of the vessel influences the numerical results obtained by MULDIF. It is expected that a theoretical description of the wave-current interaction effects is given, and that the numerical results and comparisons with full scale and/or experimental data are followed by a discussion that will be useful for validation of the MULDIF code. Task 2: Investigate the possibility to use CFD analysis to study the viscous flow pattern and possible scale effects on the coupled motions of a semisubmersible in wave-current interaction. The work should include a literature study in order to find available 2- or 3-dimensional model and/or full scale experiments in waves and current relevant for validation of MULDIF. Available literature on numerical studies on similar topics that may be used for verification of MULDIF should also be investigated, as well as identifying available CDF solvers to possibly be used to carry out comparisons with MULDIF, assuming 2-dimensional flow due to CPU-time limitations.

4 Describe the theoretical and numerical features of MULDIF, and apply the code to some cases to verify proper use and to understand the output variables and the effect of current on the motion responses. Finally, the work should also contain a description of the suggested approach to be used in a CFD study to investigate the coupling between the modes of motions, and a discussion on how this approach can be used to assess the possible scale effects observed in the model tests. In a 2-dimensional study, the semisubmersible can be modelled as two pontoons. In order to investigate the coupling between the modes of motion, a possible approach is to estimate the added mass coupling terms (such as A13, A31, A35, A53 as well as A33 and A55). If for example A35 and A53 are zero, there is no coupling between heave and pitch. A 2-dimensional CFD analysis of the two pontoons subjected to a forced harmonic motion in current and calm water can be used to find the added mass coefficients. In order to investigate the scale effects, at least two scales must be analysed, model scale and preferably full scale. It would also be useful to apply the CFD code without viscous effects in order to compare the results with results due to potential flow only. Contact at Marintek: Dr. Elin Hermundstad <Elin.Hermundstad@marintek.sintef.no>

5 Master thesis topics suggested by DNV-GL Topic 1 Nonlinear 2D wave models There exists a variety of wave models applicable for different water depths and degree of nonlinearity. DNV has recently worked on the interfacing of wave models with its forward speed wave loads tool Wasim. The traditional wave models used are based on a sum of sinusoidal waves, or Stokes waves up to 5 th order, but Wasim is quite flexible in accommodating other models. Examples are Boussinesq, G-n wave theory, higher order spectral method (HOSM), NLS, conformal mapping or wave input based on a boundary element solution. Depending on the choice of application, the candidate may focus on one or more wave models and work on implementation, verification and validation. Another interesting study could focus on implementation of wave models used to initiate and provide upstream wave input for a computational fluid dynamics simulation. Topic 2 Nonlinear wave reconstruction Quite often good measurement data exists for the free surface elevation at one or more points when model tests of marine structures have been carried out. However, in order to evaluate loads and response, it is required to evaluate the temporal and spatial kinematics, e.g. pressure and velocity in a defined part of the fluid domain and wave elevation at other points than provided by the probes. DNV has initiated work to improve current methods, and the results so far are promising. A thesis would support the developments and draw on the experience already gained. The method should be tested out on an application relevant for the offshore industry. Topic 3 Application of CFD to study roll damping of ships advancing in waves Estimation of roll motions of ships is inherently difficult, and current state-of-the-art typically involves use of empirical formulae that may or may not be valid for the studied vessel geometry. Previous studies include detailed review of the physical processes involved in roll damping as well as some limited computational fluid dynamics analyses. DNV proposes to expand on these to conduct careful CFD simulations focusing on e.g. controlled 2D simulations looking at use of turbulence models, wall law, convergence of grids, forced motions vs. free decay, importance of memory effects. It should also be viable to go on looking at 3D effects or roll damping in calm water versus in waves. Topic 4 Simplified slamming models DNV has worked on simplified 2D slamming models, greatly improving on the generalized Wagner program used previously. The new approach needs to be extended to oblique sea conditions, e.g. asymmetric ship sections and three degrees of freedom motions (heave, sway and roll). In addition, the 2D tool integrated in the DNV seakeeping tool Wasim should be replaced. As a part of this, it is important to focus on consistency, i.e. double accounting should be kept at a minimum. Some inconsistencies are unavoidable considering the fundamental differences of the seakeeping and slamming problems and solutions. Topic 5 Harmonic Polynomial Cell numerical wave tank

6 A promising new method to solve nonlinear potential fluid-structure interaction problems has been published by Yanlin Shao and Prof. Odd M. Faltinsen. The Harmonic Polynomial Cell method is currently tested in DNV for numerical wave tank applications focusing on generation of nonlinear, irregular waves in finite water. This is important to generate controlled nonlinear wave events for design load analyses using computational fluid dynamics. The HPC method should also be sufficiently efficient to study statistical properties of nonlinear waves as well as accurate wave kinematics. Alternatively, a more mathematical twist could be to implement a HPC method in 2D and study numerical aspects for different types of cell elements with varying polynomial order. Topic 6 Added resistance of objects with idealized geometries DNV strives to improve methods and tools to estimate added resistance in waves. A study focusing on idealized geometries facilitates comparison with analytical results. This would provide new insights regarding the tool s capabilities to capture different physical effects. Alternatively or as a second phase, the available tools may be used in a systematic study of ships where good experimental data exist. DNV has access to a huge data-base of test results through the Cooperative Research Ships JIP. The aim of the study would still be to understand the applicability and possible shortcomings of the tools, based on a discussion of underlying assumptions and limits in physical modeling. Topic 7 Improving coupled analysis of floating structures The classical approach for handling coupled analysis in Sesam is to run a frequency domain hydrodynamic analysis with Wadam and then read the hydrodynamic coefficients into DeepC or SIMA. The motions of the floater and the loading in risers and mooring lines are then computed in these tools. This approach has two limitations: It is not possible to obtain the pressure loads on the floater The hydrodynamic analysis is linear The purpose of this topic is to try to address both of these limitations by transferring the forces from the moorings and risers back from DeepC or SIMA to the time domain solver Wasim and then recompute the vessel response in Wasim. If Wasim is run in linear mode the motions predicted by Wasim should in principle be the same as the motions predicted by DeepC/SIMA, provided that the incoming wave is the same. If Wasim is run in non-linear mode the motions will change and it may be necessary to do additional iterations by running DeepC/SIMA again using the motions predicted by Wasim as input and then computing corrected loads from the moorings and risers which are again taken back into Wasim. Three specific problems will be addressed as parts of this work: Establish a method for translating the wave time history in the DeepC/SIMA analysis into wave input for Wasim. The wave input to Wasim is always a set of harmonic wave components. Check that the incoming waves in the Wasim simulation are the same as in DeepC/SIMA. Perform a linear Wasim analysis and compare the resulting motion with the motion predicted by DeepC/SIMA. If there are significant differences these should be investigated and explained. Perform a non-linear Wasim analysis and test if the Wasim DeepC/SIMA iteration will converge. If this turns out to be too much work the topic can be limited to the first two problems and the last investigation can be handled in a separate work later. The last problem is only of interest if the second problem is successfully solved. Contact at DNV-GL: Dr. Olav Rognebakke <Olav.Rognebakke@dnvgl.com>

7 Topics proposed by SINTEF Fisheries and Aquaculture 1. Extreme wave-loads on an aquaculture feed barge Figure 1: The feed barge at ACE - Rataren The size of feed barges of fish farm systems for salmon aquaculture is getting bigger and bigger, as the need for feed storage capacity increases with increasing production. An example of a large modern feed barge is shown in Figure 1. The hull of a typical feed barge is box shaped with low freeboard in fully loaded condition. The superstructure is often similar to storehouses, with garage doors or hatchways. There is a trend that production is moved to more exposed sites where the biological capacity for sustainable production is higher. A consequence of this is that aquaculture structures will be exposed to rougher weather conditions with higher waves and stronger currents. Large waves combined with low freeboard of the feed barge may lead to incidents with water-on-deck and slamming on the superstructure. If slamming occur on weaker parts of the superstructure, like the hatchways, local structural damage can be the result. This might cause subsequent water filling and reduced stability of the barge. This problem is relevant with several incidents reported. A suggested topic for project and master thesis could be: Project Collect information about used feed barges in terms of dimensions and geometry, operational conditions and expected changes in the working environment. Perform a literature investigation of experimental, theoretical and numerical studies relevant for this problem. Identify a numerical method that could be suitable to investigate the occurrence of water on deck and slamming. One possibility could be using a potential seakeeping-solver combined with a shallow-water approximation for the shipped water but viscous effects providing damping at resonance should be modelled and accounted for in the model. Restoring forces due to a representative mooring system should also be included. Set a plan for the studies to carry on within the master thesis work, also considering available experimental studies for possible comparison. Perform if possible preliminary numerical investigations.

8 Master Summarize the outcomes and decisions from the project work and finalize the modelling if not fully completed. If, from the literature study, relevant barge experiments are available use them to validate the solver identified during the project work. Use this method to perform the investigation of occurrence of water on deck and slamming as planned in the project work so to identify possible critical scenarios. Investigate the possibility to use a local slamming model so to analyze the slamming loads during the impact of water with the superstructure. A visit to a feed barge can be arranged. Contact at SINTEF Fisheries and aquaculture: David Kristiansen, Research manager Aquaculture structures, david.kristiansen@sintef.no

9 2. Effects of surface texture and material on the flow past cylindrical structures This project/thesis may include drag measurements on a number of clean nylon nets and/or nets strands or ropes without and with copper-coatings. Nets from the same production batch without and with copper coating may be used and changes in strand diameter and surface structure may be documented. The results can be used to evaluate the effect of such coatings and may allow the implementation of a "coating-factor" into existing numerical models describing the behavior of flexible cylindrical structures and nets in a current. Background A wide range of cylindrical structures are commercially used, for example pipelines and risers in the petroleum industry or cables and power lines for large electrical networks. The air or water flow past such structures leads to aerodynamic or hydrodynamic loads, which affects their stability and/or movement. Fences, ropes and nets are special cases for structures made from a number of cylindrical structures and a number of numerical models treat nets as a network of stiff cylinders and connection nodes. A good understanding of the interaction of fish nets and water flow is of great importance for the fish farming industry, as it defines the behavior of fish cages in the sea, the water quality inside fish cages, forces on mooring lines and anchors and the deformation of fish cage nets. A number of studies have aimed on finding formulations for the connection between the solidity of nets (the relationship between mesh openings and net strand sizes) and the drag in a given flow. Today different deterministic models exist and some of these differ substantially (see Figure 1). 14,000 13,000 12,000 11,000 10,000 9,000 Drag coefficient 8,000 7,000 6,000 5,000 4,000 3,000 2,000 1,000 0, ,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 solidity Milne, knotted net Milne, knotless net Aarsnes et al. Danilov, Re = 4,39e2 Løland 2 Zhan Balash et al., d=0,0027 Balash et al., d=0,0010, D=0,0023 Figure 1 : Overview of empirical and semi-empirical drag formulations versus the solidity. Some differences are due to different types of nets being used as basis for the formulations of the drag coefficient in dependence of the net solidity, but it is likely that also the small scale structure of net strands can have an important impact on the drag. Net strands are not perfect cylinders, but are a weave from many small filaments. The surface structure of net strands is therefore at a significant size in relation to the strand diameter. The structure of net strands may change the drag on nets very much in the same way as small structural changes of risers or industrial pipes may affect the drag and the shedding of eddies (strakes, fairings, ect.). Often aquaculture nets are coated with copper-paints (see example in Figure 2) and results from a recent study indicate that such coatings may affect the drag of nets, which may be due to a change of the surface

10 characteristic of the net. The ability to understand and parameterize the effect of changes in the net structure, for example as caused by coatings and paints, will allow the implementation and optimization of numerical models describing the behavior of cylindrical structures like nets. Figure 2. Nylon net without (white parts) and with copper coating (red part). The coating builds up a relatively smooth layer over the nylon threads. Heat treatment may shrink threads, but the layer of coating often compensates for the loss in thread thickness. Contact at SINTEF Fisheries and aquaculture: Dr. Lars Gansel Lars.Gansel@sintef.no

11 3. An experimental investigation of fouling and angle of attack on the drag on nets in a uniform flow This project/thesis may include drag measurements on a number of clean nylon nets and nets with fouling attached. The nets are positioned at different angles in a uniform flow and the drag on the nets is measured. Clean net results will be compared with data available from literature. The effect of fouling on net drag has recently been investigated for some fouling organisms, but only for the water flow being perpendicular to nets. There is no good information about the effect of fouling at different angles of attack of the water, and it is this information that is needed to parameterize fouling so that the effect of fouling on hydrodynamic loads can be estimated using existing numerical models. This information is important when designing new fish farms, and especially for the dimensioning of moorings and anchors. Organization of the work: 1) Project thesis: a literature study is performed of what already is done on this topic and on possible relevant experimental and theoretical studies. Based on this study a proper experimental setup is defined and an appropriate NTNU facility for the experimental study is chosen. Finally, the experiments are prepared and necessary parts are constructed. 2) Master thesis: tests are conducted as specified in the project thesis. The data is analyzed, including an error analysis and the clean net results are compared with existing models. The fouled net data is used to suggest a new model for fouled nets or for the effect of fouling on drag (the variables will include at least flow speed, angle of attack and net solidity/amount of fouling). Possible next steps from the present work are identified and further work is proposed Contact at SINTEF Fisheries and aquaculture: Dr. Lars Gansel Lars.Gansel@sintef.no