Browsing by Author "Parmasto, Oliver"

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  • Development of simplified non-linear response analysis procedure for ship structures
    (2020-12-14) Ekman, Essi
     Insinööritieteiden korkeakoulu | Master's thesis
    This thesis presents a simplified method that can be used in assessing the static structural response of global ship model including the nonlinear effects caused by local elastic buckling. The method consists of modelling the response of a stiffened panel under uniaxial compressive loading. This is done by developing a laminate element where the plate, stiffener web, and stiffener flange are all represented with one laminate layer. The changes in axial stiffness caused by elastic buckling are considered by assigning nonlinear material properties for the plate layer. An investigation into stiffened panel load-shortening behavior is conducted using geometric nonlinear analysis. Based on the results, assumptions about the properties for the laminate layers are made. The stiffeners are found to behave linearly under compressive stress when adequately strong stiffeners are chosen, and the loss of stiffness in the panel appears due to local elastic buckling in the plate layer. For perfect structures, a bilinear elastic modulus is found adequate. The developed laminate is used in a box girder longitudinal bending analysis and the results are compared to a full 3D model of the same structure. The laminate model is found to predict the deflections in the box girder well. Additionally, the same box girder analysis was conducted with initial imperfections where the plate layer stress-strain curve was determined form stiffened panel model with same geometry, imperfection shape and amplitude as the box girder upper and lower sides. The results of these analyses match well. However, in the post elastic buckling region, the laminate is slightly stiffer. The method differs from previous methods by modelling the nonlinear response of a stiffened panel so that it is easily implemented into commercial FE-software. Future work could include extending the method to more complicated loading conditions and to ultimate strength evaluation.
  • Effect of thin deck plates on comfort during wave-induced vibration on a cruise ship
    (2018-08-20) Tissari, Alexandra
     Insinööritieteiden korkeakoulu | Master's thesis
    This thesis investigates the effect of using thin plates as deck plates on the superstructure of a large cruise ship when the transient vibration response is studied. Wave excitation is used as the external loading, since it is a major possible excitation source especially for ships of large size. Comfort is used as the basis for evaluation, as it is closely related to the primary function of a cruise ship. Usage of thin plates is of interest due to both economical and ship stability related reasons. The thesis aims to present the current knowledge related to the physical nature of wave-induced vibration and the numerical modelling of fluid-structure interaction in a concise manner. In addition, the quantitative methods of the relevant ISO standard for vibration related comfort assessment are presented. Further information concerning the health effects and the proposed limits for comfort evaluation is included in the appendices. The method used in the numerical study is one-way coupled, with separate flow and structural solvers. Additional coupling steps are used to both exchange information between the structural and flow calculation models and to apply restoring forces. The flow solver uses the finite volume method, and is based on Reynolds-averaged Navier-Stokes equations. The SST k-ω turbulence model is used. Volume of fluid method is utilized for capturing the free surface. The structural solver is based on three-dimensional finite element method, and the equations of motion for a damped vibration system are solved in time domain. The size of the equation system is reduced by using modal analysis. Three different structural models of a 350 metres long cruise ship were prepared. One model has a superstructure with 5 mm thick deck plates and in the other two models some of the continuous superstructure decks have been reduced to 4 mm plate thickness. Two refinement areas with finer meshes are implemented into each model. The doubly-curved plate profile, corresponding to the deflected shape induced by welding, is applied in refinement areas of one of the thin plate models. The other thin plate and the thick plate models have straight plate profiles. The results indicate that there is a significant difference in acceleration responses between the studied models on the local level. However, from the comfort aspect the differences cannot be considered significant as the acceleration due to rigid body motion is much larger in magnitude than the local component. The global accelerations were found to have only small differences between different models.
  • Effects of elastic buckling at subcritical loads on the load carrying mechanism of a modern passenger ship
    (2021-03-15) Jerne, Alexander
     Insinööritieteiden korkeakoulu | Master's thesis
    This work assesses the effect of local elastic buckling on the load carrying mechanism of a modern passenger ship. A qualitative description of the effects the reduced stiffness of elastically buckled structures has on the passenger ship’s load carrying mechanism is provided. The goal is to identify the scenarios in which the reduced stiffness of the load carrying structures must be accounted for. The identification is done through a global strength analysis of a passenger vessel using nonlinear finite element analysis, which better represents the stiffness deviations caused by load shedding in the elastic range, and to compare the results with those obtained from a classic linear static analysis. The results show how the resulting nonlinearities can significantly alter the load carrying mechanisms at various structural hierarchy levels, even as load levels far below the ultimate limit state, and that simple linear static finite element solutions are unable to capture these effects. The study differs from previous ones by investigating nonlinear effects on the hull girder at subcritical loads, instead of trying to find the ultimate strength when these nonlinearities are included. This work could be expanded in the future by reaching subcritical load levels closer to the ultimate limit state in order to capture greater global effects of these phenomena, or by implementing more efficient modelling techniques capable of greatly reducing the computational problem size.
  • Fatigue analysis of ship's balcony opening corner
    (2019-06-17) Teder, Ardi
     Insinööritieteiden korkeakoulu | Master's thesis
    Fatigue strength has yet remained a major limiting factor in the application of high strength steels in ship structures, despite research showing increased resistance to crack initiation in comparison to mild steels. This indicates potential benefits to be achieved in terms of fatigue strength, with the implementation of the material. One example, where the issue is relevant, are the balcony opening corners on cruise ships, which feature large number of fatigue critical details. Utilization of higher strength steels there would allow for notable economical gains through reduced material thicknesses. In the current thesis, novel strain-based approach for fatigue assessment combined with non-linear finite element method for load analysis is used to evaluate fatigue strength of these details. This methodology allows to make clear distinction between fatigue crack initiation and propagation mechanisms and consider more explicitly the influence of material properties and loading response of the structure. The objective of the thesis is to determine, whether significant advantages could be made in terms of fatigue strength by using higher strength steel and how it is affected by production quality of the structure. The results obtained prove the potential applicability of high strength steels in fatigue critical details. Fatigue strength of the analyzed detail is can be greatly increased in comparison to rule values, if the effect of notches due to surface defects is minimized. Calculations also show the increased dependence on the notch effect, with increase of material yield strength. The method of assessment is validated by analyzing crack growth behavior parameters, which are in good agreement with experimental studies conducted by others. In the thesis, the results are obtained by numerical calculations, using material properties largely based on values presented in existing research and empirical formulae obtained by experimental studies. Therefore, in order to validate the results, experimental work in terms of fatigue tests are required together with material property assessment from full-scale specimens.
  • Mechanics of the passenger ship structure with non-longitudinal-load-carrying accomodation decks
    (2012) Parmasto, Oliver
     School of Engineering | Master's thesis
    The current thesis investigates a cruise ship structural concept which enables to use large interchangeable modules for interior outfitting. The structure has non-longitudinal-load carrying accommodation decks and a narrow deckhouse. The hull-deckhouse interaction and performance of the proposed structure under vertical bending is determined and compared to a conventional cruise ship structure which has internal longitudinal bulkheads for carrying the shear forces in the superstructure. The investigation is conducted by implementing Finite Element Method. The results indicate that the removal of the decks from the conventional cruise ship structure does not change the nature of the hull-deckhouse interaction. The responses of the compared structures were strongly affected by shear lag hull-deckhouse interaction induced secondary effects. It is also shown that the proposed structure can achieve the same stiffness under vertical bending as the conventional cruise ship structure while achieving smaller weight and the height of the vertical centre of gravity of the steel structure. The investigation assures that even at the early design phase, the Finite Element Method is the only reliable way to evaluate the response of the modern cruise ship structures.
  • The use of FSI models for the prediction of wind-induced vibration of modern cruise ship deck outfitting structures
    (2019-06-17) Kivelä, Eetu
     Insinööritieteiden korkeakoulu | Master's thesis
    Recent market trends in the cruise industry aim to provide traditionally land-based attractions on cruise liners. This leads to the integration of special architectural features, such as water parks and amusement rides, in way of the cruise ships’ upper decks. Deck amusements are ideally lightweight structures that comprise of slender beams which aims to reduce the added weight on top decks. To ensure safety it is critical to understand the influence of wind loading introduced by Vortex-Induced Vibration (VIV) on the dynamic structural response. This thesis aims to determine the differences between one- and two-way coupled Fluid-Structure Interaction (FSI) analyses in the context of ship deck outfitting structures subjected to VIV. Accordingly, a large deck amusement structure is idealized as an aluminum portal frame, subject to a constant head wind. Transient one- and two-way coupled FSI simulations, based on Reynolds-Averaged Navier-Stokes (RANS) fluid dynamics model and linear elastic 3D FEA, are conducted using the commercial CFD software STAR-CCM+. Results are assessed and compared against quasi-static and quasi-dynamic beam element idealizations solved by NX Nastran. The investigation carried out reveals that vortex shedding remains at the original shedding frequency in the one-way coupled solution. However, the two-way coupled simulation demonstrates a clear lock-in of the vortex shedding to the portal frame’s natural frequency. Consequently, the dynamic loading experienced by the portal frame is significantly increased and the structure experiences resonant vibration when full two-way FSI coupling is considered. Neither the one-way coupled nor the quasi-dynamic analysis are able to capture these effects.