Vibration of passenger ship structures by length-scale separation and inertia-induced interaction

Aleksi Laakso

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Comfort is an essential quality for new passenger ships, some of which have as their main purpose to provide comfortable settings for enjoyable holidays. Ship owners and classification societies define acceptable limit values for noise and vibrations on the basis of international standards for comfort. Vibration response analysis by means of the Finite Element Method is an essential part of the structural design of these ships. Several uncertainties limit the accuracy of the predictive calculations, especially when the response frequency range is extended above first-order excitations. These include uncertainties in excitation forces, such as waves, propulsion, and engines, but also limitations on the accuracy of the structural analysis model. This thesis focuses on improving the structural model. A method is introduced to analyze the free vibration of thin-walled structures where dynamic inertia-induced interaction occurs between structural length-scales. A global length-scale model that uses Finite Elements with structurally homogenized mass and stiffness is used to solve the free vibration eigenvalue problem. The relative response on a local length-scale is analyzed separately by applying base excitation. Analytical equations are then applied to define the kinetic and strain energies of the combined system. Finally, the natural frequency of the mode under study is altered by iteration so that the mechanical energy of the system is conserved in the time domain. The method is found to be accurate in terms of natural frequencies in case studies representing typical ship structures of 1D, 2D, and 3D global domains. It is applicable to modal response analysis, because the link between the mode shape and generalized mass and stiffness properties remains. In the response analysis of a typical deck, the accurate frequency range of homogenized finite elements is extended up to about 35-40 Hz. The method provides fine mesh-like accuracy with the computational cost of a coarse homogenized model.
Translated title of the contributionMatkustajalaivan rakenteiden värähtely-mittakaavojen erottaminen ja inertian aiheuttama vuorovaikutus
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Romanoff, Jani, Supervising Professor
  • Remes, Heikki, Thesis Advisor
Publisher
Print ISBNs978-952-64-0818-7
Electronic ISBNs978-952-64-0819-4
Publication statusPublished - 2022
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • free vibration
  • energy method
  • finite element method
  • homogenization
  • equivalent element
  • length-scale interaction
  • inertia
  • thin-walled structures
  • ship structures
  • passenger ship

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