Local Ship Vibrations – a Methodology for Their Evaluation and Control from the Early Design Phase

Local Vibrations represent one of the most critical parameters for the comfort evaluation on board of the ship. Normally a limit value of vibration is defined in the ship specification and the shipyard has the responsibility to assure that the specified values are not exceeded. Depending mainly on the extension, vibration phenomena could be divided in Global Hull vibrations and local vibrations. Hull vibrations interest all the ship as a beam in its natural modes; local vibrations interest only parts of the ship. Local and Global vibrations could be studied and controlled modelling the ship with FEM 3D model in an advanced design phase, but Global and Local Vibrations could also be investigated in an early design phase when the ship is not yet modelled, with simplified models. The paper presents a methodology, developed by Fincantieri and experimentally tested on previous constructions, for the risk evaluation of local vibration exceeding the limit curve due to the main machinery excitations. Starting on the assumed machinery excitation spectra and depending on the their distance from main ship spaces, on the characteristics of the elastic connections, and on the type of structures, an evaluation of the deck vibrations amplitude is obtained. On the base of these data, when a risk of excess is noted, a deeper analysis on the evaluation of the deck natural frequency could be made in order to investigate more in detail the risk of resonance and in case of coincidence between the main machinery excitation and the natural frequency, structural countermeasures shall be taken.