Vibrations resonance estimation in marine structures

Dynamic analyses of marine structures are one of the most important during design process as well as during exploitation. Typically, two parts of vibrations calculations are performed. Firstly, natural vibrations frequencies and modes are determined. Forced vibrations analysis is the second step of calculations. Natural vibrations give us information about possibility (e.g. at which main engine speed) of dangerous vibrations amplitudes (resonances threat). Exact values of vibrations amplitudes are results of forced vibrations analysis. Confidence level (error value) of forced vibration calculations is much lower in comparison to uncertainty of natural vibration analysis. What is more, form engineering practice point of view, natural vibrations frequencies estimation (resonance avoidance) is the most important during dynamic analysis of marine structures with propulsion systems. In the paper some examples of dynamic characteristics estimation and their changing methods (possibility) was presented. Utility of relatively simple models, with low degree of freedom number, was discussed. Modelling methods depending on the analysis target of marine slow-speed crankshafts was presented. Advantages and disadvantages of subcritical and supercritical design of marine propulsion system were discussed. An example of propulsion system natural torsional vibrations simplified analysis was presented. Location of the main torsional resonance is one of the most important decisions during design process of the marine propulsion system. Methods of fast and easy estimation as well as changing possibility of natural vibrations frequencies of marine structures is the main objective of presented paper.