The focus of the present study is on the prediction and control of the vibration levels in the structure of an AHTS (Anchor Handling Tug Supply) at a design stage.

The analysis is performed using a three-dimensional finite element model representing the entire vessel in a specific loading condition, that was selected as representative for the operating life of the unit. The model accounts for a detailed representation of the structural geometric and material parameters. Particular attention was devoted to a proper description of the structural and non structural mass distribution. Fluid-structure interaction effects are also considered, as well as damping effects in the structure and other elements.

Both free and forced vibration analyses are carried out: in the former one, the natural frequencies and mode shapes representing the coupled hull girder-local structure vibration are determined, while the latter analysis is aimed at predicting the vibration levels distribution on board. As exciting sources, those coming from the propulsion plant are modeled, with particular attention to propeller and shaft excitations.

The prediction allowed to identify a critical area on board, where vibration level standards would not be met. On this basis, a modification in the structure was hypothesized. An analysis on the modified structure showed significant improvements and compliance to requirements.