ABSTRACT

The rapid growth in world trade and the economics of marine transport has driven industry to consider much larger vessel designs. The resulting greater powering requirements have led to the need for larger propulsion systems and bigger engines. Subsequently, the unavoidable greater excitation forces, such as propeller induced unsteady forces/moments and main engine unbalanced forces & moments require consideration in design. In addition, as the vessel dimension increases, the structure natural frequencies tend to be lower than those associated with smaller ships. The lower natural frequencies associated with low to medium speed propulsion system design has increased the potential of structural resonance that causes further concerns on shipboard vibratory performance. Therefore it is critical that an appropriate vibration analysis be performed during early design stages. ABS has developed an internal document of "Vibration Analysis Procedure Guide" (ABS, 2005). The objective of the Guide is to provide the guidance for an analysis engineer and to ensure the quality of the analysis results. This paper provides the technical background of the analysis procedure and its applications to a number of large commercial vessels.

INTRODUCTION

As the ship designs become larger, the potential vibration problems tend to be significant due to larger engine and larger propulsion system. Subsequently, the design and construction of a ship free of excessive vibration continues to be a major concern and, as such, it is prudent to control vibration problems, through simple empirical calculation as well as detail first principle based Finite Element (FE) analysis, in the early design stage.

The hull structure responds as a free-free beam (both ends free) sits on series of elastic foundations (buoyancy springs) when subjected to dynamic loads. The vibration induced by propulsion system is a common source of shipboard vibration. The vibration from this source manifests itself in several ways.

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