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1 Marine Technology Transactions. Technika Morska

1 2003

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Dynamic structural analysis of a fishing vessel

Salas M., Luco R., Villavicencio R.

Marine Technology Transactions. Technika Morska

2003

Vol. 14

209-226

Traditional methods to assess ship loadings are mostly based on static or quasi-static methods. When dynamic fluid loads are incorporated they are usually restricted to those originated in head seas, therefore only symmetric loads, such as vertical bending, are allowed into the analysis. It is a fact that in conventional monohulls major stresses arise from symatric loads, however when a ship operates in oblique seas the entire hull experiences combined symmetric and atisymmetric loads such as vertical benging (symmetric) and horizontal bending and torsion (antisymmetric). From the safety point of view, when a stress analysis of certain areas of the hull is carried out, in addition to symmetric loads it is desirable to include antisymmetric loads too. For instance, the direct longitudinal stress at the hull-deck shell junction will have contributions from both, vertical and horizontal bending moments. The hydroelastic analysis of marine structures allows any type of loadings to be included in a stress investigation. The theory is traditionally separated into two parts a "dry analysis" where a Finite Element Modal Analysis is carried out to determine the dynamic "in vacuo" characteristics of a structure. In this stage, a study of modal stresses can reveal potentially dangerous areas due to high stresses being concentrated at hull discontinuities. Having established the dynamic characteristics (natural frequencies, modal loads and shapes) of the "dry hull" a "wet analysis" is conducted to introduce all fluid effects such as wave loadings and hydrodynamic damping and added mass. This paper shows the modal stress analysis of a typical Chilean fishing vessel. Particular attention is given to highly stressed areas. As a general rule, antisymmetric loads have been traditionally neglected in a dynamic structural analysis, however, in this paper it is shown that they could contribute to increase stresses at specific places of the hull.