Influence of Added Water Mass on Ship Structures Vibration Parameters in Virtual and Real Conditions

Abstract

Modeling of ship structures in a virtual environment is now standard practice. Unfortunately, many engineers forget about considering the influence of added water on the frequency values and the amplitude of natural vibrations. The article presents the effect of water damping on the frequency values of the individual natural vibration modes. The tests were carried out in two stages, first, the mentioned values were determined using FEM and then the values obtained in this way were compared with the parameters measured during laboratory tests. For the needs of laboratory measurements, structural elements made of ship steel in one of the Polish shipyards were used. All welds of the test objects have been verified in terms of their correctness. Irregularities in the execution of welded joints could result in a measurement error that is difficult to identify. As a result of the tests, the percentage differences in the frequency of occurrence of natural vibrations of individual modes were determined according to added water mass considerations. Importantly, the research concerned a real structural element of the hull, and the obtained results confirm the need to take into account the mass of accompanying water during the hull's FEM analysis. A number of more detailed research results were obtained, the most important of which is the fact that the finite element method is a valuable method for assessing the dynamics of wetted structures - the error in determining the vibration frequency did not exceed 5% for basic modes. The method of modeling the tested structures was almost equally important - the discrepancy of the results reached 4% depending on the modeling method. When designing marine SHM systems, it is essential to consider the effect of added water mass, since the frequency variations of a damaged structure relative to an undamaged one, are of the same order as the effect of added water.

Keywords

• Natural vibrations
• ship constructions
• water damping
• FEM modelling