Study On Additional Water Mass's Influence In The Natural Frequency Of Ship Hull's Overall Vibration

In the course of working, ships may encounter different kinds of prompting, such as propeller, engine, waves and so on. Under the effect of these prompting force, the ship hull will come into different degrees of vibration or even harmful vibration. The occurrence of harmful vibration will not only influence the normal operation of equipments and instruments, but also make the crew and passengers feel tired and uncomfortable. Sometimes it also destroy the structure of the ship itself. Therefore, it is necessary for us to take accurate forecasting for the natural frequencies of ship hull in the design phase in order to choose the type of engine and the propeller correctly to avoid resonance.When carrying through the ship hull's overall vibration prediction, we usually use the Lewis Theory to calculate the influence of the ship hull's outboard water. The method is simple and practical, but it is not suitable for the calculation of the additional water mass for torsional vibration.Thus,this paper will discuss this problem. The ship hull's overall vibration theory and the Lewis additional water theory are introduced firstly. Then based on these theory, we use the finite element analysis software ANSYS to forecast the overall vibration of a 11800t containership, the results gained by foregoing calculation lay a foundation for the following study.Based on the fluid-structure coupling theory, namely, the structure and outboard water are divided by FEM when calculating the ship's overall vibration.This paper carry through modal analysis of 11800t container ship,174000t bulk carrier, and 76000t oil tanker. The results of which are compared with the natural frequences calculated by using the Lewis additional water theory. By this analysis and comparison, a correction coefficient for natural frequency of ship hull's overall vibration calculated by Lewis method is put forward.This paper also discusses the selection of fluid element's range in the fluid-structure coupling calculation. As a result, we not only verify the test data in reference, but also get a more reasonable value of the fluid element's range and improve the computational efficiency. All the three examples indicate that the correction coefficient put forward by this paper is accurate and reliable, applying the conclusions of this paper can improve the efficiency and accuracy of ship hull's overall vibration.