Ship vibration : the application of the full integral method to a ro-ro vessel

Abstract

In today's world, ships are being designed to cater for particular markets. Such market requirements have seen the birth of Liquid Natural Gas (LNG) carriers, Passenger Ships, Roll-on Roll-off (ro-ro) vessel, etc. Some vessels have hence moved away from the 'typical' symmetric hull shape such as Bulk Carriers, General Cargo Ships, Tankers, etc. Ro-ro vessels are a case in point. What has not (and will not) changed is the constant search to reduce the vibration levels induced by excitation forces, generally due to either the propeller or to onboard machinery. Vibration presents problems both to the structure itself and to equipment operating on board, it is also a nuisance to people more so in certain ship types, such as passenger ships. The aim of such research is to eliminate or minimise vibration to acceptable levels. The problem may initially be tackled by determining the frequency at which unacceptable vibration levels occur and their source. At the University of Malta, two theses have already been prepared in connection with ship hull vibration; In Ship Vibration Analysis, the authors performed static tests to elliptical ship shapes to determine fundamental frequencies of the models in different loaded conditions, magnitude of vibration, effect of entrained water, effect of shallow water at moderate and a comparison of the various mathematical methods used in the testing of the models. In Analysis of Stiffering Effects On Ship Vibrations, the authors tested a true ship shaped model to observe the effects of stiffening upon acceleration and velocity readings for different frequencies. In this thesis, the author dares to go further i.e. complementary to the other two theses, this thesis is based upon an ACTUAL FULL SIZE SHIP i.e. MV ZEBBUG. MV ZEBBUG is a ro-ro vessel and was chosen since it represents a category of ships whose shape is not symmetric. The major aim of this thesis was to determine both the two-node and three-node vertical frequencies of the vessel using the Full Integral Method, and to study what effect would the lack of symmetry have upon the mathematical method being used and upon the frequencies. Upon determination of these frequencies the author was to investigate and hence find out whether any external sources could induce the ship to resonate at these frequencies. External sources imply either the environment (ie the sea) or the propellers. The investigation was to be aimed at the propellers and this was to be carried out by taking actual vibration readings on board the vessel while moving at its lowest speed in sheltered waters (ie calm water). Within broad terms of external sources, a further test was to be carried out by investigating the resultant vibration levels due to a procedure known as 'anchor drops'. Anchor drops are a practical test used on ships to induce the lowest mode of vibration. As the name implies, the anchor is actually dropped, and the chain links hitting the ship side induce the lowest frequency mode of vibration.