Development of safe operational criteria for ships based on vibration analysis

Abstract

The origin of shipboard vibrations is dominantly determined by vessels propulsion system, main machinery, auxiliary systems, pumps, breaking of the waves at the ship hull, etc. All of these peripherals and systems origin the static and dynamic induction of vibration onboard a ship while various analytical and statistical methods may be declared to assess the levels of vibration in order to declare a combined safe operational criteria for ships depending on the spectrums. Few similar researches had been completed on identification and quantified estimation of the noise or the multiple vibrations resulted from ship and machinery during passage. However, those researches were addressed mainly towards developing a relationship for a single new ship or category particularly prior to commissioning. This research was focused on studying the shipboard transferred vibration (of the vessel's structure and to sea) and vibration spectra generated by shipboard machinery / structures using portable vibration analysers for in-service ships. Data was recorded onboard classes of ships/ craft (new and old) through a series of sea trials to establish a common relationship for ships and to comprehend use this methodology as a tool in understanding complex patterns of ship vibration spectra to predict possible defects. The study commenced on 05th May 2014 under the patronage and guidance of the Department of Mechanical Engineering, University of Moratuwa to identify the unique patterns of shipboard vibration signatures generated whilst vessels are on sail, paying attention to the external forces and to develop a data base with existing vibration levels after being identified as safe operational limits (depending on the observations based on datum readings). The research data was collected using two modern vibration analysers (Make: Areva, Model: Oneprod MVP -2C, Country of Origin: France, Frequency Range: 0 – 40,000 Hz) integrated with Areva XPR 300 software system uploaded computers for the spectrum analysing task. Sea trials at following conditions had been carried out. a. Sea State : 3 and below; b. Ship Loading Condition : Full Load, Half Load; c. Wind Condition : Moderate < 15 knots; d. Sea Direction : Ahead, Astern Numerical relationships and probabilities were established using practical outcomes of the sea trials. Data analysis and Spectrum datum comparison were used to identify and quantify the safe operational vibration limits with respect to individual classes of ships being the primary outcome. The similar defect patterns and respective statistics were averaged to find out a common relationship to identify a figure named “Shipboard Machinery Vibration Safety Index (Sv)”, which was the secondary outcome of the study.