Use of operational modal analysis in solving ship vibration issues

Abstract

Vibration in ships can cause crew and passenger discomfort and induce structural cracking. The mitigation of excessive vibration requires knowledge of both excitation and response. Observation techniques and advanced Computational Fluid Dynamics have been applied successfully to investigate and characterise propeller excitation. Operational modal analysis (OMA) has proven to be a useful practical technique for investigating and solving vibration issues on ships. The use of these techniques is discussed in the context of two case studies: 1. The investigation of cracking in freshwater tanks due to vibration, and the use of OMA in tracing the transfer of energy from the propeller to the tank panels, are described. Observations of propeller cavitation and measurements of the pressure caused by its development are presented. Solutions to reduce the excitation energy and to shift the natural frequencies of the tank panels are discussed. 2. An example of vibration of a navigation bridge and accommodation block causing crew discomfort is presented. The use of OMA identified several modes that were excited by the main engine and the propeller during normal service conditions. This improved the understanding of vibration of the accommodation whilst reducing the risks associated with the uncertainty of implementing solutions.