Operational Modal Analysis Of Torsional Vibration Of The Large Ships' Propulsion Shafting

With the trend of ships' growing large,marine low-speed diesel engines' power per cylinder increases,which leads to the larger and unevener torque transmitted by propulsion shafting.This will cause heavy vibration which seriously influences ships' normal operation.It is essential to carry out the real-time condition monitoring of propulsion shafting to guarantee its proper running.Fault symptoms of shafting generally appear with its inherent nature(stiffness or damping)changed,which normally leads to changed natural frequencies.Therefore,natural frequencies can be used as one important indicator during condition monitoring.It is of great engineering significance to identify the natural frequencies of the large ship's propulsion shafting and apply them in condition monitoring with operational modal analysis(OMA)which doesn't encounter problems of insufficient excitation often met in experimental modal analysis(EMA),and only deals with the responsive vibration signal under the shaft's running condition,and performs easily and fast,and can realize online measurement.The thesis proposes using the data-driven stochastic subspace identification technique in identifying the torsional natural frequencies of the large ship's propulsion shafting and preliminarily investigate the feasibility and effectiveness of OMA application on the shafting vibration real-time condition monitoring on board.First,the research status of torsional vibration and OMA is generalized.Second,the excitation condition of the large ship's propulsion shafting is analyzed,the important concept of white noise is discussed,and the possibly encountered harmonics problems,including their cuase,influence,identification and elimination,when OMA is applied onto rotational machines are discussed;Third,to identify modal parameters of the large ship's propulsion shafting with OMA,it is planned to eliminate the trend from the torsional vibration signal initially,then identify stable frequencies and model orders with the data-driven stochastic subspace identification technique and the stabilization diagram,and finally determine the harmonics and inherent frequrencies with the help of probability distribution function curves;an OMA program based on above processes is written and proven valid through simulations;Forth,EMA and OMA tests under many load conditions(the shaft line is different at different load conditions)are carried out on the ship propulsion shafting test bed;results of EMA and OMA under the same load condition and those of OMA under different load conditions are analyzed to investigate the varying pattern of torsional natural frequencies as the shaft line changes and thus verify the feasibility and effectiveness of applying OMA in the identification of the torsional natural frequencies of the large ship's propulsion shafting and in the shafting condition monitoring.