In order to solve the severe vibration problems of an ocean engineering ship with a full-revolving propulsion system, the navigation tests, including forced vibration response test and modal test, are carded out in its stem. It is concluded from the comparison of the time-domain waveform and spectrum from different measurement points that three main factors lead to a high-level stern vibration. Firstly, the specific dynamic stiffness of a water tank is relatively small compared with its neighbor hold, which makes it act like a vibration isolator preventing vibrational energy transmitting to the main hold. Secondly, there exists high-density local modes in the working frequency range of the main engine and thus the local resonance occurs. Thirdly, the abnormal engagement of gears caused by the large deflection of the shaft bearing due to its low mounting rigidity leads to violent extra impulse excitations at high speeds. Then the modification against the dynamic defects is given by simply improving the specific stiffness of the water tanks. And the effect is validated by the FEM calculation. Some important experience is obtained with the problems being solved, which is useful in the design of ships with the same propulsion system. It is also believed that the dynamic consideration is as important as the static analysis for the ships, and that most of the vibration problems may be avoided with a proper acoustic design.
Because of propeller hydrodynamic influence, the shafting vibration is a coupled vibration which includes torsional, longitudinal and whirling vibrations. It is unsuitable to analyze different vibrations of propulsive shafting systems with development of shipbuilding technologies. To overcome the shortages of traditional marine standards, we establish a new numerical model of the shafting coupled vibration. And we put forward shafting coupled vibration calculation to ensure better reliability of main propulsion system. The shafting system is modeled into two sub-systems, a continuous one and a discrete one. Wave approach and transit matrix method are used to investigate displacement and stress fields in continuous and discrete sub-systems, respectively. And vibrations of different modes in both sub-systems are coupled by using dynamic equilibrium and continuity condition to deduce the global equations governing the motion of shafting. The coupling calculation is then used to research the reason of a very large crude carrier(VLCC) stern hull vibration. It is shown by the comparison of the results from both coupling and dependent vibration calculations that vibration in deferent directions will cause deformation in the same mode, which leads to extra stress and displacements on shafting, especially as the resonant frequencies of different vibration modes match each other. This is helpful to prevent ship stern vibration due to poor shafting vibration calculation.
With the rudder angles getting larger and larger,the moment and force on propeller shafts,which are caused by complex flowing field,become more and more.They influence the shafting alignment greatly.Stress analysis of propeller shafts has been done under increasing rudder corner conditions with complex hydrodynamics simulation for a great domestic liquified natural gas(LNG) vessel,which is with dual propulsion systems.The improved three-moment equation is adopted in the process of dual propulsive shafting alignment.The calculated results show that the propeller hydrodynamic characteristics,which affect dual propulsive shafting alignment greatly,must be considered under large rudder angle conditions.Shafting accidents of Korean LNG vessels are interpreted reasonably.At the same time,salutary lessons and references are afforded to the marine multi-propulsion shafting alignment in the future.
