Theoretical and practical backgrounds of monitoring system of ship power transmission systems\u2019 torsional vibration

Abstract

Reliability of the marine propulsion system is closely correlated with the safety of navigation at sea. Typical ship’s powertrain has a lot of advantages (mainly efficiency) but is the source of high vibration level. What is more, rough sea conditions can be a source of additional ship vibration, especially for big container ships. Vibrations may have a dangerous influence on ship equipment’s strength and consequently on the ship’s safety. Torsional vibrations of the marine power transmission system are usually the most dangerous for the shaft line and the crankshaft. The power of propulsion system is quite often measured by commercial measuring device based on instantaneous angular speed (IAS). The angular speed measurements are performed using two optical sensors for reading the IAS, mounted at shaft line. The authors try to use existing apparatus for torsional vibrations’ continuous monitoring. Designing monitoring methodology consists several analytical methods. Simplified method of torsional vibration calculation is a first one. Simplified calculation method gives us a determination possibility of torsional vibrations in typical and emergency working conditions. Natural frequency value (resonance location) as well as vibration amplitudes can be estimated on the base of the method. The presented calculation method was verified by comparison with the detailed finite element method calculation and measurements on real ships. The second part of the monitoring system contains methodology of monitoring of piston engine’s crankshaft torsional vibrations by measurement of IAS at free and power output ends of the engine’s crankshaft. It is assumed that calculation of differential value between both ends shall give the picture of torsion angle magnitudes and phases of the peak values. Analysis method of recorded signals (e.g., recalculation of angular distance sampling into constants time function and frequency base by FFT analysis) is also developed. Description of the measurements of the crankshaft torsional vibration for marine engine with some simulated malfunction (a leak of fuel injection pump, and relayed on mounting of sets of injection valves with different nozzles characteristics—spraying nozzle angle) is presented. Presented results of experiment derive from test cycle carried out using laboratory stand of Gdynia Maritime University equipped with 3-cylinder self-ignition engine, powering electric generator. The planned monitoring system should have detecting possibility of torsional vibration changes (propulsion system malfunction), the ability to assess the degree of danger of failure and eventually determination of the risk of damage and the causes of propulsion system’s threat failure.