Impact Noise Investigation On Offshore Vessels
Scope of Work and Methodology Used
PCMS were asked to investigate the location and cause of impact noise from the gearbox area on the port-side Azipull Pod on an oil-industry support ship.
Whilst no technical data was available on the bearings or gear teeth, technical data on the gearbox was used to develop an appropriate database template for the collection of data from the following key points within the accessible areas of the gearbox, ie the Motor, Coupling and Upper Gearbox areas.
- Non-Drive and Drive Ends of the Motor
- Both ends of the Input Shaft of the upper Gearbox
Additional readings were taken from the gearbox with reduced Fmax to facilitate longer capture of data, thus ensuring that events from the slower-moving components within the lower gearbox would be captured.
The input speed during data collection was set to 450 rpm with 0 pitch. The speeds of the drive components were calculated using the gear ratios available as follows:
| Motor Input speed | 450 rpm |
| Upper gearbox ratio | 1: 2.16 |
| Upper gearbox output speed | 450/2.16 = 208.3333 rpm |
| Lower gearbox ratio | 1: 2.64 |
| Output speed from Lower gearbox | 208.3333/2.64 = 78.9141 rpm |
Findings
Review of the data from all points collected showed no cause for concern with visible gear mesh and bearing frequency activity being acceptable. However, the data collected with the low Fmax for capturing the lower gearbox events clearly showed an impacting taking place in the time waveform data (see Graph 1).
The speed reference for the graph has been set at 79 rpm to match the output speed of the lower gearbox.
Note the clear impacting taking place once per revolution of the lower gearbox output speed. This is further visible and shows the position per revolution of the impacting in the Orbital Plot (see Graph 2).
Conclusion
The impacting once per revolution of the output shaft is characteristic of damaged teeth or a crack in a gear. This was further confirmed by the lack of problematic frequencies i.e. imbalance, alignment, bearing or gear mesh, etc, in the rest of the data collected. The impacting taking place at a rate equal to the lower gearbox output speed gives the location of the impacting to the gear on the output shaft.