Offshore Support Vessel Main Propulsion Gearbox, PTO Shaft Bearing.
1) Background
The propulsion configuration on this vessel is as follows:
4 x Nohab Polar V12 at 3060 BHP each, driving CP propellers through 2 Lohmann and Stolterfoht Gearboxes. Each gearbox has a shaft generator driven by a PTO shaft from the outboard engines.
2) Monitoring History
As a lead in to this incident, all bearings on the Shaft Generator, PTO shaft and No.4 Input shaft had been changed in January 2001. These bearing changes had been recommended following the first vibration recording in December 2000. Surveys subsequent to the bearing changes revealed the presence of outer race damage to one or both of the PTO shaft cylindrical roller bearings. Indications were consistent with fitting damage and showed no sign of deterioration, no maintenance actions were recommended.
In the early part of 2002 the vessel was working out of Namibia and a docking had been planned for July 2002 in South Africa. Routine recordings were taken by ships staff and forwarded for analysis just before the docking. The recording from the after PTO shaft was very “noisy”. Throughout the spectra a strong harmonic series at the inner race fault frequency for a FAG NU230E bearing was evident with PTO shaft sidebands. (shown above). The indications were of significant damage to the inner race and a recommendation to change the PTO shaft bearings was made.
3) Failure Analysis
The centre figure is of the after PTO shaft bearing inner race following removal, There is an elongated crater on the roller (first picture) and is the only damage evident on all of the rollers. The large spalled area on the inner race would more than likely have started from a similar marking.
A possible cause for this type of damage was considered to be a stray current such as from welding in the proximity of the gearbox. This has not been confirmed and is only a theory. No metallurgical examination of the bearing was conducted.
Regardless of the cause, vibration analysis had detected this fault before it caused a major gearbox failure. No unplanned down time was necessary and the vessel was back on hire on time and fully operational.
For information, The outer race of the forward bearing is also shown above. Indentations at roller spacing typical of true brinelling are evident. This is consistent with the fitting damage reported following the first overhaul in January 2001.
4) Cost Savings Calculation
SAVINGS ESTIMATION Off Hire Costs Cost of Repair Day rate for AHTS In service failure typically results in secondary damage Mid-Range Savings $75,000 $200,000 TOTAL SAVINGS $275,000
> 6000hp
A$20,000 –A$25000
Depends on spare availability.
- A$100,000-A$300,000
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