PROJECT APPLICATION
FROM FAILURE ANALYSIS TO GEARBOX RE-DESIGN by Robert Shandro, Principal Consultant, Cetim-Matcor A logical failure analysis approach is more than just a diagnosis. It is an appropriate method of observation and analysis, based on specific knowledge and extensive experience, that leverages a global vision of the failure to provide advice for improvement and redesign.
Robert Shandro
Oceanic Phoenix. Image: Wikimedia Commons/Halley Pacheco de Oliveira.
Oceanic Phoenix, owned by CGGVeritas, is an offshore scientific research vessel used for the acquisition of seismic data from the seabed. The vessel faced a significant problem a few years ago. The tapered roller bearings of the gearboxes in the power transmission system were subjected to an alarming level of vibrations - 20 mm/s RMS instead of the recommended 7 mm/s. These vibrations caused the failure of the bearings. The propulsion system is composed of two shaft lines and a variable pitch propeller. Each shaft is driven by two 2 MW electric motors via a gearbox that reduces the rotation speed of the motors from 900 RPM to 130 RPM. This redundancy in the architecture was expected to allow the vessel to operate in a degraded mode, in the event of a failure of one of the two systems. Despite this, defects occurred in both gearboxes.
No shutdown during onsite operation Two on-board missions conducted during technical downtime of the vessel helped to identify the cause of the failure. The teeth extremities of the input pinions were damaged (in the tooth root region) by micropitting and pitting. The maximum density of the surface degradation corresponded to the area with maximum intensity of the surface contact pressure (the localised tooth contact pattern). Also, one tooth of the input 32
THE SINGAPORE ENGINEER October 2020
Broken tooth of input pinion. The cracking was initiated under the case hardened layer and it then propagated parallel with the tooth flank.
Spalling on the running path pattern of the outer ring of the tapered roller bearing.
pinion was found to be broken at the extremity. A large part of the tooth was detached due to crack propagation along the case/core transition area. Also, the middle shaft’s tapered roller bearings were more damaged than the other bearings. Significant spalling was observed on the outer rings of the bearing, specifically localised at the loaded area. The input shaft bearings had a running path pattern, varying in width on
