UK: Schaeffler Completes Modifications on Subsea 7 Pipe-lay Vessel
Schaeffler has successfully completed the removal and replacement of a drive-end bearing on the pipe-storage and deployment reel on a Subsea 7 pipe-lay vessel, the ‘Seven Navica’.
The replacement required Schaeffler engineers to conduct a survey on the condition of the bearing before removing the existing bearing from the starboard side of the main reel and then replacing this with a new bearing, mounted in the existing pedestal housing.The condition of the new bearing was again monitored before being recommissioned.
The main reel of the Seven Navica has a diameter of 25m and is supported by two FAG spherical roller bearings supplied by Schaeffler. These bearings were originally supplied preloaded into the existing pedestal housings. Each custom-designed spherical roller bearing weighs around three tonnes and has an outer diameter of 1.58m.
Schaeffler conducted an acoustic-emission monitoring survey on the main reel bearings in order to ascertain their condition. This highlighted a static fault in the drive-end (starboard) bearing, which on closer inspection turned out to be a small fracture in the bearing outer ring.Acoustic-emission (AE) monitoring equipment is suitable for slow-rotating (less than 80rpm) machines or components.AE has a high sensitivity to machine faults and filters out audible noise and low-frequency background vibration.
AE monitoring systems use surface-mounted sensors to detect these stress waves, which lie within the 25kHz to 1MHz frequency range.
In order to conduct an AE monitoring survey on the main reel bearings, Schaeffler engineers mounted four acoustic sensors to the drive-end bearing, which were connected to Schaeffler’s AE-Pro semi-portable acoustic-emission monitoring system.
The bearing was then rotated in the forward and reverse modes in order to produce an AE baseline signature condition for the replacement bearing.
AE-Pro is a semi-portable acoustic-monitoring tool that enables engineers to conduct one-off investigations on complex pieces of critical plant or machinery. The functionality and performance are said to be superior to a simple, handheld AE device, as much more detailed analysis data can be provided on exactly where the damage is located.
This gives maintenance engineers valuable information on how the problem can be addressed. In December 2008, fitters from FAG Industrial Services mobilised to the vessel in Dusavik, Norway, to perform the bearing replacement.
Following the removal of the housing, it was found that the journal was oval and therefore the dismounting tools could not be used. This is because they rely on pressure, which could not be maintained due to the gap between the journal and the bearing. The bearing had to be cut off and the journal built up and re-machined prior to fitting a new Schaeffler bearing from Subsea 7 stock. In order to ensure the accuracy and safety of the rebuild, Schaeffler’s fitters were able to use their own mounting equipment and tooling, including a bespoke induction-heating coil device that was used to help speed up the heating and mounting of the replacement bearing.
Schaeffler also provided the replacement bearing with its LOAD 220 lubricant for re-lubrication purposes. To further guarantee the operational integrity of the replacement bearing, Schaeffler conducted another AE survey.
This provided Subsea 7 engineers with a benchmark of the bearing’s condition on start-up.
Source: Schaeffler,May 18, 2010;