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.
Subsea 7 is one of the world’s leading subsea engineering and construction companies in the oil & gas sector. With more than 5,000 personnel worldwide, Subsea 7’s global offshore operations are supported out of Africa, Asia Pacific, Brazil, North America and the North Sea. With a length of 108.5m and a width of 22m, the Seven Navica was built in 1999 and operates as a pipe-lay vessel. She can work in water depths of up to 2,000 metres and is capable of installing both rigid steel and flexible umbilical pipe, from a single deck mounted 2,500 tonne storage and deployment reel.
The main reel has a diameter of 25m and is supported by two FAG spherical roller bearings supplied by the Schaeffler Group. 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. The main pipe reel on an ocean-going pipe-lay vessel is a critical piece of equipment. Minimising the vessel’s downtime and time in port is essential.
In August 2008, following reports of an unusual noise when the reel was turning while the Seven Navica was in port at Subsea 7’s North Sea pipeline fabrication and spoolbase at Vigra, Norway, engineers from 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 ideal 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. Acoustic emissions are the high frequency stress waves generated by the rapid release of strain energy that occurs within a material during crack growth, plastic deformation or phase transformation. 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 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.
Craig Wilson, Equipment Superintendent for the Seven Navica at Subsea 7 commented: “At the time, we needed to minimise the time the ship remained in port, to maximise the amount of time available for pipe-lay operations. After the results of the acoustic emission monitoring survey highlighted a fault in the bearing that could cause a lot of downtime if it failed during operations when the reel was loaded, we elected to change the main reel bearing on the starboard side, which would also give us an opportunity to examine more closely the condition of the outside diameter of the journal, which we suspected to be slightly oval.”
In December 2008, specialist fitters from FAG Industrial Services, the maintenance management and condition monitoring division within the Schaeffler Group, 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, therefore the dismounting tools could not be used, as these 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, which 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. Seven Navica Technical Support Engineer, Iain Fleetwood, who was onboard for the complete workscope commented: “The use of the induction heater and assistance of the FAG engineers was beneficial to the outcome. It was fundamental in keeping the workscope to schedule on this high risk repair that we were required to complete prior to the vessel’s departure to Brazil for pipe-lay operations.”
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. “As a result of a smooth, trouble-free bearing replacement, Subsea 7 will certainly look to Schaeffler as a possible supplier of bearing products and services in the future,” said Craig Wilson.
