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Predicting Fatigue Life and Monitoring Performance of Gimbal Bearings

( 03/05/2020 ) Written by: Anonymous

Gimbal bearings are subjected to enormous stress. As the interface between a drilling rig’s riser system and the vessel, they bear a great deal of weight, accommodate motion and endure extreme environmental conditions. The elastomer within the pads eventually degrade over GIMBAL_single_elastomer_pack.110.jpgtime due to fatigue, but when designed properly, exhibit visual indicators that replacement is due long before loss of performance. While Parker LORD gimbal pads provide superior performance and longer life compared to competitor’s pads, an important part of system maintenance is predicting the pads’ fatigue life and monitoring their performance. Understanding a few key points will maximize pad service time while minimizing risks.

Visual Monitoring

The extent of wear on traditional pads can be seen if the elastomer is crumbling; reduced height is also indicative of wear. Parker LORD gimbal pads feature a high-performance coating (HPC) that, in addition to protecting the elastomer from environmental stressors, can be colored to provide visual contrast to the underlaying elastomer. This enables visual conditioning monitoring—for example, the black color of the elastomer crumbs are more easily visible against the silver HPC coating, or a crack in the coating may be more visible when the pad is loaded. 

Frequent visual inspections may be difficult to accomplish, depending upon the installed gimbal’s location, but having a long interval between inspections increases risk of bearing failure. Furthermore, for systems that use metal bearings instead of elastomeric ones, visual assessments of fatigue are not as easy or reliable—which may lead to seizing of the metal bearing or rapid crack fracture.  

For gimbal bearings that are hard to access, most oil and gas companies establish an inspection schedule that is determined using test results and proprietary fatigue data. Our engineers routinely collaborate with customers to determine optimal inspection frequency. It must be remembered that inspections need to become more frequent as a pad nears the end of its expected life. For example, a pad may be in service for 1000 hours before its first inspection is required. But the inspection interval may need to be shortened to 200 - 500 hours once the bearing has signs of damage. Replacement pads can be ordered, and service schedule created, once damage is observed since pad performance is sustained far beyond the initial signs of fatigue damage. It is also recommended to inspect elastomeric parts after storms or large rotation events which can accelerate damage.

Software Monitoring

Parker LORD has recently adapted a proprietary software tool—a multi-bearing system level stiffness calculator—to quickly and accurately analyze existing and future gimbal configurations. This tool has been used for over half a century in the aerospace field to design focalized engine mounts and other bearing arrays. Now it is being used to predict the rotational stiffness of a gimbal in all planes of rotation, assessing stresses from the riser system during pitch and roll. The software can also calculate other system stiffnesses in all vessel directions, including heave, yaw, sway and surge. The calculator can run multiple scenarios in seconds; for example, it can assess changes to system stiffness caused by a damaged pad in the field; accommodate retrofits or redesigns with a different number of pads; examine individual pad stiffnesses, positions, or orientations; and more. 

The stiffness calculator looks at the gimbal system as a whole, so rotational stiffness can be tailored and reduced in primary rotating directions without compromising load carrying capability. This results in reduced riser stresses and extended gimbal bearing service life. 

Every gimbal pad experiences slightly different loading conditions—but understanding and predicting pad service life is critical. By implementing measures mentioned in this blog, and connecting with one of our team members to design a solution tailored to your needs, you’re sure to achieve optimal service, without the worry of premature systems updates.   

 

Resources

Interested in learning more about our gimbal bearing assemblies and pads? Here are a few resources to check out:

 

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