Gimbal bearings provide an interface between an offshore drilling rig’s riser system and the vessel. The bearings protect the equipment from excessive stresses and damage. The traditional gimbal bearing uses multiple elastomeric pads in a circular array to support the weight of the drilling riser while accommodating the motions of the vessel and riser. The extreme loading and environmental conditions these pads experience in service can lead to severe elastomer degradation and bond separation, shortening the pad life.
Each pad on a rig will be strained differently under rotation. Therefore, each pad’s fatigue life should be considered separately. Rotational stiffness, center of rotation, compression stiffness and strains are all interconnected, but can be independently adjusted through pad geometry, materials, position and orientation. It’s possible there’s no bigger topic than safety. Every endeavor, every environment and every manufacturer must address safety—and every component within a larger system must achieve its own safety metrics for the overall system to be safe.
Case Study: Choosing from Options to Solve an Existing Problem
In one use case, a competitor’s gimbal pads were shearing completely apart during campaigns, requiring costly repairs and increased risk. The customer needed a solution that provided improved fatigue life but that fit within the same space and had the same performance characteristics. To arrive at a solution, our engineers studied the entire system (not just individual parts), exploring the limitations of existing elastomers and geometry.
To solve the customer’s problem, the “lowest hanging fruit” was to trade the existing, fatigued pad for a drop-in replacement that was fitted with LORD proprietary natural rubber, which provides greater fatigue life than the nitrile traditionally used. Our gimbals utilize another LORD proprietary material: high performance coating (HPC), which provides superior resistance to ozone, fluid exposure and sunlight. After prolonged exposure to petroleum fluid, HPC-coated natural rubber retains all mechanical properties while exhibiting less swell than uncoated nitrile. The HPC can be colored to provide contrast to the underlaying elastomer, enabling visual condition monitoring. For more on this topic, read “Predicting Fatigue Life and Monitoring Performance of Gimbal Bearings.”
Because a drop-in replacement pad did not solve the underlying issue of high elastomeric strains in rotation due to the part geometry, our engineers developed a highly engineered solution to optimize part performance and fatigue life using improved gimbal pads. Building on the benefits included in the drop-in replacement, this pad offered a smaller footprint with greater axial stiffness and load carrying capability—as well as double the buckling stability with fewer pads.
There was also a third option available for our customer: a U-Joint Gimbal Assembly. This product offers a fundamental paradigm shift, significantly improving fatigue resistance and loading capability. Unlike traditional gimbals, which are designed in a way that routinely pushes their elastomers past their practical limits, the geometry of the U-joint keeps transmitted loads and motions well within a range the elastomer can tolerate, avoiding extreme strain.
The U-joint gimbal has the lowest rotational stiffness and highest load carrying capability possible, along with a higher rotational angle capability. The system stiffnesses are much more “tunable” than traditional gimbal. Furthermore, compared to metal bearings, our elastomeric bearings are impervious to seizure due to contaminants or corrosion.
Parker LORD gimbal solutions can meet time and cost constraints while offering a range of solutions that are based not simply on replacing a part, but on analyzing the needs of the overall system.
Interested in learning more about our gimbal bearing assemblies and pads? Here are a few resources to check out:
- Product Details: More About Gimbal Bearing Assemblies and Pads
- Blog: Predicting Fatigue Life and Monitoring Performance of Gimbal Bearings