Tech Focus: Jackup Rig Gearbox
Award winning gearbox begins 20 year life on the high seas
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Oil rig drilling platforms are extremely high cost capital investments. So it is understandable that the designers and end users of such massive structures have reliability and availability at the very top of their list of ‘must haves’ when they consider much of the critical equipment.
And for Jack-up platforms, that are designed to float to a location and then have the drilling platform ’jacked up’ off the surface of the sea, one piece of critical equipment is surely the jacking gearbox. Jacking gearboxes provide the power to lower the platform legs onto the sea floor and then lift the enormous weight of the drilling platform high enough above the water so there is no interference to drilling operations from wave motion, even in stormy conditions.
Hyosung Corporation of Korea claim that their jacking gearboxes give both reliability and power. And SKF is proud to contribute to that assertion by providing 23 bearings in each of their latest designs of gearbox, including the SKF CARB toroidal roller bearing which delivers the highest carrying power in the smallest package of any bearing type.
Any system is only as strong as the key components in the system. If any single components should fail, the entire system fails. It was these thoughts in mind that Won-Cheol Hong, senior designer at Hyosung Industrial Machinery began his project to design a smaller, lighter, more powerful jack-up gearbox for oil drilling platforms, than his company had delivered before.
One major component in the design is the bearings that support the shafts and gears as they lift the enormous weight of the platform at 1.5 feet per minute. The sheer weight of the platform and the very demanding operating conditions mean that minor deflections of the shafts cannot be avoided. And to prevent that they cause premature bearing failure in gearbox applications the bearings selected must be able to accommodate these deflections in all conditions throughout the life of the gearbox. Hong’s final selection included SKF tapered roller bearings, spherical roller bearings, and CARB bearings for the very demanding load carrying in the planetary gears. This selection resulted from thorough advanced modeling of the gearbox together with sophisticated bearing life and load calculations, carried out by SKF.
By the time he finished the project Hong’s design had been selected for innovative product development award. More than 500 sets have been delivered for their hard work in the harsh environments of offshore drilling and will be incorporated in new platforms being commissioned in the next few years.
The first drilling platform equipped with these gearboxes started drilling off the coast of China in January 2009 and will stay there for three years. The oil rig, operated by an American company, uses 54 Hyosung reducers to jack up the drilling platform, having 18 gearboxes on each of its 3 support legs.
Considering the rig platform has a maximum drilling variable deck load of 3,673 tonnes it is easy to understand why Hyosung and the rig operator consider that such gearboxes need to give reliability and power. Add to that the fact that the oil drilling platform is expected to be active for 20 years, and so are the gearboxes, then reliability really means reliability. Especially if the operator wants to have efficient and trouble free operation during the anticipated total of 2000 hours period of lifting and lowering of the platform once it is positioned, after the vessel is moved to various drilling sites during those 20 years. The first gearboxes produced have a lifting capacity of 440 kips (kilo-pounds) and a holding capacity of 700 kips in normal conditions, extending to 1000 kips in severe storming conditions. Later models had a 550 kips carrying capacity with proportionally more holding and severe storming capacities.
While Hong is proud of his work in designing the gearbox he says “I am very happy with the technical support I received from SKF in this project. As well as full support from local SKF Korea engineers I was visited by European SKF engineers who had applied CARB bearings in other heavy industry applications such as steel mills, pulp and paper plants and wind turbines. They gave deep technical presentations to me and my staff and that gave us the confidence to go further in the development. The use of CARB, and its self aligning ability, allowed us to design low profile gears that are wider, stronger and give greater torque capacity than previous designs. This allowed us to downsize the gearbox with confidence, and this was proven in tests when we loaded the gearbox planetary gears to three times the expected loads with no problems. CARB’s ability to take up minor misalignments and heavy loads improves the gear meshing and this also improves the gearbox efficiency”.
CARB bearing focus:
CARB is a self aligning radial bearing with an inner ring that moves independently of the outer ring, enabling the shaft to move smoothly without inducing axial loads. It therefore accommodates misalignment like a spherical roller bearing and axial displacement like a cylindrical roller bearing. Additionally it carries extremely high loads due to relatively long and barrel shaped rollers.
Because the rollers are barrel shaped, and the inner and outer rings are correspondingly concave and symmetrical, the bearing will always position itself in the raceway for optimum load carrying performance.
The CARB design eliminates the problem of induced axial loads resulting from thermal expansion of the shaft. This enables two major design options:
1. Keep the current bearing dimensions and radically increase bearing service life.
2. Downsize and achieve the same or better operational reliability.