Offshore and subsea applications make specific demands of bearings

Offshore and subsea applications make specific demands of bearings
Subsea conditions represent some of the harshest and most challenging in terms of bearing technology. Combatting them is something that has forced bearing manufacturers to develop highly innovative and robust solutions using a variety of materials and designs.

Subsea screw pumps, for instance, which help enable oil extraction as low as 3,000m, operate in extreme conditions that test the performance limits of standard steel, journal and magnetic bearings.

Clearly, one of the major requirements of a bearing at such depths is that they are robust, with high levels of corrosion- and fatigue resistance. SKF addressed this need with a bearing arrangement for each rotor consisting of two CARB bearings to accommodate shaft deflection and axial displacement. A four-point angular contact ball bearing is also used to locate the shaft axially.

The bearings are constructed from a super-tough stainless steel, which features high amounts of nitrogen for superior fatigue and corrosion resistance. NoWear coated stainless steel rolling elements and a specially-designed cage also contribute to the solution's robustness and reliability to accommodate a wide range of loads and speeds.

Additionally, this special arrangement can be augmented with bearing design upgrades to meet specific application demands. These include SKF hybrid bearings with ceramic rolling elements to reduce inertial forces and heat. Special SKF cage designs offer further resistance to corrosive environments and SKF NoWear coated rolling elements can help reduce friction, while SKF INSOCOAT offers protection from electric currents.

Another product to have seen successful service in offshore applications is from SKF Group Company S2M, which has won a two-year contract to supply SKF magnetic bearings and associated electronics for subsea compression trains used in the Åsgard gas field in the Norwegian Sea. The SKF magnetic bearings will be fitted on the motor compressor shaft systems on the compressor units that maintain gas flow pressure level. These innovative bearings are an oil-free, frictionless and virtually maintenance-free alternative to mechanical bearings that provide optimum efficiency, reliability and performance vital for smooth operation in deep sea operations.

The magnetic bearings,will provide a high performance alternative to conventional mechanical bearing technology. The Åsgard gas field project will involve placing two hermetically sealed compressor units installed at a depth of between 200 and 300 metres on the seabed. The units function by maintaining discharge pressure in the pipeline and compensate for the falling suction pressure as the gas field is depleted, thereby boosting gas flow.

Used in highly specialised applications and industries such as offshore, these bearings can operate at high speeds, require no lubrication and, as there is no component wear, provide virtually maintenance-free long life. In a magnetic bearing system the rotating drive shaft is supported and held in position by electromagnetic forces generated in radial and axial directions. These forces are generated and controlled by sophisticated electronics and copper-wired electromagnets to provide contactless, wear-free operation.

Another intriguing solution to the stresses of offshore applications has been seen in the development of advanced elastomeric spring bearings for the oil and gas industry by Trelleborg Offshore. One application of such a bearing recently used the expertise gained in the oil and gas market to improve the stability of the 90-monopole wind turbine foundations at the Sheringham Shoal Wind Farm. In this instance, the gap between the inner foundation pipe and outer transition piece of the wind turbine is filled with cement grouting, which must withstand the vertical weight of the tower and the lateral wind loading forces. Following reports of grout failure on various wind farm projects, DNV, the certifying authority for the Sheringham Shoal project, radically reduced the acceptable loads on the grouted connections.

However, the Trelleborg Offshore bearings enabled the project owners to significantly reduce the load on the grouted connection and keep the project on schedule. Each wind turbine foundation comprises a 50m long, 5m diameter steel monopile pipe, which is piled 30m into the seabed. Below water level, a larger diameter transition piece pipe is fitted over the foundation for 7-8m and extends above water to a flange to attach the wind turbine tower.

Paul Fanning

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