CRANE BEARINGS Eddy current system eases pain of crane inspection cost

Hoesch Rothe Erde's eddy current monitor To many offshore operators, safety legislation costs are unavoidable, but still frustrating. Take cranes, for example. By law, bearings for cranes used on offshore installations in the UK must be taken out and inspected every five years. The exercise of jacking the crane up to pull out the bearing can itself cost 80-120,000, and unless an expensive spare is available to slot into place, the crane will then be out of action for a week - assuming the

To many offshore operators, safety legislation costs are unavoidable, but still frustrating. Take cranes, for example.

By law, bearings for cranes used on offshore installations in the UK must be taken out and inspected every five years. The exercise of jacking the crane up to pull out the bearing can itself cost 80-120,000, and unless an expensive spare is available to slot into place, the crane will then be out of action for a week - assuming the inspected unit is in a fit state to be returned.

Hoesch Rothe Erde has developed a far less costly alternative: a bearing-integrated eddy current system that provides continuous condition monitoring of the raceways and radii of large diameter crane bearings. The system has been approved by both DnV and Lloyd's, allowing the five-year change-out rule to be waived where it is installed. A big advantage is that the system can also be retrofitted to bearings already in use.

The Rothe Erde contactless monitoring system consists of eddy current modules, a power pack and a telemetric module. The eddy current sensor emits a high-frequency magnetic field which induces eddy currents in the surface of the raceway and radii.

The resultant magnetic field is affected by changes in distance between the sensor and the surface being measured, as well as by pitting and spalling in the raceway: this causes sensor output to change. The voltage change is then transmitted by telemetry into a data logger. Non-metallic materials such as lubricants do not influence the signal.

Values measured are stored on a computer and evaluated so that the exact position and length of a defect can be determined. The number of sensors in a large-diameter bearing depends on the degree if monitoring required. If needed, all the raceways and radii can be monitored.

Loggs retrofit

Last year Hoesch Rothe Erde's UK subsidiary in Peterlee, Roballo Engineering, retrofitted eddy current systems to two Liebherr cranes used on Conoco's Loggs complex offshore Lincolnshire. New installations include BHP's Douglas platform in Liverpool Bay - a Haegglunds Kenz crane - and a Stother and Pitt crane for British Gas' Armada complex.

The eddy current system complements Re-Use, a probe intended mainly for new slewing ring systems. Re-Use monitors three-row roller bearings for crack formation on the radius of the raceway by means of ultrasonic testing.

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