Bolt flips visible indicator when tension is lost

Tom Shelley reports on a development that will do much to reduce the cost of ensuring safety in bolted structures

A new tension indicating bolt flips a yellow bar on its head to give a clear visual indication of loss of tension that can be seen up to 25m away.

The RotaBolt "Vision" uses the same concept of a sensing pin in a hole drilled in a bolt to detect tension, but with the addition of a sprung cap with a prominent yellow bar that flicks round through 90 deg under spring force if tension falls below a pre-set value. The conventional RotaBolt requires operators to physically grasp the cap on the pin head and see whether it is free to rotate.

Sparrows Offshore have piloted the new bolts over 12 months on the connecting flanges of a main A frame on the Marathon Oil Brae Bravo Platform. The company has now placed a further order to fit them on the booms of the East Brae cranes.

Checking of bolt tension on cranes on offshore platforms is mandatory. With conventional bolts, it is necessary to loosen and tighten each one with a torque wrench. Hence although the new load indicating bolts are not cheap - the conversions cost from £3 to £150 each for large sizes and special materials, they typically reduce the cost of bolt tension assessment on a structure from around £15,000 to £4,000 according to marketing and business development director Diederik Neeb. The company guarantees that the flip over will take place at pre-set tension accuracy of {{plusminus}} 5 per cent but says that in practice the figure is usually nearer {{plusminus}}1 per cent. The standard bolts have been salt spray tested for 28 hours without loss of performance. They are equally suitable for use in pipelines and on pumps and compressors, provided their size is M18 or larger.

James Walker

Tom Shelley

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