Dec 272010
 
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Damaged lifeboats are expensive.

A company investigation into the parting of a wire fall during the recovery of a freefall lifeboat highlights the hazards created when wedge socket arrangements are improperly fitted. Note that in this case the wedge socket arrangement was installed while the ship was being built, so just because something is ‘factory-fresh’ does not mean it has been put together properly.

Wedge sockets are popular because they are easy to install and are used where end termination can be made only after the wire rope has been reeved into place.

Says an internal report by the ship manager: “During a routine boat drill the Free-fall lifeboat had been lowered to the water during the drill using the A-Frame and was being recovered when the fall wire parted causing the boat to fall some nine metres to the water.

Fortunately no-one was in the boat during recovery and no injuries incurred but damage to the lifeboat was extensive.

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Fall broken near top splicing

The cause of the fall wire failure has been investigated by the manufacturer’s representative and the wedge socket arrangement at the termination of the wire fall at the davit head was found to be improperly fitted. This incorrect fitting had been made during the building of the ship.

According to the service company which attended as part of the investigation, an incorrect installation will mean that undue stresses are put on the wire construction and ultimately the wire will break.

As the diagram below shows, the incorrect installation forced the wire fall around a sharp angle.

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MAC adds to the warning that some fitters will try to force the wire fall into place using a hammer. This can result in damage to the wire, weakening it.

What is sometimes seen as an additional safety measure is clipping the dead end of the wire rope to the live end to keep the rope from accidentally working out of the socket or releasing the wedge from the socket. Technical groups warn that the dead end should never be clipped to the live line.

This paper from Ontario’s Infrastructure Health And Safety Association makes the point well: “Tests repeatedly demonstrated that the capacity of a wedge socket attachment is lowered when the dead end of the wire rope is clipped to the live line… Ropes clipped in this way had a lower breaking capacity than ropes with the live line left free. Testing also  showed that clipped ropes failed at the clip, where load was concentrated”.

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