UV exposure weakens web bridle

Lookout! Issue 36, December 2016

When a lifting bridle broke, as a rescue boat was lowered from a ferry, efforts to repair it were hampered by further problems with an emergency stop button on the winch.

A crew member on the ship had to use a knife to activate the proximity switch and stop the winching during the repair. But the abrupt halt caused the rescue boat to tip 90 degrees with a crew member hanging on, while standing on the gunwale.

The series of failures happened when the ferry master ordered a boat drill earlier this year. The starboard rescue boat was prepared and lowering began with no crew on board, as outlined in the ship’s SOLAS training manual. The drill is designed to check that the vessel, and the brake and lowering apparatus are all in order before anyone boards.

Rescue boat.
Maritime New Zealand ©2019
The rescue boat with replacement steel and chain strops – installed after the webbing bridle broke.

At about three metres above the waterline one of the four nylon strops broke on the lifting bridle, and the transom and outboard engine ended up submerged in the sea with the bow facing aft.

Crew lowered it further to allow the boat to float on an even keel. It was decided to lower the starboard lifeboat to transfer someone on board the rescue vessel to carry out a repair. Two crewmen on the lifeboat pulled the painter on the rescue boat, to bring the vessel closer so one could transfer onboard.

He then started cutting away the snapped webbing to rig a replacement strop in the aft starboard position. The crew member called on the winch operator to raise the vessel slightly, to assist with the rigging. The electric winch is operated by a push button switch adjacent to the remote brake release handle.

When the crew member on the ship took his finger off the button it remained stuck in the ‘on’ position, and could not be freed. The rescue boat continued to rise at a rate of 18 metres a minute, with the man onboard hanging on to the bridle rings.

The switch could not be freed, so a knife was used to activate the inductive proximity switch. This caused a sudden stop, which unbalanced the boat further. It tilted nearly 90 degrees, with the cockpit facing inwards to the ship.

The crew member stood on the gunwale and the boat was eventually raised the final two metres, so he could step out safely to a deck of the ship.

The operator has since put out a safety bulletin for webbing strops, bridles and straps to ensure they are frequently checked for sun exposure and other signs of wear and tear.

The strops on both the port and starboard rescue boats have been changed to steel wire and chains, after it was found that the webbing bridle and sling showed extensive colour fading and wear and tear due to long term UV exposure. The bridle was three months short of the manufacturer’s five-year replacement date.

New emergency stop systems, in weather- proof boxes, have also been added at the winch controls, for both rescue vessels. The motor hoist switch face was found to have corroded around the switch plate which caused the stop button to stick.

LOOKOUT! Points

  • This incident shows the importance of regular maintenance checks for equipment and lifting gear – especially if it is exposed to salt, water and sunlight, and the effects of other harsh weather such as wind and rain.
  • After the incident the operator’s internal investigation included inspection of the webbing of the lifting bridle for signs of wear and tear - such as the colour of the sling, the texture and pliability, and the structural integrity. The colour had faded from purple to pale white, the sling was stiff and lacked pliability, and a slap test against a hard surface created a fine fibre dust indicating severe UV degradation.
  • An expert’s report also highlighted that salt absorption causes wear on webbing fibres.
  • The standards of recommended servicing vary among manufacturers. Operators need to implement their own guidance for crew on when lifting bridles and other such equipment should be replaced, and how to inspect regularly for signs of deterioration.
  • The failure of the motor hoist switch was a secondary issue that left a crew member in a precarious position during an uncontrolled hoist. Progressive erosion affected the face plate causing the hoist switch to stick in the ‘on’ position.
  • The switch should have been fitted with a protective rubber boot to shield it from the effects of weather and sea water – as had been done at some stage on the port side switch.
  • Load testing of the three remaining bridle slings revealed they all broke at less than a third of the 7000kg weight they were designed to hold – despite still being three months within the manufacturer’s 5 year warranty period.
  • Regular maintenance checks on all equipment should help detect such signs of corrosion.

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