A CORRODED wire rope caused a crane failure on the cargo ship Thorco Basilisk in July last year, the US National Transportation Safety Board has found.

An NTSB incident investigation report suggests undetected corrosion and wear on the hoisting wire rope caused the shipboard crane to fail while it was offloading part of a wind turbine.

The component dropped onto the vessel’s cargo cold tween deck, causing between US$3 million and US$5 million in damages to the ship and the cargo. No injuries were reported.

Thorco Basilisk was at the Greensport Terminal on the Houston Ship Channel in Texas at the time.

At the conclusion of the investigation NTSB highlighted the importance of maintaining wire ropes.

An examination of the hoisting wire rope showed significant external corrosion and wear, but visible signs of external corrosion were not clear until the grease on the rope was removed.

NTSB said the ropes were surveyed annually, but the surveys mostly relied on visual inspections to look for wear and would not have identified the underlying corrosion below the grease.​

While the hoisting wire rope had been in use for nine years – still within the standard 10-year period of use – a post casualty examination found the wire rope was near the end of its service life and  should have been discarded.

​“Saltwater and humid ocean air cause corrosion of metals, presenting challenges for the maintenance of high-strength steel wire ropes on vessels,” the report said.

“A deteriorated wire rope directly affects a crane’s ability to safely and reliably handle loads up to its rated capacity (safe working load).

“Therefore, diligent inspection, maintenance, and management of wire ropes are essential. Working wires should be changed at recommended intervals, or more frequently, depending on operating conditions and use.”

The operating company has since updated their planned maintenance system to require crane wire rope replacement every five years, NTSB said.