The National Transportation Safety Board (NTSB) has released an investigation report on a crane wire failure onboard cargo ship Thorco Basilisk that took place in July 2022 in Houston, Texas.
The incident
On the day of the casualty, the stevedores were using crane no. 1 to off-load a wind turbine nacelle from the Thorco Basilisk when the crane’s hoisting wire rope parted, causing the nacelle to drop and fall onto the cargo hold tween deck. The wind turbine nacelle weighed 69 metric tons (76 US tons), or 86% of crane no. 1’s SWL of 80 metric tons (88 US tons) at no greater than 19 meters (62 feet) from the crane base (as certified by DNV-GL in 2019).
During the lift, the nacelle was not hoisted outside of cargo hold no.1 and was therefore within the 19-meter radius prescribed for the SWL by DNV-GL in the 2019 load test certificate. Additionally, crane no. 1 had completed two identical lifts just before the casualty lift without incident. A sample of the wire rope had previously been tested to a maximum breaking strength of 1,374 kilonewtons, or 140 metric tons (154 US tons). The manufacturer stated that the wire rope as originally installed had a safety factor of 4.44 at the crane’s 80-metric-ton SWL.
Therefore, crane no. 1 should have been able to complete the lift of the nacelle load. Shock loading can occur when a sudden movement, or jerk, of a suspended load causes rapid acceleration and deceleration, creating a dynamic force that exceeds the static load. Shock loading may overcome the SWL, or load limitations, of a wire rope and crane.
The crane operator did not note any issues with the crane, nor did he observe any sudden movements of the load. Additionally, immediately following the casualty, stevedores inspected the internal connections and rigging components for failures (which could have introduced a shock load) and found no issues with any of the equipment. Therefore, there is no evidence that the hoisting wire rope or the crane were shock-loaded—either through sudden crane movement or failure of nacelle rigging—in such a way as to create a dynamic load on the wire rope during the lift.
After the casualty, samples of crane no. 1’s hoisting wire rope were examined to determine what had caused the wire rope to fail. The examination noted a localized increase in wire rope diameter near the eye and stated this increase would warrant discard (replacement).
However, based on stevedores’ and vessel crew’s statements, investigators determined that the increase in diameter near the eye occurred postcasualty when the lifting block fell through the fiberglass housing of the nacelle unit and therefore had not contributed to the failure of the wire rope.
The examination also found significant external corrosion, as well as roughness and pitting, and “uniform corrosion of internal surfaces” of the hoisting wire rope. This corrosion and wear caused some of the individual wires comprising the strands of the hoisting wire rope to part (fail) when crane no. 1 lifted the nacelle unit, subsequently causing the strand and then the remaining wire strands to become overloaded and fail.
The postcasualty wire rope analysis report stated that visible signs of external corrosion could not be fully appreciated until the grease was removed. DNV-GL surveyors completed annual surveys of the Thorco Basilisk cargo cranes and their associated wire ropes, but these surveys primarily involved visual inspections limited to obvious indications of wear (such as broken wires, visible corrosion, or observable degradation to outer strands and surfaces) and therefore would not have identified the corrosion.
The wire rope had been regularly maintained in accordance with the operating company’s PMS, and the required daily and monthly checks had been performed according to maintenance records; however, the vessel’s PMS did not requirethe removal of grease from the wire rope (as recommended by the manufacturer). Without removing the grease to examine the wire rope, the corrosion on the wire rope could not be detected.
The hoisting wire rope was 14 years old and had been in use for 9 years. Although the wire rope was still within the 10-year period of use prescribed in DNV-GL standard DNV-ST-0377, the postcasualty failure report stated, “the wire rope was near the end of its service life and probably should have been discarded.” After the casualty, the operating company updated their PMS to require crane wire rope replacement every 5 years.
Probable Cause
The National Transportation Safety Board determines that the probable cause of the failure of the hoisting wire on the cargo ship Thorco Basilisk’s crane was undetected corrosion and wear in strand wires.
Lessons Learned
Maintenance of Wire Ropes
Saltwater and humid ocean air cause corrosion of metals, presenting challenges for the maintenance of high-strength steel wire ropes on vessels. A deteriorated wire rope directly affects a crane’s ability to safely and reliably handle loads up to the crane’s 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.
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Source: NTSB
