Offshore tank container rigging failure

A member has reported two incidents in which the rigging of an offshore tank container failed as a result of a cold fracture. In both cases a tank container was rearranged on deck and damage was observed prior to actually lifting the container. There was no other damage than to the link itself.

An approved offshore container is outfitted with an associated rigging set that stays attached for handling. The container and sling are re-certified on an annual basis. For both sets of failed rigging the certification was found to be in order.

• Both containers were lifted in static conditions (deck to deck) in good weather conditions;
• Both containers were full at the time of lifting and the weight of the container did not exceed the safe working load;
• There was no deformation in the link or chain observed in either case; they were so-called cold fractures;
• In both cases it was the master link in a corner fitting of the container that failed.

Following the first incident, the chain link was sent to a laboratory to establish the cause of the failure. It was, at the time, concluded that the most likely scenario that caused the fast sudden fracture was a forging defect in the master link.

Following the second incident some seven months later, the similarities between the two incidents were apparen’t and it was established that both rigging sets were purchased from a single batch. With reference to similar incidents in the industry, hydrogen induced cracking or manufacturing process errors could not be ruled out. As this failure mechanism could not be determined by non-destructive examination methods, it was decided to replace all rigging sets from this batch (32 of) with new rigging sets.

Laboratory results are awaited on these quarantined rigging sets and the broken link for further action as appropriate.