Dynamic loading contributes to deck slewing crane failure

Transport Malta’s MSIU issued an investigation report on the slewing deck crane failure onboard the Maltese-flagged product tanker ‘Bozdag’, while the ship was in the port of Tallinn, on 28 November 2017. The report revealed that the dynamic loading and the sudden halt of the free falling loads were two major contributing factors to the failure of the deck slewing crane.

The Incident

A regular, five-yearly compulsory test on the ship’s deck slewing crane was planned for 28 November 2017. Given that the safe working load (SWL) was 10 tons, the test was planned to ‘overload’ the crane by 25%. The test plan was the lifting of two large bags, with about 10 tons of water.

During the test, it was observed that the cable seemed to be slipping, with the weight dropping to about 2 meters. It was recalled that the crew operating the deck slewing crane attempted to lift the bags of water again but suddenly the weight dropped further 2 meters. It was during this time that a very loud impact noise was heard, and the crane’s jib collapsed and rested against the bulwark on the port side.

Two crew members, who were inside the deck slewing crane’s cabin, were seriously injured.

Conclusions

1. The dynamic loading and the sudden halt of the free falling loads were two major contributing factors to the failure of the deck slewing crane.
2. The estimated force, generated by the sudden drop of the weights, was in the region of 400 tons.
3. It was not excluded that the mechanical brake slipped as a result of hydraulic fluid leakage past the oil seals and less than optimal contact between the friction surfaces of the outer and inner lamella parts.
4. The brake assembly inspection / testing had not been included in the maintenance programme.
5. There was no reference to the tell-tale opening on the maintenance programme and any leaking hydraulic oil went undetected.

Recommendations

The company was recommended to review its maintenance programme for the deck slewing crane and ensure that procedures are included for the rocking test of the slewing bearing and the inspection of the braking system assembly.

More on the accident can be found on the following report: