Inspection of the ship-to-shore container crane and determination of the cause of plastic deformation of the its boom
Plastic deformations have been detected in the boom structure of the ship-to-shore crane. The crane's warranty period hadn't expired yet. The crane was manufactured in 2012. The manufacturer stated that the damage was the maintenance staff fault. As they turned off the limit switch on the boom lift mechanism, which led to the boom bending when it rest against A-frame bumpers.
The Specialized Laboratory of the Odessa National Maritime University inspected the boom in order to find out the cause of plastic deformation.
Analysis of the boom possible damage according to the manufacturer's scenario
On the first stage of the analysis the manufacturer's scenario of the boom deformation was simulated. For this the moment of the boom rest against A-frame buffers in the upper position simulated using the finite element method. As a result of the simulation the following was determined:
If the limit switch hadn't worked and the boom still had been bending against the bumpers pulled by the rope of the boom lifting mechanism, the plastic deformation would have been far more severe. But during our survey we faced just the local plastic deformation.
Our assumption regarding the boom plastic deformation of the STS crane
Having analysed the boom damage we have come up with the following assumption according the deformation mechanism:
As a result of the loose inner forestay of the boom, the entire load was taken by the outer forestay. Thus, the middle part of the boom left beam do not have any support. And when the loaded trolley comes over this part of the boom, plastic deformation occurs and extends with the crane's growing operation cycle.
To validate our assumption, we have measured forces in the forestays, during the loaded trolley moving along boom. Measurements showed that the inner forestay from the side of plastically deformed boom beam almost did not take any load during testing.
Thus, plastic deformation was caused by inadequate setting up of the forestays lengths.
- Simulation showed that according to the manufacturer's scenario development of plastic deformations in the STS crane boom the plastic deformation would occur in the upper and lower flanges and partially in the webs of the boom beams, which would lead to the boom total deformation However, upon inspection of the boom only local plastic deformations were found, which does not support the manufacturer's version.
- Based on the information obtained during structural inspection, it was determined that the most probable cause of deformation was uneven distribution of forces in forestays of the waterside of the crane, which resulted in gradual accumulation of the plastic deformation during the crane's service life.
- The residual life assessment shows elements for which the residual life is less than 5%. For further trouble free operating it was suggested to repair/reinforce the detected critical elements.
- The suggested hypothesis was supported by the measurement of forces distribution between forestays, which proved that the inner forestay of the boom almost does not take any load during crane operation.
Thus, the most probable cause of the boom's plastic deformation was inadequate setting of forestay lengths during installation of the boom on the crane.