During survey several fatigue cracks were found in way of the elliptic curved corner of the opening in way of cargo hatch ways on the main deck. A shedder plate had been welded to the main deck in way of the hatch opening corner. This is believed to be the most probable cause of the crack in the deck plate.
Welding carried out near the free edge of the main deck curvature at the hatch corners will create a heat affected zone with hardening and built in welding stresses in the material. This will cause additional stress concentrations in a highly stressed area which was subject to special consideration at the design stage. In this case the cracks seem to have started from one of the lower shedder plate terminations and into the deck plating, and it is likely that the cracks are caused by dynamic stresses due to a combination of torsion of the hull girder and longitudinal hull girder stresses, and the local stress concentration factor due to the weld.
Further propagation of the cracks in the main deck may seriously affect the vessels structural integrity if not dealt with in time.
Lessons learned
- No welding should be carried out on the connection between shedder plates and main deck. The same recommendation applies to welding of rounded bars to the curved edge.
- The shedder plate may be welded to the hatch coaming, but the lower 100 mm or more should be left with no weld and sealed with plastic steel, mastic or similar, to avoid ingress of water and dirt.
- An alternative to the shedder plate could be build up of concrete or epoxy filler in the corners in order to serve the purpose of drainage of cargo from the hatch corners.
Source: DNV
Officer of the Watch 
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