Crack in Deck Plating

During survey of cargo centre tanks, cracks were found in some of the deck longitudinals in a amidships. The cracks had penetrated through the flange, into the web of the deck longitudinal, and further into the deck plating at one location. The most probable cause of the cracking was fatigue due to longitudinal hull girder stresses combined with unfavourable location of welding in a knuckle.

Some of the cargo centre tanks of the vessel were designed for partial filling, and some outer deck longitudinals on either side had been reinforced in orde to account for sloshing loads when partially filled tanks. The cracks had occurred in aft part of centre tank at the transition between the smaller deck longitudinals in centre tank and the bigger deck longitudinals in an adjacent centre tank. The welding of the web and flange of these deck longitudinals were both located in the knuckle at the step in scantlings, and the crack had started in the weld of the flange, propagated through the web and into the deck plating. There was no significant corrosion in the tank.

When the deck structure is subject to longitudinal hull girder stresses, the force acting in the knuckle, together with the location of the welding, is believed to be the most probable cause of the fatigue cracks. The step in scantlings is in centre tanks, but the cracks occurred in the aft part of tank where the cracks occured since this is amidships and thus subject to the highest longitudinal bending stresses.

In order to improve the damaged detail, the structure in way of the knuckle was in this case modified so that no welds were allowed in way of knuckle. Further the cracked part of the deck plating had to be renewed. Remaining transitions between ordinary and increased deck longitudinals in centre tank (where the cracks occured) were also subject to modification in order to avoid similar cracks occurring in the future. An additional support bracket in way of the force “R” was not considered necessary in this case, since no welds of the flange were allowed in the knuckle.

The lessons to be learned from such an incident can be summarized as follows:

During design, one should be aware that welds in way of knuckles in longitudinal structures may be unfavourable, in particular where significant longitudinal stresses are present.
During construction, such knuckles should be subject to special attention with respect to fitting and welding.
In the operational phase, welds located in way of knuckles in longitudinal deck structure and also in longitudinal bottom structure should be subject to thorough examination during owner’s inspection and during class surveys, in particular in the amidships area. Relevant information is amended to the survey programme for the main hull survey for the ship in question, and for sister vessels