REASONS FOR BULK CARRIER LOSSES

In the last decade a large number of bulk carriers were lost. From 1990 to mid 1997 a total number of 99 bulk carriers were lost, with the death of 654 people. This forced IMO to think about the safety of bulk carriers and a new chapter was included in SOLAS which is chapter XII. What were the reasons which caused so many losses of bulk carriers? We will discuss the reasons one by one.
1) AGE OF SHIP:- Age is a contributing factor in the loss of bulk carriers. Statistically ,bulk carriers 20 years or older exhibit a greater chance of total loss than their younger counterpart
2) CORROSION:-Certain products including coal, phosphate and raw sulphur transported by bulk carriers can rapidly corrode the hold side frames and promote fracture. For a capsize bulk carrier carrying coal and iron ore it has been recorded that a hull web frame with an original thickness of 10mm can corrode to only 3 to 5mm along bottom portion of the hold in a short period. Corrosion increases the structure’s susceptibility to fatigue and buckling. As web thickness is not sufficient to support the heavy cargo being transported, as a result frame can detach from side shell.
3) FATIGUE:-Bulk carriers are susceptible to many modes of cyclic forces that combine with other forces acting upon vessel’s structure. Cyclic wave pressure acts upon the side frames of the vessel in a constant cycle of loading and unloading forces. For bulk carrier carrying high density cargo, such as iron ore, the side frames do not have an internal pressure to counteract the external forces and the side shell is forced inward by the unbalanced forces. This can result in weakening of side frames. Conversely internal pressure created by lower density cargo impose load in opposite direction when a wave trough is encountered. This pressure can also fluctuate and react with the motion of vessel.
            Bulk carriers with cargo loaded in an alternate hold pattern experience high level of still water shear forces as the weight of the holds loaded with cargo are pushing down and the buoyant forces are pushing up the empty holds. These upward and downward acting still water forces combine with the shear forces that fluctuate with the wave motion to impose fluctuating stresses on the side frames.
               Similarly the longitudinal framing of the upper and lower wing tanks is subjected to fluctuating stresses due to the external wave action as well as internal pressure from the ballast tank.

4) COMMERCIAL PRESSURE :- Commercial pressure to reduce the number of cargo transfer passes, resulted in the use of large grabs followed by bulldozers and hydraulic hammers to collect remainder cargo. These caused extensive damage to the inner bottom, lower hopper brackets and shell plating.
5) LARGE HATCHES:- Presence of large hatchway opening over the cargo area reduced the torsional resistance of the hull structure and added focal points for stress concentration at the corners of hatchways.
6) GREEN WATER:- The flooding of spaces below the main deck due to hatch cover or securing device failure, or failure of other deck fittings has been linked to green water loads. This also caused structure failure and finally loss of the ship. Also the flooding of foremost hold resulted in collapse of bulkhead between this hold and adjacent hold leading to progressive flooding and finally sinking of vessels.
7) SPONTANEOUS COMBUSTION:- Transportation of coal and fishmeal can cause spontaneous combustion. Coal emits combustible methane. If temperature in the hold is not checked it can lead to spontaneous combustion and can cause fire.
8) MOISTURE IN CARGO:- Earlier the shippers were not bound to declare moisture in the cargo. So, extra moisture in the cargo initially or moisture generated during the voyage led to green water accumulation and free surface effect in the hold resulting in virtual loss of metacentric height and making the vessel less stable. These had caused the vessels to heel dangerously and finally capsize.
9) LOADING PATTERN:- Sometimes the ships broke on the berth while loading only. This happened because of the casual approach towards loading by the ship’s crew. They didn’t follow the loading pattern and because of this structural failure took place and caused the ship to break while loading only.
                  Above are some reasons due to which a large number of bulk carriers were lost. All these .factors were examined in detail by IMO and additional safety measures were introduced in chapter XII of SOLAS. The measures proved to be effective and these reduced the structural failure to a greater extent.