More than just tonne per area
When selecting a suitable vessel to transport project cargo, we often receive a very basic specification for the ship itself, informing us of the ship’s main particulars and hold size. A key factor that must be contemplated at this stage is the hold dimension; i.e. whether the project cargo will fit. Sometimes the global deck load capacity is considered, but this only tends to be with respect to the cargo overall footprint area and weight vs the ship’s deck capacity in tonnes per square metre.
Consider a hypothetical cargo weighing 200 Te with a foot print of 10m x 8m (or 80m2). This would show a static deck load of 2.5Te/m2, acceptable for most cargo vessels, regardless of whether the cargo is stowed on the tank-top, tween deck or hatch covers.
However, issues can arise once the vessel has been booked and further details of the cargo are then later released. Suddenly it is noted that the cargo has bearing pads or support feet (4 off at 0.3m X 0.3m each) and it is not permitted to support the cargo anywhere else. Now we realise that the static weight of 200Te is supported on a total contact area of 0.36m2; the total area of the supporting feet.
It should be clear that the static loading has now increased from 2.5 Te/m2 to 555.5Te/m2 a significant increase in local deck load.
In addition to the above, the loads due to the motion of the ship are often-overlooked, i.e. the combination of static weight, heave acceleration and either roll or pitch accelerations and motions. The ships motions will increase these local deck loads further.
If not properly engineered, there is risk that the ship’s deck structure becomes damaged due to extreme loads, which could lead to the cargo being damaged or lost.. However, the issue can be mitigated through structural analysis of the vessel’s deck structure in way of the supports and implementing load-spreading where required.
In any case, you should always look to engage a highly-skilled engineer – such as a structural engineer or naval architect – who can perform such checks on the vessels deck structure where there are concerns over the deck strength.
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