Congestion in bulk load ports is an increasing problem. At some Brazilian and Australian ports, ships may wait offshore for as long as two weeks for their turn to load (estimated average in Brazil 6-8 days, and in Australia 10-14 days by year end 2007).
In an attempt to counteract this problem, some ports have warned that some ships may be blocked from trading, or else subject to pressure for faster loading. Reports from Australia indicated that about 70 ships will need to increase their de-ballasting pace or be blocked by rejection of nominations.
Port Waratah Coal Services at Newcastle has explicitly told bulk operators to speed up their de-ballasting times so that its Kooragang coal loaders could operate at a speed closer to their full 10,500 tons/hour capacity. Some CVRD iron ore terminals in Brazil have been reported to request Capesize to follow a nominal loading rate of 16 000 tons/hour.
The high speed loading rate of iron ore especially for Capesize ships may cause some concerns. However, the loading rate in tons/hour is not necessary the critical factor. The critical factor is how much you fill in the cargo hold relative to what you have in the neighboring hold and the draught. The shear force in way of a transverse bulkhead is ruled by the difference you have in downward acting forces from the cargo hold forward of the bulkhead relative to the upward acting force from the cargo hold aft of the bulkhead. The down/upward force is ruled by the net pressure, i.e. pressure on inner bottom plating from the cargo minus the external sea pressure on the bottom plating within that cargo hold, and the safety is really about how you are able to monitor the loading related to this pressure difference.
The same effect may apply in the engine room area – one may experience high shear force in way of engine room bulkhead may during loading the vessel. The fuel tanks are often located in the engine room area. These tanks are often almost empty at arrival, whereas the aft cargo hold is loaded. When approaching full draught, you have large upward acting force in the engine room area and the opposite in the aftermost cargo hold.
In both cases described above, the loading speed is not the major concern, what is the governing factor is really the vessels’ loading flexibility. Such loading flexibility should be taken into consideration during design and approval and should thus not represent any hazards during operation.
The loading flexibility was increased by the introduction of IACS unified requirement, URS25 - “Harmonized class notation and standard loading conditions”, in 2003. Also, in 1998 all IACS members introduced comprehensive requirements to ensure that the vital design limits are to be controlled during loading and unloading. Many captains seems to be unfamiliar with how the cargo hold loading diagrams are now mandated as part of the loading manual for bulk carriers. Hence, the loading computer will check towards the limits for not only the global still water bending moment and shear force, but also the local strength diagram for the hold. The local strength diagram checks the limits with respect to:
- Allowable cargo intake in each individual cargo hold as a function of the actual draught
- Allowable cargo intake for two adjacent cargo hold as a function of the actual draught
However, the cargo mass, ballast and draught must in most cases be given as input to the loading computer by the user.
DNV are fully aware of the pressure from terminals to undertake the loading quick and have therefore introduced a new class notation called Easy Loading EL. This is a voluntary notation, which will provide a more automated control of the critical design parameters described above, and allow for easier and more flexible loading sequences. In order to get the EL notation, the following must be complied with:
- The EL notation can only be given to bulk carriers with BC-A or BC-B class notation, with additional GRAB[X] notation
- Relevant loading sequences, with average loading rates shall be given
- Each step in the loading sequences, from commencement of cargo loading to full deadweight is reached, step-wise synchronized with the de-ballasting operation, shall be documented
- Sufficient de-ballasting capacity to meet the requirements for the average loading rates
- The ship must be designed such that minimum 50% of maximum permissible cargo intake per cargo hold can be loaded in one pour
- Automatic draft reading system shall be fitted
- An on-line ballast tank level monitoring system, linked to the loading computer, shall be fitted
In conclusions: vessels that can undertake the loading/unloading faster than other modern bulk carriers are those who have been designed with this in mind. This is the choice of the ship-owner ordering new vessels – he may specify the loading and unloading sequence, with specified loading rates, in such a way that the sequence will give a competitive loading time.
Such loading sequences may then have to be considered as design conditions strength-wise. Requirements should also be considered for redundancy, accessibility, remote monitoring and operability of all parts of the ballast system in order to ensure that full de-ballasting capability is available during loading.