Cargo handling covers all the activities related to safe transferral, stowing, lashing and securing, taking into account the typical behaviour and characteristics of the different cargoes to be transported.
According to recent statistical information from P&I clubs, claims due to cargo contamination as a result of improper cargo hold cleaning are second in magnitude only to those due to water ingress from leaking hatch covers or bilge wells with associated piping systems, etc.
In general, the requirements as to the cargo hold condition prior to loading depend on the previous cargoes carried, the next cargo to be carried, charterers’ and shippers’ requirements and the requirements of the authorities at the ports where the loading and unloading take place. Although it is common practice today for cargo holds to be inspected by the receiver prior to loading and, in the case of grain cargoes, for a fitness certificate to be issued by an independent surveyor, cargoes may still be rejected at the port of discharge.
As a general rule and regardless of the previous cargo carried, all cargo holds should be thoroughly cleaned prior to loading the next cargo. This is particular true for some dry bulk cargoes which are used as food or feedstuff, but also for other dry bulk cargoes (e.g. Kaolin, Alumina) used as a basis for semi-finished or finished products do not allow any contamination from residues of a previous cargo.
With respect to cargo flexibility, the Handysize and Handymax segments are those with the greatest variety as minor bulk cargoes are mainly carried by them. Consequently, the cleaning requirements after discharging are most stringent within these segments.
From a cleaning point of view, there is a distinction between:
Clean cargoes (e.g. grain & wheat, fertilizers, mineral sands)
Dirty cargoes (e.g. coals, petcokes)
Corrosive cargoes (e.g. sulphur, salt)
Others (e.g. cement, clinkers, phosphate rock, ore concentrates, scrap)
Basically, a ship’s operator has several cargo hold cleaning options depending on the previous cargo and the next one to be carried:
Standard cargo hold cleaning comprises the simple sweeping of the hold bottom combined with a wash down using manually operated ordinary wash hoses.
Portable cargo hold cleaning makes use of combined high-pressure water/air-jet washing units to wash away cargo residues.
Automatic cargo hold cleaning consists of fixed or portable automatic hold washing machines similar to those used for cleaning cargo tanks in oil tankers.
With respect to cost-benefit, the portable one might be considered the most efficient.
Effective cleaning using a water/air-jet unit requires a proper impact, in particular at the shell, in order to get the shadow sides of structural elements in the cargo hold (e.g. flanges of hold frames) cleaned by ‘back-flushing’.
The selection of the appropriate water/air-jet unit depends on the height of the cargo hold. For ships up to Panamax size, a unit with an effective washing height of about 25 m might be sufficient, while for bigger ships an effective washing height of about 50 m is advisable. In any case, the efficiency of these units totally depends on the availability of shipboard water and the air pressure supply. The working pressure of bigger units is in general 7 to 10 bar for both water and air supply, whereas a pressure of 7 bar is sufficient for smaller ones. Water consumption is in the range of 80 to 120 m³/h for the bigger units and 18 to 20 m³/h for the smaller ones.
Consequently, to allow the most effective operations, it might be necessary to up-grade service pumps, particularly if it is intended to use several units simultaneously. This also applies to the service air compressor which has to be sized such that at least a sufficient air volume at rated pressure is supplied to one water/air-jet unit and an air-driven diaphragm pump working simultaneously.
For dirty cargoes such as petcoke, it might not be sufficient to use the abovementioned water/air-jets alone. At least for the upper part of the cargo hold, it is rather necessary to make use of high pressure cleaning, with pumps providing 300 bar and more from a crane or cherry-picker basket. In addition, chemicals are needed to get rid of the stains.
The chemicals are generally applied in foam form, which allows a longer reaction time. The choice of chemicals must be carefully considered, as odour and caustic effects will affect the next cargo. When loading dirty cargoes, a sufficient amount of chemicals and application equipment should be ordered in time or provided on board to avoid delays.
It should be ensured that the chemicals used are environmentally acceptable for marine use and safe for the crew to apply. The crew should be familiar with the relevant safety data sheet and, if necessary, personal protection equipment is to be provided.
It is worth mentioning that, for some cargoes, the cleaning process can be speeded up if a slip coat is applied prior to loading dirty cargoes, as this forms a chemical barrier between the steel structure and cargo.
As the washing process makes use of sea water, it should be common practice to rinse the cargo hold with fresh water to minimise the effects of corrosion and prevent salt from contaminating the next cargoes. Prior to starting with the fresh water rinse, the supply line is to be flushed through to remove any residual salt water.
In the majority of today’s operated bulk carriers, water and air are supplied from the deck through open hatches using loose hoses. The handling of long pressurised hoses might require more crew than having permanently installed branch supply lines through the lower bulkhead stool. It is therefore advisable to consider upgrading by fitting separate supply lines.
When cargo hold cleaning is carried out during the voyage and outside environmentally sensitive areas, the washing water is normally ejected from the bilge wells by either a bilge pump or, more commonly, an ejector driven by a service pump in the engine room, and is discharged directly into sea. If the vessel has to perform cargo hold cleaning in port prior to loading a ‘clean cargo’, the washing water has to be stored in either a ballast tank or, more preferably, a dedicated wash water holding tank if provided.
Using the standard cargo hold bilge system for extracting the washing water may lead to a risk of blocking in-line valves with larger cargo residue particles. Also, abrasive types of cargo might be harmful to the bilge pump.
Therefore, it is advisable to discharge the washing water from cargo holds by means of portable air-driven diaphragm pumps with a capacity adjusted to suit the water/air-jet units that are used.
Using portable ejectors for bilging is effective when discharging directly overboard but is less preferable if the wash water is to be transferred into a dedicated wash water holding tank due to the amount of water needed to operate the ejector. Later on, it can be seen that ejectors have a rather low lifting capacity compared to that of a pump.
To enable convenient handling of the cleaning equipment, a portable cargo residue davit should be supplied to the ship.
The effective cleaning of cargo holds can be performed provided
the ship is technically fit and prepared to carry the intended types of cargoes;
the ship is well equipped with cleaning devices, arrangements and materials;
the crew is well trained in performing the necessary cleaning steps; and
the time needed to clean the cargo holds is considered when planning voyages.