HEAT DAMAGE TO AGRICULTURAL CARGOES ON BOARD
Published: 25 September 2023
Updated: 20 October 2023
Fuel consumed on board a ship requires heating to pump it from its storage to its settling or service tanks. However, some cargoes are extremely heat sensitive and therefore heating of tanks adjacent to cargo holds should be done with caution.
The International Maritime Solid Bulk Cargoes Code (IMSBC) identifies the following as heated ship structures: steam pipes, heating coils, top or side walls of heated fuel and cargo tanks, and bulkheads of machinery spaces. A heated ship structure is where the surface temperature is liable to exceed 55°C1.
That said, certain types of agricultural cargo suffers heat damage even at lower temperatures of around 40°C to 55°C. When steam is supplied to fuel oil tanks, the temperature within the tank rises and heat is either transferred by a conduction process or radiated from the tank to the adjacent cargo holds.
A significant number of claims have been linked to heat-sensitive agriculture products such as soya bean, wheat, maize, corn, barley, sunflower pellets, canola, bananas, flower bulbs, soybean meal and fish meal. Processed cargoes have an even higher moisture and oil content, making them more susceptible to microbiological activity and self-heating, which can lead to severe heat damage with the feed cargoes.
The extent of cargo damage may differ due to the extent of the exposure period, properties and/or inherent temperature of the cargoes within the hold. Heat transfer mostly accelerates the self-heating process which often leads to cargo deterioration and can result in heavy economic losses to the carriers and shippers. Soya bean, as a high value cargo, has amounted to more than half of those claims in recent years.
Since agriculture products are heat-sensitive cargoes, the master and chief engineer should consider the location of heated fuel oil tanks and fuel consumption sequences as part of their cargo stowage planning alongside the ship’s stability.
SELF-HEATING DAMAGE AND RISKS ASSOCIATED WITH FUEL TANK HEATING
The moisture and oil content within agriculture cargoes dictates the speed of naturally occurring microbiological activities which can lead to mould, fungal spores, bacteria and yeast cells forming. This can cause germination, fermentation or putrefaction. However, drying the cargo prior to loading can slow these microbiological activities down so the deterioration is less likely to occur during the voyage.
When agriculture cargoes are loaded with sufficiently high moisture content, it usually promotes the migration of moisture from higher to lower temperature areas within the cargo. When moisture content and temperature are mostly uniform, the rate of moisture migration is usually low.
Therefore, greater temperature differences within the cargo favours mould development and exacerbates microbiological self-heating. This self-heating is usually slow to begin with, but heat from the vessel or fuel oil tanks could aggravate the process.
Further self-heating could then cause the bulk temperature of the cargo to rise to a point where the residual oil in the cargo begins to oxidise. This promotes possible spontaneous ignition and may even result in rancidity, discolouration or blackening of the cargoes. The cargo’s deterioration could get worse with long sea passages or when vessels encounter any delays at the discharge port.
Moreover, when cargo becomes mouldy it often emanates malodour and forms long filaments that intertwine, creating visible layers of agglomeration (caking). This contributes to product quality degradation, storage instability and impacts the nutritive value of the cargo.
Therefore, one of the mitigation processes is to dry the cargo prior loading to slow down the thermal conductivity which in turn slows the heat transfer within the cargo. The smaller the chance of moisture migration means cargo deterioration will be less likely to occur during the voyage. However, this may not be within the control of the vessel. The vessel should communicate with the shippers at load port and/or employ surveyors to conduct quality cargo control inspection.
Sometimes, it is not possible to detect whether the cargo is dry or wet at load port. In such instances, the Master could request for the moisture content and temperature of the cargo to be recorded in the certificate of quality. This may be beneficial against future claims and will also give clarity to the Master when it comes to ventilation practices during the voyage.
In one instance, soya bean loaded from Brazil to the Far East did not result in dispute during initial lightering process. However, subsequent weeks of delays at the anchorage had exacerbated microbiological deterioration of the remaining cargo on board and partial cargo was eventually rejected by the receiver. Therefore, it is very important to communicate with the receiver in regards to the berthing schedule and report the condition of the cargo to the shipper whenever there is a delay.
The cargo’s insulating properties will at some point prevent the heat from dissipating further. Surface cargo may appear to be in good condition whereas some of the cargoes at deeper levels, especially those near the cargo hold bulkhead or tank top may already be burnt.
As the damage is caused by microbiological processes within the cargo, it may extend well beyond the heat penetration from fuel tanks. Consequently, the cargo may be discoloured and scorched up to several metres in depth when it is immediately above the double bottom fuel oil tanks. This will be apparent during discharging.
In another recent claim, the heating of the aft double bottom fuel oil tank below the cargo hold resulted in significant heat damage to the soybean meal. As the vessel received last minute instructions from the charterer to plan for the loading of soybean meal, the chief officer then decided to load the parcel in no.7 hold due to the availability of space. The cargo hold is directly above the fuel oil tank and situated in front of the engine room bulkhead.
Furthermore, the constant heat generated from the exhaust gas economiser (EGE) was recirculated back to the fuel oil tanks to disperse excessive heat energy. The ship’s crew member did not monitor the temperature of the fuel oil tank heating throughout the voyage even though the tanks were fitted with temperature sensors.
On discharge, some 3,500 metric tons of cargo was allegedly found damaged by heat and rejected by the receiver. The claim was estimated at a value of a quarter of a million dollars.
The service and settling tank temperature can heat up to more than 80°C in the process of fuel transfer. This means the cargo hold situated in front of the engine room bulkhead will be exposed to heat radiation if the bulkhead is not properly insulated. Some modern vessels are designed with cofferdams and insulations between the cargo hold bulkhead and the engine room. Therefore, vessels without this design should plan their stowage of valuable yet vulnerable cargo with this consideration in mind and/or prioritise the loading of non-agricultural products in this after hold.
Some post incident investigations also revealed that crew members did not maintain any temperature records of the cargo holds and fuel oil tank heating. This could prove counter-productive in defending claims against the vessel’s owners.
Very often, recipients at discharge ports may reject the whole parcel of the cargo when they find alleged heat damaged cargo has been mixed with sound cargo. This can result in substantial claims of up to several hundred thousand dollars and some ports may even refuse to discharge these unsound cargoes in the discharge port. Subsequently, the vessel may have to proceed to another nearby port to dispose of the cargo as their last option, which may result in additional cost and delays.
It is important for the master or their representatives to inspect the cargo condition at load port and the following recommendations may provide some assistance to the master in planning and transportation of these type of cargoes:
- Vessels should consult the provisions scheduled by the IMSBC Code for stowage precautions, especially for temperature-sensitive cargo during planning stages
- High value yet temperature-sensitive cargoes should be loaded away from the fuel oil tanks and/or uninsulated engine room bulkhead. It is preferred to load these cargoes adjacent to non-essential fuel oil tanks as heat from fuel tanks may trigger rapid deterioration even if the cargo would be otherwise suitable for carriage
- When planning fuel storage management for a voyage, master and chief officer should liaise with the chief engineer about the nature and type of cargoes prior to loading and discuss fuel oil management for the voyage
- Prioritise fuel consumption for fuel oil tanks that are not adjacent to cargo hold with heat-sensitive cargo
- Consider using lower viscosity fuel oil when carrying temperature-sensitive cargoes as low viscosity oil requires lower temperature heating
- Carry out fuel heating gradually by increasing the steam capacity in the steaming coil, limiting the rise in temperature to a minimum extent, until it is sufficiently heated but not unnecessary (ideally tanks should not be heated more than 40°C)
- Remember that the heating lines exposed above the surface of the fuel in the storage tank may transfer heat to tanktop plates even faster than if they were submerged. To minimise the heat transfer, leave a sufficient amount of slack fuel in the tank
- Ensure effective working conditions for the heating coils in the fuel tanks and shut off valves, steam traps and temperature sensors in the bunker tanks as per the vessel’s Planned Maintenance System (PMS)
- Update the PMS maintenance records thoroughly as these may serve as valuable evidence in the future
- Ensure the steam supply valves are shut when emptying fuel oil tanks
- Retain temperature records of heating fuel oil tanks carried out throughout the voyage to protect interests of the Members
- If accessible, carry out temperature monitoring inside the cargo holds and ventilate as practiced by relevant temperature rules
- Be aware of surplus steam from the Exhaust Gas Economiser (EGE) being recirculated to the fuel oil tanks.
By and large, bunker management is often neglected and may not always be included in cargo stowage and stability planning processes, especially prioritising the usage of fuel tank sequences. This is particularly important for heat-sensitive agriculture products. In short, management on board the vessel should plan cargo stowage concurrently with fuel management and conduct proactive discussions to better understand the characteristics of the cargoes to be loaded. Finally, the goal is to protect Member’s interests and achieve a claims-free outturn at discharging ports.
For more information on the carriage of grain read our ‘Carriage of Grain and Oilseed Cargoes – An Overview for the Maritime Sector’ report.