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 favour mould development and exacerbate microbiological self- heating. This self-heating is usually slow to begin with, but the 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 oxidize. This promotes possible spontaneous ignition and may even result in rancidity, discoloration or blackening of the cargo. The cargo’s deterioration could get worse with long sea passages or when vessels encounter any delays at the discharge port. When the 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.
One of the mitigation processes is to dry the cargo prior to 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 destined for the Far East did not result in dispute during the 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 about 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.
Case study
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 economizer (EGE) was recirculated back to the fuel oil tanks to disperse excessive heat energy. The ship’s crew members 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
THE REPORT | DEC 2024 | ISSUE 110 | 85
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