DNV GLs guidelines for mitigating actions
DNV GL has published a guideline for the design and operation of vessels with bulk cargo that may liquefy. The guideline aims to raise the awareness of the risks of liquefaction and describes mitigating actions to reduce these risks.
The following causes are the most common causes for liquefaction-related incidents, along with DNV GLs guidelines for mitigating actions:
1. Wrong cargo name
The name of the cargo should be described by using the Bulk Cargo Shipping Name (BCSN) as detailed in the IMSBC Code. Sometimes shippers use trade or commercial names instead. The trade or commercial name may be used as a supplement to the BCSN, but must not be used as a substitute. The consequence of not using the proper name could be that the risks of the cargo are not correctly detected.
Guideline: The Master and shipper/operator should always make sure the cargo is correctly identified before loading. |
2. Cargo not listed in IMSBC Code
If the cargo is not listed in the IMSBC Code, such as bauxite with high moisture content, the shipper must provide the competent authority of the port of loading with the characteristics and properties of the cargo. Based on the information received, the port authority will assess the acceptability of the cargo for safe shipment. After consulting with the port of unloading and the flag state, the loading port authority will provide to the Master a certificate stating the characteristics of the cargo and the required conditions for carriage and handling of this shipment.
Guideline: The Master should always make sure proper documentation of the cargo is received before loading. |
3. TML
In order to get reliable TML values, representative samples of the cargo have to be tested in laboratories, as explained in Section 2.3. For shippers of processed mineral concentrates, this is usually not a problem, since it is sufficient to do testing every sixth month due to the uniform nature of the cargo. On the other hand, in case of unprocessed ore cargoes, such as iron ore fines, bauxite or nickel ore, it may be difficult to get the cargo tested in a suitable lab. The properties of the cargo may vary significantly, so every cargo being shipped should be tested.
TML testing is a specialized task, and worldwide there are not many competent and independent laboratories. In some of the main ore exporting countries, there are few or none such labs. The testing problem is amplified by the fact that many mines are not easily accessible due to their remoteness. It is therefore difficult for independent surveyors or experts to visit the mines and take samples of the cargo. The mines usually have their own laboratories, but it is questionable whether all of them are properly equipped or whether the procedures of the IMSBC Code are always followed.
It is the responsibility of the shipper to declare the cargo as a liquefaction hazard and provide a TML certificate. Unfortunately it is difficult for the Master to verify or independently assess the TML value other than with the highly inaccurate can test, as mentioned in Section 2.3. The Master should be updated on any known problems with a specific cargo. In addition, it is recommended to appoint an independent surveyor or cargo specialist for advice
Guideline: It is recommended to appoint an independent surveyor or cargo specialist for advice. |
4. Moisture content prior to loading
The shipper has to present a declaration of the average moisture content of the cargo before loading. In this process there are many potential sources of error, and again the unprocessed ore cargoes are most vulnerable.
Guideline:
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It is sometimes reported that the guidelines mentioned above are not adhered to. As with TML, it is difficult for the Master to verify or independently assess the moisture content, other than when the cargo is so wet that the moisture can be seen on the surface in the form of wet mud or puddles. However, especially during monsoon season or other periods with heavy rain, the Master should be vigilant. Again, it is recommended to appoint an independent surveyor or cargo specialist for advice.
5. Increased moisture during loading and during voyage
Even if a reliable measurement of the moisture content is provided before loading, there are still some areas of concern regarding moisture content. Needless to say, the moisture content will increase in case of heavy precipitation during loading. If barges are used for transportation of the cargo from the stockyard to the ship, rain and waves may add water.
During the voyage the moisture content may vary due to drainage and possible water ingress into the cargo hold. Even if the average moisture is below TML, there may be local variations in the cargo pile due to moisture migration. The moisture from the top layer is likely to migrate downwards, resulting in an increasing moisture level towards the bottom of the hold. This may cause partial liquefaction or sliding of the cargo.
Guideline: To avoid increased moisture content, care should be taken if loading is carried out during heavy rain. Cargo hatches should be kept closed, except when opened for loading. Both during loading and voyage, the cargo in the holds should be monitored for excess water or other signs of liquefaction risk, such as flattening of the cargo or fluid flow. |
6. Vessel motions and GM
The metacentric height of the vessel should be carefully considered when carrying cargoes that may liquefy. An excessive GM value results in shorter rolling periods and high accelerations which may trigger liquefaction.
Guideline:
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7. Trimming of load to avoid shifting/ sliding
Trimming the cargo is a well-known method for reducing the risk of cargo shift or cargo sliding. In addition, the stability and the weight distribution are improved. On the other hand, trimming increases the required time and cost for loading.
Guideline: When carrying cargoes that may slide, the cargoes should be trimmed as necessary to ensure that they are reasonably level. |
Source: DNV GL