Skuld P&I Club issues pocket guide re bulk mineral cargoes
The carriage of Bulk Mineral Cargoes can present unique challenges to Masters, Crews and Vessels, which if not handled appropriately may lead to catastrophic consequences, including the loss of life and of the vessel. Skuld P&I Club has produced a Pocket Reference Guide which sets out the key information needed to assess the safety of a bulk cargo operation against the risks of liquefaction of the cargo.
Shipowners will be aware of the continuing need for vigilance when carrying such cargoes, as a number of high profile casualties over recent years have shown.
The Pocket Guide produced by Skuld along withwith experts Brookes Bell contains the following topics:
|
Hazards of shipping bulk mineral cargoes
The IMSBC Code draws attention to hazards associatedwith shipments of wet/damp solid bulk mineral cargoes,as follows:
1. Mineral cargoes are often high density. Therefore,their distribution in the holds must be consideredas improper distribution of the cargo may leadto the ship being structurally damaged.
2. They are often loaded at a very fast pace; with theresult that loading may stress the hold plating.
3. The loss of, or reduction in, the stability of the shipduring the voyage, that may be due to:
a. A shift of cargo due to inadequate trimming orimproper distribution of the cargo.
b. A wet cargo liquefying under the stimulus andvibration or motion of the ship, then sliding orflowing to one side of the cargo compartment.
Types of Solid Bulk Cargoes – The Groups
The characteristics of solid bulk cargoes have beendivided into three groups; Group A, Group B and Group C.
- Group A – cargoes which may liquefy if shipped withmoisture content in excess of their transportable moisture limit.
- Group B – cargoes which possess a chemical hazardand could cause a dangerous situation on the ship.
- Group C – cargoes which are neither likely to liquefy(Group A) nor possess chemical hazards (Group B).
- Some mineral cargoes can be both Group A and Group B, i.e., they may liquefy and have a chemical hazard.
Transportable Moisture Limit (TML) – the name saysit all, it is the maximum moisture content, that isconsidered safe for carriage of Group A cargoes. It isgiven as a percentage.
The moisture content (which may be referred to as ‘MC’)of the cargo, and also given as a percentage, must beestablished by the shipper within 7 days of loading.Provided the moisture content is less than the TML, thecargo meets the carriage requirements of the Code.
If shippers report a Flow Moisture Point (FMP), the TMLwill be 90% of this value. It is always worth checkingshipper’s calculations on this matter.
Why do some cargoes liquefy?
The Code lists a number of mineral cargoes and theirproperties. Many mineral cargoes are fine-grained andloaded while wet/damp. They may be wet due to theindustrial processing conducted by the shippers, orstockpiles that have been exposed to rain prior to loading.If the particles of the mineral cargo are in contact witheach other, friction will hold them together.
Large particles, commonly known as ‘lumps’, are bigenough for contact between them to be maintainedregardless of any vibrations and/or ship motion duringthe voyage. This contact allows water to drain. Thereforecargoes consisting of lumps often require the bilges to bepumped during a voyage and are Group C cargoes.
Small particles, commonly known as ‘fines’ or’concentrates’ often do not drain as quickly as lumps, theymay not drain at all. If these cargoes are loaded with amoisture contents above their TML, then they may settle,and become fully saturated. When this happens they mayliquefy.When the particles are surrounded by water and losecontact with each other, the whole cargo can behave like a dense liquid and will flow.
This is what makes them GroupA. Some fine-particled cargoes that drain allow a ‘wet base’to form. This information should be provided on theshippers’ declaration.
Click on image to enlarge
(Image Credit: Skuld P&I Club)
Why is liquefaction dangerous on a ship?
Because most of the mineral cargoes are dense and likelyto liquefy when the moisture content is above their TML,they can ‘walk up the hold plating’ when the vessel rolls,i.e., they flow towards the downside of the roll, but donot necessarily flow back. On each subsequent roll, thecargo might move further to the same side. This is unlikewater and grain (which flow back), and can result in alarge amount of heavy cargo resting against the hold sideplating.
If a liquefied cargo moves during a voyage, the ship mightsuffer a loss of or reduction in stability due to the cargoshifting. There is a risk that the ship will list as a resultof the cargo shifting. If the cargo continues to shift to oneside, the ship will list more heavily to that side and, if theshift is excessive, there will be down-flooding of sea waterinto the ship’s tanks and the ship might capsize and sink.
Group A cargoes should only be carried when themoisture content of the cargo is less than, or equalto, the Transportable Moisture Limit of the cargo. TheTransportable Moisture Limit (TML) can be calculated as90% of the Flow Moisture Point (FMP), depending on thetest method used.
Discharging very wet cargo can be difficult
(Image Credit: Skuld P&I Club)
For more information, read the Skuld P&I ClubPocket guide to Bulk Mineral Cargo Liquefaction