Dry cargo hold systems: The Basics
The carriage of dry cargo can be just as hazardous as for liquids and in some cases the problems are more difficult to deal with. Issues such as shifting cargoes, angles of repose and liquefaction are matters that no specialist equipment can deal with but rely on crew expertise and proper care taken before and during loading and stowage.
Other problems with dry cargo such as spontaneous combustion, ship and cargo sweat, and flooding can however be prevented or monitored using equipment some of which may be mandatory.
In the latter part of the 20th century it became established practice to build bulk carriers and other large ships without a raised fo’cs’le. This design feature was blamed for the flooding and loss of several ships including the *Derbyshire*. As a consequence of the various official enquiries, two initiatives were put in place.
At MSC 76 in 2002, the IMO issued resolution MSC.134(76) which contained a requirement for means of alerting of water ingress in bulk carriers.
Dry cargo - Water ingress alarms
Since 1 July 2004, water level alarms giving audible and visual indication on the navigation bridge must, under SOLAS XII Regulation 12 and SOLAS Chapter II-1 Part B Regulation 23-3, be provided on all bulk carriers of 500gt and above.
The alarms must detect water at 0.5m and 15% dry cargo hold depth (but not more than 2m) at the aft end of each cargo hold; at a level corresponding to 10% capacity of any ballast tank forward of the collision bulkhead and at 0.1m above the deck of any dry or void space (other than a chain locker) which extends entirely or partially forward of the foremost dry cargo hold and exceeds 10% of the ship’s maximum displaced volume.
Monitoring water in the lower portions of the ship is nothing new and has been something done regularly by seafarers since ancient times. However, visual monitoring is not possible in a loaded dry cargo hold and in any case the visual checks would not provide continuous monitoring needed to prevent a tragedy like the Derbyshire. The two level arrangement of the alarms will give an indication of the speed of water ingress before it reaches a critical level allowing for emergency measures to be taken.
The regulation is not prescriptive as to the design of the alarm system and various solutions are in use. Some manufacturers use simple float switches, others may have other methods of detecting water. It is even permissible to install alarm points at the levels prescribed in the regulation in remote tank sounding systems.
The alarm panel must indicate where the ingress has been detected and indicate whether an alarm is a “Pre-alarm” and “Main alarm”. It must also indicate that the alarm system is live and has power connected. The panel must be positioned on the bridge together with the audible alarms associated with it. Repeater panels may be placed in other spaces but are not compulsory. For single hold ships, the requirement for an alarm indicating water level is maintained but obviously the alarm panel will be simpler. As a major alarm, the system should be connected to and its status recorded on the ship’s VDR.
Dry cargo - Controlled atmosphere
All seafarers and officers concerned with cargo matters must be well aware of the hazards of the commodities they are carrying. Much of their knowledge should have come from their training or been acquired over time as they become more experience but there is also regulation in the form of International Maritime Solid Bulk Cargoes (IMSBC) Code, which became mandatory in 2011, under the SOLAS Convention. The Code categorises common cargoes into three groups:
- Group A – cargoes which may liquefy if shipped at a moisture content exceeding their Transportable Moisture Limit (TML)
- Group B – cargoes which possess a chemical hazard which could give rise to a dangerous situation on a ship.
- \Group C – cargoes which are neither liable to liquefy (Group A) nor possess chemical hazards (Group B). Cargoes in this group can still be hazardous.
Some cargoes – coal is a good example – may fall into two categories so extra care need be taken. As already mentioned Group A cargoes do not need any particular systems on board for safe carriage but those coming in Group B and C may need the ship to be equipped with special systems.
This article is not intended to detail the hazards of all bulk cargoes but to explain the systems used to deal with them. Of the chemical hazards fire and explosion and toxic fumes are the main problems. Depending on the cargo, precautions against fire and explosion may include ventilating or inerting the dry cargo holds and the enclosed spaces adjacent to the holds: in some cases the ventilators will need to be explosion proof. The atmosphere in the dry cargo holds and the enclosed spaces adjacent to the holds may also need to be monitored with an appropriate gas detector.
The inerting system for dry cargo ships is basically the same as for tankers but is used less often. Ships which only occasionally carry a cargo that requires inerting may prefer to hire nitrogen generators rather than have a specialist system installed on board.
Ventilation is needed far more often than inerting and for many types of cargoes. Even with relatively hazard free cargoes such as steel and timber, ventilation is aimed at reducing ship and cargo sweat to avoid damage to the cargo. Many ships have only natural ventilation systems with air circulating in the hold and vented to the atmosphere through mushroom ventilators that are open when necessary and closed when the outside air is at the wrong temperature or humidity or in periods of bad weather. The flow of air in natural ventilation systems is caused by temperature differences, by headwinds and by general movement of the ship.
To get more effective ventilation, mechanical or forced systems are fitted as part of the ships equipment. In these a duct passes from the hold to a box containing a powered fan. The fan can run in either direction to push air in or extract it from the hold. If the air coming from the hold contains toxic or explosive gases then care must be taken on deck around the ventilators.
Dry cargo - Dehumidifiers
Humidity in the hold of a ship is usually high when at sea and when carrying cargoes that are easily damaged then a supply of dry air may be an essential. Bringing humidity levels down to appropriate levels will require the use of a dehumidifier which may or may not be equipped with a recording function that registers the humidity level on a constant basis.
Dehumidifiers draw the air from the hold passing it through a desiccant such as silica gel to absorb moisture. Dehumidifiers may also be used after hold washing to speed drying in preparation for the next cargo.